June 1998 ML4824* Power Factor Correction and PWM Controller Combo GENERAL DESCRIPTION FEATURES The ML4824 is a controller for power factor corrected, switched mode power supplies. Power Factor Correction (PFC) allows the use of smaller, lower cost bulk capacitors, reduces power line loading and stress on the switching FETs, and results in a power supply that fully complies with IEC1000-2-3 specification. The ML4824 includes circuits for the implementation of a leading edge, average current, “boost” type power factor correction and a trailing edge, pulse width modulator (PWM). ■ Internally synchronized PFC and PWM in one IC ■ Low total harmonic distortion ■ Reduces ripple current in the storage capacitor between the PFC and PWM sections ■ Average current, continuous boost leading edge PFC ■ Fast transconductance error amp for voltage loop ■ High efficiency trailing edge PWM can be configured for current mode or voltage mode operation ■ Average line voltage compensation with brownout control ■ PFC overvoltage comparator eliminates output “runaway” due to load removal ■ Current fed gain modulator for improved noise immunity ■ Overvoltage protection, UVLO, and soft start The device is available in two versions; the ML4824-1 (fPWM = fPFC) and the ML4824-2 (fPWM = 2 x fPFC). Doubling the switching frequency of the PWM allows the user to design with smaller output components while maintaining the best operating frequency for the PFC. An over-voltage comparator shuts down the PFC section in the event of a sudden decrease in load. The PFC section also includes peak current limiting and input voltage brownout protection. The PWM section can be operated in current or voltage mode at up to 250kHz and includes a duty cycle limit to prevent transformer saturation. BLOCK DIAGRAM *Some Packages Are Obsolete 16 VFB POWER FACTOR CORRECTOR 3.5kΩ + 2.5V 13.5V + IEA + + 2.7V – –1V + – IAC – 2 GAIN MODULATOR VRMS 4 – 7.5V REFERENCE S Q R Q S Q R Q S Q R Q VREF 14 PFC OUT 3.5kΩ ISENSE VCC VCCZ OVP VEA – 15 13 1 IEAO VEAO PFC ILIMIT 12 3 RAMP 1 OSCILLATOR 7 (-2 VERSION ONLY) RAMP 2 x2 DUTY CYCLE LIMIT 8 8V VDC 6 1.25V + VCC SS – – 50µA 5 + VFB – 2.5V + VIN OK 1V 8V – + PWM OUT 11 DC ILIMIT DC ILIMIT 9 PULSE WIDTH MODULATOR VCCZ UVLO 1 ML4824 PIN CONFIGURATION ML4824 16-Pin PDIP (P16) 16-Pin Wide SOIC (S16W) IEAO 1 16 VEAO IAC 2 15 VFB ISENSE 3 14 VREF VRMS 4 13 VCC SS 5 12 PFC OUT VDC 6 11 PWM OUT RAMP 1 7 10 GND RAMP 2 8 9 DC ILIMIT TOP VIEW PIN DESCRIPTION PIN NAME FUNCTION 1 IEAO PFC transconductance current error amplifier output 2 IAC PFC gain control reference input 3 ISENSE Current sense input to the PFC current limit comparator 4 VRMS 5 NAME FUNCTION 9 DC ILIMIT PWM current limit comparator input 10 GND Ground 11 PWM OUT PWM driver output 12 PFC OUT PFC driver output Input for PFC RMS line voltage compensation 13 VCC Positive supply (connected to an internal shunt regulator) SS Connection point for the PWM soft start capacitor 14 VREF Buffered output for the internal 7.5V reference 6 VDC PWM voltage feedback input 15 VFB PFC transconductance voltage error amplifier input 7 RAMP 1 Oscillator timing node; timing set by RTCT 16 VEAO PFC transconductance voltage error amplifier output 8 2 RAMP 2 When in current mode, this pin functions as as the current sense input; when in voltage mode, it is the PWM input from PFC output (feed forward ramp). PIN ML4824 ABSOLUTE MAXIMUM RATINGS Absolute maximum ratings are those values beyond which the device could be permanently damaged. Absolute maximum ratings are stress ratings only and functional device operation is not implied. VCC Shunt Regulator Current .................................. 55mA ISENSE Voltage ..................................................–3V to 5V Voltage on Any Other Pin ... GND – 0.3V to VCCZ + 0.3V I REF ............................................................................................ 20mA IAC Input Current .................................................... 10mA Peak PFC OUT Current, Source or Sink ................ 500mA Peak PWM OUT Current, Source or Sink .............. 500mA PFC OUT, PWM OUT Energy Per Cycle .................. 