CM6800 PFC and PWM chip Vgnd point on computer PSU

[lordvader88]

On all the computer PSUs I've worked on, they have a PFC boost converter stage, that charges a bulk capacitor. Between the mains FWR neg. terminal and the bulk cap. neg. terminal is/are 1/2/3 fuseable and/or flameproof pink resistors, with sub-ohm resistance.

It seems most things on the PCB hotside, are referenced to the neg. terminal of the bulk cap, not the slightly lower voltage of the FWR(-)

And for instance the CM6800 PFC/PWM chips seem to always be on a sister PCB, and have their pin10GND on the Cbluk(-)


Look at CM6800datasheet and design guide
http://www.champion-micro.com/datasheet/Analog%20Device/CM6800.pdf
http://www.bannerspan.com.tw/technical/Champion%20Micro/CM6800%20Design%20Guide.pdf

There's a lot of beyond 1st year EE that IDK to be able to really follow along with these. IDk what internal opamps can sink current, or what hanging a tank circuit on the output really does to pin1 IEAO transconductance error amp, really. And IDK the transfer function eq's they are using to program these.

I broke 2 smd caps on a TL431 3.3V regulating circuit. Very soon I better figure out what their values should be, besides checking other similar PCBs

So if the CM6800 is floating slightly over the FWR(-) and yet the current hence voltage across those 1/2/3 resistors is changing all the time, the CM6800 calculations will be WAY HARDER, and none of the datasheets include any amount of formula's (or did I miss them?)


And on some of the internal opams like "PFC Current limit", it has -1V as the non-inv. ref, where is that being generated relative to Cblk(-) Is the chip using internal LC circuits to make -1V Or what am I missing/forgetting/not understanding ??

[lordvader88]

I mixed up the pins, I corrected my post. Also the chip is 16pin, and the sister PCB is 10pin, with 1 un-used. The sister PCB is doubled sided, and has about 80 resistors/caps/diodes/transistors, and a LM393 doing something idr-atm, but being powered off only the Vref of the CM6800

[lordvader88]

I mapped out most of it