Adlsong Forward Converter. 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

adlsong

Forward Converter

Forward Converter

Gate Q1

VDS Q1

IDS Q1

Im Reset

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

1 Conventional clamp & reset technical – Magnetizing current reset by an extra winding that in parallel with Pri-winding

2 RCD type clamp & reset technical

3 Loss less snubber - LCD Snubber Circuit

4 Self resonant reset

5 Soft Switching – Active Clamp/Reset

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

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 Vin single switches, and up to 1kW in double switch structures.

Single switch vs. double switch forward

Forward Converter

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 thedemagnetization 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

Forward Converter

Forward Converter

Power Switch: VDDS > Vinmax(1+n)+ leakage inductance spikeIDrms > 1.2Pin/VinminDmax

IDrms>1.2Pin/VinminDmax1/2

Rectifiers: Forward D1:VRRM > Vinmax/n +leakage inductance spikeIF(AV)>IoDmaxFreewheeling D2:VRRM > Vinmax(Vo+VFD)/VinminDmaxIF(AV)>IoDemagnetization D3:VRRM >(1+N3/Np) Vinmax(Vo+VFD)/VinminDmaxIF(AV)>ImagpeakDmax/2

Forward Converter

Double switch forward, also called asymmetrical half bridge forward, themagnetizing 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 > VinmaxIDrms > 1.2Pin/VinminD1/2

Rectifiers: Forward D1:VRRM > Vinmax(Vo+VFD)/VinminDmaxIF(AV)>IoDmaxFreewheeling D2:VRRM > Vinmax(Vo+VFD)/VinminDmaxIF(AV)>Io

Forward Converter

Dual Switches ConvertersSelf 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

Forward Converter

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

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 mostcommon 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 itssimplicity and flexibility. Output Ripple Frequency FRelative 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

辅助绕组复位

• 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

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

RCD复位

• 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

Forward Converter

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

ZC Reset

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

LCD Reset

• 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

Self Resonant Reset

要高压的MOSFET

T

Cc

D

R

Sa

T

Cc

Sa Da

Np

T

Cc

D

R

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

T Vo

+-

Vin

C1

Cc Co RL

Sa

S1

Da

Dc

D1

Dr

Np Ns

Lf

Sa: auxiliary switchS1: main switchDa: intrinsic parasitic diode inside SaD1: intrinsic parasitic diode inside S1C1: intrinsic parasitic capacitor inside Sa and external capacitor


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 switches3. Resonant Transition mode need control the inductance of the main transformer.4. Active switch need select high voltage MOSFET because the energy store in Cc5. 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 switching7. Reduced EMI/RFI via soft switching8. Operate at fixed switching frequency9. Duty cycles beyond 50% max are obtainable10. Actively resets main transformer to third quadrant of BH curve


t0 t1 t2 t3 t4 t5 t6 t7

S1

Sa

vCs

ip

im

iCc

Vin VCc


M1: t0~t1S1/Dr is on and Sa/Dc is off


Lf is so high that it can be acted as constant current source Io

M2: t1~t2S1/Sa/Dc is off and Dr is on

n

II

L

ttV

n

Iii o

mm

inomp

)max(

0 )(

sM DTttT 011

)( 11

1 ttCn

Ii

u

om

c

Normal PWMIm: Rise from – to 0 to

+

21 tatVu inc

2max tatim

ResonantIm: Rise

T

+-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo

T

+-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo

M3: t2~t3S1/Sa/Dr is off and Dc is onVNp 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.


ResonantIm: Down

ResonantIm: Down

M4: t3~t4S1/Dr is off and Da/Dc is onVNp is –VCc so Im is demagnetized to 0 and Da is turned off naturally at t4. Sa is ZVS during t3~t4.

T

+-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo

T

+-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo

T

+-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo

T

+-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo

M5: t4~t5Sa/Dc is on and Sa/Dr is offVNp is -VCc and Im increases inversely from 0. Sa is turned off at t5.


ResonantIm: Rise inversely

M6: t5~t6S1/Sa/Dr is off and Dc is onLm and C1 resonate and C1 discharge and VC1 declined to Vin at t6.

ResonantIm: Rise inversely

T

+-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo

NpDrNs niii

M7(very short): t6~t7Sa/S1 is off and Dr/Dr is onVNp 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.


DcDro iii

)( 61 tt

L

VVii

k

CinLkNp

)6(61 )( tLm

m

CinLm Itt

L

VVi

Lm

Np

Lk

Ns

+

-

Vin

C1S1 D1

LkCLm iii 1

Lm

Np

Lk

Ns

+

-

Vin

C1S1 D1


M8(very short): t7~t8S1/Sa is off and Dr/Dc is onD1 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.

T

+

-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo

NpDrNs niii

DcDro iii

)7(7 )( tLkk

inLkNp Itt

L

Vii

)7(7 )( tLmm

inLm Itt

L

Vi

LkDLm iii 1

M9(very short): t8~t9S1/Dc/Dr is on and Sa is offS1 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.


T

+-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo Lm

Np

Lk

Ns

+

-

Vin

C1S1 D1

NpDrNs niii

DcDro iii

LkSLm iii 1

)8(8 )( tLkLk

inLkNp Itt

L

Vii

)8(8 )( tLmm

inLm Itt

L

Vi

Documents

Adlsong Forward Converter. 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