SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 FEATURES D Combines PFC and Downstream Converter D D D D D D D D D Controls Controls Boost Preregulator to Near-Unity Power Factor Accurate Power Limiting Improved Feedforward Line Regulation Peak Current-Mode Control in Second Stage Programmable Oscillator Leading-Edge/Trailing-Edge Modulation for Reduced Output Ripple Low Start-up Supply Current Synchronized Second Stage Start-Up, with Programmable Soft-start Programmable Second Stage Shutdown DESCRIPTION The UCC2850x family provides all of the control functions necessary for an active power-factorcorrected preregulator and a second-stage dc-todc converter. The controller achieves near-unity power factor by shaping the ac input line current waveform to correspond to the ac input-line voltage using average current-mode control. The dc-to-dc converter uses peak current-mode control to perform the step-down power conversion. The PFC stage is leading-edge modulated while the second stage is trailing-edge synchronized to allow for minimum overlap between the boost and PWM switches. This reduces ripple current in the bulk-output capacitor.In order to operate with over three-to-one range of input-line voltages, a line feedforward (VFF) is used to keep input power constant with varying input voltage. Generation of VFF is accomplished using IAC in conjunction with an external single-pole filter. This not only reduces external parts count, but also avoids the use of high-voltage components, offering a lower-cost solution. The multiplier then divides the line current by the square of VFF. The UCC2850x PFC section incorporates a low offset-voltage amplifier with 7.5-V reference, a highly-linear multiplier capable of a wide current range, a high-bandwidth, low offset-current amplifier, with a novel noise-attenuation configuration, PWM comparator and latch, and a high-current output driver. Additional PFC features include over-voltage protection, zero-power detection to turn off the output when VAOUT is below 0.33 V and peak current and power limiting. The dc-to-dc section relies on an error signal generated on the secondary-side and processes it by performing peak current mode control. The dc-to-dc section also features current limiting, a controlled soft-start, preset operating range with selectable options, and 50% maximum duty cycle. The UCC28500 and UCC28502 have a wide PFC-UVLO threshold (16.5 V/10 V) for bootstrap bias supply operation. The UCC28501 and UCC28503 are designed with a narrow UVLO range (10.5 V/10 V) more suitable for fixed bias operation. The UCC28500 and UCC28501 have a narrow UVLO threshold for PWM stage (to allow operation down to 75% of nominal bulk voltage), while the UCC28502 and UCC38503 are configured for a much wider operation range for the PWM stage (down to 50% of bulk nominal voltage). Available in 20-pin N and DW packages. 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'!! &"&#+ www.ti.com 1 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 absolute maximum ratings over operating free-air temperature (unless otherwise noted)† Supply Voltage VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 V Gate Drive Current Continuous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.2 A Pulsed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 A Input Voltage ISENSE1, ISENSE2, MOUT, VSENSE, OVP/ENBL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 V CAI, MOUT, CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 V PKLMT, VERR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 V Input Current RSET, RT, IAC, PKLMT, ENA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 mA VCC (no switching) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 mA Maximum Negative Voltage GT1, GT2, PKLMT, MOUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.5 V Power Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 W † Stresses beyond those listed under “absolute maximum ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under recommended operating conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability. ‡ Currents are positive into, negative out of the specified terminal. Consult Packaging Section of Databook for thermal limitations and considerations of packages. All voltages are referenced to GND. AVAILABLE OPTIONS PFC THRESHOLD TJ –40°C 40°C to 85°C 0°C to 70°C PACKAGED DEVICES PFC UVLO TURN–ON THRESHOLD (V) UVLO2 HYSTERESIS (V) PLASTIC DIP (N) SMALL OUTLINE (DW) 16 1.2 UCC28500N UCC28500DW 10.5 1.2 UCC28501N UCC28501DW 16 3.0 UCC28502N UCC28502DW 10.5 3.0 UCC28503N UCC28503DW 16 1.2 UCC38500N UCC38500DW 10.5 1.2 UCC38501N UCC38501DW 16 3.0 UCC38502N UCC38502DW 10.5 3.0 UCC38503N UCC38503DW The DW package is available taped and reeled. Add TR suffix to device type (e.g. UCC38500DWTR) to order quantities of 2000 devices per reel. N PACKAGE (TOP VIEW) VAOUT RT VSENSE OVP/ENBL CT GND VERR ISENSE2 VCC GT2 2 1 20 2 19 3 18 4 17 5 16 6 15 7 14 8 13 9 12 10 11 DW PACKAGE (TOP VIEW) VREF VFF IAC MOUT ISENSE1 CAOUT PKLMT SS2 GT1 PWRGND VAOUT RT VSENSE OVP/ENBL CT GND VERR ISENSE2 VCC GT2 www.ti.com 1 2 3 4 5 6 7 8 9 10 20 19 18 17 16 15 14 13 12 11 VREF VFF IAC MOUT ISENSE1 CAOUT PKLMT SS2 GT1 PWRGND SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 electrical characteristics TA = 0°C to 70°C for the UCC3850X, –40°C to 85°C for the UCC2850X, TA = TJ, VCC = 12 V, RT = 22 kΩ, CT = 330 pF (unless otherwise noted) supply current PARAMETER TEST CONDITIONS Supply current, off VCC turn-on threshold –300 mV Supply current, on VCC = 12 V (no load on GT1 or GT2) MIN TYP MAX UNITS 150 300 µA 4 6 mA undervoltage lockout PARAMETER TEST CONDITIONS VCC turn-on threshold (UCCx8500/502) MIN 15.4 UVLO hysteresis (UCCx8500/502) TYP MAX 16 UNITS 16.6 V 5.8 6.3 15.4 16.2 17.0 V VCC turn-on threshold (UCCx8501/503) 9.7 10.2 10.8 V VCC turn-off threshold 9.4 9.7 V UVLO hysteresis (UCCx8501/503) 0.3 0.5 V Shunt voltage (UCCx8500/502) IVCC = 10 mA V voltage amplifier PARAMETER TEST CONDITIONS 0°C ≤ TA ≤ 70°C Inp t voltage Input oltage –40°C ≤ TA ≤ 85°C VSENSE bias current Open loop gain High-level output voltage Low-level output voltage MIN TYP MAX UNITS 7.387 7.500 7.613 7.35 7.50 7.65 V 50 200 nA V VAOUT = 2 V to 5 V 50 90 ILOAD = –150 µA ILOAD = 150 µA 5.3 5.5 5.6 dB V 0.00 0.05 0.15 V overvoltage protection and enable PARAMETER TEST CONDITIONS MIN TYP MAX UNITS VREF + 0.480 VREF + 0.500 VREF + 0.520 V Hysteresis 300 500 600 mV Enable threshold 1.7 1.9 2.1 V Enable hysteresis 0.1 0.2 0.3 V Over voltage reference current amplifier PARAMETER Input offset voltage Input bias current Input offset current Open loop gain Common–mode rejection ratio High-level output voltage Low-level output voltage Gain bandwidth product TEST CONDITIONS VCM = 0 V, VCM = 0 V, VCAOUT = 3 V VCAOUT = 3 V VCM = 0 V, VCM = 0 V, VCAOUT = 3 V VCAOUT = 2 V to 5 V VCM = 0 V to 1.5 V, VCAOUT = 3 V ILOAD = –120 µA ILOAD = 1 mA See Note 1 MIN –6 TYP MAX UNITS 0 6 mV –50 –100 nA 25 100 nA 90 dB 90 dB 5.6 7.0 7.5 0.1 0.2 0.5 2.5 V V MHz NOTES: 1. Ensured by design. Not production tested. 