SMPS fails while connecting to UPS input

The most important thing about a UPS power source is that many have a square wave as the AC output.   Other types may have a PWM output, which may or may not be filtered.

So they may have a lot of harmonics on the AC output.  This can cause interference with some circuits, although most SMPS power supplies are not affected.

It would probably be best to see your PWB layout.

Also, what do you refer to as "failure?".  is the device damaged?   Or does the circuit not work but device is good?

Thank you for the interest shown.

As you see my schematic, I have added a didoe in snubber ckt. This gets failed all the time when I connect to UPS. Please note the fail of diode is not immediate. The diode pn P6KE200A. Once the diode fails PS become hickup.

In another case without the diode at snubber ckt, the TOPSW itself get failed.

Please suggest.

Best Regards

I do not see the P6KE200A in the original schematic, I see a normal FR diode only.. But I assume the P6KE200A is across the capacitor.

The P6KE200A is 191V minimum breakdown.  If the reflected voltage is too high, then the dissipation in P6KE200A can be too much.  If dissipation is not very much over the rating, the device may not fail immediately.

If the P6KE200A is connected as I think it is, then I am not sure why the UPS input should cause the problem.

However, if the TOPswitch fails,  That is usually due to high voltage above the limit.

If the resistor in parallel with the capacitor in the snubber is too small, then the capacitor may not be discharged to a safe voltage every cycle.  The voltage may continue to rise..

Even though the theoretical reflected voltage according to transformer turns ratio may be low, if the leakage inductance in the transformer is too much, then a lot of energy is returned to the snubber.  If the resistor cannot dissipate that energy, the voltage on the capacitor will build up and may cause a failure.  The "extra" voltage may be much more than the theoretical voltage.

The P6KE200A will limit the "extra" voltage, but if it does, then it will dissipate more power, and may fail.

So I think of the transformer as possibly the cause of this problem.   Make sure the transformer does not have too much leakage inductance.  Leakage inductance is inductance  of primary winding which is not coupled to the secondary.

It is possible that the UPS has higher peak voltage and so DC bus voltage is higher with UPS.  Then some extra energy (volt seconds) may be stored in the leakage inductance.  That energy is returned to the snubber, and will raise the snubber dissipation. 

If the snubber discharge resistor is too high a value, the energy will not be all dissipated each cycle, and the voltage on snubber capacitor will continue to rise until  something fails.

I said

 "If the resistor in parallel with the capacitor in the snubber is too small, then the capacitor may not be discharged to a safe voltage every cycle. "

What I meant was if the CURRENT DRAWN BY the resistor is too small.....   which is the same as the resistor value being too high. Sorry for any confusion

Hi,

Would you please let me know what is the failure of the power supply? Is the IC broken? Or the power supply goes to auto-restart?

Thanks 

Hi,

I would recommend that you verify the SMPS design first. As some of the other contributors have indicated, the snubber circuit is sized based on the leakage reactance of the transformer. Please verify if the leakage reactance is within the limits predicted by PIExpert.

From what you are describing, it appears that the drain voltage of the TOPSwitch is indeed exceeded when you operate the unit from the UPS. Can you describe the type of UPS being used? Is it a Sine wave UPS or a Quasi-Square wave UPS? Either UPS should not be a problem since the SMPS recfies and filters the UPS output and as such does not depend on the waveshape of the supply. Some Quasi-Square wave UPS have a tendency to go unstable with a capacitive load such as an SMPS being connected hence the question.

Can you furnish the waveform of the AC supply from the UPS especially under the conditions when you see the failure?

This should help us to investigate this further and help.

thanks and Regards,

Regards PI-Sarek

Hi,

The TOP SW IC is getting broken. If I add P6KE200A didoe that get failed as well lot of heat generates which goes up to 140degree C. 

The Transformer primary leakage inductance is around 34 uH while testing at 1KHz where as the datasheet recommended by PI is expected to have 13.74uH but not sure at what frequency. However PS is designed for 66KHz.

Regards SIVA 

34 uH is a lot....    at almost 3x the inductance the snubber is designed for, it is no wonder there are problems.

The energy per cycle stored is (I^2*L )/2, where "I" is the peak primary current and "L" is the primary side leakage inductance.

The wattage to be snubbed is the joules per cycle * frequency, resulting in joules/second, or watts.  It sounds as if that will be more than double the expected energy.

The transformer may need to be re-designed for lower leakage, or the snubber made to accept more power without damage.   The snubber resistors, and capacitor value might need to be changed to accept the energy without exceeding the voltage, or a series connection of lower voltage zener transient absorbers might be used to increase the power capability at same snubbing voltage.

I'd certainly try to change the transformer, because that will lower the losses.

I understood. Thank you for the details. However I am using the recommended transformer  from PI expert.

I would like to underline this 34uH is tested under 1KHz. Could you please suggest transformer if any.

Thanks & Regards

Siva 

Hi,

You can test the transformer around 100kHz which is closer to the switching frequency.