How to theoretical analyze the zener diode circuit?

[electrowhiz]

Hi all,

Please find the circuit and the simulation result in the attachment the attachment
I am struggling to theoretically calculate the at which source voltage the zener diode will enter the breakdown region.The zener diode used is BZX84C15L. In the circuit, voltage applied is 17V. The voltage across the zener is around 14.6V and the current through the circuit is 80uA. Whether 80uA is the leakage current?

How to theoretically determine the voltage across the zener for different supply voltage?

Thanks

[iMo]

You cannot easily calculate where exactly comes to the zener/avalanche effect.

It depends on many factors, from chemical's doses used during fabrication, size and shape of the n-p regions, temperature, etc.

You would need the manufacturer's know-how to do that with still some percentage of precision error.

Also the zener's diode "knee" is rather wide.

[MrAl]

Hi all,

Please find the circuit and the simulation result in the attachment the attachment
I am struggling to theoretically calculate the at which source voltage the zener diode will enter the breakdown region.The zener diode used is BZX84C15L. In the circuit, voltage applied is 17V. The voltage across the zener is around 14.6V and the current through the circuit is 80uA. Whether 80uA is the leakage current?

How to theoretically determine the voltage across the zener for different supply voltage?

Thanks

Hi,

Zeners can usually be replaced by a voltage reference, which is far more stable over input and temperature.

The standard way to analyze a zener circuit is to assume the zener voltage is constant.  Then calculate the range of current that flows though the zener and make sure it stays within the operating range of the zener for decent regulation.
To do this, you can replace the zener with a voltage source (battery) and calcualte the current into the battery as if to charge it.  That's your zener current.

To get a better approximation you have to have the zener curve.  You would then do a similar thing except the zener voltage would change with the zener current.
To get a first approximation you can assume a straight line curve, to get better you have to use the actual curve which may not be very straight line.

[Ian.M]

There's a paper on modelling Zener diodes in a SPICE simulator here:
https://people.eecs.berkeley.edu/~hu/PUBLICATIONS/Hu_papers/Hu_Melvyl/Hu_Melvyl_91_01.pdf
If you absolutely had to, you could solve it manually using mesh or nodal analysis and the  Shockley diode equation.  Fitting the model to a specific part number Zener is a whole other problem - the datasheets are typically rather lite on accurate parameters so unless you put a specific diode on a curve tracer there's not a lot of data to work from.

OTOH if you've put the Zener on a curve tracer, and aren't needing a model to sim it, you don't need to model it any more!