2. See Figure 6 for reference variation. 3. See Figure 5 for reference variation for VCC < 10.8 V. www.ti.com 3 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 electrical characteristics TA = 0°C to 70°C for the UCC3850X, –40°C to 85°C for the UCC2850X, TA = TJ, VCC = 12 V, RT = 22 kΩ, CT = 330 pF (unless otherwise noted) voltage reference PARAMETER TEST CONDITIONS TA = 0°C to 70°C TA = –40°C to 85°C Inp t voltage Input oltage Load regulation Line regulation IREF = –1 mA to –2 mA, VCC = 10.8 V to 15 V, Short circuit current VREF = 0V MIN TYP MAX UNITS 7.387 7.500 7.613 V 7.35 7.50 7.65 V See Note 2 0 10 mV See Note 3 0 10 mV –50 mA –20 –25 oscillator PARAMETER Frequency, initial accuracy TEST CONDITIONS Frequency, voltage stability TA = 25°C VCC = 10.8 V to 15 V Frequency, total variation Line, Temp MIN TYP 85 MAX 100 115 UNITS kHz –1% 1% 80 120 kHz Ramp peak voltage 4.5 5 5.5 V Ramp amplitude voltage (peak to peak) 3.5 4 4.5 V peak current limit PARAMETER TEST CONDITIONS MIN TYP MAX UNITS PKLMT reference voltage –15 0 15 mV PKLMT propagation delay 150 300 500 ns multiplier PARAMETER TEST CONDITIONS MIN TYP MAX UNITS IMOUT, high-line low-power output current IAC = 500 µA, VFF = 4.7 V, 0°C ≤ TA ≤ 85°C VAOUT = 1.25 V, IMOUT, high-line low-power output current IAC = 500 µA, VFF = 4.7 V, –40°C ≤ TA ≤ 85°C VAOUT = 1.25 V, IMOUT, high-line high-power output current IMOUT, low-line low-power output current IAC = 500 µA, VFF = 4.7 V, IAC = 150 µA, VFF = 1.4 V, VAOUT = 5 V –10 –19 –50 IMOUT, low-line high-power output current IMOUT, IAC-limited output current IAC = 150 µA, VFF = 1.4 V, IAC = 150 µA, VFF = 1.3 V, VAOUT = 5 V –268 –300 –345 VAOUT = 5 V –250 –300 –400 Gain constant (K) IAC = 300 µA, VFF = 2.8 V, IAC = 150 µA, VFF = 1.4 V, VAOUT = 2.5 V 0.5 1 1.5 VAOUT = 0.25 V 0 –2 IAC = 500 µA, VFF = 4.7 V, IAC = 500 µA, VFF = 4.7 V, 0°C ≤ TA ≤ 85°C VAOUT = 0.25 V 0 –2 µA 0 –3 µA IAC = 500 µA, VFF = 4.7 V, –40°C ≤ TA ≤ 85°C VAOUT = 0.5 V, 0 –3.5 µA IAC = 150 µA, VFF = 1.4 V, VAOUT = 5 V –420 –485 µW IMOUT, zero current Power limit (IMOUT × VFF) VAOUT = 1.25 V 0 –6 –20 0 –6 –23 –70 –90 –105 VAOUT = 0.5 V, –375 µA 1/V zero power PARAMETER Zero power comparator threshold TEST CONDITIONS Measured on VAOUT NOTES: 1. Ensured by design. Not production tested. 2. See Figure 6 for reference variation. 3. See Figure 5 for reference variation for VCC < 10.8 V . 4 www.ti.com MIN 0.175 TYP 0.330 MAX 0.500 UNITS V SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 electrical characteristics TA = 0°C to 70°C for the UCC3850X, –40°C to 85°C for the UCC2850X, TA = TJ, VCC = 12 V, RT = 22 kΩ, CT = 330 pF (unless otherwise noted) PFC gate driver PARAMETER GT1 pull up resistance GT1 pull down resistance GT1 output rise time GT1 output fall time TEST CONDITIONS TYP CLOAD = 1 nF, VGT1 from 0.7 V to 9.0 V CLOAD = 1 nF, VGT1 from 9.0 V to 0.7 V MAX UNITS 5 12 Ω 2 10 Ω RLOAD = 10 Ω 25 50 ns RLOAD = 10 Ω 10 50 ns 95% 100% IOUT from –100 mA to –200 mA IOUT = 100 mA Maximum duty cycle Minimum controlled duty cycle MIN 93% f = 100 kHZ 2% second stage undervoltage lockout (UVLO2) PARAMETER TEST CONDITIONS MIN TYP MAX UNITS PWM turn-on reference (UCCx8500/501) 6.30 6.75 7.30 V Hysteresis (UCCx8500/501) 0.96 1.20 1.44 V PWM turn–on reference (UCCx8502/503) 6.30 6.75 7.30 V 2.4 3 3.6 V Hysteresis (UCCx8502/503) second stage soft-start PARAMETER TEST CONDITIONS SS2 charge current Input voltage (VERR) SS2 discharge current MIN TYP –7.3 IVERR = 2 mA,UVLO = Low ENBL = High, UVLO = Low, SS2 = 2.5 V MAX –10 3 UNITS –12.5 µA 300 mV 10 mA second stage duty cycle clamp PARAMETER TEST CONDITIONS Maximum duty cycle MIN TYP MAX 44% UNITS 50% second stage pulse-by-pulse current sense PARAMETER Current sense comparator threshold TEST CONDITIONS VERR = 2.5 V measured on ISENSE2 MIN 0.94 TYP MAX 1.05 1.15 UNITS V second stage overcurrent limit PARAMETER TEST CONDITIONS Peak current comparator threshold MIN 1.15 Input bias current TYP MAX 1.30 1.45 50 UNITS V nA second stage gate driver PARAMETER GT2 pull up resistance GT2 pull down resistance TEST CONDITIONS IOUT from –100 mA to –200 mA IOUT = 100 mA CLOAD = 1 nF,RLOAD = 10 Ω VGT2 from 0.7 V to 9.0 V GT2 output fall time CLOAD = 1 nF,RLOAD = 10 Ω VGT2 from 9.0 V to 0.7 V NOTES: 1. Ensured by design. Not production tested. 2. See Figure 6 for reference variation. 3. See Figure 5 for reference variation for VCC < 10.8 V . GT2 output rise time www.ti.com MIN TYP MAX UNITS Ω 5 12 3 10 Ω 25 50 ns 25 50 ns 5 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 pin assignments CAOUT: (current amplifier output) This is the output of a wide bandwidth operational amplifier that senses line current and commands the PFC pulse width modulator (PWM) to force the correct duty cycle. This output can swing close to GND, allowing the PWM to force zero duty cycle when necessary. CT: (oscillator timing capacitor) A capacitor from CT to GND sets the oscillator frequency according to: f+ 0.725 ǒRT C T Ǔ GND: (ground) All voltages measured with respect to ground. VCC and VREF should be bypassed directly to GND with a 0.1-µF or larger ceramic capacitor. The timing capacitor discharge current also returns to this pin, so the lead from the oscillator timing capacitor to GND should be as short and direct as possible. GT1: (gate drive) The output drive for the PFC stage is a totem pole MOSFET gate driver on GT1. Use a series gate resistor of at least 10.5 Ω to