AN2447 Application note

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Application note Quasi-resonant flyback converter for low cost set-top box application Introduction This application note describes how to implement a complete solution for a 17 W switch mode power

1 Application note Quasi-resonant flyback converter for low cost set-top box application Introduction This application note describes how to implement a complete solution for a 17 W switch mode power supply for set-top box applications using a dedicated evaluation board based on the primary controller L6565, working in quasi resonant (QR) mode. Two versions of the evaluation board are available: STEVAL-ISA016V1 accepts European input voltage (185 to 265 V ac ) and delivers 6 outputs. STEVAL-ISA017V1 accepts US input voltage (88 to 135 V ac ) and delivers 6 outputs. Power supply for set-top box applications March 2007 Rev 2 1/22

2 Contents AN2447 Contents 1 Specifications Power supply description Input section Output section Experimental results Output voltages Start-up behavior at full load Conducted EMI measurements Transformers Board layout Conclusions Reference Revision history /22

3 List of figures List of figures Figure 1. Converter schematic Figure 2. Drain voltage and drain current - Vin = 185 V ac Figure 3. Drain voltage and drain current - Vin = 230 V ac Figure 4. Drain voltage and drain current - Vin = 265 V ac Figure 5. Drain voltage and drain current - Vin = 88 V ac Figure 6. Drain voltage and drain current - Vin = 110 V ac Figure 7. Drain voltage and drain current - Vin = 135 V ac Figure 8. European version start-up - 12 V V V at 230 V ac full load Figure 9. European version start-up - 5 V - 8 V V at 230 Vac full load Figure 10. US Version start-up - 12 V V Vac full load Figure 11. US Version start-up - 5 V - 8 V Vac full load Figure 12. Conducted EMI V ac - line 1 and line Figure 13. Conducted EMI V ac - line 1 and line Figure 14. Schematic diagram Figure 15. Board layout - top view Figure 16. Board layout - bottom view /22

4 List of tables AN2447 List of tables Table 1. Output specifications Table 2. Part list for 230 V ac board Table 3. Part list for 110 V ac board Table 4. Output voltage and efficiency - European version - Vin = 185 V ac full load Table 5. Output voltage and efficiency - European version - Vin = 230 V ac Table 6. Output voltage and efficiency - European version - Vin = 265 V ac Table 7. Output voltage and efficiency - US version - Vin = 88 V ac Table 8. Output voltage and efficiency - US version - Vin = 110 V ac Table 9. Output voltage and efficiency - US version - Vin = 135 V ac Table 10. Output voltage and efficiency - European version - Vin = 230 V ac no load Table 11. Output voltage and efficiency - US version - Vin = 110 V ac Table 12. Transformers - electrical specifications Table 13. Revision history /22

5 Specifications 1 Specifications The main specifications of the SMPS are listed below: Input: Vin: 185 to 265 V ac (European version) 88 to 135 V ac (US version) mains frequency: 50/60 Hz Output: Table 1. Output specifications Voltage Maximum current Stability Output 1 12 V 0.5 A ±10% Output 2 8 V 0.04 A ±15% Output 3 5 V 1 A ±5% Output V 0.45 A ±2% Output V 1.7 A ±2% Output V 0.7 A ±2% EMC: EN55022 Class B on conducted emission Safety: UL60950 and EN60950 The schematic of the converter is shown in Figure 1. 5/22

6 Power supply description AN Power supply description The power supply has been designed according to the specs given in Chapter 1: Specifications. The choice of having two versions of the board, optimized for each single input voltage range, is due to lower cost as well as size considerations. The chosen topology is the Flyback operated in valley commutation by the primary controller L6565, performing variable frequency control. The primary switch is a MOSFET, STD3NK80Z (SuperMESH 800 V-3.8 Ω typ.) for European boards and STD5NK50Z (SuperMESH 500 V Ω typ.) for US boards, in DPAK packages. The voltage spike on the drain due to leakage inductance is clamped by the network R7+R8+C4+D2, minimizing the EMI impact of the RCD clamp. The controller L6565 is activated by a couple of dropping resistors (R5+R6) that draws current from the DC bus and charges the supply capacitor C3. During normal operations the controller is powered by the auxiliary winding of the transformer, via the diode D3 and the resistor R12. Voltage feed-forward is given by R2+R3 and R4 in order to control the power capability change vs. the input voltage. Pulse-by-pulse current limit is automatically adjusted on input voltage value. The current in the primary of the transformer is detected by the resistor R14. The output rectifiers have been chosen in accordance with the maximum reverse voltage and power dissipation. The rectifiers for 3.3 V and 5 V outputs are Schottky diodes, STPS5L25B and STPS5L40 respectively, in order to achieve low forward voltage drop and lower power dissipation with respect to standard types. The rectifier for 12 V output is a fast recovery diode, SMBYW /22