1.5µJ Junction Temperature .............................................. 150°C Storage Temperature Range ..................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) ..................... 260°C Thermal Resistance (θJA) Plastic DIP ....................................................... 80°C/W Plastic SOIC ................................................... 105°C/W OPERATING CONDITIONS Temperature Range ML4824CX ................................................. 0°C to 70°C ML4824IX .............................................. –40°C to 85°C ELECTRICAL CHARACTERISTICS Unless otherwise specified, ICC = 25mA, RT = 52.3kΩ, CT = 470pF, TA = Operating Temperature Range (Note 1) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 7 V VOLTAGE ERROR AMPLIFIER Transconductance 0 VNON INV = VINV, VEAO = 3.75V Feedback Reference Voltage Input Bias Current 50 85 120 µ 2.46 2.53 2.60 V -0.3 –1.0 µA Note 2 Output High Voltage Ω Input Voltage Range 6.0 Output Low Voltage 6.7 0.6 V 1.0 V Source Current ∆VIN = ±0.5V, VOUT = 6V –40 –80 µA Sink Current ∆VIN = ±0.5V, VOUT = 1.5V 40 80 µA 60 75 dB 60 75 dB Open Loop Gain Power Supply Rejection Ratio VCCZ - 3V < VCC < VCCZ - 0.5V CURRENT ERROR AMPLIFIER Transconductance –1.5 VNON INV = VINV, VEAO = 3.75V Input Offset Voltage V 130 195 310 µ 0 8 15 mV –0.5 –1.0 µA Input Bias Current Output High Voltage 2 Ω Input Voltage Range 6.0 Output Low Voltage 6.7 0.6 V 1.0 V Source Current ∆VIN = ±0.5V, VOUT = 6V –40 –90 µA Sink Current ∆VIN = ±0.5V, VOUT = 1.5V 40 90 µA 60 75 dB 60 75 dB Open Loop Gain Power Supply Rejection Ratio VCCZ - 3V < VCC < VCCZ - 0.5V 3 ML4824 ELECTRICAL CHARACTERISTICS (Continued) SMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Threshold Voltage 2.6 2.7 2.8 V Hysteresis 80 115 150 mV Threshold Voltage –0.8 –1.0 –1.15 V ∆(PFC ILIMIT VTH - Gain Modulator Output) 100 190 OVP COMPARATOR PFC ILIMIT COMPARATOR Delay to Output mV 150 300 ns 1.02 1.07 V Input Bias Current ±0.3 ±1 µA Delay to Output 150 300 ns DC ILIMIT COMPARATOR Threshold Voltage 0.97 VIN OK COMPARATOR Threshold Voltage 2.4 2.5 2.6 V Hysteresis 0.8 1.0 1.2 V IAC = 100µA, VRMS = VFB = 0V 0.36 0.55 0.66 IAC = 50µA, VRMS = 1.2V, VFB = 0V 1.20 1.80 2.24 IAC = 50µA, VRMS = 1.8V, VFB = 0V 0.55 0.80 1.01 IAC = 100µA, VRMS = 3.3V, VFB = 0V 0.14 0.20 0.26 GAIN MODULATOR Gain (Note 3) Bandwidth IAC = 100µA Output Voltage IAC = 250µA, VRMS = 1.15V, VFB = 0V 10 MHz 0.74 0.82 0.90 V 71 76 81 kHz OSCILLATOR Initial Accuracy TA = 25°C Voltage Stability VCCZ - 3V < VCC < VCCZ - 0.5V Temperature Stability Total Variation Line, Temp 1 % 2 % 68 Ramp Valley to Peak Voltage 84 2.5 kHz V Dead Time PFC Only 270 370 470 ns CT Discharge Current VRAMP 2 = 0V, VRAMP 1 = 2.5V 4.5 7.5 9.5 mA Output Voltage TA = 25°C, I(VREF) = 1mA 7.4 7.5 7.6 V Line Regulation VCCZ - 3V < VCC < VCCZ - 0.5V 2 10 mV Load Regulation 1mA < I(VREF) < 20mA 2 15 mV REFERENCE Temperature Stability 4 0.4 Total Variation Line, Load, Temp Long Term Stability TJ = 125°C, 1000 Hours 7.35 5 % 7.65 V 25 mV ML4824 ELECTRICAL CHARACTERISTICS (Continued) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 0 % PFC Minimum Duty Cycle VIEAO > 4.0V Maximum Duty Cycle VIEAO < 1.2V Output Low Voltage IOUT = -20mA 0.4 0.8 V IOUT = -100mA 0.8 2.0 V IOUT = 10mA, VCC = 8V 0.7 1.5 V Output High Voltage 90 95 % IOUT = 20mA 10 10.5 V IOUT = 100mA 9.5 10 V 50 ns Rise/Fall Time CL = 1000pF Duty Cycle Range ML4824-1 0-44 0-47 0-50 % ML4824-2 0-37 0-40 0-45 % IOUT = -20mA 0.4 0.8 V IOUT = -100mA 0.8 2.0 V IOUT = 10mA, VCC = 8V 0.7 1.5 V PWM Output Low Voltage Output High Voltage Rise/Fall Time IOUT = 20mA 10 10.5 V IOUT = 100mA 9.5 10 V 50 ns CL = 1000pF SUPPLY Shunt Regulator Voltage (VCCZ) 12.8 13.5 14.2 V ±100 ±300 mV 14.6 V VCCZ Load Regulation 25mA < ICC < 55mA VCCZ Total Variation Load, Temp Start-up Current VCC = 11.8V, CL = 0 0.7 1.0 mA Operating Current VCC < VCCZ - 0.5V, CL = 0 16 19 mA 12.4 Undervoltage Lockout Threshold 12 13 14 V Undervoltage Lockout Hysteresis 2.7 3.0 3.3 V Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst-case test conditions. Note 2: Includes all bias currents to other circuits connected to the VFB pin. Note 3: Gain = K x 5.3V; K = (IGAINMOD - IOFFSET) x IAC x (VEAO - 1.5V)-1. 