UC1854 UC2854 UC3854 High Power Factor Preregulator FEATURES DESCRIPTION • Control Boost PWM to 0.99 Power Factor • Limit Line Current Distortion To <5% • World-Wide Operation Without Switches • Feed-Forward Line Regulation • Average Current-Mode Control • Low Noise Sensitivity • The UC1854 provides active power factor correction for power systems that otherwise would draw non-sinusoidal current from sinusoidal power lines. This device implements all the control functions necessary to build a power supply capable of optimally using available power-line current while minimizing line-current distortion. To do this, the UC1854 contains a voltage amplifier, an analog multiplier/divider, a current amplifier, and a fixed-frequency PWM. In addition, the UC1854 contains a power MOSFET compatible gate driver, 7.5V reference, line anticipator, load-enable comparator, low-supply detector, and over-current comparator. Low Start-Up Supply Current • Fixed-Frequency PWM Drive • Low-Offset Analog Multiplier/Divider • 1A Totem-Pole Gate Driver • Precision Voltage Reference BLOCK DIAGRAM The UC1854 uses average current-mode control to accomplish fixedfrequency current control with stability and low distortion. Unlike peak current-mode, average current control accurately maintains sinusoidal line current without slope compensation and with minimal response to noise transients. The UC1854’s high reference voltage and high oscillator amplitude minimize noise sensitivity while fast PWM elements permit chopping frequencies above 200kHz. The UC1854 can be used in single and three phase systems with line voltages that vary from 75 to 275 volts and line frequencies across the 50Hz to 400Hz range. To reduce the burden on the circuitry that supplies power to this device, the UC1854 features low starting supply current. These devices are available packaged in 16-pin plastic and ceramic dual in-line packages, and a variety of surface-mount packages. UDG-92055 6/98 UC1854 UC2854 UC3854 ABSOLUTE MAXIMUM RATINGS Supply Voltage VCC . . . . . . . . . . . . . . . . . . . . GT Drv Current, Continuous . . . . . . . . . . . . . GT Drv Current, 50% Duty Cycle. . . . . . . . . . Input Voltage, VSENSE, VRMS . . . . . . . . . . . . . Input Voltage, ISENSE, Mult Out . . . . . . . . . . . Input Voltage, PKLMT . . . . . . . . . . . . . . . . . . Input Current, RSET, IAC , PKLMT, ENA . . . . . Power Dissipation . . . . . . . . . . . . . . . . . . . . . Storage Temperature . . . . . . . . . . . . . . . Lead Temperature (Soldering, 10 Seconds) . . . . . . . . . . . . . . . . . 35V . . . . . . . . . . . . . . . 0.5A . . . . . . . . . . . . . . . 1.5A . . . . . . . . . . . . . . . . 11V . . . . . . . . . . . . . . . . 11V . . . . . . . . . . . . . . . . . 5V . . . . . . . . . . . . . . 10mA . . . . . . . . . . . . . . . . 1W . . . . . –65oC to +150oC . . . . . . . . . . . . . +300oC Note 1: All voltages with respect to Gnd (Pin 1). Note 2: All currents are positive into the specified terminal. Note 3: ENA input is internally clamped to approximately 14V. Note 4: Consult Unitrode Integrated Circuits databook for information regarding thermal specifications and limita- CONNECTION DIAGRAMS DIL–16 & SOIC-16 (Top View) J, N & DW Packages PLCC-20 & LCC-20 (Top View) Q & L Packages PACKAGE PIN FUNCTION FUNCTION PIN N/C Gnd PKLMT CA Out ISENSE N/C Mult Out IAC VA Out VRMS N/C VREF ENA VSENSE RSET N/C SS CT VCC GT Drv 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Unless otherwise stated, VCC=18V, RSET=15k to ground, CT=1.5nF to ground, PKLMT=1V, ENA=7.5V, ELECTRICAL CHARACTERISTICS VRMS=1.5V, IAC=100µA, ISENSE=0V, CA