Feedback compensation for current-mode switching power supplies

That is so 'horrible' someone should be shot at dawn for it....

The closest I might care to guess is the 'idiot' what wrote it has managed to derive an equation for a PWM gain in the current sense section based on the level of ripple current in the output inductor reflected to the primary as it appears across the current sense resistor which ends up incorporating the other equations he has used to calculate a value for the filter inductor with. Look at his Eq

Oh no....... In fact once upon a time I provided a similar derivation but for a single switch flyback converter using a UC38xx type controller in order to develop a linear model. It works and is necessary there because of the way a flyback converter operates.

Ooooh dear..... you don't think someone read that and has applied it, incorrectly, to a Forward Converter?

Perhaps that's why I recognised the possibility.

Arse

In fact, since it is now dawn here I shall go out and shoot myself.

:-)

DNA

I'm sure some Googling will turn up various references for you. One example is Unitrode's ap note U-97 (as slua101.pdf on the TI web site).

Have you done a simulation of your circuit? That may lead to valuable insights too.

Cheers, Tom

Hi Tom,

Thanks, I'll check it out.

Happily I'm using a Linear Tech IC in my power supply so, yes, I have done a simulation with LTspice. However, I'd prefer to work out some theoretical values and then do tweaking in a simulator rather than trying to do the "design" in a simulator and not really know what, e.g., the phase margin is other than by inferring it from the step response.

Also, since simulating the switcher itself is a bit slow, I am in the process of doing some simple linear (AC) simulations as well... but of course to do that I have to be able to input models of the system, hence the desire to derive that app note's equation for control to output gain.

Since last night I've found that equation pops up in Marty Brown's book as well (not exactly in the same class as Pressman or Billing's books, but it is still useful).

---Joel

yeah that's right

You obviously start with Vout=Vin x D x Ns/Np, then arrange that in terms of the error input to the error amplifier and differentiate wrt the error as an independent variable. That gets you the required gain for compensation purposes. D=ton x Fsw, and ton is obviously derived from Vin/Lpri x ton + Iload x Ns/Np = Vsense, start from there. Iload has a ripple to it as function of (Ns/Np x Vin -Vout)/L's too.

Responding to myself here...

According to Marty Brown's book (pg. 203), (Vin-Vout)^2/(Vin*Ve)*Ns/Np is the control to output gain for a *flyback* power supply, whereas for a *forward* converter the gain is still the usual Vin/Ve*Ns/Np.

I'm tempted to believe Marty since -- other than tacking on a few more constants and bundling them into "Ve" -- my derivation shows the same result for a forward converter. I suppose that if I get ambitious I should derive the results for a flyback and check it, though.

---Joel