1. adlsong Forward Converter

2. 1 开关管导通 LdiL/dt = Vin – Vo  I = (Vin – Vo) ton / L 2 开关管关断 LdiL/dt = – Vo  I = – Vo toff / L  I : 输出电感伏秒平衡. 输出电压 : Vo =  Vin

3. 1 Conventional clamp & reset technical – Magnetizing current reset by an extra winding that in parallel with Pri-winding 2 RCD type clamp & reset technical 3Loss less snubber - LCD Snubber Circuit 4Self resonant reset 5 Soft Switching – Active Clamp/Reset

4. ADVANTAGES -- drain current reduced by the ratio of Ns/Np -- low output voltage ripple -- supports multiple outputs DISADVANTAGES -- poor transformer utilization -- poor transient response -- transformer design is critical because of reset winding -- transformer reset limits duty ratio -- high switch voltage required -- high input ripple current Forward Converter

5. The forward converter transfers directly the energy from the input source to the load during the on-timeof the powerswitch. During off-time of the power switch, the energy is freewheeling through the output inductor and the rectifier D2, like in a chopper A forward regulator can be realized with a single switch structure or witha doubleswitch structure, according to the way the energy stored in the transformer primary inductance is demagnetized. Forward converters are commonly usedfor output power up to 250W N Vin Vout = d V in single switches, and up to 1kW in double switch structures. Single switch vs. double switch forward Forward Converter

6. In the single switch forward, the magnetizing energy stored in the primary inductance is restored to the input source by a demagnetization winding Nd. Most commonly, the primary and the demagnetization windings have the same number of turns. So, at turn-off, the power switch has to withstand twice the input voltage during the demagnetization time, and then, once the input voltage. The demagnetization and primary windings have to be tightly coupled to reduce the voltage spike - morethan the theoretical 2Vin - occuring at turn-off across the power switch. Forward Converter

9. Power Switch: VDDS > Vinmax(1+n)+ leakage inductance spike IDrms > 1.2Pin/V inmin D max IDrms>1.2Pin/VinminDmax 1/2 Rectifiers: Forward D1: VRRM > Vinmax/n +leakage inductance spike IF(AV)>IoDmax Freewheeling D2: VRRM > Vinmax(Vo+VFD)/VinminDmax IF(AV)>Io Demagnetization D3: VRRM >(1+N3/Np) Vinmax(Vo+VFD)/VinminDmax IF(AV)>ImagpeakDmax/2 Forward Converter

10. Double switch forward, also called asymmetrical half bridge forward, the magnetizing energy stored in the primary inductance is automatically returned to the bulk capacitor by the two demagnetization diodes D1 and D2. The two power switches and demagnetization diodes have to withstand only once the input voltage Vin. As for the double switch fly back, the asymmetrical half bridge needs a floating gate drive for the high side switch. Power Switch: VDDS > Vinmax IDrms > 1.2Pin/V inmin D 1/2 Rectifiers: Forward D1: VRRM > Vinmax(Vo+VFD)/VinminDmax IF(AV)>IoDmax Freewheeling D2: VRRM > Vinmax(Vo+VFD)/VinminDmax IF(AV)>Io Forward Converter

11. Dual Switches Converters Self Reverse Voltage Clamp & Magnetism Current Reset Max Duty been limits and not exceeds 50% because of the magnetizing current reset Lm = LP on dual switches converters Forward Converter

13. The Forward Converter A Review of Transformers: Voltage applied across primary is transformed into a voltage across the secondary, with polarity following the dotted terminals, in accordance with the relation: Vp/Vs = Np/Ns Current going into the dotted primary terminal is transformed and goes out of the secondary terminal, following the relation: Ip/Is = Ns/Np An ideal transformer does not store energy, hence: Pin = Pout or Pp = Ps. Forward Converter

14. The forward topology is one of the most commonly used, and has several variations, the most basic of which is shown in Figure 1.9. The forward converter is essentially an isolated version of the buck converter operating in the direct mode and the basic single switch version shown can be successfully operated over a wide power range. Due to the transformer, the forward topology can be used as either an up or a down converter, although the most common application is down conversion. The power transferred to the secondary during the on-time is conducted through diode D1 to the output LC filter. During the off-time, the secondary current circulates through diode D2. The transformer is reset during the offtime by means of the auxiliary winding, Naux, and diode D3. The main advantages of the forward topology are its simplicity and flexibility. Output Ripple Frequency F Relative Cost Low One common variation on the forward converter is the two transistor forward. This configuration adds another switch element on the other side of the transformer primary and two clamping diodes, one from each side of the primary to the opposite input voltage terminal. In exchange for this additional complexity the two transistor forward reduces the voltage stress on the switch elements. Forward Converter

15. 辅助绕组复位

16. 开关管关断时激磁能量由复位绕组传输到输入电容, 无功率损耗. 简单, 复位电路只要一个二极管和一个复位绕组 最大占空比小于 50% 保证变压器复位, 因而最大输入电压时占空比小, 输入电 压范围窄. 最大占空比 50%, 开关管额定电压 VDS= 2VIN 由于复位绕组使漏感大 输出短路时复位二极管大有电流和电压应力

17. 减小 Dmax 可以开关管的最大电 压, 但会导致次级二极管的电压应 力大. 宽电压输入时 : Dmax=0.45 and Np=Nr. 开关管的最大电压 最大占空比