Out=3.5V, VA Out=5V, VSENSE =7.5V, no load on SS, CA Out, VA Out, REF, GT Drv, –55oC<TA<125oC for the UC1854, –40oC<TA<85oC for the UC2854, and 0oC<TA <70oC for the UC3854, and TA=TJ. PARAMETER OVERALL Supply Current, Off Supply Current, On VCC Turn-On Threshold VCC Turn-Off Threshold ENA Threshold, Rising ENA Threshold Hysteresis ENA Input Current VRMS Input Current TEST CONDITIONS MIN TYP MAX UNITS 1.5 10 2.0 16 mA mA ENA=0V 14.5 9 2.4 0.2 –5.0 16 10 2.55 0.25 –0.2 17.5 11 2.7 0.3 5.0 V V V V µA VRMS=5V –1.0 –.01 1.0 µA 8 500 mV nA dB V mA µA ENA=0V VOLTAGE AMPLIFIER Voltage Amp Offset Voltage VA Out=5V VSENSE Bias Current Voltage Amp Gain Voltage Amp Output Swing Voltage Amp Short Circuit Current VA Out=0V SS Current SS=2.5V –8 –500 70 –36 –20 2 –25 100 0.5 to 5.8 –20 –14 –5 –6 UC1854 UC2854 UC3854 Unless otherwise stated, VCC=18V, RSET=15k to ground, CT=1.5nF to ground, PKLMT=1V, ENA=7.5V, ELECTRICAL VRMS =1.5V, IAC=100µA, ISENSE=0V, CA Out=3.5V, VA Out=5V, VSENSE=7.5V, no load on SS, CA Out, CHARACTERISTICS VA Out, REF, GT Drv, –55oC<TA<125oC for the UC1854, –40oC<TA <85oC for the UC2854, and 0oC<TA<70oC for the UC3854, and TA=TJ. PARAMETER CURRENT AMPLIFIER Current Amp Offset Voltage ISENSE Bias Current Input Range, ISENSE, Mult Out Current Amp Gain Current Amp Output Swing Current Amp Short Circuit Current Current Amp Gain-BW Product REFERENCE Reference Output Voltage VREF Load Regulation VREF Line Regulation VREF Short Circuit Current MULTIPLIER Mult Out Current IAC Limited Mult Out Current Zero Mult Out Current RSET Limited Mult Out Current Multiplier Gain Constant OSCILLATOR Oscillator Frequency TEST CONDITIONS Peak GT Drv Current GT Drv Rise/Fall Time GT Drv Maximum Duty Cycle CURRENT LIMIT PKLMT Offset Voltage PKLMT Input Current PKLMT to GT Drv Delay TYP MAX UNITS 4 mV –120 500 –36 110 0.5 to 16 –20 –5 nA V dB V mA 400 800 7.4 7.35 –15 –10 –50 7.5 7.5 5 2 –28 7.6 7.65 15 10 –12 V V mV mV mA –220 –2.0 –280 –50 –38 –165 –250 –95 –200 –0.2 –255 –42 –27 –150 –225 –80 –1.0 –180 2.0 –220 –33 –12 –105 –150 –60 µA µA µA µA µA µA µA µA V 46 86 4.9 0.8 55 102 5.4 1.1 62 118 5.9 1.3 kHz kHz V V 13 12 14.5 12.8 0.9 1.0 0.1 1.0 35 18 V V V V V A ns –4 –500 –0.3 to 2.5 80 CA Out=0V TA=25oC (Note 6) IREF=0mA, TA=25oC IREF=0mA, Over Temp. –10mA<IREF<0mA 15V<VCC<35V REF=0V IAC=100µA, RSET=10k, VRMS=1.25V IAC=0µA, RSET=15k IAC=450µA, RSET=15k, VRMS=1V, VA Out = 6V IAC=50µA, VRMS=2V, VA=4V IAC=100µA, VRMS=2V, VA=2V IAC=200µA, VRMS=2V, VA=4V IAC=300µA, VRMS=1V, VA=2V IAC=100µA, VRMS=1V, VA=2V (Note 5) RSET=15k RSET=8.2k CT Ramp Peak-to-Valley Amplitude CT Ramp Valley Voltage GATE DRIVER Maximum GT Drv Output Voltage 0mA load on GT Drv, 18V<VCC<35V GT Drv Output Voltage High GT Drv Output Voltage Low, Off GT Drv Output Voltage Low MIN –200mA load on GT Drv, VCC=15V VCC=0V, 50mA load on GT Drv 200mA load on GT Drv 10mA load on GT Drv 10nF from GT Drv to Gnd 1nF from GT Drv to Gnd VCA Out=7V PKLMT=–0.1V PKLMT falling from 50mV to –50mV Note 5: Multiplier Gain Constant (k) is defined by: IMult Out = k × IAC × (VA Out−1) VRMS2 Note 6: Guaranteed by design. Not 100% tested in production. 