Zener Failure Issue

Thread Starter

panditshashank

Joined Dec 4, 2019
3
Dear All Members,
I am facing issue of Failure of 200V, 5W Zener Diode (1N5388B) connected across primary winding of transformer in SMPS TOP 224Y based design. Switching frequency of Power supply is 100 KHz. When tested Zener for failure issue i observed that it is heating more then 60Deg. C but less then 75Deg. C. in just 30sec. I have done some trials for finding cause,
1. Current through Zener measured is 7mA which is less then 23.6mA mentioned in datasheet.
2. Changed voltage rating of Zener by replacing with another Zener diode with higher voltage rating of 270V, but that too is heating.
3. During this i observed that Transformer is also heating too much. Transformer used is E30/15/7, 12Pin horizontal.
4. Leakage Inductance when measured at primary by shorting all secondary is found around 140 Micro H. Primary inductance when all secondary open is measured to be 3.563mH, this was measured by LCR meter at 25KHz.
5. Supply is designed for Universal Input (85VAC - 265VAC), but it is not switching On below 110VAC. Below this Zener gets blown.
Power supply is designed for 6W and 80% efficiency.
The only change in our supply and that mentioned in application note of power integration if the use of Zener, In there application Note AN 22 they mention TVS diode of rating 200V and we are using Zener diode of 200V.
It is also not that all the instruments are failing but the failing rate is about 10%, so it is of concern to us.
Please help.
Regards,
Shashank
 

BobaMosfet

Joined Jul 1, 2009
1,767
Dear All Members,
I am facing issue of Failure of 200V, 5W Zener Diode (1N5388B) connected across primary winding of transformer in SMPS TOP 224Y based design. Switching frequency of Power supply is 100 KHz. When tested Zener for failure issue i observed that it is heating more then 60Deg. C but less then 75Deg. C. in just 30sec. I have done some trials for finding cause,
1. Current through Zener measured is 7mA which is less then 23.6mA mentioned in datasheet.
2. Changed voltage rating of Zener by replacing with another Zener diode with higher voltage rating of 270V, but that too is heating.
3. During this i observed that Transformer is also heating too much. Transformer used is E30/15/7, 12Pin horizontal.
4. Leakage Inductance when measured at primary by shorting all secondary is found around 140 Micro H. Primary inductance when all secondary open is measured to be 3.563mH, this was measured by LCR meter at 25KHz.
5. Supply is designed for Universal Input (85VAC - 265VAC), but it is not switching On below 110VAC. Below this Zener gets blown.
Power supply is designed for 6W and 80% efficiency.
The only change in our supply and that mentioned in application note of power integration if the use of Zener, In there application Note AN 22 they mention TVS diode of rating 200V and we are using Zener diode of 200V.
It is also not that all the instruments are failing but the failing rate is about 10%, so it is of concern to us.
Please help.
Regards,
Shashank
The zener can only withstand 5W in reverse breakdown. Test conditions show 1.2V @ 1.0A in forward- which is likely your safe limit for 25C - because the TEST CONDITIONS show you the real values the mfg expects you to use.

Look at your thermal derating for temp, and it probably needs a heatsink. What would be most useful is for you top provide a schematic, and because you mentioned a transformer, you're working with A/C, correct?
 

Ian0

Joined Aug 7, 2020
2,162
Is D1 (diode in series with the zener) still OK? Is it the right way round?
Did you make your own transformer? Does it have too much leakage inductance (which leads to a lot of dissipation in the clamp)?
Is the phase of the secondary correct?
Did you see the primary waveform before it failed?
 

DickCappels

Joined Aug 21, 2008
7,418
As BoabMosfet indicates the Zener might not be able to handle the power.

If trying the recommended TVS does not solve the problem you may want to get with your transformer manufacturer and see whether they can reduce the leakage inductance.
 

Ian0

Joined Aug 7, 2020
2,162
Try putting 47nF 250V in parallel with the zener. That might keep it alive long enough for you to really see what is happening.
 

Thread Starter

panditshashank

Joined Dec 4, 2019
3
Is D1 (diode in series with the zener) still OK? Is it the right way round?
Did you make your own transformer? Does it have too much leakage inductance (which leads to a lot of dissipation in the clamp)?
Is the phase of the secondary correct?
Did you see the primary waveform before it failed?
Thanks for your reply,
1. Diode in series never fails.
2. Transformer is made externally by our vendors as per our datasheets. Lekage Inductance measured is 140Micro Henry, measured at primary by shorting all secondary. Primary Inductance is 3.5mili Henry, measured when all secondary winding is open.
3. No sir we have not seen the waveform.
Regards
 
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