7 Power supply description Figure 1. Converter schematic 7/22

8 Power supply description AN Input section The input section consists of a bridge rectifier and EMI filter. In particular, the EMI filter features two electrolytic capacitors, C1 and C2, and a common mode choke, L1, in between. An NTC limits the inrush current of the input capacitors at plug-in. Either a standard fuse or a fuse resistor (not flammable) is used for either the US or the European version of the board. This is due to standard compliance which is UL for the US and IEC for Europe. 2.2 Output section The output voltage regulation is performed by secondary feedback on the 3.3 V output, while the other outputs are regulated by the transformer coupling. The feedback network is based on the shunt regulator TS2431 which drives an optocoupler, ensuring the required insulation between primary and secondary. The opto-transistor drives directly the COMP pin of the controller. The 8 V output is linearly post-regulated from the 12 V output, using a 9.1 V zener regulator, D6, a resistor, R15, and a small signal NPN transistor, T2. The 1.23 V output is postregulated from the 5 V output using a DC-DC converter, L5973D, in order to have maximum efficiency. The 2.5 V is linearly post-regulated from the 3.3V output using a very low drop voltage regulator (typ. 0.4 V at 800 ma), LD29080DT25. A small LC filter has been connected on the 5 V and 3.3 V outputs, in order to have very low voltage ripple with reasonable output capacitor value and quality. Table 2. Part list for 230 V ac board Reference Value Description F1 1 A Fuse resistor R1 16 Ω NTC R2 0 Ω R3 1.5 MΩ 1/2 W R4 10 kω 1/2 W R5 0 Ω R6 270 kω 1/2 W R7 150 kω 1/4 W R8 1 kω 1/4 W R9 1 kω 1/8 W R kω 1/8 W R11 10 Ω 1/2 W R12 33 Ω 1/4 W R13 47 kω 1/8 W R Ω 1/2 W R Ω 1/8 W R16 75 Ω 2 W 8/22

9 Power supply description Table 2. Part list for 230 V ac board (continued) Reference Value Description R Ω 1/8 W R kω 1/8 W 1% R kω 1/8 W 1% R20 10 kω 1/8 W 1% R kω 1/8 W R kω 1/8 W R23 47 kω 1/8 W C1 10 µf Electr. 400 V C2 22 µf Electr. 400 V C3 47 µf Electr. 25 V C4 10 nf Ceramic 200 V C5 10 nf Ceramic 25 V C6-C7 1 nf Ceramic Y1 250 V ac C8-C9-C10-C11-C µf Electr. 16V TEAPO SEK 105 C C13 n.c. Ceramic C14-C15-C16-C µf Electr. 16V TEAPO SEK 105 C C18 47 nf Ceramic 25 V C µf Electr. 16V TEAPO SEK 105 C C pf Ceramic 25 V C nf Ceramic 25 V C22-C µf Electr. 16V TEAPO SEK 105 C C24 10 µf Electr. 16V 105 C D1 1 A V Diode bridge D2 STTH108 1 A 800 V D3 1N A 75 V D4 SMBYW Rectifier 2 A 200 V D5 9.1 V Zener D6 STPS5L40 Schottky 5 A 40 V D7 STPS5L25B Schottky 3 A 40 V D8 STPS340 Schottky 3 A 40 V L1 2 x 1 mh Epcos CM choke L2 1 µh Coilcraft DR0810 L3 1 µh Coilcraft DR0608 L4 10 µh Coilcraft DR0608 IC1 L6565 STMicroelectronics 9/22

10 Power supply description AN2447 Table 2. Part list for 230 V ac board (continued) Reference Value Description IC2 PC817-B SHARP IC3 TS2431LT STMicroelectronics IC4 L5973D STMicroelectronics IC5 LD29080DT25 STMicroelectronics T1 STD3NK80Z STMicroelectronics T2 BC847 STMicroelectronics TR1 Flyback transformer HICAL MAR 5146 In Table 3 the different part references for 110V ac (US version) board are listed. Table 3. Part list for 110 V ac board Reference Value Description F1 1 A Fuse R Ω 1/2 W C1 22 µf Electr. 200 V C2 4 µf Electr. 200 V T1 STD5NK50Z STMicroelectronics TR1 Flyback transformer HICAL MAR /22

11 Experimental results 3 Experimental results All measurements have been performed at ambient temperature (about 25 C), with 185 V ac V ac -265 V ac input voltage (88 V ac -110 V ac -135 V ac for the US version). In Figure 2 to Figure 7 some waveforms during normal operations at full load are shown. In particular, the drain voltage and current at minimum, nominal and maximum input mains voltage are shown, both for the European and the US versions of the board. The drain peak voltage at full load and maximum input mains voltage is about 770 V for European versions and 484 V for US versions. Figure 2. Drain voltage and drain current - Vin = 185 V ac Figure 3. Drain voltage and drain current - Vin = 230 V ac 11/22

12 Experimental results AN2447 Figure 4. Drain voltage and drain current - Vin = 265 V ac Figure 5. Drain voltage and drain current - Vin = 88 V ac Figure 6. Drain voltage and drain current - Vin = 110 V ac 12/22

