Topswitch smps keeps resetting and higher output voltage
hi,
I have designed 24volt - 5amp smps with help of PI expert design tool. i am attaching my schematic below. I have checked my pcb and components including transformer turns ratio. All things seems to be as schematic. When i first power up my board with 220 ohm resistor at output load it gave me 35 volt in the output instead 24 volt. so i decreased my output resistor value and suddenly it went into resetting state.
Now no matter at what load it keeps resetting.
i have debugged following things
1) control pin signal : it goes from 4.8 to 5.5 volt (checked in CRO)
2)primary winding - every 16 pulse after it gives one switching output (CRO)
i haven't put up Y capacitor yet!
What should i do now? how should i debug my circuit? pls let me know if more of the information required.
Comments
hi,
Before i check transformer winding orientation i found problem with FR257 (D1) diode in my circuit. When i replaced it, Circuit started to working but i am still getting 35 volt as output instead of 24 volt. When i take my load current to above 3 amp my D1 gets blown and i have to replace it every time.
What do you think about this?
I put up a pot on R13(RLED) and by decreasing that pot value i was able to set output voltage at 24 volt as i wanted. But when i run my circuit at full load means 5 amp FR257 diode gets blown away every time.
How should i debug this problem?
Hi Kd,
D1 is just a snubber for the Drain to source of the MOSFET in the TOPswitch. You can remove this if you suspect that this is the problem. Just make sure to probe the MOSFET of the Topswitch as not to exceed the voltage rating across when you operate to higher input voltage range.
If it worked properly, then here are few suggestions.
1. VR1- remove this. You can return this one later but optimize the clamping voltage as this might have a loading effect.
2.R3 and R4- Parallel resistance is just 18K. This is way too low. Total Parallel resistance should be around 100K-300K.
3.C3-reduce this value to 1nF and slightly increase as needed.
Regards.
Hi,
i will try removing D1 and probe the MOSFET . Out of curiosity dont PI expert's schematic is optimum in itself. i just want to understand that is this software's problem or our problem?
Regards.
Hi kd,
That should be the case. But in this snubber circuit, the program would be assuming leakage inductance which would not be the real case. It is better to have this snubber circuit be optimized since this are designed to clamp noise which are generated by parasitics which can vary from board to board.
In any case let us verify first if our assumption is correct.
Regards.
Hey,
i ve been away so couldnt reply. Just yesterday i came back and made a new pcb from scratch with the same schematic and astonishingly i got 24 volt with same components as of earlier pcb.
Now the catch is when i load the output with 1.5 amps after certain times ic goes into thermal reset, And this interval decreased as i increased my load. When load reaches 3 amps it immediately goes into reset state. As soon as load is reduced it turns back on automatically.
i have used heat sinks on all places wherever it is required. Still my topswitch ic , transformer and diode D3(LQA10T300) gets pretty hot.
here's temp at 2 amp loading
diode = 70*C (on heat sink)
transformer = 60*C
ic = 60*C (on heat sink)
Can you tell me the reason for it?
Regards,
Keval desai
Hi Kd,
You are not experiencing thermal shutdown in this case since your IC temp measurement is well below thermal shutdown(135-150*C). I think you have to dig deeper on this.
Regards
Hey,
Only thing came to my mind apart from thermal shutdown is external current limit which i removed from my pcb. But i am still having the same problem.
If not a temperature what could it be that makes it turn off. Even everything works normally when i reduce the load .
Regards
Hi Kd,
Can you send update schematic.
If you say you are still using the same values as your first attachment I think you will gonna have a problem. Your primary snubber is just too heavy. You can check this by checking the input power during no load. If the input power is substantially high at no load then you can suspect this.
Regards
Hi PI-DATA,
Only change in the schematic is instead of two 39K(2W), i have put up 100K(2W-parallel) as you suggested before.
All other things are the same. What are your suggestion to make more lighter snubber. circuit.
Meanwhile i will check my power consumption.
Regards
Hi,
Make it lighter. 200Kohm single resistor and decrease C3 to 2.2nF.
Make a table on power consumption as you increase the load until it gets autorestart.
Regards.
Regards.
Hi kd,
Most of the time in this kind of problem, the transformer winding orientation is the culprit.
Verify again if secondary and auxiliary winding are in reverse with respect to primary. You may use inductance check to verify start and finish of the transformer.
Regards,