Topic 2 Incorporating Active-Clamp Technology to Maximize Efficiency in Flyback and Forward Designs Bing Lu
Agenda 1. Basic Operation of Flyback and Forward Converters 2. Active Clamp Operation and Benefits 3. Active Clamp Flyback Design 4. Active Clamp Forward Design 5. Test Results Texas Instruments 2010 Power Supply Design Seminar 2-2
Basic Power Stages D2 D3 L1 R1 C1 D1 + V OUT _ R1 C1 D1 D4 + V OUT _ Flyback Transformer stores energy R1 dissipates leakage and some magnetizing energy Typically 2 to 5% of output power Forward Transformer transfers energy Storage is in L1 R1 dissipates magnetizing plus leakage energy Typically 3 to10% of output power How can we avoid loss in R1? Texas Instruments 2010 Power Supply Design Seminar 2-3
Secondary Winding Currents Forward Secondary Current (A) I OUT Flyback Secondary Current (A) I OUT D T (1 D) T Assuming 50% duty cycle and CCM Synchronous rectifiers force CCM T RMS flyback current = 2 X RMS forward current For low voltage/high current output, forward is best choice Texas Instruments 2010 Power Supply Design Seminar 2-4
Output Capacitor Currents Forward Capacitor Current (A) Flyback Capacitor Current (A) I OUT D/(1 D) I OUT D T (1 D) T T Flyback output capacitors see much higher current Higher RMS current increases heating Higher peak current requires much lower ESR Result is more, higher quality capacitors in flyback Texas Instruments 2010 Power Supply Design Seminar 2-5
Agenda 1. Basic Operation of Flyback and Forward Converters 2. Active Clamp Operation and Benefits 3. Active Clamp Flyback Design 4. Active Clamp Forward Design 5. Test Results Texas Instruments 2010 Power Supply Design Seminar 2-6
Active Clamp Operation L mag L mag L mag L mag L leakage L leakage L leakage L leakage OFF ON OFF OFF ON OFF OFF OFF L mag and L leakage Current commutes Current resonates, Current commutes are energized to body diode changes direction to body diode or C oss D T sw t delay (1-D) T sw tdelay Texas Instruments 2010 Power Supply Design Seminar 2-7
Active Clamp Configurations L mag L mag L leakage L leakage P-Channel Clamp N-Channel Clamp + Easy to drive clamp FET - Higher capacitor voltage - P-channel FET - Floating gate drive + Lower capacitor voltage + N-channel FET Texas Instruments 2010 Power Supply Design Seminar 2-8
Active Clamp Benefits RCD Clamp Most of leakage energy is dissipated as heat Hard switching results in power losses More difficult implementation of self-driven synchronous rectifiers with Forward Voltage spike on drain at turn off can be EMI issue Active Clamp Most of leakage energy is reclaimed Zero voltage switching reduces losses Simple Implementation of selfdriven synchronous rectifiers with forward No voltage spike on drain at turn off Nearly lossless recovery of magnetizing energy in forward Texas Instruments 2010 Power Supply Design Seminar 2-9
Agenda 1. Basic Operation of Flyback and Forward Converters 2. Active Clamp Operation and Benefits 3. Active Clamp Flyback Design 4. Active Clamp Forward Design 5. Test Results Texas Instruments 2010 Power Supply Design Seminar 2-10
Active Clamp Flyback Design + V OUT _ 1 ON OFF V DS_ +V clamp L leakage Q3 2 OFF V DS_ ON V clamp 4 I pri (1 A/div) Time (1 µs/div) Operation same as RCD flyback during on time No leakage spike at turn off; leakage energy controlled by clamp Magnetizing energy transferred to secondary Leakage current is steered by body diodes during dead times Reduces switching losses Dead times occur between ON and OFF states Texas Instruments 2010 Power Supply Design Seminar 2-11