13 Experimental results Figure 7. Drain voltage and drain current - Vin = 135 V ac 3.1 Output voltages In the following tables the output voltage cross regulation and the efficiency of the converter are given. All the output voltages are measured on the output connector pins. Test condition: full load Table 4. Output voltage and efficiency - European version - V in = 185 V ac full load Iout [A] Vout [V] Pout [W] Efficiency Pin = 24.6 W η = 67.61% Table 5. Output voltage and efficiency - European version - V in = 230 V ac Iout [A] Vout [V] Pout [W] Efficiency Pin = 24.3 W η = 68.3% /22

14 Experimental results AN2447 Table 6. Output voltage and efficiency - European version - V in = 265 V ac Iout [A] Vout [V] Pout [W] Efficiency Pin = 24.3 W η = 68.25% Table 7. Output voltage and efficiency - US version - V in = 88 V ac Iout [A] Vout [V] Pout [W] Efficiency Pin = 25.5 W η = 64.54% Table 8. Output voltage and efficiency - US version - V in = 110 V ac Iout [A] Vout [V] Pout [W] Efficiency Pin = 24.6 W η = 66.82% 14/22

15 Experimental results Table 9. Output voltage and efficiency - US version - V in = 135 V ac Iout [A] Vout [V] Pout [W] Efficiency Pin = 24.2 W η = 67.8% Test condition: no load (see Table 10 and Table 11 for details) Table 10. Output voltage and efficiency - European version - V in = 230 V ac no load Iout [A] Vout [V] Pout [W] Efficiency Pin = 1.1 W η = 18.07% Table 11. Output voltage and efficiency - US version - V in = 110 V ac Iout [A] Vout [V] Pout [W] Efficiency Pin = 1.3 W η = 56.59% 3.2 Start-up behavior at full load In this paragraph the output voltage waveforms at start up are shown. In Figure 8 to Figure 11, the rising slopes for all output voltages at nominal input mains voltage are shown, respectively for the European and US input voltage range. 15/22

Experimental results AN2447 Figure 8. European version start-up - 12 V - 3.3 V - 1.23 V at 230 V ac full load Figure 9.

23 V @110 V ac full load Figure 11. US Version start-up - 5 V - 8 V - 2.5 V @110 V ac full load 3.

for European and US boards, in case of nominal input voltage, and the QP and average limits.

16 Experimental results AN2447 Figure 8. European version start-up - 12 V V V at 230 V ac full load Figure 9. European version start-up - 5 V - 8 V V at 230 V ac full load Figure 10. US Version start-up - 12 V V V ac full load Figure 11. US Version start-up - 5 V - 8 V V ac full load 3.3 Conducted EMI measurements The following figures show the conducted emission measurements using the peak detector at full load, for European and US boards, in case of nominal input voltage, and the QP and average limits. The measurements have been performed using a 50 Ω LISN and a spectrum analyzer. As shown in Figure 12 and Figure 13, the SMPS is compliant with the standard EN55022 CLASS B, with a large margin from the limits (~10dB/µV). 16/22

17 Experimental results Figure 12. Conducted EMI V ac - line 1 and line 2 17/22

18 Experimental results AN2447 Figure 13. Conducted EMI V ac - line 1 and line 2 18/22

19 Transformers 4 Transformers The winding arrangement and pin out of the transformers are shown in Figure 14. The core geometry is E16 and the ferrite grade is N87. Figure 14. Schematic diagram HICAL MAR 5146 HICAL MAR 5145 Table 12. Transformers - electrical specifications Electrical specifications Part Parameter Test conditions Terminals Value MAR 5146 Inductance 1 khz, 300 mv 2,1 1.5 mh+/-15% MAR 5145 Inductance 1 khz, 300 mv 2, mh+/-15% 19/22

Board layout AN2447 5 Board layout The board is made with a double layer CEM1 PCB with 0.

In Figure 15 the top view of the board is shown while Figure 16 shows the bottom view.

20 Board layout AN Board layout The board is made with a double layer CEM1 PCB with 0.35 um of copper thickness. In Figure 15 the top view of the board is shown while Figure 16 shows the bottom view. The maximum size of the board is 50 x 115 mm, and the maximum height is 25 mm, given by the input bulk capacitor. Figure 15. Board layout - top view Figure 16. Board layout - bottom view 20/22

21 Conclusions 6 Conclusions A low cost SMPS (switch mode power supply) for set-top box applications has been designed and optimized for either European or US input voltage ranges, making the transformer quite small for the considered output power level. Finally, the proposed solution performs well in all test conditions, confirming the suitability of such a solution for very low cost consumer applications. 7 Reference 1. L6565 datasheet; 2. AN L6565 Quasi-Resonant Controller; 3. L5973 datasheet; 4. AN Designing with L5973D, up to 2.5 A High Efficiency DC/DC Converter. 8 Revision history Table 13. Revision history Date Revision Changes 19-Dec First issue 13-Mar Minor text changes 21/22

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UM0920 User manual. 4 W non-isolated, wide input-voltage range SMPS demonstration board based on the VIPer16. Introduction

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