Designing the Power Train of a 00W Power Supply with PFC Michael Weirich Global Power Resource Center, Europe www.fairchildsemi.com
Design issues for 00W PSU System Specification AC Input Voltage: 85 65 Vrms Power-Factor: > 0.95 Total Output Power: 00W Three DC Outputs: 5V/0.3A (standby) V/6A, 4V/5A Height limit: 5mm Target application: LCD TV Design Issues System partitioning Low profile boost inductor PFC MOSFET dissipation Using single switch forward tools to design two-switch forward converters Two-switch forward drive Second post-regulated output for a forward converter Auxiliary power supply Layout
Control Component Overview PFC/two switch forward circuit FAN4800 combined CCM PFC and current mode controller FAN738 dual high and low side driver Post regulation circuit Low cost MC34063A controller with external MOSFET Auxiliary power supply FSD0 Fairchild Power Switch 3
System Partitioning 85V 65VAC input Isolation Boundary Rectifier PFC Stage Two Switch Forward Primary Side Forward Transformer Forward Output 4V, 6A 4V,.5A PFC/PWM Combo controller Buck Postregulation V, 5A Flyback Auxiliary Primary Side Bias Power Flyback Transformer Flyback Output 5V, 0.3A 4
Rectifier dimensioning and tips Output power Output power is 00W Efficiency assumptions: PFC stage: 90% Forward converter: 90% Maximum input current (rms) is.9a 00W P In = = 47W 0.9 0.9 I 47W 85V In, RMS = =. 9 A Bridge rectifier Rectifiers with larger current ratings typically have lower forward voltages We chose GBU6K (6A/800V): Vf=0.45V Estimate the equivalent resistance from the datasheet: R S =0.03 ohm P Loss, BR = 4 ( I = 4 ( I I In, + = 4.7W AVG, D In, RMS RMS V F, D + I RMS, D 0.45 0.8V 0.03Ω) R S, D ) Conclusion 4.7W dissipation ºC/W heatsink needed R Θ, BR T = J,max P T Loss, BR A,max 50 C -50 C 0.75 C = 4.7W W C W 5
PFC inductor dimensioning Pin = 47W, Input rms current =.9A Inductor inductance value FAN4800 PFC set to switch at 00kHz di, the percentage current ripple is set to 0% Inductor peak current The peak value of the input rms current is added to half of the ripple current L I = ( ) V V Out V Out f S In,min V di P (400V 85V ) 85V = 400V 00kHz 0% 47W =.08mH Peak, L = I In, RMS + I In Ripple In,min Conclusion mh inductor 4.5A peak current = = = I I In, RMS In, RMS 4.5A + 0. I. In, RMS 6
PFC inductor design () Input rms current =.9A, peak = 4.5A Wire thickness Copper wire: 5A/mm for rms current 0.58mm copper wire area (ACu) Three or four strands required: skin effect Design used three strands of 0.5mm wire Core size Use the area product Ap to estimate core size from Ae (magnetic cross section) and Aw (winding cross section) Bpeak is 0.35 Tesla for standard ferrite core Conclusion Area product is 494 mm4 Skin depth: δ = P π μ μ σ f 0 A = 494mm r Proximity effect: useful area of a conductor is reduced due to magnetic field generated by adjacent layers A P = A e A 4 w = L I B Peak Peak f A Cu Cu 7
PFC inductor design () Area product is 494 mm4 General selection Normally Ae and Aw are equal, so as a starting point, select Ae as mm >> no 5mm height bobbins available Specific selection Look for cores meeting area product and size requirements: EER354 core (Ae=07mm,Aw=54mm) Turns Aw is 54mm, ACu is 0.58mm Assuming a fill factor of 50%, we can get 30 turns.. Gap A gap is needed to get the required Inductance. AL = mh/30^ s 0.4 π A e A L A s is the gap size, AL,0 is the AL value for an ungapped core Adjust the turns to get a standard gap size Conclusion Gap is mm, 4 turns, mh inductance mh 4.5A Bmax = = 0. 34T 4 07mm L,0 8
