UC3843 smps design frequency problem

[engineer2017]

Hi,

I want to design smps with UC3843 but I have a problem. When the input voltage is changed from 60 to 40 Vdc, frequency is changes. The output voltage does not change.I added smps circuit and switching pulses. Yellow is switching pulse, blue is output of RT/CT pin. What is the problem?

 

[KlausST]

Hi,

Yes, what is the problem?

If you want a constant frequency, use a crystal oscillator.
If you want a constant voltage power supply then use an SMPS. And that's what it does.

The picture shows a pulse skipping mode ... apperantly you use the SMPS with no or low load.

Klaus

 

[dr pepper]

The 3842 series do change frequency a little with voltage I'd guess 10 - 15% for what you said, it probably isnt a major problem in a lot of supplies as the control loop makes up for it.
The chip is able to be synced to another oscillator, its in the datasheet.
If you need a little more precision like not as good as an xtal osc regulate the supply to the chip.