prevent interaction between the gate impedance and the GT1 output driver that might cause the GT1 to overshoot excessively. Some overshoot of the GT1 output is always expected when driving a capacitive load. Refer to Figure 4 for gate drive resistor selections. GT2: (gate drive) Same as output GT1 for the second stage output drive. Limited to 50% maximum duty cycle. IAC: (input ac current) This input to the analog multiplier is a current. The multiplier is tailored for very low distortion from this current input (IAC) to MOUT, so this is the only multiplier input which should be used for sensing instantaneous line voltage. Recommended maximum IAC is 500 µA. ISENSE1: (current sense) This is the non-inverting input to the current amplifier. This input and the inverting input MOUT remain functional down to and below GND. ISENSE2: (current sense) A resistor from the source of the lower FET to ground generates the input signal for the peak limit control of the second stage. The oscillator ramp can also be summed into this pin, for slope compensation. MOUT: (multiplier output and current sense amplifier inverting input) The output of the analog multiplier and the inverting input of the current amplifier are connected together at MOUT. As the multiplier output is a current, this is a high impedance input so the amplifier can be configured as a differential amplifier to reject ground noise. Multiplier output current is given by: I MOUT + ǒVVAOUT * 1.0Ǔ K ǒVVFFǓ I IAC 2 Connect current loop compensation components between MOUT and CAOUT. OVP/ENBL: (over-voltage/enable) A window comparator input which disables the PFC output driver if the boost output is 6.67% above nominal or disables both the PFC and second stage output drivers and reset SS2 if pulled below 1.9 V. This input is also used to determine the active range of the second stage PWM. PKLMT: (PFC peak current limit) The threshold for peak limit is 0 V. Use a resistor divider from the negative side of the current sense resistor to VREF to level-shift this signal to a voltage corresponding to the desired overcurrent threshold across the current sense resistor. PWRGND: Ground for totem pole output drivers. RT: (oscillator charging current) A resistor from RT to GND is used to program oscillator charging current. A resistor between 10 kΩ and 100 kΩ is recommended. Nominal voltage on this pin is 3 V. 6 www.ti.com SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 pin assignments (continued) SS2: (soft-start for PWM) SS2 is at ground for either enable low or OVP/ENBL below the UVLO2 threshold conditions. When enabled, SS2 charges an external capacitor with a current source. This voltage is used as the voltage error signal during start-up, enabling the PWM duty cycle to increase slowly. In the event of a disable command or a UVLO2 dropout, SS2 quickly discharges to disable the PWM. VAOUT: (voltage amplifier output) This is the output of the operational amplifier that regulates output voltage. The voltage amplifier output is internally limited to approximately 5.5 V to prevent overshoot. VCC: (positive supply voltage) Connect to a stable source of at least 20 mA between 12 V and 17 V for normal operation. Bypass VCC directly to GND to absorb supply current spikes required to charge external MOSFET gate capacitances. To prevent inadequate gate drive signals, the output devices are inhibited unless VCC exceeds the upper under-voltage lockout threshold and remains above the lower threshold. VERR: (voltage amp error signal for the second stage) The error signal is generated by an external amplifier which drives this pin. This pin has an internal 4.5-V voltage clamp that limits GT2 to less than 50% duty cycle to ensure transformer reset in the typical application. VFF: (RMS feed forward signal) VFF signal is generated at this pin by mirroring one-half of IAC into a single pole external filter. At low line, the VFF voltage should be 1.4 V. VSENSE: (voltage amplifier inverting input) This is normally connected to a compensation network and to the boost converter output through a divider network. VREF: (voltage reference output) VREF is the output of an accurate 7.5-V voltage reference. This output is capable of delivering 10 mA to peripheral circuitry and is internally short-circuit current limited. VREF is disabled and remains at 0 V when VCC is below the UVLO threshold. Bypass VREF to GND with a 0.1-µF or larger ceramic capacitor for best stability. www.ti.com 7 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 block diagram VERR ISENSE2 7 8 SS2 VCC GND 9 6 13 7.5 V REFERENCE SECOND STAGE SOFT START OVP/ENBL + 4 1.9 V ILIMIT 4.5 V 1.5 V R R + 1.3 V PWM + 8.0 V + 3 0.33 V ÷ MULT OSC CLK1 X + Q PWM LATCH R R CLK2 (VFF )2 CLK2 12 GT1 11 PWRGND 14 PKLMT CLK1 OSCILLATOR ILIMIT + 18 17 16 MOUT ISENSE1 8 PWM 2ND STAGE SECTION S + MIRROR 2:1 IAC S PFC SECTION CURRENT AMP 7.5 V 19 GT2 Q VCC PWM + X + VFF 10 VCC PFCOVP ZERO POWER 1 VOLTAGE ERROR AMP VSENSE VREF CLK2 PWM 2ND STAGE SECTION PFC SECTION UVLO 16 V/10 V 10.5 V/10 V ENABLE + VAOUT 20 UVLO2 6.75 V 15 2 5 CAOUT RT CT www.ti.com UDG–98189 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL CHARACTERISTICS MULTIPLIER OUTPUT CURRENT vs. VOLTAGE ERROR AMPLIFIER OUTPUT MULTIPLIER GAIN vs. VOLTAGE ERROR AMPLIFIER OUTPUT 1.5 300 1.3 IAC = 150 µ A IAC = 150 µ A 250 Multiplier Gain – K IMOUT - Multiplier Output Current – µA 350 200 IAC = 300 µ A 150 100 1.1 0.9 IAC = 300 µ A IAC = 500 µ A 0.7 50 IAC = 500 µ A 0.5 0 0 1 2 3 4 1 5 2 Figure 1 (VFF × IMOUT) – µW 400 VAOUT = 5 V 300 VAOUT = 4 V 200 VAOUT = 3 V 100 VAOUT = 2 V 0 3 4 5 RGATE - Recommended Minimum Gate Resistance – Ω 500 2 5 Figure 2 MULTIPLIER CONSTANT POWER PERFORMANCE 1 4 VAOUT – Voltage Error Amplifier Output – V VAOUT – Voltage Error Amplifier Output – V 0 3 RECOMMENDED MINIMUM GATE RESISTANCE vs. SUPPLY VOLTAGE 17 16 15 14 13 12 11 10 9 8 10 12 14 16 18 20 VCC – Supply Voltage – V VFF – Feedforward Voltage – V Figure 3 Figure 4 www.ti.com 9 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 REFERENCE VOLTAGE vs. REFERENCE CURRENT REFERENCE VOLTAGE vs. SUPPLY VOLTAGE 7.510 VREF – Reference Voltage – V VREF – Reference Voltage – V 7.60 7.55 7.50 7.45 7.505 7.500 7.495 7.490 7.40 9 10 11 12 13 14 5 10 