5 ML4824 250 200 200 Ω TRANSCONDUCTANCE (µ ) 250 Ω TRANSCONDUCTANCE (µ ) TYPICAL PERFORMANCE CHARACTERISTICS 150 100 50 0 1 0 2 4 3 150 100 50 0 –500 5 0 500 IEA INPUT VOLTAGE (mV) VFB (V) Voltage Error Amplifier (VEA) Transconductance (gm) Current Error Amplifier (IEA) Transconductance (gm) VARIABLE GAIN BLOCK CONSTANT - K 400 300 200 100 0 0 1 2 3 4 5 VRMS (mV) Gain Modulator Transfer Characteristic (K) 16 1 IEAO VEAO VFB 15 VEA – OVP + IEA 3.5kΩ + 2.5V + + 4 ISENSE – – IAC – 2 VRMS 2.7V –1V GAIN MODULATOR S Q R Q S Q R Q + – PFC OUT 3.5kΩ PFC ILIMIT 3 RAMP 1 7 OSCILLATOR Figure 1. PFC Section Block Diagram. 6 12 ML4824 FUNCTIONAL DESCRIPTION The ML4824 consists of an average current controlled, continuous boost Power Factor Corrector (PFC) front end and a synchronized Pulse Width Modulator (PWM) back end. The PWM can be used in either current or voltage mode. In voltage mode, feedforward from the PFC output buss can be used to improve the PWM’s line regulation. In either mode, the PWM stage uses conventional trailingedge duty cycle modulation, while the PFC uses leadingedge modulation. This patented leading/trailing edge modulation technique results in a higher useable PFC error amplifier bandwidth, and can significantly reduce the size of the PFC DC buss capacitor. The synchronization of the PWM with the PFC simplifies the PWM compensation due to the controlled ripple on the PFC output capacitor (the PWM input capacitor). The PWM section of the ML4824-1 runs at the same frequency as the PFC. The PWM section of the ML4824-2 runs at twice the frequency of the PFC, which allows the use of smaller PWM output magnetics and filter capacitors while holding down the losses in the PFC stage power components. In addition to power factor correction, a number of protection features have been built into the ML4824. These include soft-start, PFC over-voltage protection, peak current limiting, brown-out protection, duty cycle limit, and under-voltage lockout. POWER FACTOR CORRECTION Power factor correction makes a non-linear load look like a resistive load to the AC line. For a resistor, the current drawn from the line is in phase with and proportional to the line voltage, so the power factor is unity (one). A common class of non-linear load is the input of most power supplies, which use a bridge rectifier and capacitive input filter fed from the line. The peak-charging effect which occurs on the input filter capacitor in these supplies causes brief high-amplitude pulses of current to flow from the power line, rather than a sinusoidal current in phase with the line voltage. Such supplies present a power factor to the line of less than one (i.e. they cause significant current harmonics of the power line frequency to appear at their input). If the input current drawn by such a supply (or any other non-linear load) can be made to follow the input voltage in instantaneous amplitude, it will appear resistive to the AC line and a unity power factor will be achieved. To hold the input current draw of a device drawing power from the AC line in phase with and proportional to the input voltage, a way must be found to prevent that device from loading the line except in proportion to the instantaneous line voltage. The PFC section of the ML4824 uses a boost-mode DC-DC converter to accomplish this. The input to the converter is the full wave rectified AC line voltage. No bulk filtering is applied following the bridge rectifier, so the input voltage to the boost converter ranges (at twice line frequency) from zero volts to the peak value of the AC input and back to zero. By forcing the boost converter to meet two simultaneous conditions, it is possible to ensure that the current which the converter draws from the power line agrees with the instantaneous line voltage. One of these conditions is that the output voltage of the boost converter must be set higher than the peak value of the line voltage. A commonly used value is 385VDC, to allow for a high line of 