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TOP 250Y failure

Posted by: abish.g.nair on 02/24/2022

Sir,

sir,
We are purchased TOP250Y IC for the use of Adapter making,still some batches ICs continuously failed from the field.we are using the same design in the TOP250 datasheet circuit.Please help to findout the actual problem of that ICs.
Design details:
input voltage     :150 to 240 Vac
output voltage  : 12v DC
Out put Current : 10A max
Adapter used to : Analyzer machine, resistive load device,
                                one stepper motor used ( Current below 0.600A)
                                One coil heater ( Used max.6A at 4 second 'ON')

Comments

Submitted by PI-Wrench on 03/01/2022

I am assuming in this case that your power supply was adapted from the 250W design in the TOPSwitch-GX datasheet. Please let me know if this is not the case. Since the design is used at lower output voltage and higher output current than the original design, special care must be taken with the transformer design and output layout to minimize stray inductance. The output capacitors will need to be sized to handle the output ripple current, which will be in excess of 10A. In the 250W design, an oversize transformer core was deliberately used to minimize the number of transformer turns required for the design, minimizing the transformer leakage inductance, as the leakage inductance scales as turns squared.

It would be helpful in this case if you shared your actual schematic and transformer design. Also, what sort of initial peak output current is encountered when driving the motor and the cold heater coil?

We have recently experienced problems with other customers encountering counterfeit TOP-GX devices from unauthorized sources. Would it be possible to share some photographs of  representative samples of the devices you are using?

Submitted by abish.g.nair on 03/02/2022

Sir, 

Thank you for your valuable replay, 

Only one thing confusing is that when we replaced the damaged TOP250Y, the power supply was working fine and no more failures were reported. If it was a design related issue, then how does it work after replacement ?

Submitted by PI-Wrench on 03/02/2022

I would be better able to answer your question if you shared a schematic.

Submitted by abish.g.nair on 03/05/2022

Sir,

Please check the attached file below, is our SMPS design.
if problem found please let me know.

Submitted by PI-Wrench on 03/07/2022

Please re-submit your schematic file - I don't see an attachment here.

Submitted by PI-Wrench on 03/09/2022

Attachment failed again. Please re-try using a pdf or image file

Submitted by PI-Wrench on 03/14/2022

I see the schematics now - thanks. I will be doing evaluation of the design soon.

Submitted by PI-Wrench on 03/16/2022

OK, I was able to run the design through a legacy version of PIXLS as a check, and the news is not too good.

The transformer as designed saturates heavily under peak power conditions, which will stress the TOPSwitch with high peak current.

The low value of input bulk capacitor (47 uF) causes the TOPSwitch to run into duty cycle limit at peak output, which will cause the output voltage to fall out of regulation.

In the case of the primary snubber, paralleling TVS clamps and snubber diodes is not a good idea, as the device with the lowest voltage drop will take all the current. It is better to use a 3A rated part for the snubber diode and to use two 100V TVS clamps in series, which will spread out the power dissipation between the two TVS diodes.

The transformer will need to be redesigned to avoid saturation, with either more turns or a larger core size. Since the peak power condition is of relatively short duration, you may be able to keep the same core size and just increase the number of turns. This will depend to a large extent on the duty cycle of the peak power condition.

The size of the input bulk capacitor should be increased if you want to keep the output voltage from dropping out at peak power. Using 2 pieces of 47uF instead of one may be sufficient.. 

I am appending a copy of the evaluation spreadsheet for your reference The transformer inductance in the spreadsheet is 404 uH rather than 390 uH, but this is close enough for evaluation purposes.

I will run another spreadsheet with changes to remove the problem areas, and add it later.

Attachment Size
TOP250_12V_120W_ETD34 .pdf 102.86 KB
Submitted by PI-Wrench on 03/16/2022

I was able to run another spreadsheet and apply some corrective measures. Adding another 47uF cap to the input (using a single 100uF cap would also work) and adjusting the transformer turns cleared up the most egregious problems. I would also suggest to add another capacitor or two to the output, as the two caps presently in place will be severely stressed during peak power conditions, shortening their lifetime.

Attachment Size
TOP250_12V_120W_ETD34_corrected.pdf 102.71 KB
Submitted by abish.g.nair on 03/18/2022

sir,

thank you for your valuable information.