3 kHz 1.5 2.2 0.4 95 –10 –200 % 10 –100 175 mV µA ns UC1854 UC2854 UC3854 PIN DESCRIPTIONS (Pin Numbers Refer to DIL Packages) Gnd (Pin 1) (ground): All voltages are measured with respect to Gnd. VCC and REF should be bypassed directly to Gnd with an 0.1µF 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 also be as short and as direct as possible. VRMS (Pin 8) (RMS line voltage): The output of a boost PWM is proportional to the input voltage, so when the line voltage into a low-bandwidth boost PWM voltage regulator changes, the output will change immediately and slowly recover to the regulated level. For these devices, the VRMS input compensates for line voltage changes if it is connected to a voltage proportional to the RMS input line voltage. For best control, the VRMS voltage should stay between 1.5V and 3.5V. PKLMT (Pin 2) (peak limit): The threshold for PKLMT is 0.0V. Connect this input to the negative voltage on the current sense resistor as shown in Figure 1. Use a resistor to REF to offset the negative current sense signal up to Gnd. REF (Pin 9) (voltage reference output): REF is the output of an accurate 7.5V voltage reference. This output is capable of delivering 10mA to peripheral circuitry and is internally short circuit current limited. REF is disabled and will remain at 0V when VCC is low or when ENA is low. Bypass REF to Gnd with an 0.1µF or larger ceramic capacitor for best stability. CA Out (Pin 3) (current amplifier output): This is the output of a wide-bandwidth op amp that senses line current and commands the pulse width modulator (PWM) to force the correct current. This output can swing close to Gnd, allowing the PWM to force zero duty cycle when necessary. The current amplifier will remain active even if the IC is disabled. The current amplifier output stage is an NPN emitter follower pull-up and an 8k resistor to ground. ENA (Pin 10) (enable): ENA is a logic input that will enable the PWM output, voltage reference, and oscillator. ENA also will release the soft start clamp, allowing SS to rise. When unused, connect ENA to a +5V supply or pull ENA high with a 22k resistor. The ENA pin is not intended to be used as a high speed shutdown to the PWM output. ISENSE (Pin 4) (current sense minus): This is the inverting input to the current amplifier. This input and the non-inverting input Mult Out remain functional down to and below Gnd. Care should be taken to avoid taking these inputs below –0.5V, because they are protected with diodes to Gnd. VSENSE (Pin 11) (voltage amplifier inverting input): This is normally connected to a feedback network and to the boost converter output through a divider network. Mult Out (Pin 5) (multiplier output and current sense plus): The output of the analog multiplier and the non-inverting input of the current amplifier are connected together at Mult Out. The cautions about taking ISENSE below –0.5V also apply to Mult Out. As the multiplier output is a current, this is a high impedance input similar to ISENSE, so the current amplifier can be configured as a differential amplifier to reject Gnd noise. Figure 1 shows an example of using the current amplifier differentially. RSET (Pin 12) (oscillator charging current and multiplier limit set): A resistor from RSET to ground will program oscillator charging current and maximum multiplier output. Multiplier output current will not exceed 3.75V divided by the resistor from RSET to ground. SS (Pin 13) (soft start): SS will remain at Gnd as long as the IC is disabled or V CC is too low. SS will pull up to over 8V by an internal 14µA current source when both V CC becomes valid and the IC is enabled. SS will act as the reference input to the voltage amplifier if SS is below REF. With a large capacitor from SS to Gnd, the reference to the voltage regulating amplifier will rise slowly, and increase the PWM duty cycle slowly. In the event of a disable command or a supply dropout, SS will quickly discharge to ground and disable the PWM. IAC (Pin 6) (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 Mult Out, so this is the only multiplier