Change of zener voltage with age?

[grizedale]

Hi,

We are using BZX84-C30 zener to regulate our power supply ao 238V.

BZX84-C30 DATASHEET
https://www.fairchildsemi.com/ds/BZ/BZX84C30.pdf


We worry because i heard that zener voltage increases with the age of the zener.
-so do you know what is the rate of change of zener voltage with time?


The electrolytic output caps are 250V rated.

 

[goldsmith]

Dear grizedale
Hi
Take a look at TL431 please ! i think it can solve your problem as well .
Best Wishes
Goldsmith

 

[albbg]

I think you could be interested in the following article:

http://www.imeko2009.it.pt/Papers/FP_670.pdf

 

[grizedale]

TL431 draws 1ma of bias current and that will flow through the upper divider resistor in our setup and mess up the output voltage like that.

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[mister_rf]

In your place I’ll be more concerned about their temperature coefficient, check the datasheet, their temperature drift renders them useless in precision applications. :roll:
So if you want less temperature drift, choose the temperature compensated zener reference diodes, those are ideal for applications requiring long term voltage stability over a specified temperature range.

 

[grizedale]

to be honest we wont really have more than 2mA of current in our zener, so we should escape the temperature problem

 

[mister_rf]

How come? :-D Can you provide a solution to obtain a stable temperature environment for that circuit?

 

[goldsmith]

humm ! you completely confused me ! what do you want to do exactly ? why this high value of precision needed ? if you tell us a bit more explanations , perhaps we can help you in better way .
Anywhere , if temperature is pretty important for you , why you don't get feedback from temperature ? then you can affect it , on your values .
Best Wishes
Goldsmith :roll:

 

[grizedale]

its just a simple zener circuit, whereby when the smps gets to the regulation vout level, the zener breaks over, switches on a bjt, which grounds the nfet gate and so no more switching.
The base of the npn needs very little current , so its always less than 2mA in the zener.

I dont need good accuracy, but our vout is reg'd to 238V, and if it ever gets to 250V, then its "goodnight Vienna" from the 250V electrolytic caps.

 

[edpalmer42]

Zeners certainly do drift with time, but AFAIK there is no way to predict the direction or magnitude of the drift. Voltage references are often spec'd in terms of % drift per 1000 hours, but they don't give a direction. I suggest that you review your design to see if you can increase the voltage rating of the capacitors. 250V rating on a 238V line doesn't give you much room for degradations due to drift, temperature, noise spikes, and of course, Mr. Murphy!

Ed