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smps output filter capacitor turning everything off

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usama14

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Hello All. I made a dc to dc converter(Full Bridge) from 48VDC TO 330VDC. The problem Im facing is that whenever I connect the output of the transformer to the rectifier, everything shuts down. Plz help.
 

Which features of your converter may cause a "shut down"? Intentional over current protection or just failing processor supply. In the latter case you may want to introduce better filtering of the internal power supply.

Can it be a problem of output filter inrush current? In this case, you could try to slowly ramp-up the inverter duty cycle (e.g. using the slow start feature of the switch mode converter).
 

what do you mean by "output filter inrush current"?
 

I believe inrush current is a well-known technical term. "Output filter" means all capacitors connected to the DC bus.

Showing a schematic would enable us to determine if there's a large capacitance with respective inrush current problem or not.
 

okay I understood. So I should increase the duty cycle in small steps of say 5%? Is that what you're suggesting?
 

Yes. You can also precharge the output circuit by a different power supply for test. This would check for possible output shorts.
 

Here is my schematic of dc-dc converter. The output filter is RC based and C=1uF for now. Switching frequency is 25kHz. And the total duty cycle is 40%. Input rail dc voltage is 48VDC and output is 330VDC.
122.png

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Actually last time I had the same issue, and I just increased the duty cycle from 20 to 40% and the matter solved. Now it is again creating problems. I'll try doing what you said and share the results. Thankyou! :)
And by "precharge" you mean I should charge the output capacitor to some known value first and the use it?
 

1 µF is a rather small capacitor and shouldn't cause large inrush currents. The other (less pleasant) explanation is that the rectifier diodes aren't suited for the used operation frequency.

You also didn't tell yet how the converter is shutting down.
 

The diodes Im using are DSEI20-12A fast switching diodes.
I already told you that whenever the output of the transformer is connected to the filter, the converter shuts down. Im still in shadows about the reason behind it. Not to mention all confused. :/
 

I am not implementing the feedback right now. Only the main circuit is attached. :/
 

you may have a wiring fault, i.e. one diode the wrong way around, or you may need snubbers on your diodes, as even lightly loaded there will be ringing on the diodes causing RFI getting into your control and upsetting it - do you have a photo of your set up?

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also not having a soft start (PWM going from 0 - 40% over 1 second) at power up puts largish pulses in the fets and diodes causing noise issues.

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p.s. you do not have an RC o/p filter, just a C.
 

Easy peasy said:
you may have a wiring fault, i.e. one diode the wrong way around, or you may need snubbers on your diodes, as even lightly loaded there will be ringing on the diodes causing RFI getting into your control and upsetting it - do you have a photo of your set up?

- - - Updated - - -

also not having a soft start (PWM going from 0 - 40% over 1 second) at power up puts largish pulses in the fets and diodes causing noise issues.

- - - Updated - - -

p.s. you do not have an RC o/p filter, just a C.
Click to expand...

Sorry for the late reply, was going through the testing phase. :/
Here is what I've done in these 2 days.
I double checked everything. There's no wiring fault. I solved the issue by replacing the microcontroller. I precharged the 1uF capacitor and then used it, it worked too. But cant understand why everything still got turned OFF whenever I connect any bigger capacitor(I also tried charging them too but didnt work).
Also is using only the Capacitor enough for filtration? As opposed to the RC one?
 

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