input that should be used for sensing instantaneous line voltage. The nominal voltage on IAC is 6V, so in addition to a resistor from IAC to rectified 60Hz, connect a resistor from IAC to REF. If the resistor to REF is one fourth of the value of the resistor to the rectifier, then the 6V offset will be cancelled, and the line current will have minimal cross-over distortion. CT (Pin 14) (oscillator timing capacitor): A capacitor from CT to Gnd will set the PWM oscillator frequency according to this relationship: F= VA Out (Pin 7) (voltage amplifier output): This is the output of the op amp that regulates output voltage. Like the current amplifier, the voltage amplifier will stay active even if the IC is disabled with either ENA or VCC. This means that large feedback capacitors across the amplifier will stay charged through momentary disable cycles. Voltage amplifier output levels below 1V will inhibit multiplier output. The voltage amplifier output is internally limited to approximately 5.8V to prevent overshoot. The voltage amplifier output stage is an NPN emitter follower pull-up and an 8k resistor to ground. 1.25 RSET × CT VCC (Pin 15) (positive supply voltage): Connect VCC to a stable source of at least 20mA above 17V for normal operation. Also bypass V CC directly to Gnd to absorb supply current spikes required to charge external MOSFET gate capacitances. To prevent inadequate GT Drv signals, these devices will be inhibited unless VCC exceeds the upper under-voltage lockout threshold and remains above the lower threshold. 4 UC1854 UC2854 UC3854 PIN DESCRIPTIONS (cont.) GT Drv (Pin 16) (gate drive): The output of the PWM is a totem pole MOSFET gate driver on GT Drv. This output is internally clamped to 15V so that the IC can be operated with VCC as high as 35V. Use a series gate resistor of at least 5 ohms to prevent interaction between the gate im- pedance and the GT Drv output driver that might cause the GT Drv output to overshoot excessively. Some overshoot of the GT Drv output is always expected when driving a capacitive load. TYPICAL CHARACTERISTICS at TA = TJ = 25°C Current Amplifier Gain and Phase vs Frequency Voltage Amplifier Gain and Phase vs Frequency 120 Phase Margin degrees 120 Phase Margin degrees 100 80 100 80 60 60 40 Open-Loop 40 20 Gain dB 0 Open-Loop 20 Gain dB 0 -20 0.1 1 10 100 1000 -20 0.1 10000 1 10 Frequency kHz Gate Drive Rise and Fall Time 1000 10000 Gate Drive Maximum Duty Cycle 100% 700 600 95% Rise Time 500 ns 100 Frequency kHz 90% Fall Time 400 Duty 85% Cycle 300 80% 200 75% 100 70% 0 0 0.01 0.02 0.03 0.04 1 0.05 10 100 RSET, k Ω Load Capacitance, µF Multiplier Output vs Voltage on Mult Oscillator Frequency vs RSET and CT 1000 600 Mult Out=3V 500 Mult Out=1 Mult Out=2V Mult Out=0V 400 100pF Frequency kHz 100 Multiplier Output 300 µA 200pF VRMS=2V, VA Out=5V 500pF 200 1nF 100 10nF 5nF 3nF 2nF 0 10 0 100 200 300 400 500 600 700 1 800 IAC, µA 10 RSET, k Ω 5 100 UC1854 UC2854 UC3854 TYPICAL CHARACTERISTICS at TA = TJ = 25oC (cont.) Multiplier Output vs Multiplier Inputs with Mult Out=0V 600 250 VRMS=1.5V VRMS=3V VA Out=5V 500 200 VA Out=3.5V 400 150 300 VA Out=3V VA Out=2.5V Mult Out µA Mult Out µA 100 200 VA Out=2V VA Out=1.25V 50 100 VA Out=1.25V 0 0 0 100 200 300 400 500 0 100 200 IAC, µA 300 VRMS=4V VA Out=5V 140 500 IAC, µA 140 160 400 VRMS=5V 120 VA Out=5V 120 100 VA Out=4V 100 Mult Out µA 80 80 Mult Out, µA VA Out=3V VA Out=3V 60 60 VA Out=2V 40 40 VA Out=1.5V VA Out=1.25V 20 20 0 0 0 100 200 300 400 0 500 100 200 IAC, µA 300 400 500 IAC, µA APPLICATIONS INFORMATION cycle of this output is simultaneously controlled by four