200V Zener failure Issue.

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

Please reply to my post I have no Idea how to approve my post.
Reagrds,
Shashank

Shashank Pandit said:

I have no Idea how to approve my post.

Click to expand...

The forum software decided to put your post into the approval queue.
The post has to be approved by a moderator before it can be viewed by anyone.
I have approved your post, welcome to ETO.

JimB

A TVS diode does not conduct in either direction below it's rated voltage but a zener starts to conduct at it's rated voltage in one direction but behaves like a normal diode in the other direction, (I.E it conducts in that direction at 0.6 to 0.7 volts) Why did you choose a zener diode instead of the recommended TVS diode ?
EDIT. I have just been reading some information on TVS diodes. I was wrong to say that all TVS diodes are bi directional. Some are not. You would need to give the part number of the recommended TVS diode to clarify if this is the case.

Les.

What is the output voltage of the supply?
What is the number of turns of all windings of your transformer?

This info is to determine what the reflected voltage is for the transformer primary.

Also, I hope that you have the fast recovery diode in series with the zener. Without that, neither a 200V zener or TVS will survive.

It would also be good to post your complete and detailed schematic.

Thanks for your reply,

TVS Diode suggested in Power Integration Application Note AN 22 is P6KE200 unidirectional legend name VR1 in attached pdf.
Regards,
Shashank

We have multiple supply output, . Designed for reflected voltage of 135V. Efficiency of Supply is 80%.Winding details are as follows:
Primary: 124Turns, 32SWG, Input(85 - 265VAC).
Feedback / Biased: 15Turns, 32SWG for 15V@6mA
Sec1: 26Turns, 32SWG for 27V@100mA
Sec2: 8Turns, 32SWG for 8V@150mA
Sec3: 21Turns, 32SWG for 22V@40mA
Sec4: 8Turns, 32SWG for 8V@150mA
Switching device used is TOP224Y.
we are using diode Ultrafast Plastic Rectifier UF4007 in series with Zener diode.
Is there any issue with transformer heating also as it is also getting heated. So is there any issue with the design. But it should not be any issue with the design as not all the instruments are failing, approx. 10% of instruments are failing but it is of major concern to us.

use transil instead, or RDC snubber network

MacIntoshCZ said:

use transil instead

Click to expand...

Sorry not understood

transil is tvs

Thank you Sir,
But do you find any issue in using Zener diode? Why you recommend the Transil please elaborate and what rating should i use of transil. Is there any part number which you recommend for my design.
Regards,
Shashank

transils are more fast, more power durable and also for higher voltage than zener diodes...
As you mention P6KE200 could work...