270VACrms. The other condition is that the current which the converter is allowed to draw from the line at any given instant must be proportional to the line voltage. The first of these requirements is satisfied by establishing a suitable voltage control loop for the converter, which in turn drives a current error amplifier and switching output driver. The second requirement is met by using the rectified AC line voltage to modulate the output of the voltage control loop. Such modulation causes the current error amplifier to command a power stage current which varies directly with the input voltage. In order to prevent ripple which will necessarily appear at the output of the boost circuit (typically about 10VAC on a 385V DC level) from introducing distortion back through the voltage error amplifier, the bandwidth of the voltage loop is deliberately kept low. A final refinement is to adjust the overall gain of the PFC such to be proportional to 1/VIN2, which linearizes the transfer function of the system as the AC input voltage varies. Since the boost converter topology in the ML4824 PFC is of the current-averaging type, no slope compensation is required. PFC SECTION Gain Modulator Figure 1 shows a block diagram of the PFC section of the ML4824. The gain modulator is the heart of the PFC, as it is this circuit block which controls the response of the current loop to line voltage waveform and frequency, rms line voltage, and PFC output voltage. There are three inputs to the gain modulator. These are: 1) A current representing the instantaneous input voltage (amplitude and waveshape) to the PFC. The rectified AC input sine wave is converted to a proportional current via a resistor and is then fed into the gain modulator at IAC. Sampling current in this way minimizes ground noise, as is required in high power switching power conversion environments. The gain modulator responds linearly to this current. 2) A voltage proportional to the long-term rms AC line voltage, derived from the rectified line voltage after scaling and filtering. This signal is presented to the gain modulator at VRMS. The gain modulator’s output is inversely proportional to VRMS2 (except at unusually low values of VRMS where special gain contouring takes over, to limit power dissipation of the circuit components under heavy brownout conditions). The relationship between VRMS and gain is called K, and is illustrated in the Typical Performance Characteristics. 7 ML4824 FUNCTIONAL DESCRIPTION (Continued) VREF 3) The output of the voltage error amplifier, VEAO. The gain modulator responds linearly to variations in this voltage. The output of the gain modulator is a current signal, in the form of a full wave rectified sinusoid at twice the line frequency. This current is applied to the virtual-ground (negative) input of the current error amplifier. In this way the gain modulator forms the reference for the current error loop, and ultimately controls the instantaneous current draw of the PFC from the power line. The general form for the output of the gain modulator is: IGAINMOD = IAC ´ VEAO VRMS 2 16 ´ 1V (1) . V ) ´ I AC IGAINM OD = K ´ ( VEAO - 15 of V-1. 1 IEAO VEAO VFB 15 VEA – IEA + 2.5V + + – IAC More exactly, the output current of the gain modulator is given by: where K is in units PFC OUTPUT – 2 VRMS 4 GAIN MODULATOR ISENSE 3 Figure 2. Compensation Network Connections for the Voltage and Current Error Amplifiers Note that the output current of the gain modulator is limited to ≅ 200µA. Current Error Amplifier Overvoltage Protection The current error amplifier’s output controls the PFC duty cycle to keep the average current through the boost inductor a linear function of the line voltage. At the inverting input to the current error amplifier, the output current of the gain modulator is summed with a current which results from a negative voltage being impressed upon the ISENSE pin (current into ISENSE ≅ VSENSE/3.5kΩ). The negative voltage on ISENSE represents the sum of all currents flowing in the PFC circuit, and is typically derived from a current sense resistor in series with the negative