SMPS for 100W Class D amplifier supply

[treez]

Hello,
As you know, such an SMPS, (for class D supply) must be able to go from no load to full load very quickly.

(This smps is isolated and uses optocoupler in the feedback loop.

When you are in no load, how do you stop the opto transistor from saturating?.....as its this saturation which makes the amplifier slow when coming out of no load, and causes overshoot.

 

[godfreyl]

how do you stop the opto transistor from saturating?

With a cascode, as you showed in your other thread.

 

[treez]

The cascode does not stop the opto transistor from saturating in no load conditions.

https://www.onsemi.com/pub_link/Collateral/AND8273-D.PDF

On page 9 (bottom right) of the article above, they say that the opto transistor does not saturate because of the biasing of the base of the NPN, Q1 (pse see fig 13 on same page) to 1.5V.
But i've simulated it and it does saturate when the smps goes into no load.
The BJT base-emitter diode effectively "shorts out" the lower resistor, R11, and then the opto transistor does saturate.

How can i stop the opto transistor from saturating?
(i need a fast no load to full load response)

How do you stop a BJT from saturating?,( if its say a 2N3904), how long does it take to come out of saturation?, and if it does saturate, how can you make it come quicker out of saturation?

 

[godfreyl]

The BJT base-emitter diode effectively "shorts out" the lower resistor, R11, and then the opto transistor does saturate.

OK so Q1 saturates first, then the bias voltage at it's base collapses, causing the opto transistor to saturate.

You could just stiffen the bias voltage at Q1's base. That would prevent the opto transistor saturating, but Q1 would still saturate.

https://www.onsemi.com/pub_link/Collateral/AND8273-D.PDF

On page 9 (bottom right) of the article above, they say....

BTW: That article discusses how to make an SMPS with an NCP1562. However the circuit you showed uses an LT1241, a completely different chip.

I don't understand your circuit at all. It looks like you're connecting the feedback pin directly to ground, and applying the optocoupler output to the compensation pin.:???:

Are you sure the circuit you showed is correct? I noticed in a recent thread that you waited until post 9 before mentioning that the circuit you showed in post 1 was not actually the circuit you're using. Meanwhile three different people were trying to help you based on the wrong circuit. That just wastes everyone's time.

 

[mtwieg]

If the phototransistor is saturating then the biasing network of Q1 should be a lower impedance. If Q1 saturating is still a problem then you'll probably need a more complicated solution (like using an op amp TIA instead of a cascode).

 

[treez]

I don't understand your circuit at all. It looks like you're connecting the feedback pin directly to ground, and applying the optocoupler output to the compensation pin.

This is the way its done with the LT1241, which is pin for pin like the UCC38C42 chips.

I garantee that schem is absolutley right. -readers familiar with smps will say that the cct in the top post is fine.