15 20 IVREF – Reference Current – mA VCC – Supply Voltage – V Figure 5 10 0 Figure 6 www.ti.com 25 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION The UCC38500 series is designed to incorporate all the control functions required for a power factor correction circuit and a second stage dc-to-dc converter. The PFC function is implemented as a full-feature, average-current-mode controller integrated circuit. In addition, the input voltage feedforward function is implemented in a simplified manner. Current from IAC input is mirrored over to the VFF pin. By simply adding a resistor and capacitor (to attenuate 120-Hz ripple) a voltage is developed which is proportional to RMS line voltage, eliminating the need for several components normally connected to the line. The UCC3850x uses leading-edge modulation for the PFC stage and trailing-edge modulation for the dc-to-dc stage. This reduces ripple current in the output capacitor by reducing the overlap in conduction time of the PFC and dc-to-dc switches. Figures 7 and 8 depict the ripple current reduction in the boost switch. In addition to the reduced ripple current, noise immunity is improved through the current error amplifier implementation. Please refer to the UCC3817 datasheet (TI Literature No. SLUS395) for a detailed explanation of current error amplifier implementation. UDG–97130–1 Figure 7. Simplified Representation of a 2–Stage PFC Power Supply iCBST iCBST = iD1 – iQ2 Figure 8. Timing Waveforms for Synchronization Scheme www.ti.com 11 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION The UCC3850x is optimized to control a boost PFC stage operating in continuous conduction mode, followed by a dc-to-dc converter (typically a forward topology). The dc-to-dc converter is transformer isolated and therefore its error amplifier is located on the secondary side. For this reason the UCC3850x is configured without an internal error amplifier for the second power stage. The externally generated error signal is fed into the VERR pin typically through an opto coupler. The UCC3850x can be configured for voltage-mode control or current-mode control of the second stage. The application figure shows a typical current-mode configuration. For voltage-mode control, the ramp generated by CT can be fed back into the ISENSE2 pin through a voltage divider. One of the main system challenges in designing systems with a PFC front end is coordinating the turn-on and turn-off on the dc-to-dc converter. If the dc-to-dc converter is allowed to turn on before the boost converter is operational, it must operate at a much-reduced voltage and therefore represents a large current draw to the boost converter. This start-up sequencing is handled internally by the UCC3850x. The UCC3850x monitors the output voltage of the PFC converter and holds the dc-to-dc converter off until the output is within 10% of its regulation point. Once the trip point is reached the dc-to-dc section goes through a soft start sequence for a controlled, low stress start-up. Similarly, if the output voltage drops too low (two voltage options are available) the dc-to-dc converter shuts down thereby preventing overstress of the converter. For the UCC38500 and UCC38501, the dc-to-dc converter shuts down when the PFC output falls below 74% of its nominal value, while for the UCC38502 and UCC38503, the threshold is lowered to 50%. design example: an off-line, 100-W, power converter The following design example shows how to implement the UCC38500 in an off-line 100-W power converter. The system requires the converter to operate from a universal input of 85 VRMS to 265 VRMS with a 12-V, 100-W, dc output. This design example is divided into two parts. The first part is the PFC stage design and the second section is the dc-to-dc power stage design. The design goal of the system is to achieve an efficiency of approximately 80%. This is accomplished by requiring the boost regulator to be designed for an efficiency of 95% and the dc-to-dc power stage to be designed for 85% efficiency. The efficiency of the boost converter is designated by variable η1 and the efficiency of the dc-to-dc converter is designated by variable η2. Figure 9 shows the schematic of the typical application upon which this design example is based. The UCC38500 control device is chosen for this design because of it’s self-biasing scheme and minimum input voltage requirements of the dc-to-dc power stage. 12 www.ti.com C5 D1 VCC BIAS CIRCUIT VAC 85–265V RMS C4 T2 Q1 R1 C3 GT2 D2 L1 PGND R2 L2 PWR GND D3 Vout + D8 T1 + D4 AC–L C29 R22 R23 – Q3 D11 C26 C2 GT1 R33 12V 10A C30 R34 GT2 Vout – Q2 R12 PGND2 R11 ISENSE2 R20 R5 D6 R21 www.ti.com C7 PGND R14 R19 PKLIMIT 4 C23 PGND OVP/ENBL GT1 C25 R15 3 VSENSE GT2 10 1 VAOUT VCC 9 16 ISENSE CT 5 R28 R29 R7 PGND GT1 12 R6 VCC GT2 D10 C21 VREF 17 MOUT VCCBIAS CIRCUIT R24 D12 15 CAOUT C22 RT 2 GND 6 R17 PGND PGND SGND PGND 18 IAC PWRGND 11 19 VFF SS2 13 D9 C18 R25 VREF C19 PKLIMIT R39 C28 C24 R13 C16 14 PKLIMIT R30 VERR 7 ISENSE2 8 D16 20 VREF Q5 C17 D14 R36 Q5 ISENSE2 SGND VREF D15 C27 D5 SGND R31 PGND2 R16 5 R38 VCC C8 C15 2 C14 SGND 4 C6 D7 H11AV1 1 C13 R10 C12 L1 6 R35 3 U3 C20 R27 D13 SGND PGND2 UDG–99138 13 R3 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 Figure 9. Typical Application Circuit AC–N R18 R4 UCC38500 U1 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION I. PFC Boost Power Stage LBOOST (L1 in Figure 9) The boost inductor value is determined by the following equations: P DI + ǒ0.25Ǔ OUT h1 h2 D+1* Ǹ2 , V IN (min) (1) Ǹ2 V IN (min) L BOOST + V BOOST , (2) V IN (min) Ǹ2 DI fS D (3) where ∆I, the inductor current ripple was set to approximately 25% of the peak inductor current. In this design example ∆I is approximately 505 mA. D represents the duty cycle at the peak of low line voltage, VIN(min) is the minimum RMS input voltage, and VBOOST is the controlled output voltage of the PFC stage. VBOOST for this design is selected to be 385 V to ensure the PFC stage regulates for the full input voltage range. Variable fS represent the switching frequency. The switching frequency was selected to be 100 kHz for this design. The calculated boost inductor required for this design is approximately 1.7 mH. CBOOST (C2 in Figure 9) Two main