advertisement Single IC, Power Factor Corrected, Off-Line Supply Design Note 143 Kurk Mathews An ever increasing number of off-line power supplies now include power factor correction (PFC) in order to reduce input current and meet future regulatory requirements. Switching power supplies that incorporate a bridge rectifier followed by bulk capacitance create harmonic currents. These harmonics only increase the supply’s RMS input current, while contributing nothing to real power. The typical solution to this problem has been to add a PFC preregulator and a separate controller to an existing design. The LT®1508 (voltage mode) and LT1509 (current mode) eliminate the need for separate controllers by combining the PFC and a pulse width modulator (PWM) function in a single 20-pin IC. PFC is achieved by programming the input current of a boost regulator to follow the input line voltage. This results in a near-unity power factor compared to 0.5 to 0.7 for a typical capacitive input switcher. Linear’s unique architecture maintains 0.99 power factor over a 20:1 load range. Figure 1 shows input current with output powers of 30W, 150W and 300W. A number of issues associated with a 2-IC approach are addressed within the LT1508 and LT1509, resulting in a 2A/DIV 5ms/DIV DN143 F01 Figure 1. Input Current at 30W, 150W and 300W 12/96/143 lower part count and improved PC board layout. Start-up is controlled by separate PFC and PWM soft start pins. The PWM Soft Start pin is held low, disabling the PWM output until the PFC stage is in regulation. The PWM will remain enabled as long as the PFC output voltage stays above 73% of its preset value (typically 280V out of 383V for universal input). A separate overvoltage protection pin can be connected to the output through an independent resistor divider. This ensures overvoltage protection during safety agency abnormal testing conditions, such as opening the main feedback path. The two stages are synchronized and the PWM turn-on is delayed for 50% of the oscillator cycle. This minimizes noise and conducted emission problems. 2A peak current gate drivers and a 1.2V optoisolator offset on the VC pin further simplify the design. A universal input, 24VDC, 300W converter using the LT1508 is shown in Figure 2. The circuit’s user benefits include low cost, customizable footprint and off-the-shelf magnetics designed to meet UL and EC safety standards. Following the PFC boost preregulator is a 2-transistor forward converter that features low voltage (500VDC) switches, low peak currents and automatic nondissipative core reset. Under worst case conditions (low line, full power), the PFC and PWM stages have efficiencies of 90% and 92% respectively. See the graph in Figure 2 for typical overall efficiency versus input line and output power. The LT1508’s low start-up current (250µA) minimizes start-up resistor power dissipation. An overwinding on T1 provides the bootstrapped chip supply. The intermediate bus voltage of 382V is well controlled, simplifying the post regulator and increasing capacitor holdup time compared to a typical off-line converter. This results in lower transformer primary current and simplified magnetics design. Different or additional outputs are easily accommodated with this topology by modifying T2 and L1. , LTC and LT are registered trademarks of Linear Technology Corporation. 499k 1% 70 75 80 20k 1% 0.047µF EMI FILTER 9 11 132 3 2 180 VRMS 0.001µF 30W 100W 264 1µF FILM LT1508 13 SS2 4.02k 1% R1 0.15Ω 5W 15V + 16 SS1 200µF 17 VCC 1µF FILM – + + + 20 VREF 2k 18 VC ILIM 19 GTDR2 + 330µF 35V FUJI ERA82-004 0.6A/40VR 0.01µF CNY17-3 100pF 1N5819 20k 15V 1N5819 20Ω 2.2µF 50V 2.2µF 50V 6 1 CAOUT GTDR1 ERA82-004 ERA82-004 T1 20k 0.047µF 7 ISENSE 0.001µF 300pF 4.02k 1% 91k 2W T1 GND-F COMP 6 2 1 COLL LT1431 2.2k + C1 1µF 400V 33Ω 0.068µF 1k 470Ω IRFP450 MURH860CT (DUAL) – + GND-S 5 3 V+ RTOP 10Ω 7 REF 8 RMID 4 20Ω 10:15 TURNS 0.1µF 10Ω 1µF FILM T3 470µF 450V 382VBUS Figure 2. 24V, 300W Off-Line PFC Supply NOTE: UNLESS OTHERWISE SPECIFIED 1. ALL RESISTORS 1/4W, 5% 2. ALL CAPACITANCE VALUES IN MICROFARADS RT1 = KETEMA SG57 SURGE GARD T1 = COILTRONICS CTX02-13141-X2, (407) 241-7876 T2 = COILTRONICS CTX02-13063-X3 T3 = BI TECHNOLOGY HM41-11510, (714) 447-2345 C1 = ELECTRONIC CONCEPTS 5MP12J105K R1 = JW MILLER/FUKUSHIMA MPC71 BR1 = GENERAL INSTRUMENTS KBU6J L1 = COILTRONICS CTX02-13140-X2 15k 15 RSET 5 8 PKLIM MOUT 10k 12 VREF 200W/300W 4 CSET 10 0.1µF 1.8k BR1 VREF 0.001µF VIN RT1 VAOUT 330k Efficiency GND1 GND2 IAC OVP 14 VSENSE 0.47µF 0.0047µF 90 85 90 20k 1% 499k 1% 499k 1% 382VBUS 499k 1% VIN EFFICIENCY (%) 90VAC TO 264VAC + 6A FAST Danger!! Lethal Voltages Present In This Section + (408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977 – Linear Technology Corporation – For literature on our Power Products, call 1-800-4-LINEAR. For applications help, call (408) 432-1900, Ext. 2360 LT/GP 1296 185K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 LINEAR TECHNOLOGY CORPORATION 1996 3.4k 1% 30.1k 1% 470µF, 50V NICHICON PL12,5X25 (× 3) + 4700pF "Y" L1 MUR150 IRF840 OUTPUT COM 1µF 63V FILM + + 20k 2200pF "Y" 20k 15V 1N965 (× 2) 2000pF 10Ω, 2W 0.51Ω, 2W RG ALLEN RPS2 (× 2) IRF840 MUR150 G1 FEP 30DP (DUAL) 1000pF 10Ω 1W 24VOUT 12.5A T2