The whole Two Transistor Thing

[rievax_60]

Re: The whole Two Transistor Thing

This is the way I understand it.
The regulation loop controls the ON time of the chopper transistor during the forward period to control how much energy is sent through the transformer during the following flyback period. The small control transistor only takes notice of the opto transistor's conduction state during the forward period.
When ripple develops at the 5V output because of the capacitor going high ESR, the ripple troughs correspond to the forward period, diode reversed biased and the capacitor only supplying load current.
The ripple peaks correspond to the flyback period, diode forward biased and charging the capacitor.
So what tends to happen is the regulation loop regulates the ripple troughs to about 5V, but ignores the high ripple peaks during flyback. This causes the average voltage to be much higher than 5V, although the 431 is conducting heavily through the opto's LED during flyback.
The high ripple peaks also get reflected back to the chopper transistor's Collector/Drain, causing voltage stress and possible failure.

[everell]

Re: The whole Two Transistor Thing

In a bad 5vsb circuit, those peaks and ripple trouphs are only part of the story. Those bad 5vsb circuits are loaded with varying frequency oscillations. They will even sing to you. Try a Fast Fourie Analysis on one of those bad circuits!

[socketa]

Re: The whole Two Transistor Thing

Near the beginning of this thread, in post #14, there is photos of a Seasonic SS-250FS
https://www.badcaps.net/forum/attach...0&d=1249818980
Here is one another so-called SS-250FS,
Same brand of capacitors - PCE-TUR,
but i don't see any sign this one being a 2-Transvestite design.
It's a different board (and appears to me to made later)
Looks like the 5VSB is controlled by a TDA16822 PWM Controller chip
Just goes to show that you can't always judge a book by it's cover
Also highlights the subjectivity of convention.

[Behemot]

Re: The whole Two Transistor Thing

I guess it is single switcher forward, for 250 W it is enough.