criteria, the capacitance and the voltage rating, dictate the selection of the output capacitor. The value of capacitance is determined by the holdup time required for supporting the load after the input ac voltage is removed. Holdup is the amount of time that the output stays in regulation after the input has been removed. For this circuit, the desired holdup time is approximately 16 ms. Expressing the capacitor value in terms of output power, output voltage, and holdup time is described in equation (4): C BOOST + 2 P OUT Dt ǒVBOOSTǓ 2 * ǒVBOOST (min)Ǔ 2 (4) In practice, the calculated minimum capacitor value may be inadequate because output ripple voltage specifications limit the amount of allowable output capacitor ESR. Attaining a sufficiently low value of ESR often necessitates the use of a much larger capacitor value than calculated. The amount of output capacitor ESR allowed is determined by dividing the maximum specified output ripple voltage by the capacitor ripple current. In this design, holdup time is the dominant determining factor and a 100 µF, 450 V aluminum electrolytic capacitor from Panasonic, part number ECOS2TB101BA, is used. The voltage rating and the low ESR of 0.663 Ω make it an ideal choice for this design. 14 www.ti.com SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION power switch selection (Q3 in Figure 9) As in any power supply design, tradeoffs between performance, cost and size are necessary. When selecting a power switch, it is useful to calculate the total power dissipation in the switch for several different devices at the switching frequencies being considered for the converter. Total power dissipation in the switch is the sum of switching loss and conduction loss. Switching losses are the combination of the gate charge loss, drain source capacitance of the MOSFET loss and turnon and turnoff losses: P GATE + Q GATE V GATE P COSS + 1 C OSS ǒV OFFǓ 2 P SW + 1 V OFF 2 2 fS (5) fS (6) ǒt ON ) tOFFǓ IL fS (7) Where QGATE is the total gate charge, VGATE is the gate drive voltage, fs is the switching frequency, COSS is the drain source capacitance of the MOSFET, tON and tOFF are the switching times (estimated using device parameters RGATE, QGD and VTH) and VOFF is the voltage across the switch during the off time, in this case VOFF = VBOOST. Conduction loss is calculated as the product of the RDS(on) of the switch (at the worst case junction temperature) and the square of RMS current: P COND + R DS(on) K ǒI RMSǓ 2 (8) where K is the temperature factor found in the manufacturer’s RDS(on) vs junction temperature curves. Calculating these losses and plotting against frequency gives a curve that enables the designer to determine either which manufacturer’s device has the best performance at the desired switching frequency, or which switching frequency has the least total loss for a particular power switch. For this design example an IRFP450 HEXFET from International Rectifier is chosen because of its low RDS(on) and its VDSS rating. The IRFP450’s RDS(on) of 400 mΩ and the maximum VDSS of 500 V makes it an ideal choice. A comprehensive review of this procedure can be found in the Unitrode Power Supply Design Seminar SEM–1200, Topic 6, TI Literature No. SLUP117. More recently, faster switching insulated gate bipolar transistors (IGBTs) have become widely available. Depending on the system power level (and the switching frequency), use of IGBTs may make sense for the power switch. boost diode selection (D3 in Figure 9) In order to keep the switching losses to a minimum and meet the voltage and current requirements, a HFA08TB60 fast recovery diode from International Rectifier is selected for the design. This diode is rated for a maximum reverse voltage of 600 V and a maximum forward current of 8 A. The typical reverse recovery of 18 ns made this diode ideal for this design. www.ti.com 15 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION peak current limit Resistor divider R14 and R29 along with current sense resistor R5, devise the peak-limit comparator of the UCC38500 and are used to protect the boost switch Q3 from excessive currents. Proper preparation of this comparator requires that it not interfere with the boost converter’s power limit or the forward converter’s pulse-by-pulse current limiting. For this design example the forward converter is selected to go into pulse-by-pulse current limiting at approximately 130% of maximum output power. The power limit of the boost converter is set at 140% of the maximum output power. The peak current limit for the boost stage was selected to engage at 150% of the maximum output power to ensure circuit stability. The following equation is used to select the current-sense resistor R5, where the current-sense resistor is selected to operate over a 1-V dynamic range (VDYNAMIC). The current-sense resistor required for the design needed to be approximately 0.43 Ω. R5 + R SENSE + V DYNAMIC ^ 0.43 W I PK ) (0.5) DI (9) The following equation is used to size resistor R14 properly by first selecting R29 to be a standard resistance value. For this design resistor R29 was selected to be 10 kΩ. With a typical reference voltage (VREF) of 7.5 V gives a calculated value of approximately 1.91 kΩ for resistor R14. R14 + ǒ Ǔ P OUT 1.5 Ǹ2 ) DI V IN (min) h1 h2 R5 R29 V REF (10) multiplier The output of the multiplier of the UCC38500 is a signal representing the desired input line current. It is an input to the current amplifier, which programs the current loop to control the input current to give high power-factor operation. As such, the proper functioning of the multiplier is key to the success of the design. The inputs to the multiplier are VVAOUT, the voltage amplifier output, IIAC, a representation of the input rectified ac line voltage, and an input voltage feed forward signal, VVFF. The output of the multiplier, IMOUT, can be expressed: I MOUT + ǒVVAOUT * 1Ǔ I IAC K ǒVVFFǓ 2 (11) Where K is a constant typically equal to 1 / V. The IIAC signal is obtained through a high-value resistor connected between the rectified ac line and the IAC pin of the UCC3850X. This resistor (RIAC) is sized to provide the maximum IIAC current at high line. For the UCC3850X the maximum IIAC current is about 500 µA, and a higher current can drive the multiplier out of its linear range. A smaller current level is functional, but noise can become an issue, especially at low input line. Assuming a universal line operation of 85 VRMS to 265 VRMS gives a RIAC value of 750 kΩ. Because of voltage rating constraints of the standard 1/4-W resistor, this application requires a combination of lower value resistors connected in series to give the required resistance and distribute the high voltage amongst the resistors. For this design example two 383 kΩ resistors are used in series. 