Flyback Clamp Circuit + V OUT _ C clamp L leakage Q3 4 V DS C oss C oss I clamp (2 A/div) 1 f clamp 1 = 2 π L C leakage clamp Time (1 µs/div) Peak current is I I pri clamp = Ipri _ pk cos(2 π fclamp t) RMS current much higher 1D 1 2 than forward I F clamp_ rms (D) = I clamp (t) dt T 0 Clamp current must flow from source to drain in when turns on Reverse recovery of body diode Choose C clamp for f clamp 20% lower than switching frequency Texas Instruments 2010 Power Supply Design Seminar 2-12
Flyback Soft Switching Turn-Off + V OUT _ 1 V DS_ 2 I (2 A/div) L leakage Q3 3 V DS_ 4 I (2 A/div) Turns Off Delay Turns On Magnetizing energy transfers to secondary Delay from turn-off to turn-on Leakage energy flows in body diode Zero voltage switching of Not load or line dependent Texas Instruments 2010 Power Supply Design Seminar 2-13
Flyback Soft Switching Turn-On Heavy Loads + C clamp L leakage C oss C oss Q3 V OUT _ 1 2 3 V DS_ I (2 A/div) V DS_ 4 I (2 A/div) Turns Off Delay Clamp current flowing source to drain in Turns On turns off, clamp current flows in C oss Partial zero voltage switching of Load dependent Texas Instruments 2010 Power Supply Design Seminar 2-14
Flyback Soft Switching Turn-On Light Loads + V OUT _ V DS_ C clamp L leakage Q3 1 2 I (2 A/div) C oss C oss 3 V DS_ Less energy in leakage Voltage is higher on at turn on Farther away from ZVS 4 I (2 A/div) Turns Off Delay Turns On Texas Instruments 2010 Power Supply Design Seminar 2-15
Flyback Synchronous Rectifiers Narrow Input Range Q3 + V OUT _ Wide Input Range T1 D2 R2 Q3 + V OUT R2 D5 C13 330 pf D5 D6 R7 4.99 kω Q5 Self-driven from transformer winding for low cost Leakage inductance limits shoot-through current Sync FET must turn off fast Conditioning circuit Some adjustment of turn-on and turn-off times Texas Instruments 2010 Power Supply Design Seminar 2-16
Agenda 1. Basic Operation of Flyback and Forward Converters 2. Active Clamp Operation and Benefits 3. Active Clamp Flyback Design 4. Active Clamp Forward Design 5. Test Results Texas Instruments 2010 Power Supply Design Seminar 2-17
Active Clamp Forward Design L mag Q5 + V OUT _ 1 2 OFF V DS_ V DS_ ON ON OFF Q4 No Load 4 I pri (1 A/div) Time (1 µs/div) Reflected primary voltage during reset time allows self driven sync rectifiers No leakage spike at turn off Primary current resets to third quadrant resulting in better core utilization Unlike flyback, clamp resonant frequency is determined by magnetizing inductance and C clamp Texas Instruments 2010 Power Supply Design Seminar 2-18
Forward Clamp Circuit V hump f clamp 1 = 2 π L C magnetizing clamp V V D (1 D) in hump = 2 8 Lmagnetizing fsw Cclamp I _RMS V D 1 D (Peak current is I in mag ; = RMS clamp current is 2 3 Lmagnetizing fsw much less than flyback) Texas Instruments 2010 Power Supply Design Seminar 2-19
Forward Soft Switching Turn-Off L mag Q5 + V OUT _ 1 V DS_ I (1 A/div) 2 Q4 3 V DS_ 4 I (1 A/div) Magnetizing and reflected load current flowing in Transfers to body diode Turns Off Delay Turns On Delay from turn-off to turn-on Zero voltage switching of Not load or line dependent Texas Instruments 2010 Power Supply Design Seminar 2-20
Forward Soft Switching Turn-On Light Loads L mag Q5 + V OUT _ 1 2 V DS_ I (1 A/div) Q4 3 4 V DS_ I (1 A/div) Turns Off Turns On Delay No current in Q4 or Q5 during delay time Allows to achieve ZVS Texas Instruments 2010 Power Supply Design Seminar 2-21
Forward Soft Switching Turn-On Heavy Loads L mag + 0 V Q5 + V OUT _ 1 2 V DS_ I (1 A/div) Q4 3 4 V DS_ I (1 A/div) Turns Off Turns On Delay Current flows in body diodes of Q4 and Q5 during delay time drain voltage = when turns On Partial zero voltage switching Texas Instruments 2010 Power Supply Design Seminar 2-22