Low Profile Boost Inductor Specification PFC Choke Winding Details Name Pins (Start End) Layers Strands x Wire ø Turns Construction Material W 8 6 6 3 x 0.5 mm 4 perfect solenoid CuLL Electrical Characteristics Parameter Pins Specification Conditions Inductance 8 6 000 uh +/- 5% 0kHz, 00mV Core and Bobbin Core: EER354 Material: PC40 (TDK) or equivalent Bobbin: EER354 / 6 Pin / Horizontal e.g. Pin Shine P-3508 Gap in center leg:approx..0 mm for an A L of 65 nh/turns 9
PFC MOSFET Selection Voltage and current ratings 500V MOSFET needs external surge protection 600V MOSFET preferred Peak current same as inductor (4.5A) Power dissipation and heatsink FCP6N60 chosen: RDSon=0.45ohm, Coss,eff=0pF Cext=50pF, (estimated parasitic capacitance) Conclusions Power dissipation: 9W 0ºC/W heatsink needed I RMS, Q P P P = I In, RMS =.9A =.5A = I 8 V 3 V Out 8 85V 3 π 400V =.8W R In, Min Cond Loss, Q RMS, Q DSONmax, Q Cap Loss,Q Cross Loss, Q 0. 5 PLoss rr, Q ( C + C ) OSS,eff ext V Out = 0. 5 60 pf 400V 00kHz =. W 0. 9 I In,RMS VOut tcrossover fs 6 = 0. 9. 9A 400V 0. 5 50ns 00kHz =. 6W W f S 0
FAN4800 PFC Circuit - + ~ ~ BR GBU6K D5 FDLL448 D ISL9R460P R8 0K D0 N448 D N448 C 470nF D N448 Ieao Iac Isense 3 Vrms 4 Ss 5 Vdc 6 Ramp 7 Ramp 8 Ilim 9 GND 0 Vo Vo Vcc 3 Ref 4 Vfb 5 Veao 6 IC FAN4800 C6.nF Rc 390K R4 7k C6 680pF R3 0K R7b M Ra 390K D6 FDLL448 C3 00nF Ra 60K Rb 60K + C5c 8uF 450V C 470nF 50V C4 00nF 5V R 47K C 680nF 400V C3 470nF NTC R A C48 nf R7a 60K + C5b 8uF 450V Rb 390K R4 8K R 80K C 680nF X C7 0pF R3 4.7K LF 0mH C5 nf R5 0.5 W C8 00nF + C5a 8uF 450V R35 00 8 6 L.0mH L3 FERRITE BEAD J GSF.00.3 C3 00nF C9 0nF R5 Q FCP6N60 Vcc Vbus 0
Two-switch forward design with PFC front-end Use the forward design spreadsheet Enter 84V to emulate PFC input Use a very large DC link capacitor to account for the PFC stage Set the maximum duty cycle to 0.45 to ensure demagnetisation The ratio Np/Nr can be ignored as there is no need for a reset winding. Define specifications of the SMPS Minimum Line voltage (V_line.min) Maximum Line voltage (V_line.max) Line frequency (fl) For forward converter with reset winding Blue cell Red cell is the input parameters is the output parameters 84 V.rms 84 V.rms 50 Hz Vo Io Po KL st output for feedback 4 V 8.5 A 03 W 00 nd output 0 V 0 A 0 W 0 3rd output 0 V 0 A 0 W 0 4th output 0 V 0 A 0 W 0 Maximum output power (Po) = 0.8 W Estimated efficiency (Eff) 90 % Maximum input power (Pin) = 5.3 W A single switch forward would need 803V rating without any safety factor two-switch forward designs need half of this. Determine DC link capacitor and the DC voltage range DC link capacitor 000 uf DC link voltage ripple = 4 V Minimum DC link voltage = 397 V Maximum DC link voltage = 40 V 3. Determine the maximum duty ratio (Dmax) Maximum duty ratio 0.45 Turns ratio (Np/Nr) > Maximum nominal MOSFET voltage = 803 V 0.8