separate inputs to the chip: A 250W PREREGULATOR The circuit of Figure 1 shows a typical application of the UC3854 as a preregulator with high power factor and efficiency. The assembly consists of two distinct parts, the control circuit centering on the UC3854 and the power section. The power section is a "boost" converter, with the inductor operating in the continuous mode. In this mode, the duty cycle is dependent on the ratio between input and output voltages; also, the input current has low switching frequency ripple, which means that the line noise is low. Furthermore, the output voltage must be higher than the peak value of the highest expected AC line voltage, and all components must be rated accordingly. In the control section, the UC3854 provides PWM pulses (GT Drv, Pin 16) to the power MOSFET gate. The duty INPUT PIN # FUNCTION VSENSE........................ 11 .......... Output DC Voltage IAC .................................6 .......... LineVoltage Waveform ISENSE/Mult Out .........4/5 .......... Line Current VRMS .............................8 .......... RMS Line Voltage Additional controls of an auxiliary nature are provided. They are intended to protect the switching power MOSFETS from certain transient conditions, as follows: INPUT PIN # FUNCTION ENA ............................10 .......... Start-Up Delay SS ...............................13 .......... Soft Start PKLIM ...........................2 .......... Maximum Current Limit 6 UC1854 UC2854 UC3854 APPLICATIONS INFORMATION (cont.) ISENSE/Mult Out (Line current): The voltage drop across the 0.25 ohm current-sense resistor is applied to pins 4 and 5 as shown. The current-sense amplifier also operates with high low-frequency gain, but unlike the voltage amplifier, it is set up to give the current-control loop a very wide bandwidth. This enables the line current to follow the line voltage as closely as possible. In the present example, this amplifier has a zero at about 500Hz, and a gain of about 18dB thereafter. PROTECTION INPUTS ENA (Enable): The ENA input must reach 2.5 volts before the REF and GT Drv outputs are enabled. This provides a means to shut down the gate in case of trouble, or to add a time delay at power up. A hysteresis gap of 200mV is provided at this terminal to prevent erratic operation. Undervoltage protection is provided directly at pin 15, where the on/off thresholds are 16V and 10V. If the ENA input is unused, it should be pulled up to VCC through a current limiting resistor of 100k. VRMS (RMS line voltage): An important feature of the UC3854 preregulator is that it can operate with a three-toone range of input line voltages, covering everything from low line in the US (85VAC) to high line in Europe (255VAC). This is done using line feedforward, which keeps the input power constant with varying input voltage (assuming constant load power). To do this, the multiplier divides the line current by the square of the RMS value of the line voltage. The voltage applied to pin 8, proportional to the average of the rectified line voltage (and proportional to the RMS value), is squared in the UC3854, and then used as a divisor by the multiplier block. The multiplier output, at pin 5, is a current that increases with the current at pin 6 and the voltage at pins 7, and decreases with the square of the voltage at pin 8. SS (Soft start): The voltage at pin 13 (SS) can reduce the reference voltage used by the error amplifier to regulate the output DC voltage. With pin 13 open, the reference voltage is typically 7.5V. An internal current source delivers approximately -14µA from pin 