Forward Synchronous Rectifiers Output Voltage PRI:SEC Turn Ratio MAX Sync FET V DS Stress Sync FET V DS Rating 3.3 V 6:1 12.5 V 20 V 5 V 4.5:1 17 V 30 V 12 V 1.88:1 40 V 60 V Turn ratios and voltages for telecom 35- to 75-VDC input FET gate rating of 20 V or less 3.3-V output can be driven directly from transformer winding D7 DRV R3 Q6 D8 Outputs >3.3 V require gate protection Texas Instruments 2010 Power Supply Design Seminar 2-23
Agenda 1. Basic Operation of Flyback and Forward Converters 2. Active Clamp Operation and Benefits 3. Active Clamp Flyback Design 4. Active Clamp Forward Design 5. Test Results Texas Instruments 2010 Power Supply Design Seminar 2-24
Test Results 3.3-V/7.6-A Sync Flyback Efficiency (%) 100 90 80 70 60 50 40 30 20 10 0 0 Active Clamp RCD Clamp 1 2 3 4 5 Output Current, I OUT (A) =48V 6 7 Active clamp increases efficiency 4% at 7.5-A load 1.37-W of extra power to deliver to load Texas Instruments 2010 Power Supply Design Seminar 2-25
Test Results 12-V/2.1-A Flyback Efficiency (%) 100 80 60 40 20 0 RCD Clamp with Schottky Rectifier Active Clamp with Synchronous Rectifier 0 0.5 1 1.5 2 Output Current, I OUT (A) Efficiency same at maximum load Low leakage transformer and no current shoot through RCD better at light loads No circulating clamp current, DCM No current shoot through =48V Texas Instruments 2010 Power Supply Design Seminar 2-26
Test Results 3.3-V/7.6-A Sync Forward 94 92 Efficiency (%) 90 88 86 84 82 Active Clamp Forward with Synchronous Rectifier 80 Active Clamp Flyback with 78 Synchronous Rectifier 76 0 1 2 3 4 5 6 7 8 Load Current (A) Efficiency with active clamp and sync rectifiers 2 to 3% better than flyback More difficult implementation of self-driven synchronous rectifiers with RCD clamp Texas Instruments 2010 Power Supply Design Seminar 2-27
Test Results 12-V/2.1-A Forward 95 90 Active Clamp with Synchronous Rectifier Efficiency (%) 85 80 75 RCD Clamp with Diode Rectifier 70 65 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Load Current (A) Active clamp increases efficiency 5% at 2.1-A load Gate conditioning circuit limits efficiency gain Texas Instruments 2010 Power Supply Design Seminar 2-28
Flyback versus Forward Efficiency Efficiency (%) 94 92 90 88 86 84 82 80 78 76 74 12 V 5V 3.3 V 0 10 20 30 40 50 Output Pow er (W) Synchronous Forward Converter with Active Clamp Synchronous Flyback Converter with Active Clamp Diode-Rectified Flyback Converter with RCD Clamp Diode and RCD clamp for low power, low cost Either topology is option for mid-range power, with forward delivering better efficiency Flyback good for multiple outputs Forward can be scaled to higher output power with same results Texas Instruments 2010 Power Supply Design Seminar 2-29
Flyback vs. Forward Size Flyback with active clamp and sync FETs Forward with active clamp and sync FETs Flyback: larger transformer and clamp FET, more output capacitors Forward: bias inductor, larger output inductor Texas Instruments 2010 Power Supply Design Seminar 2-30
Active Clamp Forward 3.3-V/7.6-A, 25-W POE Converter Using TPS23754 33 to 57 VDC + + V OUT 3.3 V at 7.6 A PoE Input from Diode Bridges Texas Instruments 2010 Power Supply Design Seminar 2-31
Summary Adding active clamp and sync rectifiers improves efficiency of flyback and forward up to 5% (Efficiencies >90%) Forward provides best efficiency due to lower conduction losses than flyback Forward can be scaled to higher output power with similar results Flyback for multiple outputs or when cost is most important Diode rectified flyback with RCD clamp for low power and low cost Texas Instruments 2010 Power Supply Design Seminar 2-32