16 www.ti.com SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION The current into the IAC pin is mirrored internally to the VFF pin where it is filtered to produce a voltage feed forward signal proportional to line voltage. The VFF voltage is used to keep the power stage gain constant and to providing input power limiting. Please refer to Texas instruments Application Note on Power Limiting with Sinusoidal Input TI Literature No. SLUA196, for detailed explanation on how the VFF pin provides power limiting. The following equation is used to determine the VFF resistor size (RVFF) to provide power limiting where VIN(min) is the minimum RMS input voltage and RIAC is the total resistance connected between the IAC pin and the rectified line voltage. R VFF + ǒ 1.4 V Ǔ V IN (min) 0.9 2 R IAC ^ 28.7 kW (12) Because the VFF voltage is generated from line voltage it needs to be adequately filtered to reduce total harmonic distortion caused by the 120-Hz rectified line voltage. Refer to Unitrode Power Supply Design Seminar, SEM–700 Topic 7, Optimizing a High Power Factor Switching Preregulator, TI Literature No. SLUP093. A single pole filter is adequate for this design. Assuming that an allocation of 1.5% total harmonic distortion from this input is allowed, and that the second harmonic ripple is 66% of the input ac line voltage, the amount of attenuation required by this filter is: 1.5% + 0.022 66% (13) With a ripple frequency (fR) of 120-Hz and an attenuation of 0.022 requires that the pole of the filter (fP) be placed at: f P + 120 Hz 0.022 ^ 2.6 Hz (14) The following equation is used to select the filter capacitor (CVFF) required to produce the desired low pass filter. C VFF + 2p 1 R VFF fP ^ 2.2 mF (15) This results in a single-pole filter, which adequately attenuates the harmonic distortion and provides power limiting. The RMOUT resistor is sized to provide power limiting for the circuit. The power limit is set to 140% of the maximum output power. This is done so that the power limit of the PFC stage does not interfere with power limiting of the dc-to-dc converter, which is set to 130% of the maximum output power. The following equations are used to size the RMOUT resistor, R19. In these equations PLIMIT is the power limit level, POUT is the maximum output power. IMOUT(max) is the maximum multiplier output current, IIAC@VIN(min) is the minimum current into the IAC pin at low line and VVAOUT(max) is the maximum voltage amplifier output voltage. For this design R19 and R15 need to be approximately 3.57 kΩ. P LIMIT + P OUT 1.4 h1 h2 I MOUT(max) + (16) I IAC @ V IN(min) K ǒVVAOUT(max) * 1 VǓ ǒVFFǓ 2 (17) www.ti.com 17 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION R MOUT + P LIMIT Ǹ2 R SENSE V IN (min) I MOUT(max) (18) current loop The UCC38500 current amplifier has the input from the multiplier applied to the inverting input. This change in architecture from previous Texas Instruments PFC controllers improves noise immunity in the current amplifier. It also adds a phase inversion into the control loop. The UCC38500 takes advantage of this phase inversion to implement leading-edge duty cycle modulation. Please refer to Figure 10 for the typical configuration of the current amplifier. The following equation defines the gain of the power stage, where VP is the voltage swing of the oscillator ramp, 4 V for the UCC38500. G ID(s) + V BOOST R SENSE s L BOOST V P (19) In order to have a good dynamic response the crossover frequency of the current loop was set to 10% of the switching frequency. This can be achieved by setting the gain of the current amplifier (GCA) to the inverse of the current loop power stage gain at the crossover frequency. This design requires that the current amplifier have a gain of 2.581 at 10 kHz. G CA + 1 + 2.581 G ID(s) (20) RI is the RMOUT resistor, previously calculated to be 3.57 kΩ (refer to Figure 10). The gain of the current amplifier is RF/RI, so multiplying RI by GEA gives the value of RF, in this case approximately 9.09 kΩ. Setting a zero at the crossover frequency and a pole at half the switching frequency to roll off the high-frequency gain completes the current loop compensation. CZ + 2p 1 CP + 2p 18 1 RF RF fC (21) ǒf2 Ǔ s (22) www.ti.com SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION C P C Rf Z RI – CAOUT + Figure 10. Current Loop Compensation voltage loop The second major source of harmonic distortion is the ripple on the output capacitor at the second harmonic of the line frequency. This ripple is fed back through the error amplifier and appears as a 3rd harmonic ripple at the input to the multiplier. The voltage loop must be compensated not just for stability but also to attenuate the contribution of this ripple to the total harmonic distortion of the system (refer to Figure 11). Cf VOUT CZ Rf R IN – + RD VREF Figure 11. Voltage Amplifier Configuration The gain of the voltage amplifier, GVA, can be determined by first calculating the amount of peak ripple present on the output capacitor VOPK. The peak value of the second harmonic voltage is given by equation (23), where fR is the frequency of the rectified line voltage. For this design fR is equal to 120 Hz. V OPK + ǒ2 p P IN fR C BOOST V BOOSTǓ www.ti.com (23) 19 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION In this example VOPK is equal to 4 V. Assuming an allowable contribution of 0.75% (1.5% peak-to-peak) from the voltage loop to the total harmonic distortion budget sets the gain equal to: G VA + ǒDVVAOUTǓ (0.015) 2 V OPK (24) Where ∆VVAOUT is the effective output voltage range of the error amplifier (5 V for the UCC38500). The network needed to realize this filter is comprised