13. Thus a capacitor connected between that pin and ground will charge linearly from zero to 7.5V in 0.54C seconds, with C expressed in microfarads. PKLIM (Peak current limit): Use pin 2 to establish the highest value of current to be controlled by the power MOSFET. With the resistor divider values shown in Figure 1, the 0.0V threshold at pin 2 is reached when the voltage drop across the 0.25 ohm current sense resistor is 7.5V*2k/10k=1.5V, corresponding to 6A. A bypass capacitor from pin 2 to ground is recommended to filter out very high frequency noise. PWM FREQUENCY: The PWM oscillator frequency in Figure 1 is 100kHz. This value is determined by CT at pin 14 and RSET at pin 12. RSET should be chosen first because it affects the maximum value of IMULT according to the equation: CONTROL INPUTS VSENSE (Output DC voltage sense): The threshold voltage for the V SENSE input is 7.5V and the input bias current is typically 50nA. The values shown in Figure 1 are for an output voltage of 400V DC. In this circuit, the voltage amplifier operates with a constant low frequency gain for minimum output excursions. The 47nF feedback capacitor places a 15Hz pole in the voltage loop that prevents 120Hz ripple from propagating to the input current. IMULTMAX = This effectively sets a maximum PWM-controlled current. With RSET=15k, IMULT MAX = IAC (Line waveform): In order to force the line current waveshape to follow the line voltage, a sample of the power line voltage in waveform is introduced at pin 6. This signal is multiplied by the output of the voltage amplifier in the internal multiplier to generate a reference signal for the current control loop. This input is not a voltage, but a current (hence IAC). It is set up by the 220k and 910k resistive divider (see Figure 1). The voltage at pin 6 is internally held at 6V, and the two resistors are chosen so that the current flowing into pin 6 varies from zero (at each zero crossing) to about 400µA at the peak of the waveshape. The following formulas were used to calculate these resistors: RAC = −3.75V = −250µA 15k Also note that the multiplier output current will never exceed twice IAC. With the 4k resistor from Mult Out to the 0.25 ohm current sense resistor, the maximum current in the current sense resistor will be IMAX = −IMULTMAX ×4k = −4A 0.25Ω Having thus selected RSET, the current sense resistor, and the resistor from Mult Out to the current sense resistor, calculate CT for the desired PWM oscillator frequency from the equation Vpk 260VAC × √ 2 = = 910k IACpk 400µA RREF = −3.75V RSET CT = RAC = 220k 4 (where Vpk is the peak line voltage) 7 1.25 F × RSET UC1854 UC2854 UC3854 FIGURE 1 - Typical Application This diagram depicts a complete 250 Watt Preregulator. At full load, this preregulator will exhibit a power factor of 0.99 at any power line voltage between 80 and 260 VRMS. This same circuit can be used at higher power levels with minor modifications to the power stage. See Design Note 39B and Application Note U-134 for further details. UDG-92056-1 NOTE: Boost inductor can be fabricated with ARNOLD MPP toroidal core part number A-438381-2, using a 55 turn primary and a 13 turn secondary. UNITRODE CORPORATION 7 CONTINENTAL BLVD. • MERRIMACK, NH 03054 TEL. (603) 424-2410 FAX (603) 424-3460 These products contain patented circuitry and are sold under license from Pioneer Magnetics, Inc. 8 IMPORTANT NOTICE Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue any product or service without notice, and advise customers to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. 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