How does this switching PS work ?

[michel74]

Hi everyone,

Yes ! How does this switching PS works ? :-)
I understand (not in deep, of course) the switching PS principle; typically, I'd expect to get a VCC just to "feed up" the STR or the Switching regulator, but I cannot figure how this VCC is "created" (on a TV switching PSU, I often see some cascaded resistors, but here, I believe that one of the transformer input should make the permanent VCC, but I cannot figure where )
Thanks for your help. I already wrote some explanation of what I was having and did some measurements here

[PeteS in CA]

Re: How does this work ?

IC901 gets its Vcc from T901-1 and -2. Could not find datasheet

IC921 gets its Vcc from T903-2 and -3. https://html.alldatasheet.com/html-p.../1/MR4020.html

[michel74]

Re: How does this work ?

Thanks PeteS,
I got these ones, :-)
My question was more about how we get this permanent VCC? When the A/C plug is .. plugged, we should have a permanent VCC “somewhere”, no ?

[petehall347]

Re: How does this work ?

will be at rectifier .

[R_J]

Re: How does this work ?

There is likely a built in resistor or what is called a startup cell from the Drain pin that supplies initial vcc voltage to start the ic.

[PeteS in CA]

Re: How does this switching PS work ?

Page 7 of the Shindengen regulator datasheet (which I linked)explains the derivation of Vcc during start-up. There is an internal current source the supplies current through the Vcc pin, charging the external filter capacitor. Once the Vcc voltage is high enough, the regulator starts to operate, the current source is disconnected from the Vcc lin, and Vcc is supplied by the bootstrap winding.

I could not find a datasheet for the other regulator, but I expect it operates similarly.