terminal of the input bridge rectifier. In higher power applications, two current transformers are sometimes used, one to monitor the ID of the boost MOSFET(s) and one to monitor the IF of the boost diode. As stated above, the inverting input of the current error amplifier is a virtual ground. Given this fact, and the arrangement of the duty cycle modulator polarities internal to the PFC, an increase in positive current from the gain modulator will cause the output stage to increase its duty cycle until the voltage on ISENSE is adequately negative to cancel this increased current. Similarly, if the gain modulator’s output decreases, the output duty cycle will decrease, to achieve a less negative voltage on the ISENSE pin. The OVP comparator serves to protect the power circuit from being subjected to excessive voltages if the load should suddenly change. A resistor divider from the high voltage DC output of the PFC is fed to VFB. When the voltage on VFB exceeds 2.7V, the PFC output driver is shut down. The PWM section will continue to operate. The OVP comparator has 125mV of hysteresis, and the PFC will not restart until the voltage at VFB drops below 2.58V. The VFB should be set at a level where the active and passive external power components and the ML4824 are within their safe operating voltages, but not so low as to interfere with the boost voltage regulation loop. Cycle-By-Cycle Current Limiter The ISENSE pin, as well as being a part of the current feedback loop, is a direct input to the cycle-by-cycle current limiter for the PFC section. Should the input voltage at this pin ever be more negative than -1V, the output of the PFC will be disabled until the protection flipflop is reset by the clock pulse at the start of the next PFC power cycle. 8 Error Amplifier Compensation The PWM loading of the PFC can be modeled as a negative resistor; an increase in input voltage to the PWM causes a decrease in the input current. This response dictates the proper compensation of the two transconductance error amplifiers. Figure 2 shows the types of compensation networks most commonly used for the voltage and current error amplifiers, along with their respective return points. The current loop compensation is returned to VREF to produce a soft-start characteristic on the PFC: as the reference voltage comes up from zero volts, it creates a differentiated voltage on IEAO which prevents the PFC from immediately demanding a full duty cycle on its boost converter. There are two major concerns when compensating the voltage loop error amplifier; stability and transient response. Optimizing interaction between transient response and stability requires that the error amplifier’s ML4824 FUNCTIONAL DESCRIPTION (Continued) open-loop crossover frequency should be 1/2 that of the line frequency, or 23Hz for a 47Hz line (lowest anticipated international power frequency). The gain vs. input voltage of the ML4824’s voltage error amplifier has a specially shaped nonlinearity such that under steady-state operating conditions the transconductance of the error amplifier is at a local minimum. Rapid perturbations in line or load conditions will cause the input to the voltage error amplifier (VFB) to deviate from its 2.5V (nominal) value. If this happens, the transconductance of the voltage error amplifier will increase significantly, as shown in the Typical Performance Characteristics. This raises the gainbandwidth product of the voltage loop, resulting in a much more rapid voltage loop response to such perturbations than would occur with a conventional linear gain characteristic. The current amplifier compensation is similar to that of the voltage error amplifier with the exception of the choice of crossover frequency. The crossover frequency of the current amplifier should be at least 10 times that of the voltage amplifier, to prevent interaction with the voltage loop. It should also be limited to less than 1/6th that of the switching frequency, e.g. 16.7kHz for a 100kHz switching frequency. There is a modest degree of gain contouring applied to the transfer characteristic of the current error