of an input resistor, RIN, and feedback components CF, CZ, and RF. The value of RIN is already determined because of its function as one-half of a resistor divider from VOUT feeding back to the voltage amplifier for output voltage regulation. In this case the value is 1.12 MΩ. This high value was chosen to reduce power dissipation in the resistor. In practice, the resistor value would be realized by the use of two 560-kΩ resistors in series because of the voltage rating constraints of most standard 1/4 W resistors. The value of CF is determined by the equation: C + F ǒ2 p 1 f R G VA R Ǔ IN (25) In this example CF equals 150 nF. Resistor RF and CF generate a pole in the voltage amplifier feedback to reduce total harmonic distortion (THD). The location of the pole is found by setting the gain of the loop equation to one and solving for the crossover frequency. The frequency, expressed in terms of input power, is calculated by the equation: f VI + ǸPIN 2p ǸDV VAOUT V OUT R IN C BOOST CF (26) fVI for this converter is 10 Hz. A derivation of this equation can be found in the Unitrode Power Supply Design Seminar SEM–1000, Topic 1, Power Factor Correction Circuit, TI Literature No. SLUP106. Solving for RF becomes: R + F ǒ2 p 1 f VI C F Ǔ (27) Or RF equals approximately 118 kΩ. Due to the low output impedance of the voltage amplifier, capacitor CZ is added to improve dc regulation. To maintain good phase margin, the zero from CZ is set to 10% of fVI. For this design, CZ is a 2.2-µF capacitor. The following equation is used to calculate CZ. 1 CZ + 2p 20 ǒ Ǔ f VI 10 R F (28) www.ti.com SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION II. Two Switch Forward DC–to–DC Power Stage A two-switch forward converter topology was selected for the second stage of this design. The two-switch forward power converter has two major advantages over a traditional forward converter, making it ideal for this application. First, the FETs used in the two-switch forward required only one-half the maximum VDS as compared to the traditional forward converter. Second, the transformer’s reset energy is returned to the input through clamping diodes for higher efficiency. transformer turns ratio Equation (29) calculates the transformer turns ratio required for the two-switch forward power converter of this design example. It can be derived from the dc transfer function of a forward converter. VOUT is the output voltage of the forward converter and is 12-V for this design. VF is the forward voltage drop of the secondary rectifier diode and is set to 1V. VBOOST(min) is the minimum input voltage to the forward converter. The level of this voltage is determined by where the control device forces the dc-to-dc converter into undervoltage lockout (UVLO). The UCC38500 control device is configured to drive the dc-to-dc power stage into UVLO at approximately 74% of the nominal boost converters output voltage. VBOOST(min) for this design is approximately 285 V. DMAX is 0.44 and is the guaranteed maximum duty cycle of the forward converter. For this design example the calculated turns ratio is approximately 0.101. Transformer Turns + N V OUT ) V F + S V BOOST(min) D MAX NP (29) output inductor The following equations can be used to calculate the inductor required for this design example. First, the minimum duty cycle DMIN, which occurs at the maximum boost voltage, needs to be calculated. The maximum boost voltage is limited by the OVP trip point, which is set to approximately 425 V. For this design DMIN is approximately 31%. The output inductor ripple current (∆IL) for this design is given at 30% of the maximum load current. Next calculate the output inductor (L), where the switching frequency (fS) is 100 kHz. The calculated output inductor for this design is approximately 38 µH. D MIN + DI L + L+ V OUT ) V F V BOOST(min) NS NP (30) P OUT 0.3 V OUT ǒVOUT ) VFǓ DI L (31) ǒ1 * DMINǓ fS (32) www.ti.com 21 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION output capacitor The following equations can be used to estimate the minimum output capacitance and the capacitor’s maximum allowable equivalent series resistance (ESR), where COUT is the minimum output capacitance and tS is the period of the switching frequency. ∆VOUT is the maximum allowable output ripple voltage, selected as approximately 1% of the output voltage. For this design, the minimum calculated output capacitance is 170 µF and the maximum allowable ESR is 96 mΩ. A Panasonic HFQ 1800-µF electrolytic capacitor with an ESR of 0.048 Ω is used. C OUT + 1 8 ESR + ǒVOUT ) VFǓ L ǒD MAX ǒt SǓ Ǔ 2 DV OUT (33) DV OUT DI L (34) RSENSE2 The dc-to-dc power converter is designed for peak current mode control. RSENSE2 is the resistor that senses the current in the forward converter. The sense resistor in Figure 9 is referred to as R4. The following equations can be used to calculate RSENSE2. Where IM is the magnetizing current of the transformer used in the step-down converter and VBOOST is the output voltage of the boost stage. D is the typical duty ratio of the forward converter. VISENSE2_peak is the peak current sense comparator voltage that is typically 1.15 V. For this design example LM is approximately 8 mH and the RSENSE2 is approximately 1 Ω. IM + V BOOST LM D fS (35) V ISENSE2_peak R SENSE2 + IM ) NS NP ǒ DI L ) I OUT(max) 2 Ǔ 1.3 (36) soft-start The UCC38500 has soft-start circuitry to allow for a controlled ramp of the second stage’s duty cycle during start-up. This is accomplished through the SS2 circuitry described earlier in this data sheet. Equation (37) calculates the approximate capacitance needed based on the designer’s soft-start requirements. Where ISS2 is the soft-start charging current, which is typically 10 µA. ∆t is the desired soft start time, which was selected to be approximately 5 ms for this example. The calculated soft-start capacitor (CSS) for this example is approximately 10 nF. C SS + I ISS2 D t 4.5 (37) slope compensation When designing with peak current-mode control, slope compensation may be necessary to prevent instability. In this design, the magnetizing current provided more than enough slope compensation. If slope compensation is needed with external components, please refer to Unitrode/Texas Instruments Application Note, Practical Considerations in Current Mode Power Supplies, TI Literature No. SLUA110. 