SMPS power supply help needed

[rob]

Hi to all.
I am building a 12 to 19V @4A convertor using an uc3843 controller
chip.
I am using an inductor , not a xfmr , so there is no isolation. The
frequency
is 300khz.
I was wondering how to calculate the compensation components , in and
around the feedback cct.Looking on the internet at various ccts using
this chip
I have seen wildly differing values. The resistor / cap combination
from the output of the error amp to the - input is 100k // 100p at the
moment. I have seen values from 220k// 100n . (yes , that is nano)
At the moment the convertor is "working" , but it is making a high
frequency
singing / hissing noise , which it should not do.
Any help would be appreciated.
Cheers
Rob

 

[Klaus Vestergaard Kragelund]

Hi to all.
I am building a 12 to 19V @4A convertor using an uc3843 controller
chip.
I am using an inductor , not a xfmr , so there is no isolation. The
frequency
is 300khz.

So a Buck converter?

I was wondering how to calculate the compensation components , in and
around the feedback cct.Looking on the internet at various ccts using
this chip
I have seen wildly differing values. The resistor / cap combination
from the output of the error amp to the - input is 100k // 100p at the
moment. I have seen values from 220k// 100n . (yes , that is nano)
At the moment the convertor is "working" , but it is making a high
frequency
singing / hissing noise , which it should not do.
Any help would be appreciated.

Three ways to go:

1. Go to the library and get a book on SMPS design - read your way through
it a do the math

2. Fiddle your way through the design by measuring the gain/phase response
at all possible load and input voltage situations. The select the
compensation circuit to get 60 degree phase margin and 10dB gain margin

3. Write down the equations for the power stage, PWM modulator and error
amp. And then workout the feedback components accroding to the 60degree/10dB
rule

Cheers

Klaus