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PSFB Transformer Design

sabu31

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Dear All,

I wanted to confirm if the design for PSFB Transformer is same as that of Full Bridge DC-DC Transformer. Are there any references for the PSFB Transformer Design.

Thanks
 

sabu31

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the pri current is higher in PSFB ...
So if we estimate the calculate the primary and secondary currents then we will be able to design transformer. What about the volt-sec. Is there any good reference for the Transformer Design of PSFB
 

Easy peasy

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the rms is higher because current circulates in the Tx and fets during the times between power pulses to the o/p
 
O

opampsmoker

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..yes , PSFB keeps a high circulating current which is needed to discharge and charge the vds caps of the pri fets so as to bring about zvs...especially zvs at lighter loads.

But yes, PSFB is like a FB otherwise.

Here is a good thread on PSFB.

It shows that a external leakage L is required...so really, consider sandwich winding the PSFB transformer since you will be likely using an external L(leak).
 

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at higher powers ( at any power level ) simple primary over secondary winding increases leakage but also AC resistance of wire - leading to quite a lot of heating at higher frequencies and currents ...
 

sabu31

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..yes , PSFB keeps a high circulating current which is needed to discharge and charge the vds caps of the pri fets so as to bring about zvs...especially zvs at lighter loads.

But yes, PSFB is like a FB otherwise.

Here is a good thread on PSFB.

It shows that a external leakage L is required...so really, consider sandwich winding the PSFB transformer since you will be likely using an external L(leak).
Thanks for reply. But can you refer me to any design note for the transformer. In Full bridge the volt-sec is calculated based on switch on time. But in PSFB there is no concept of duty ratio, Then how do we obtain volt-sec in primary for design of Transformer.
 
O

opampsmoker

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For the transformer, you can think of it being like a full bridge, btu just design for the true rms current, which includes the circulating current.
The PSFB is like a full bridge in many ways...its just that the current is kept circulating at a high level so as to allow it to be high enough to discharge the vds caps to give zvs.
Ill get you a sim of a PSFB..
--- Updated ---

download this course...
..and in there there is a phase shift full bridge folder in which there are some ltspice sims for you to play with if you want.
--- Updated ---

for a duty cycle you have an "On time" and OFF time right?
Well look at the sim, and you will see times when diagonally opposite fets are on together...that time is the ON time....the rest of the switching period is the OFF time.
 

    sabu31

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Easy peasy

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0 phase shift = full pwm applied to Tx, 180 deg phase shift = 0 pwm applied to Tx. 90 deg phase shift = 50% pwm applied to Tx
 

sabu31

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For the transformer, you can think of it being like a full bridge, btu just design for the true rms current, which includes the circulating current.
The PSFB is like a full bridge in many ways...its just that the current is kept circulating at a high level so as to allow it to be high enough to discharge the vds caps to give zvs.
Ill get you a sim of a PSFB..
--- Updated ---

download this course...
..and in there there is a phase shift full bridge folder in which there are some ltspice sims for you to play with if you want.
--- Updated ---

for a duty cycle you have an "On time" and OFF time right?
Well look at the sim, and you will see times when diagonally opposite fets are on together...that time is the ON time....the rest of the switching period is the OFF time.
Thanks for the reply. I will go through the documents.
 
O

opampsmoker

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There is a good ti.com chip that has app notes that go thru PSFB.

I coudl be wrong, but i think its only ti.com that do PSFB control chips(?)
 

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