Identify the Maximum Voltage that can be applied across Zener, MOSFET and Transistor

[AbhinavRajan]

In many Zener Diode datasheets, I have witnessed that in the Absolute Maximum Ratings section, they don't include the "Maximum Voltage" that can be applied to the Zener diode. If the applied voltage crosses the Zener breakdown voltage, then the voltage will start to conduct and will enter in the Reverse Breakdown Region.

The datasheet only include maximum power dissipation, Thermal, temperature and Maximum Forward voltage information.

I want to understand why the maximum voltage rating is not provided in the zener diode datasheet.

My questions:

1. How to find the maximum voltage that can be applied across zener? Should I reverse calculate the Maximum Zener Voltage from the Maximum Power Dissipation rating provided in the datasheet? If so, could you provide an example.

2. And in case of a transistor/MOSFET, the maximum voltage that can be applied across it, would be the Maximum Collector-Emitter / Drain-Source Voltage, right?

Please help to provide clarity

 

[KlausST]

Hi,

1. How to find the maximum voltage that can be applied across zener? Should I reverse calculate the Maximum Zener Voltage from the Maximum Power Dissipation rating provided in the datasheet? If so, could you provide an example.

It's a problem of the generated heat.
The ammount of heat relates to the power dissipation
Power dissipation (of any path. LED, diode, CE of a bjt, incandescent light, ...) is the current through this path multiplied with the voltage across the two nodes.
P = V x I.
Calculation: rearranged formula above gives a simple division. You don't need an example for this.
The formula is good for a raw estimation, because V and I depend in each other.
To take this into your estimation: you may use the V-I chart of the zener and additionally draw the "P" line (hyperbolic) into the chart.
Where both lines cross this is the limit point.

But mind: maximum power dissipation is given at dedicated conditions: ambient temperature, mounting, heat spreading (via traces), air flow....
Thus, if you change one of these conditions, you also change maximum power dissipation and maximum current.
And sadly voltage drop of a zener varies with temperature.

Hint: don't go to the extremes with current and power dissipation. In a real circuit there will be limited air flow...which becomes even more limited by dirt over the years. You don't know the end users conditions in ambient temperature and orientation...

2) there are several limits:
* absolute voltage. Limited by isolation.
* absolute current. Limited by local electrical and thermal resistances, short time temperature rise....
* temperature. Short time to long time. Limits are shown in the SOA chart .... and maximum junction temperature....

Klaus

 

[Easy peasy]

a zener is a voltage clamping device - therefore it is a bit silly to talk about a max voltage you can apply

a pure voltage source has unlimited current - so applying 30V to a 27V zener will allow a large current to flow and it will die very quickly

that is why we talk about current into a zener, not applied volts, any applied voltage sources usually have a resistor or current limiting ckt to get the desired performance out of the zener ...

zeners can be used as over voltage clamps - but if you melt the die - that's it ...

 

[AbhinavRajan]

a zener is a voltage clamping device - therefore it is a bit silly to talk about a max voltage you can apply

a pure voltage source has unlimited current - so applying 30V to a 27V zener will allow a large current to flow and it will die very quickly

that is why we talk about current into a zener, not applied volts, any applied voltage sources usually have a resistor or current limiting ckt to get the desired performance out of the zener ...

zeners can be used as over voltage clamps - but if you melt the die - that's it ...

So, from your reply what I understand is that, for a 27V Zener, we can Apply as high Voltage as possible, provided we don't send much current through the Zener (the current which can be limited by the Current Limited Resistor). Is it correct?

Please correct me if I am wrong.
For example, If for a 27V Zener, the Maximum Power Dissipation is 270mW from the datasheet, the Maximum Current that can flow through the Zener without damaging it would be I=(270mW/27V)=10mA. So, in this case, If I can apply voltage anywhere from 27V to 100V, the zener won't get damaged if I limit the current flowing through it to less than 10mA. (Which can be done by the Series current limiting resistor). Provided I should also take care that the resistor power dissipation is within its limits as the excess voltage above 27V, gets dropped across the resistor only.

 

[betwixt]

Please correct me if I am wrong.

You are correct!
But that series current limiting resistor would drop the voltage, no matter how high it is, down to 27V providing you take the maximum current and power dissipation into account. In general you don't apply a voltage across a Zener diode, you pass a current through it and the diode itself provides a constant (well - almost constant!) voltage drop across itself.

In order that the Zener conducts, you need a higher than Vz voltage to start with of course or no current would flow anyway.

Brian.

 

[d123]

Hi,

for a 27V Zener, we can Apply as high Voltage as possible, provided we don't send much current through the Zener (the current which can be limited by the Current Limited Resistor). Is it correct?

If I can apply voltage anywhere from 27V to 100V, the zener won't get damaged if I limit the current flowing through it to less than 10mA. (Which can be done by the Series current limiting resistor). Provided I should also take care that the resistor power dissipation is within its limits as the excess voltage above 27V, gets dropped across the resistor only.

Right, correct. Shunt regulators - IC versions or like a Zener diode and shunt references do the same: the resistor drops/carries the excess voltage and burns it off as heat/'wasted' energy so the shunt device only sees it's operating voltage and current (+ load current). Ohm's Law applies in that sense.