what is the optimum oscillation frequency for operation of uc3842

[Tunde53]

I am building an smps battery charger with UC3842 chip for 28volts 12ampres, but the circuit continues to ring and the output is fluctuating. Can anybody provide help? Thanks.

 

[Papabravo]

Why do you think that such an optimum frequency exists? The output current ripple is controlled by the inductor selection and the output voltage ripple is controlled by the output capacitor. Did you choose those values from a design equation, or did you just swag a value?

 

[ChrisP58]

Can you please post your schematic and circuit layout? And scope pictures of the waveforms and output fluctuations?

The UC3842 can operate over a wide range of frequencies. The frequency needs to match the characteristics of the rest of the circuit. Usually, the frequency is selected and the rest of the components are designed / selected based on that.

Ringing can be caused by many things, some only slightly related to switching frequency.

Fluctuation in the output voltage is probably due to improper compensation.

 

[schmitt trigger]

There is a MAXIMUM clock frequency, which -from my old worn out memory, is about 500 Khz. Anything below is fine for the IC itself.
But as others have mentioned, the actual switching frequency depends on a host of external components and design requirements.

 

[simonbramble]

Fluctuation in the output voltage is probably due to improper compensation.

This is probably the most likely problem. Without a Bode analyser, finding out the stability of the control loop is difficult. Check the drive to the FET. If you can see different length pulses, but with a constant load, this loop instability and the compensation components will alter this. Board layout also needs to be impecible. You cannot build these on strip board. It needs at least a 2 layer (ideally 4 layer) PCB

 

[Nigel Goodwin]

It needs at least a 2 layer (ideally 4 layer) PCB

Hardly, it's a VERY common chip used in domestic SMPSU's (TV's, set-top boxes etc.), many of which use single sided PCB's, particularly for the PSU. For set-top boxes, while the main board will be multilayer, the PSU is often separate, and only single layer.

 

[simonbramble]

You can, but your life will be made easier with a 2 layer PCB

 

[simonbramble]

... and yes I agree it is probably the most popular offline converter in the world. Less so now with the popularity of Power Integrations. Personally if I was going to build an offline converter, I would look at Power Integrations

 

[Nigel Goodwin]

You can, but your life will be made easier with a 2 layer PCB

I don't see why?, it's a simple circuit, so easy to design as single sided - although as any PCB manufacturers (such as JLCPCB etc.) do double-sided as a minimum, you may as well go double-sided.

Or single-sided, with an upper ground plane?.