Psım Full Bridge Feedback & Output Voltage Problem

[Fethiyeli]

I've simulated my full bridge circuit via Psim but i cannot get a stable 370VDC at output. I calculated my feedback network according to Switching power supply design and Power supply cookbook. I've also used Basso's book. When i set the input voltage to lower value, output also goes down, and if i set the input to higher value, output also goes up, so feedback does not work. What's wrong with my circuit ? Could u pls help and give some advice ? I also do not know how to calculate the feedback network accuretly without having any network analyser. I've also tried to get bode plot but it will get something like 8 hours to get whole bode plot.

 

[Fethiyeli]

Here is the OrCAD results;

Actually i do not know how to calculate feedback network accurately. I am really confused that which book is good for calculating. Since I've used marty brown's power supply cookbook, presmann's switching power supply design and basso's book. Pressmann's book does not tell anything about TL431 feedback with optocoupler. Marty Brown's book is pretty good but i am really confused to calculate some equations about feedback network. Basso's book is great but it needs some parameters such as needed gain and phase margin. So I've used Marty Brown's book for calculating but according this book, one resistor in compensation network in TL431 appears as 1054Mohm which is not realistic. Could you tell me which book you have been using for calculating feedback network and compensation ? I really need that. And I've also tried to simulate whole circuit via OrCAD but OrCAD always gives an error during the simulation. Here is the schematic and error.



INFO(ORPROBE-3183): Simulation running...

** Profile: "SCHEMATIC1-Full-Bridge" [ D:\Electrical & Electronics\Power Electronics\Simulation & Schematics\Orcad\full-bridge-pspi

Reading and checking circuit

ERROR(ORPSIM-16276): Can't find library

Circuit has errors ... run aborted

See output file for details

INFO(ORPROBE-3188): Simulation aborted



I am using UC3825 model which is in OrCAD's own library but it also gives an error as follows:

WARNING(ORNET-1018): Connection to unmodeled pin U8 pin 'RT'

WARNING(ORNET-1018): Connection to unmodeled pin U8 pin 'CT'

 

[gman76]

Network analysis and bode plots come much later. You need to figure out the DC feedback mechanism first. What voltage do you expect the tl431 cathode to be? How much cathode current? Looks like the resistors on the tl431 gate side are running about 1mA.

The gate driver symbols appear to be buffers, shouldn't they be inverters? I didn't see a little bubble on their outputs.

 

[KX36]

Forget the TL431 circuit for now, it won't work directly on such a high voltage anyway and its difficult to use. Use an opamp error amp such as the one in the IC model, get a stable loop with that first. I'd also recommend getting a model working as its buck converter equivalent (just transform the input voltage by the turns ratio of the transformer).

I put a simplified idealised own ic together out of an error amp, comparator, latch and ramp/clock voltage sources. I've had issues with whole IC models before just not working at all like they should or throwing errors they shouldn't.

While we're on the point, I'd also highly recommend current mode control for a full bridge converter. It will perform better and be easier to compensate through a TL431/opto than VMC