18. RCD 复位

19. Dissipative type Clamp/Reset technical Very low cost design of reset circuit with discrete components R, C and D Max. operating duty could be slightly higher than 50% (up to 55~65%), depending the magnetizing current reset by RC constant Critical point: Higher consumption because the energy storage on “C” should dissipates by passive component “R” The transformer volt/second balance should tradeoff by adjust the value of “C” and “R” for Clamp/Reset In case VDS ≤ 2Vin but very possible higher than 2VIN in worst case at open loop operating and max duty exceed 50% To avoid transformer saturation need to evaluate the Dset max of PWM in case of short circuit RCD reset

20. Forward Converter

22. The snubber capacitor voltage is fixed and almost independent of the input voltage, the MOSFET voltage stress can be reduced compared to the reset winding approach when the converter is operated with a wide input voltage range. Another advantage of RCD reset method is that it is possible to set the maximum duty ratio larger than 50% with relatively low voltage stress on MOSFET compared to auxiliary winding reset method, which results in reduced voltage stress on the secondary side. The maximum voltage stress the nominal snubber capacitor voltage

24. ZC Reset

26. Elements of primary zener clamp circuit D1 current Primary winding leakage current with reverse current pull out due to slow diode D1/C2 Leakage spike Normalized and inverted secondary winding voltage Magnetizing voltage ring at drain Magnetizing current

27. LCD Reset

28. Non dissipative type snubber with passive components L, C and D, but higher cost than RCD type clamp circuit Operating duty limits as same as RCD voltage clamp Energy recycle and clamp VDS at passive linear mode thru C to Bulk Bus during main switch turn off and LC resonant transition mode to move the energy storage of C to L during main switch turning on Critical point: Very high peak current on the leading edge of switching current of Q1 at switch turn on because the energy storage of “C” transfer to “L” thru Q1 Higher power efficiency than RCD type but lower than “Conventional” because addition one more diode dissipation To avoid DC bias on “L”, should be selected low leakage current of the Diode that in series on current loop

30. Self Resonant Reset

31. 要高压的 MOSFET

32. Add Auxiliary Switch Sa Basic RCD Clamp Remove Resistor Compared with RCD, Cc can discharge to inversely deeply magnetize the magnetization inductance via Sc. Active Clamp Reset Forward Converter

33. Sa: auxiliary switch S1: main switch Da: intrinsic parasitic diode inside Sa D1: intrinsic parasitic diode inside S1 C1: intrinsic parasitic capacitor inside Sa and external capacitor Active Clamp Reset Forward Converter

34. 1. Usually need high side drive for high side active switch. Need P channel MOSFET for Low side drive. 2. Dead time adjustment in between two switches 3. Resonant Transition mode need control the inductance of the main transformer. 4. Active switch need select high voltage MOSFET because the energy store in Cc 5. Allow much higher efficiency operation because energy of transformer magnetizing recycles and ZVT of the main switch. 6. Lower voltage clamp would reduced the voltage stress on main switch, similar power transfer to conventional square wave switching 7. Reduced EMI/RFI via soft switching 8. Operate at fixed switching frequency 9. Duty cycles beyond 50% max are obtainable 10. Actively resets main transformer to third quadrant of BH curve Active Clamp Reset Forward Converter

35. S1 SaSa v Cs ipip imim i Cc V in V Cc Active Clamp Reset Forward Converter

36. M1: t0~t1 S1/Dr is on and Sa/Dc is off Active Clamp Reset Forward Converter Lf is so high that it can be acted as constant current source Io M2: t1~t2 S1/Sa/Dc is off and Dr is on Normal PWM I m : Rise from – to 0 to + Resonant I m : Rise

37. M3: t2~t3 S1/Sa/Dr is off and Dc is on V Np is negative so Dr is off and Io can not reflected to primary side. C1 and Lm resonate and Vc1 resonated up to Vin+VCc at t3 and Da is on. Active Clamp Reset Forward Converter Resonant I m : Down M4: t3~t4 S1/Dr is off and Da/Dc is on V Np is –VCc so Im is demagnetized to 0 and Da is turned off naturally at t4. Sa is ZVS during t3~t4.

38. M5: t4~t5 Sa/Dc is on and Sa/Dr is off V Np is -VCc and Im increases inversely from 0. Sa is turned off at t5. Active Clamp Reset Forward Converter Resonant I m : Rise inversely M6: t5~t6 S1/Sa/Dr is off and Dc is on Lm and C1 resonate and C1 discharge and VC1 declined to Vin at t6. Resonant I m : Rise inversely

39. M7(very short): t6~t7 Sa/S1 is off and Dr/Dr is on V Np is positive so Dr is on. Primary current is not enough to provide load current so Dc is still on and secondary side is short circuit. Primary inductance is magnetized by Vin-VC1 and iLm increases from negative. Leakage inductance Lk is magnetized by Vin-VC1 and its current rises sharply and is reflected to secondary side. So iDr increases gradually from 0. Vc1 declines to 0 and D1 is on at t7. Active Clamp Reset Forward Converter

40. M8(very short): t7~t8 S1/Sa is off and Dr/Dc is on D1 is on and primary inductance continues to be magnetized by Vin and im continues to increase from negative. Leakage inductance Lk continues to be magnetized by Vin and its current rises sharply and is reflected to secondary side. iDr continues to increase and iDc continues to decline. iD1=0 and iLk=iLm at t8 and D1 turned off naturally. S1 is ZVS during t7~t8.

41. M9(very short): t8~t9 S1/Dc/Dr is on and Sa is off S1 is on and primary inductance continues to be magnetized by Vin and im continues to increase from negative. Leakage inductance Lk continues to be magnetized by Vin and its current rises sharply and is reflected to secondary side. iDr continues to increase and iDc continues to decline. iLm+iS1=iLk=iNp=Io/n at t9 so Dc turned off naturally and iDr=Io. Go into next cycle. Active Clamp Reset Forward Converter