Bode plots for TOPswitch HX Flyback

Posted by: treez on 07/07/2023

Hi,

We are doing a TOPswitch flyback with TOP256GN. 90-264VAC, 24Vout, 800mA out.

We are using a BJT in the feedback loop to reduce the opto current.

Please tell us the modulator and error amplifier transfer function. Also, the power stage transfer function.

LP=1.07mH, Ls = 54uH, Cout = 1mF.

We use the feedback option as detailed in the attached. (from Basso Book)

Basso says to look at his expression for Flyback in voltage mode...but that is in the form of vout(s)/verr(s)....and in his book, (attached), the expression is for "D(S)/vout(s)".

So how do we deal with these two expressions, which obviously are "non-matching"?

As you know, only power integrations knows what gain you have put in the chips....so could you please give the modulator and error amplifier transfer function, in a form that can be multiplied by the power stage transfer function, so that we can find the overall open loop gain and phase?

Files

Attachment	Size
Pg 455.pdf	779.39 KB
Pg 466.pdf	570.29 KB
Pg 467.pdf	508.77 KB

Comments

If you want a stable feedback loop, you will need to do measurements, as the TOPHX changes operating mode as the load changes. The transfer functions are protected IP. Attached is a suggested setup for using the current injection method at the TOPSwitch control pin to break the loop and obtain gain-phase plots. The analyzer shown in the attached file is the venerable HP4194A, but you can use the analyzer of your choice, as long as it is used with an isolation transformer, as the current injection circuitry is by necessity primary-referred.. We suggest using a wide-bandwidth injection transformer such as the Cine Mag CMOQ-1H for signal injection, as it is linear over a wide bandwidth.

Attachment	Size
TOPSwitch_curr_injection.pdf	242.78 KB

Thanks, as you kindly say, the TOPswitch transfer function is Power.com protected IP. Please , then, may i ask what is equations, 22, 23 ,24 and 25 of AN57?

...AN57 says that these are the very transfer function of topswitch and TL431 based error amplifier, does it not?

Not for the TOPSwitch HX/JX...