Forward Converter - go-gddq. · PDF fileThe forward converter transfers directly the energy from the input source to the load during the on-timeof the powerswitch. During off-time

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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 = δ Vin

1 Conventional clamp & reset technical – Magnetizingcurrent reset by an extra winding that in parallel with

Pri-winding

2 RCD type clamp & reset technical

• Loss less snubber - LCD Snubber Circuit

• Self resonant reset

• 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 V

in 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 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

Forward Converter

Forward Converter

Power Switch: VDDS > Vinmax(1+n)+ leakage inductance spike

IDrms > 1.2Pin/VinminDmax

IDrms>1.2Pin/VinminDmax1/2

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

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/VinminD1/2

Rectifiers:

Forward D1:


IF(AV)>IoDmax

Freewheeling D2:


IF(AV)>Io

Forward Converter

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

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 withthe relation:

• Vp/Vs = Np/Ns

•Current going into the dotted primary terminal is

transformed and goes out of the secondaryterminal, 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 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

辅助绕组复位辅助绕组复位辅助绕组复位辅助绕组复位

• 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复位复位复位复位

• 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 bypassive 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 openloop operating and max duty exceed 50%

• To avoid transformer saturation need to evaluate the Dset max of PWM incase 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 windingleakage currentwith reversecurrent pull outdue to slow diodeD1/C2

Leakage spike

Normalized and inverted

secondary winding voltage

Magnetizing voltagering 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

????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 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


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


t0 t1 t2 t3 t4 t5 t6 t7

S1

Sa

vCs

ip

im

iCc

Vin VCc


M1: t0~t1

S1/Dr is on and Sa/Dc is off


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

n

II

L

ttV

n

Iii o

m

m

ino

mp ++−

=+=−)max(

0 )(

sM DTttT =−=∆ 011

)( 1

1

1 ttC

n

Ii

u

o

m

c −

+

=

Normal PWM

Im: Rise from – to 0 to

+

21 tatVu inc =

2max tatim =

Resonant

Im: 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~t3

S1/Sa/Dr is off and Dc is on

VNp 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.


Resonant

Im: DownResonant

Im: Down

M4: t3~t4

S1/Dr is off and Da/Dc is on

VNp 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~t5

Sa/Dc is on and Sa/Dr is off

VNp is -VCc and Im increases

inversely from 0. Sa is turned off at t5.


Resonant

Im: 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

Im: Rise inversely

T

+-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo

NpDrNs niii ==

M7(very short): t6~t7

Sa/S1 is off and Dr/Dr is on

VNp 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

Cin

LkNp −−

==

)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

Active Clamp Reset Forward ConverterM8(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.

T

+

-

Vin

C1

Cc

Sa

S1

Da

Dc

D1

Dr

Np NsIo

NpDrNs niii ==

DcDro iii +=

)7(7 )( tLk

k

inLkNp Itt

L

Vii +−==

)7(7 )( tLm

m

in

Lm IttL

Vi +−=

LkDLm iii += 1

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.


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 )( tLk

Lk

inLkNp Itt

L

Vii +−==

)8(8 )( tLm

m

inLm Itt

L

Vi +−=

Documents

Forward Converter - go-gddq. · PDF fileThe forward converter transfers directly the energy from the input source to the load during the on-timeof the powerswitch. During off-time