amplifier, to increase its speed of response to current-loop perturbations. However, the boost inductor will usually be the dominant factor in overall current loop response. Therefore, this contouring is significantly less marked than that of the voltage error amplifier. This is illustrated in the Typical Performance Characteristics. For more information on compensating the current and voltage control loops, see Application Notes 33 and 34. Application Note 16 also contains valuable information for the design of this class of PFC. Oscillator (RAMP 1) The oscillator frequency is determined by the values of RT and CT, which determine the ramp and off-time of the oscillator output clock: f OSC = 1 t RAM P + t D EAD TIM E (2) The deadtime of the oscillator is derived from the following equation: FG V HV t RAMP = C T ´ R T ´ In REF REF at VREF = 7.5V: . - 125 . - 375 IJ K (3) The deadtime of the oscillator may be determined using: t DEADTIME = 2.5V ´ C T = 490 ´ C T 51 . mA (4) The deadtime is so small (tRAMP >> tDEADTIME) that the operating frequency can typically be approximated by: f OSC = 1 (5) t RAM P EXAMPLE: For the application circuit shown in the data sheet, with the oscillator running at: fOSC = 100kHz = 1 t RAM P t RAMP = C T ´ R T ´ 0.51 = 1 ´ 10 -5 Solving for RT x CT yields 2 x 10-4. Selecting standard components values, CT = 470pF, and RT = 41.2kΩ. The deadtime of the oscillator adds to the Maximum PWM Duty Cycle (it is an input to the Duty Cycle Limiter). With zero oscillator deadtime, the Maximum PWM Duty Cycle is typically 45%. In many applications, care should be taken that CT not be made so large as to extend the Maximum Duty Cycle beyond 50%. This can be accomplished by using a stable 470pF capacitor for CT. PWM SECTION Pulse Width Modulator The PWM section of the ML4824 is straightforward, but there are several points which should be noted. Foremost among these is its inherent synchronization to the PFC section of the device, from which it also derives its basic timing (at the PFC frequency in the ML4824-1, and at twice the PFC frequency in the ML4824-2). The PWM is capable of current-mode or voltage mode operation. In current-mode applications, the PWM ramp (RAMP 2) is usually derived directly from a current sensing resistor or current transformer in the primary of the output stage, and is thereby representative of the current flowing in the converter’s output stage. DC ILIMIT, which provides cycleby-cycle current limiting, is typically connected to RAMP 2 in such applications. For voltage-mode operation or certain specialized applications, RAMP 2 can be connected to a separate RC timing network to generate a voltage ramp against which VDC will be compared. Under these conditions, the use of voltage feedforward from the PFC buss can assist in line regulation accuracy and response. As in current mode operation, the DC ILIMIT input is used for output stage overcurrent protection. t RAMP = C T ´ R T ´ 0.51 9 ML4824 FUNCTIONAL DESCRIPTION (Continued) No voltage error amplifier is included in the PWM stage of the ML4824, as this function is generally performed on the output side of the PWM’s isolation boundary. To facilitate the design of optocoupler feedback circuitry, an offset has been built into the PWM’s RAMP 2 input which allows VDC to command a zero percent duty cycle for input voltages below 1.25V. PWM Current Limit The DC ILIMIT pin is a direct input to the cycle-by-cycle current limiter for the PWM section. Should the input voltage at this pin ever exceed 1V, the output of the PWM will be disabled until the output flip-flop is reset by the clock pulse at the start of the next PWM power cycle. VIN OK Comparator The VIN OK comparator monitors the DC output of the PFC and inhibits the PWM if this voltage on VFB is less than its nominal 2.5V. Once this voltage reaches 2.5V, which corresponds to the PFC output capacitor being charged to its rated boost voltage, the soft-start begins. PWM Control (RAMP 2) When the PWM section is used in current mode, RAMP 2 is generally used as the sampling point for a voltage representing the current in the