Two Switch Forward Output Inductor Output Inductor Design Current ripple should be small as possible to reduce the RMS and peak values of the primary and secondary currents. Small current ripple results in large inductors Design compromise: Current ripple is set to 4% (Note KRF used in spreadsheet is half of the current ripple) Output inductance 40uH The calculated windings will fill an EER88 core completely. Choke 40uH / 9A (L5) Winding Details Name Pins (Start End) Layers Strands x Wire ø Turns Construction Material W,,3,4,5 8,9,0,, 6 5 x 0.7 mm 5 perfect solenoid CuL Electrical Characteristics Parameter Pins Specification Conditions Inductance 5 40 uh +/- 5% 0kHz, 00mV 3
Two Switch Forward Transformer Main Transformer Specification Winding Details Pins (Start Strands x Wire Name Layers Turns Construction Material End) ø Wa 6 3 x 0.5 mm 39 perfect solenoid CuLL W3 0,, 7,8,9 3 x 0.7 mm perfect solenoid Triple insulated Wb 3 x 0.5 mm 38 perfect solenoid CuLL = 3 Layers of Tape e.g. 3M 350 3 6 W b W a 0 9 8 7 3 W b 0,, W 7,8,9 6 W a 3 Layers not to scale! Schematic Construction Electrical Characteristics Parameter Pins Specification Conditions Primary Inductance 6 3 mh +/- 30% 0kHz, 00mV, all secondaries open Leakage Inductance 3 500 uh maximum 0kHz, 00mV, all secondaries short Core and Bobbin Core: EER834 Material: PC40 (TDK) or equivalent Bobbin: EER834 / Pin / Horizontal e.g. Pin Shine P-809 Gap in center leg: 0 mm 4
Two-switch forward MOSFETs From forward calculation spreadsheet: Peak current is.6a RMS current is 0.9A Voltage and current ratings 600V MOSFET preferred Peak current same as inductor (.6A) Power dissipation and heatsink FCP7N60 chosen: Rdson=.ohm, 00ºC Coss,eff=60pF Cext=60pF, (estimated) As tr=0ns and tf=75ns in datasheet are measured at 7A scale by.6a/7a Conclusions Power dissipation: 3.7W 0ºC/W heatsink needed P P P P Cap Loss, Q05 Cross Loss, Q05 Tot Loss, Q05 = I = 0.9A = 0.9W 3.7W R Cond Loss, Sw RMS, Sw DSONmax, Sw 0.5.Ω ( C + C ) OSS, eff ext V Out = 0.5 0 pf 400V 00kHz =.W t r + t f I Peak, Q05 VOut f S 0 + 75.6 =.6A 400V ns 7 =.4W f S 00kHz 5
6 FAN4800 Two Switch Forward Converter 4V R4 n.a. C49 nf R46 33 Q FCP7N60 CONN5 BP-VH R5 R05 3k R0 0k C35 00nF R45.5K C5 nf TR Main Transformer R4 n.a. C6 680pF + C47 n.a. R33 0.47 R4 7k R40 n.a. D6 RSK + C46 680uF OC FOD74BTV R3 4.7K Ieao Iac Isense 3 Vrms 4 Ss 5 Vdc 6 Ramp 7 Ramp 8 Ilim 9 GND 0 Vo Vo Vcc 3 Ref 4 Vfb 5 Veao 6 IC ML4800IS R47 33 D0 FYP00DN R6 D4 FDLL448 C3 5nF L5 40uH 9A D4 n.a. C38 470pF C39 0nF C50 nf + C45 680uF Q FCP7N60 D7 UF5407 Q0 n.a. + C44 680uF D8 UF5407 D9 FYP00DN C 470nF 50V D3 FDLL448 VCC HIN LIN 3 COM 4 LO 5 VS 6 HO 7 VB 8 IC4 FAN738N R04 80K R34 0 C30 n.a. Vbus Vcc Vcc
Waveforms for low side switch and output diode Low side switch voltage and current Output diode voltage and current 7
Detail of High Voltage Driver Circuit Vbus Vcc D3 FDLL448 D6 RSK C35 00nF D7 UF5407 R5 Q FCP7N60 IC4 C39 0nF 3 4 VCC HIN LIN COM VB 8 HO 7 VS 6 LO 5 FAN738N R6 Q FCP7N60 C38 470pF R34 0 R33 0.47 D8 UF5407 TR Main Transformer D4 FDLL448 8
Rectifier diodes From forward calculation spreadsheet: Diode reverse output voltage is 57V Output current is 8.5A Voltage and current ratings 00V Schottky diode used: FYP00DN Average current: 4.7A Power dissipation and heatsink Rs=0.04ohm (from datasheet curves) Vf=0.5V Snubber network Schottky diodes switch fast causing oscillations RC snubber network needed to damp these Conclusions Power dissipation:.5w 0ºC/W heatsink needed Snubber network needed I P Avg, rect Loss, rect = I Out ( Dmax ) = 8.5A 0.55 = 4.7 A I Avg, rect =.5W V F,Re ct + = 4.7A 0.5V + 5.7A I RMS, rect R 0.04Ω S,Re ct 9