22 www.ti.com SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION control loop Figure 12 shows the control block diagram for the typical application shown in Figure 9. GC(s) is the compensation network’s transfer function (TF), GOPTO(s) is the opto-isolator TF, GCO(s) is the control-to-output TF, and H(s) is the divider TF. The following equations can be used to estimate the frequency response of each gain block, where fOPTO_pole is the frequency, where the optoisolator is –3 dB from its dc operating point, and VREF_TL431 is the reference voltage of the TL431 shunt regulator. RLOAD represents the typical load impedance for the design. G OPTO(s) + R13 R36 G C(s) + H(s) + s C14 1 1) (38) s R35 C14 ) 1 R31 (1 ) (s R35 R27 + R27 ) R31 G CO(s) + s 2p f OPTO_pole R13 R36 C15)) 1 1) s 2p f OPTO_pole (39) V VREF_TL431 V OUT V OUT R LOAD + VC R SENSE2 NP Ns (40) ǒ1 ) ǒs ǒ1 ) ǒs C OUT C OUT ESRǓ Ǔ R LOADǓ Ǔ (41) VBOOST VREF_TL431 Σ GC(s) VC GCO(s) VOUT H(s) Figure 12. UCC38500 Control Block www.ti.com 23 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION Figure 13 shows the circuitry for the voltage feedback loop. D13 is a TL431 shunt regulator that functions as an operational amplifier, providing feedback control. VBOOST VC = VERR VOUT GCO(s) GCO(s) Q5 VREF R36 H11AV1 6 R13 D14 1 5 R16 PGND2 R31 2 4 C14 3 C15 U3 R35 D13 SGND R27 UDG–01091 Figure 13. UCC38500 Feedback Loop Initially the designer must select the resistor values for the divider gain H(s). Equation (42) is used to determine resistor size. Selecting R27 to be a standard value of 10-kΩ requires R31 to be approximately 38.3 kΩ. R31 + R27 ǒV OUT * V REFǓ V REF (42) It is important to correctly bias the TL431 and the optoisolator for proper operation. Zener diode D14 and a depletion mode J-FET, Q5, supply the bias voltage for the TL431. Resistors R16 and R13 provide the minimum bias currents for the TL431 and the optoisolator respectively and can be calculated with the following equations. Where IOP(min) is the minimum optoisolation current, and VVERR(max) is the maximum voltage seen at the VERR pin of the UCC38500. VERR has an internal clamp that limits this pin to 4.5 V. VF is the typical forward voltage of the diode in the opto isolator, and ITL431(min) is the minimum cathode current of the TL431. For the components used in this design example R13 is calculated to be approximately 2.0 kΩ and R16 was calculated to be approximately 680 Ω. The optoisolator is configured to have dc gain of approximately 20 dB and the optoisolator –3 dB point is approximately 8 kHz. Figure 14 shows the frequency response of the optoisolator. 24 www.ti.com SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 TYPICAL APPLICATION R16 + R13 + VF I TL431 (min) (43) V REF * V VERR (max) I OP (min) (44) To compensate the loop, it is necessary to estimate or measure the control-to-output gain’s frequency response GCO(s). The frequency response for GCO(s) was measured with a network analyzer and the measured frequency response is shown in Figure 15. 40 120 GAIN 60 Gain – dB 20 0 0 –60 –20 –40 –120 PHASE 50 180 40 144 30 108 20 10 36 0 0 –10 –36 –20 –72 1k 10 k f – Frequency – Hz –180 100 k –108 –30 –40 –60 100 72 GAIN –50 100 Figure 14 Phase – Degrees 180 Gain – dB 60 POWER STAGE CONTROL-TO-OUTPUT TRANSFER FUNCTION (GAIN AND PHASE) vs. FREQUENCY Phase – Degrees OPTOISOLATOR TRANSFER FUNCTION (GAIN AND PHASE) vs. FREQUENCY PHASE 1k 10 k f – Frequency – Hz –144 –180 100 k Figure 15 After determining the frequency response of GCO(s) it is necessary to define some closed loop frequency response design goals. The following equation describes the frequency response of the loop gain (T(s)dB) of the system in decibels. Typically, the loop is designed to crossover at a frequency below one-sixth of the switching frequency. In order for this design example to have good transient response, the design goal is to have the loop gain crossover at approximately 1 kHz, which is less than one-sixth of the switching frequency. The gain crossover frequency for this design example is referenced as fC. T(s) dB + G C(s) ) G CO(s) ) H(s) (45) The compensation network that is used (GC(s)) has three poles and one zero. One pole occurs at the origin, and a second pole is caused by the limitations of the opto-isolator. The third pole is set to attenuate the high-frequency gain and needs to be set to one-half of the switching frequency. The zero is set at the desired crossover frequency. The following equations can be used to select R35, C14 and C15, where GCO(s), GOPTO(s), and H(s) are the gains in decibels (dB) of each control block at the desired fC. From the graphs in Figures 14 and 15 it can be observed at the desired crossover frequency GCO(s) is approximately 0 dB and GOPTO(s) is approximately www.ti.com 25 SLUS419C – AUGUST 1999 – REVISED NOVEMBER 2001 23 dB. Therefore the compensation circuitry needs to have a gain of –23 dB at the desired crossover frequency. For this example R35 is calculated at approximately 18.2 kΩ. Capacitor C14 is estimated to be approximately 10 nF and C15 is calculated at approximately 180 pF. H(s) + 20 log R35 + R31 C14 + C15 + ǒ2p ǒ ƪ ƫ V REF V OUT (46) 10 ǒ*G CO(s) dB)G OPTO(s) dB)H(s) dBǓ (47) 1 R35 f CǓ 1 2p R35 (48) Ǔ f SW 2 (49) Figure 16 shows the frequency response of the compensation network GC(s) and Figure 17 shows the measured frequency response of the loop gain T(s). The frequency response characteristics in Figure 17 show that fC is approximately 1.5 kHz with a phase margin of about 55 degrees. The gain margin is approximately 50 dB. 60 FEEDBACK CONTROL TRANSFER FUNCTION (GAIN AND PHASE) vs. FREQUENCY TOTAL LOOP TRANSFER FUNCTION (GAIN AND PHASE) vs. FREQUENCY 180 60 40 120 40 20 60 0 0 180 20 –20 –60 LOOP GAIN –180 100 k –60 100 –120 1k 10 k f – Frequency – Hz Figure 17 Figure 16 26 0 –40 1k 10 k f – Frequency – Hz 120 60 0 –120 –40 –60 100 –60 LOOP PHASE www.ti.com –180 100 k Phase – Degrees COMPENSATION GAIN GOPTO – Gain – dB –20 Phase – Degrees GOPTO – Gain – dB COMPENSATION PHASE IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. 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