primary of the PWM’s output transformer, derived either by a current sensing resistor or a current transformer. In voltage mode, it is the input for a ramp voltage generated by a second set of timing components (RRAMP2, CRAMP2), which will have a minimum value of zero volts and should have a peak value of approximately 5V. In voltage mode operation, feedforward from the PFC output buss is an excellent way to derive the timing ramp for the PWM stage. Solving for the minimum value of CSS: C SS = 5ms ´ 50µA = 200nF . V 125 Generating VCC The ML4824 is a current-fed part. It has an internal shunt voltage regulator, which is designed to regulate the voltage internal to the part at 13.5V. This allows a low power dissipation while at the same time delivering 10V of gate drive at the PWM OUT and PFC OUT outputs. It is important to limit the current through the part to avoid overheating or destroying it. This can be easily done with a single resistor in series with the Vcc pin, returned to a bias supply of typically 18V to 20V. The resistor’s value must be chosen to meet the operating current requirement of the ML4824 itself (19mA max) plus the current required by the two gate driver outputs. EXAMPLE: With a VBIAS of 20V, a VCC limit of 14.6V (max) and the ML4824 driving a total gate charge of 110nC at 100kHz (e.g., 1 IRF840 MOSFET and 2 IRF830 MOSFETs), the gate driver current required is: IGATEDRIVE = 100kHz ´ 100nC = 11mA (7) 20 V - 14.6V = 180 Ω 19mA + 11mA (8) R BIAS = To check the maximum dissipation in the ML4824, find the current at the minimum VCC (12.4V): ICC = 20 V - 12.4V = 42.2mA 180 Ω (9) Soft Start The maximum allowable ICC is 55mA, so this is an acceptable design. Start-up of the PWM is controlled by the selection of the external capacitor at SS. A current source of 50µA supplies the charging current for the capacitor, and start-up of the PWM begins at 1.25V. Start-up delay can be programmed by the following equation: The ML4824 should be locally bypassed with a 10nF and a 1µF ceramic capacitor. In most applications, an electrolytic capacitor of between 100µF and 330µF is also required across the part, both for filtering and as part of the start-up bootstrap circuitry. C SS = t DELAY ´ 50µA . V 125 (6) where CSS is the required soft start capacitance, and tDELAY is the desired start-up delay. It is important that the time constant of the PWM soft-start allow the PFC time to generate sufficient output power for the PWM section. The PWM start-up delay should be at least 5ms. VBIAS RBIAS VCC ML4824 10nF CERAMIC 1µF CERAMIC GND Figure 3. External Component Connections to VCC 10 ML4824 LEADING/TRAILING MODULATION Conventional Pulse Width Modulation (PWM) techniques employ trailing edge modulation in which the switch will turn on right after the trailing edge of the system clock. The error amplifier output voltage is then compared with the modulating ramp. When the modulating ramp reaches the level of the error amplifier output voltage, the switch will be turned OFF. When the switch is ON, the inductor current will ramp up. The effective duty cycle of the trailing edge modulation is determined during the ON time of the switch. Figure 4 shows a typical trailing edge control scheme. One of the advantages of this control teccnique is that it requires only one system clock. Switch 1 (SW1) turns off and switch 2 (SW2) turns on at the same instant to minimize the momentary “no-load” period, thus lowering ripple voltage generated by the switching action. With such synchronized switching, the ripple voltage of the first stage is reduced. Calculation and evaluation have shown that the 120Hz component of the PFC’s output ripple voltage can be reduced by as much as 30% using this method. In the case of leading edge modulation, the switch is turned OFF right at the leading edge of the system clock. When the modulating ramp reaches the level of the error amplifier output voltage, the switch will be turned ON. The effective duty-cycle of the leading edge modulation is determined during the OFF time of the switch. Figure 5 shows a leading edge control scheme. TYPICAL APPLICATIONS SW2 L1 + I2 I1 Figure 6 is the application circuit for a complete 100W power factor corrected power supply, designed using the methods and general topology detailed in Application Note 33. I3 I4 VIN RL SW1 DC C1 RAMP VEAO REF U3 + –EA TIME DFF RAMP OSC U4 CLK + – U1 R Q D U2 Q CLK VSW1 TIME Figure 4. Typical Trailing Edge Control Scheme. 