Output Voltage Increasing very slowly

[vkskasana]

I have been working on a SMPS for quite a while now. It is an offline flyback converter. I have attached the circuit along with the thread. As soon as i turn on the power the output of the SMPS is roughly around 8-9 volts and then after a few minutes it begins to rise until it reaches 24v. However, if i turn off the supply and right after re-start it, the output instantly reaches 24v. When i provide a certain cooling period, the smps output again takes a lot of time to reach 24 v. Please provide help.

 

[KlausST]

H,

What output voltage do expect it to be?
Where exactly did you measure the output voltage (2 points)?
What measurement tool whith which setup did you use?

Klaus

 

[cupoftea]

What is the loading when you measure this?
Have you tried it on full load?

 

[dick_freebird]

In my own designs of controllers I have seen things like the soft start failing to fully reset the SS cap to ground explicitly, so it can get "pumped" and stay at a voltage that does not do any soft-starting. Unless you give it enough bleed-down time, which can vary widely as it's not designed (whether this is the case at hand, wants investigating).

Then the question would be, is "slowly" just the designed SS ramp, and OK, or is the rise time much more than expected SS interval?

 

[cupoftea]

Also, on the schem of top post, it doesnt show where the feedback opto U4A is connected to....where is the opto_diode part of it?
As i understand it, UCC28C4X type controllers need a minimum load

 

[vkskasana]

H,

What output voltage do expect it to be?
Where exactly did you measure the output voltage (2 points)?
What measurement tool whith which setup did you use?

Klaus

I expect 24 volts output, but the output was very slowly increasing. Infact, when i saw the output of the controller, i found that the controller was not starting at this point. As soon as the controller started, the output voltage would reach 24v. On a closer look, i figured out that the R47 and C23 had some issues with them. So, i replaced them and now the supply is working absolutely fine and it slows 24v right after I turn on the power supply.

Nevertheless, i would like to thank all of you who took their time and replied to my post. I still have some issues with the power supply.

Now, what is happening is, when i give 310v as input the supply works fine, but when i give 540v as input, the output voltage is only 10v. During the turn off time, the output voltage slowly comes down from 10v to something around 6-7v and then for a flick of second the output voltage comes 24v. I don't know why this is happening. Anyone has any idea, please revert.

 

[KlausST]

Hi,

Wild guessing:
* inductor saturation
* too high minimum ON time, thus - even at minimum On time the voltage rises above 24V. This leads to the next idea:
* too low minimum load current. Try to add a small bleeding load.

In post#1 we don´t see the SMPS controller chip and it´s schematic. So we can´t verify it.
Generally I don´t think it´s the best idea to wire the primary power swtiching signals via a connector ... maybe even (long) cables...

Klaus

 

[vkskasana]

Hi,

Wild guessing:
* inductor saturation
* too high minimum ON time, thus - even at minimum On time the voltage rises above 24V. This leads to the next idea:
* too low minimum load current. Try to add a small bleeding load.

In post#1 we don´t see the SMPS controller chip and it´s schematic. So we can´t verify it.
Generally I don´t think it´s the best idea to wire the primary power swtiching signals via a connector ... maybe even (long) cables...

Klaus

It already contains a bleeder resistor. and How do I determine the ON time? Does it depend on the startup resistors?

I hope you have got my second query. The output voltage is about 10v when input is about 540v while it is 24v when the input is 310v. So, the circuit works well for 310v, but as soon as the input voltage is 540v, the input falls down to 10v and 24v is only seen for a fraction of second while the smps is discharging.

I have attached the circuit for your reference. The Controller used is UC3845.