MC34063A Post-regulation Circuit 4V R9 R3 R33 0.R 8 7 6 5 DRc Ipk Vcc FB SWc SWe Ct Gnd IC5 KA34063AD 3 4 R 680 R6 680 C9 00pF Q BC848B BC858C Q 3 Q09 FQP47P06 L6 00uH 6A D3 MBR745 + C5 470uF 35V + C8 680uF C5 00nF V R5 3K CONN4 R.5K V/6A BP-VH 0 Hysteretic buck control (or ripple regulator) 0
Inductor for Post Regulated Output Choke 00uH / 6A (L6) Winding Details Name Pins (Start End) Layers Strands x Wire ø Turns Construction Material W,,3,4,5 8,9,0,, 5 x 0.56mm 5 perfect solenoid CuL Electrical Characteristics Parameter Pins Specification Conditions Inductance 5 00 uh +/- 5% 0kHz, 00mV Core and Bobbin Core: EER88 Material: PC40 (TDK) or equivalent Bobbin: EER88 / Pin / Horizontal e.g. Pin Shine P-86 Gap in center leg:approx. 0.45 mm for an A L of 05nH/Turns
Auxiliary Power Supply using FSD0 Vbus TR Stdby Transformer L4 uh R30 75k C0.nF 3 4 8 6 D08 SB380 5V CONN6 R3 47 5 UF4007 D5 D6 N448 + C3 00uF + C33 00uF C36 0nF GND VSTR 8 GND DRAIN 7 3 GND 4 VFB VCC 5 IC6 FSD0BM R7 K R8 3.3K C nf + C 4.7uF R30 C37 0.5K 0nF R9 K D5 8V 0.5W OC3 FOD7BTV Standby ON/OFF 4 R35 Vcc 3 75 CONN OC HA87A.W C53 4.7nF
Flyback Transformer for Auxiliary Output Standby Transformer Specification Winding Details Name Pins (Start End) # of Layers Strands x Wire ø Turns Construction Material Wa 3 x 0.5 mm 9 spaced winding CuLL W3 8 6 x 0.5 mm 8 spaced winding Triple insulated Wb x 0.5 mm 9 spaced winding CuLL W 4 5 x 0.5 mm 4 spaced winding CuLL W b 4 W 5 3 4 W a W W3 8 6 W b 8 W3 6 3 W a = 3 Layers of Tape e.g. 3 M 350 = Layer Tape e.g. 3 M 350 5 Layers not to scale! Schematic Construction Electrical Characteristics Parameter Pins Specification Conditions Primary Inductance 3 5.85 mh +/- 5% 0kHz, 00mV, all secondaries open Leakage inductance 3 90 uh maximum 0kHz, 00mV, all secondaries short Core and Bobbin Core: EF 0 Material: FI35 (Vogt) or equivalent Bobbin: EF0 / 0 Pin / Horizontal / Increased creepage Gap in center leg: approx. 0. mm for A L of 77 nh/turns 3
Layout and Heatsink General Power Supply Layout Rules the enclosed area of loops with high di/dt must be as small as possible the copper area of nodes with high dv/dt must be as small as possible. avoid common impedance coupling by using star connections to ground These rules conflict with other requirements Heatsink construction at the edge of a board Star connection of all ground lines would enlarge the PCB Low cost solutions require single sided PCB s which result in longer traces Compromise Critical signals are routed to the shortest path Less critical signals give way to the large ground plane which emulates star-like connection Heatsink All devices except Q are connected to a simple heatsink made of mm aluminium bent into the form of a U Q dissipates more power so needs an additional heatsink 4
Layout and photo of finished board Dimensions: 70mm x 56mm x 5mm (L x W x H) 5
Standby power and efficiency 0.6 Standby Power [W] 0.5 0.4 0.3 0. 0. 0.0 85 0 35 60 85 0 35 60 Input Voltage [Vrms] Less than 0.5W standby power for 95V-65V Efficiency [%] 89.0 87.0 85.0 83.0 8.0 79.0 77.0 75.0 85 0 35 60 85 0 35 60 Input Voltage [Vrms] Overall efficiency target of 8% (90%x90%) met 6
Summary Design Issues System partitioning Low profile boost inductor PFC MOSFET dissipation Using single switch forward tools to design two-switch forward converters Two-switch forward drive Second output for a forward converter Auxiliary power supply Layout Performance 7