11 ML4824 SW2 L1 + I2 I1 I3 I4 VIN RL SW1 DC C1 RAMP VEAO REF U3 + –EA RAMP OSC U4 CLK VEAO + – CMP U1 TIME DFF R Q D U2 Q CLK VSW1 TIME Figure 5. Typical Leading Edge Control Scheme. 12 ML4824 AC INPUT 85 TO 265VAC F1 3.15A C1 470nF L1 3.1mH D1 8A, 600V Q1 IRF840 C4 10nF R2A 357kΩ BR1 4A, 600V Q2 R17 IRF830 33Ω C5 100µF C25 100nF T1 R1A 499kΩ R21 22Ω R2B 357kΩ D13 1A, 50V C2 470nF D6 600V R1B 499kΩ C30 330µF C21 1800µF C20 1µF D3 50V C12 10µF R14 33Ω C7 220pF C22 4.7µF Q3 IRF830 R23 1.5kΩ 2 3 4 5 C19 1µF 6 7 8 VEAO IEAO VFB IAC ISENSE VREF VRMS VCC PFC OUT SS PWM OUT VDC RAMP 1 GND RAMP 2 DC ILIMIT R6 41.2kΩ R18 220Ω R26 10kΩ R22 8.66kΩ C23 100nF R25 2.26kΩ MOC 8102 R7B 178kΩ TL431 16 15 14 13 C15 10nF 12 C16 1µF C13 100nF C14 1µF R8 2.37kΩ C31 1nF R11 750kΩ C9 8.2nF C8 82nF 11 10 D8 1A, 20V 9 ML4824 C18 470pF R20 1.1Ω R19 220Ω C6 1nF R12 27kΩ 12VDC RTN R7A 178kΩ R4 13kΩ C24 1µF R24 1.2kΩ R3 75kΩ 1 R5 300mΩ 1W L2 D11 MBR2545CT 33µH T2 R15 3Ω R28 180Ω D12 1A, 50V D7 15V R30 4.7kΩ R27 39kΩ C3 470nF D5 600V R10 6.2kΩ C17 220pF D10 1A, 20V L1: L2: T1: T2: Premier Magnetics #TSD-734 33µH, 10A DC Premier Magnetics #TSD-736 Premier Magnetics #TSD-735 Premier Magnetics: (714) 362-4211 C11 10nF Figure 6. 100W Power Factor Corrected Power Supply, Designed Using Micro Linear Application Note 33. 13 ML4824 PHYSICAL DIMENSIONS inches (millimeters) Package: P16 16-Pin PDIP 0.740 - 0.760 (18.79 - 19.31) 16 0.240 - 0.260 0.295 - 0.325 (6.09 - 6.61) (7.49 - 8.26) PIN 1 ID 0.02 MIN (0.50 MIN) (4 PLACES) 1 0.055 - 0.065 (1.40 - 1.65) 0.015 MIN (0.38 MIN) 0.170 MAX (4.32 MAX) 0.125 MIN (3.18 MIN) 14 0.100 BSC (2.54 BSC) 0.016 - 0.022 (0.40 - 0.56) SEATING PLANE 0º - 15º 0.008 - 0.012 (0.20 - 0.31) ML4824 PHYSICAL DIMENSIONS inches (millimeters) Package: S16W 16-Pin Wide SOIC 0.400 - 0.414 (10.16 - 10.52) 16 0.291 - 0.301 0.398 - 0.412 (7.39 - 7.65) (10.11 - 10.47) PIN 1 ID 1 0.024 - 0.034 (0.61 - 0.86) (4 PLACES) 0.050 BSC (1.27 BSC) 0.095 - 0.107 (2.41 - 2.72) 0º - 8º 0.090 - 0.094 (2.28 - 2.39) 0.012 - 0.020 (0.30 - 0.51) SEATING PLANE 0.005 - 0.013 (0.13 - 0.33) 0.022 - 0.042 (0.56 - 1.07) 0.009 - 0.013 (0.22 - 0.33) 15 ML4824 ORDERING INFORMATION © Micro Linear 1998. PART NUMBER PWM FREQUENCY TEMPERATURE RANGE ML4824CP-1 ML4824CP-2 ML4824CS-1 ML4824CS-2 1 x PFC 2 x PFC 1 x PFC 2 x PFC 0°C 0°C 0°C 0°C ML4824IP-1 ML4824IP-2 ML4824IS-1 ML4824IS-2 1 x PFC 2 x PFC 1 x PFC 2 x PFC –40°C –40°C –40°C –40°C to to to to 70°C 70°C 70°C 70°C to to to to 85°C 85°C 85°C 85°C PACKAGE 16-Pin PDIP (P16) 16-Pin PDIP (P16) 16-Pin Wide SOIC (S16W) 16-Pin Wide SOIC (S16W) 16-Pin PDIP (P16) 16-Pin PDIP (P16) (Obsolete) 16-Pin Wide SOIC (S16W) 16-Pin Wide SOIC (S16W) (Obsolete) is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their respective owners. Products described herein may be covered by one or more of the following U.S. patents: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; 5,652,479; 5,661,427; 5,663,874; 5,672,959; 5,689,167; 5,714,897; 5,717,798. Japan: 2,598,946; 2,619,299; 2,704,176. Other patents are pending. Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design. Micro Linear does not assume any liability arising out of the application or use of any product described herein, neither does it convey any license under its patent right nor the rights of others. The circuits contained in this data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to whether the illustrated circuits infringe any intellectual property rights of others, and will accept no responsibility or liability for use of any application herein. The customer is urged to consult with appropriate legal counsel before deciding on a particular application. 16 2092 Concourse Drive San Jose, CA 95131 Tel: (408) 433-5200 Fax: (408) 432-0295 www.microlinear.com DS4824-01