[SOLVED] 600W DC to DC Converter 12V out

[DonRose]

This design has a boost converter to boost the DC 18 to 72V input to 85V for a phase shifted full bridge to create the 12V output at 600W

The problem I am having is at 10% load the boost goes into skip cycle even with duty cycles of >40% just before entering the skip cycle mode. The problem is not the skip cycle mode it is the audible noise that it creates

The control is a UC2843 current mode controller with slope compensation.

Frequency is 70kHz and the boost inductor is 40uH.

I have changed the slope compensation and the voltage loop but can not fix this problem. I expect to be able to get duty cycle of <10%

I am down to noise or something in the layout as being the root cause.
Any Ideas?

 

[rvionics]

Check the Current F/B signal on the scope (with shortest lead length on GND- probe) for any noise riding above the signal, also check if current sense resister
is sized properly i.e. at full load you should allready reach 80~90% of the max signal on the IC PIN (1V in case of UC2843) if the resistor is very small you will not get
sufficient voltage at lower loads (& the Comp pin level will also be smallar because of that) causing jitter or instability.

 

[DonRose]

Rvionics Thanks for the reply. I do not have a problem at full load and yes the voltage on the CS pin is clean and so is the ramp pin. The comp pin voltage is 2.0V so it should be able to go lower to the 1V level as per the data sheet. I am thinking that there is to much slope compensation at higher input voltages.

 

[DonRose]

This problem is pretty much solved. An extra diode in the base of the slope comp emitter follower was the biggest problem the it was a matter of using correct value. It still skips at high line and light load but the audible noise ls no longer a factor.