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?

 

[Nigel Goodwin]

Doesn't sound like a 'problem' at all?.

It's a regulated power supply, the output should remain constant as input voltage varies, that's the entire point of it.

 

[engineer2017]

Thank you but Why the output voltage remain constant by changing duty cycle?Frequency is determined with RT/CT.

 

[Nigel Goodwin]

Thank you but Why the output voltage remain constant by changing duty cycle?Frequency is determined with RT/CT.

It depends on the specific chip some vary duty cycle, some vary frequency.

 

[engineer2017]

Frequency of triangle wave form is stable. How does the frequency change?

 

[ChrisP58]

It's not changing frequency, it's skipping cycles. Even your 40 Volt image is skipping. For the UC3843, there should be one switch pulse (yellow trace) for each oscillator cycle (blue trace.) And, if it's properly regulating, that switch pulse should end before the oscillator signal hit's it's max value.

The fact that you have switch pulses that are equal to the rise time of the oscillator shows that values in your circuit are out of balance.

You show only the schematic for the circuit that you built, which I assume that you copied from somewhere. There are a lot of other factors that are important in building a switch mode power supply. Was there any more information from the design source giving more details? Particularly about the transformer.

My first guess about the circuit behavior is that the primary inductance of the transformer is too low with respect to the operating frequency.

 

[ronsimpson]

It's not changing frequency, it's skipping cycles.

The TL431 lacks compensation. So it is using cycle skipping to regulate. Add compensation and you will see the duty cycle change to cause regulation.

 

[engineer2017]

The TL431 lacks compensation. So it is using cycle skipping to regulate. Add compensation and you will see the duty cycle change to cause regulation.

How can I Add compensation ? Do you have example circuit?

 

[ronsimpson]

link
More reading. link 2