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?
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.
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:
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.
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!