# Identify the maximum voltage that can be applied across a Zener, MOSFET and transistor

In many Zener diode datasheets, I have seen 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 a 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. For a transistor/MOSFET, the maximum voltage that can be applied across it, would be the maximum collector-emitter or drain-source voltage, right?

Edit :

• Please provide a data sheet link. Apr 26, 2020 at 8:02
• Provided the datasheets of the components. Apr 26, 2020 at 8:06

The rated Zener breakdown voltage IS the maximum voltage that can or should be applied. If you try to apply more voltage, the current will increase (theoretically) without limit, exceed the device's power rating and destroy it. The maximum current can be calculated by the Zener voltage and the maximum power rating by P = IV, or I = P/V.

A Zener diode id typically powered through a resistor or a constant-current source, and maintains its rated voltage across it, and thus is commonly used for voltage regulation.

A bipolar transistor and mosfet have three terminals, and thus have several ratings for the maximum voltages between the different terminals.

• From what I understand, If the Zener Breakdown Voltage is 36V, and its Zener Maximum Power Dissipation is 300mW, the maximum current that can flow through the device during the zener breakdown condition is 8.3mA? Apr 26, 2020 at 7:35
• @Newbie. In practice it would be prudent to derate the device, so as to provide a safety margin before failure occurs. How much derating is up to you. Suggest starting at 20% Jan 20 at 16:25

Zeners are designed to operate in breakdown. Thus hotspots must be avoided.

MOSFETS and Bipolars are not designed to operating in breakdown, tho I've seen schematics using the 2N2169 NPN (Jim Williams of LTC uses this device) in a sub-nanosecond pulse generator, where the avalanche performance (a breakdown mode) produces the fast onset of charge movement and hence the fast edge.

I suspect your performance with such a circuit will strongly depend on who manufactured the transistor, given the avalanche speed is not spec'd in datasheets.

It is not robust to specify a maximum voltage across a zener since it is designed operate in breakdown, and has a very sharp curve.

For instance, if you have a '10 V' zener, applying 9.9 V will result in negligible current, and the zener will be ineffective. Applying 10.1V would result in a very large current, possibly over the thermal rating (I*V) of the zener.

Consider that the '10 V' may change (with temperature or different samples) -- some could conduct a very large current at 10.0 V, others a negligible current. Therefore specifying voltage is not very useful.

Conversely, specifying a maximum current in a zener (which is intended to operate in breakwdown) allows accurate calculations of power dissipation, and is reasonably independent of the precise breakdown voltage (9.9, 10.0, 10.1 etc.).

On the other hand, FETs and BJTs are not generally designed or intended to operate in breakdown -- you want negligible current at high voltages across the device. So the max. V is specified at the lowest V where the junction (at any temperature and sample-sample variation) would begin to conduct.

Some power FETs are specified for unclamped inductive switching (UIS) where breakdown is allowed; generally the specific BV is not critical as long as it is greater than some minimum value (which is the specified maximum DC (continuous) applied voltage).