# Reverse polarity protection using P-CH MOSFET

#### sudharshan.panduranga

Joined Apr 10, 2008
65
Attached circuit is using in Automotive to protect reverse polarity using P-CH Enhancement type MOSFET.
MOSFET: SSM3J351R,LXHF
Zener: BZX84-B18,235
Resistor: 10K

During normal input voltage (9V to 30V) Zener diode is using to protect gate-source of the MOSFET. MOSFET VGS max is -20V/+10V as per datasheet.
Zener diode should clamp at after 18V of input voltage.

But in this case Zener diode is not reverse biasing
kindly explain the root cause.

Attached schematic portion.

#### crutschow

Joined Mar 14, 2008
34,715
But in this case Zener diode is not reverse biasing
Don't know why you say that(?).
When the normal polarity voltage is applied the Zener is indeed reverse biased.

#### sudharshan.panduranga

Joined Apr 10, 2008
65
Don't know why you say that(?).
When the normal polarity voltage is applied the Zener is indeed reverse biased.
In simulation yes it works.
But in PCB across zener is crossing 18V beyond....like 22V, 25V so on.

#### crutschow

Joined Mar 14, 2008
34,715
But in PCB across zener is crossing 18V beyond
Then it would appear there is a problem with the Zener or the circuit.

#### sudharshan.panduranga

Joined Apr 10, 2008
65
Then it would appear there is a problem with the Zener or the circuit.
Should we need to consider any Zener impedance?

#### panic mode

Joined Oct 10, 2011
2,825
Zener impedance is highly non-linear...so no...

simple and usually good enough approximation is that forward biased diode with have forward voltage drop of max about 0.7V (for any zener) and reverse biased diode it is the rated zener breakdown voltage.

if you have a zener in a circuit where you measure more than those two limits, there are few things that may be behind it such as:
the diode is busted
the meter is busted or or not connected (maybe defective meter, or probe, or low batteries etc)
the person using meter is not using it correctly
the user is misinterpreting results (wrong range, or autoranging responds too slow for changing signal etc)

#### sudharshan.panduranga

Joined Apr 10, 2008
65
Zener impedance is highly non-linear...so no...

simple and usually good enough approximation is that forward biased diode with have forward voltage drop of max about 0.7V (for any zener) and reverse biased diode it is the rated zener breakdown voltage.

if you have a zener in a circuit where you measure more than those two limits, there are few things that may be behind it such as:
the diode is busted
the meter is busted or or not connected (maybe defective meter, or probe, or low batteries etc)
the person using meter is not using it correctly
the user is misinterpreting results (wrong range, or autoranging responds too slow for changing signal etc)
I don't think these are the possibilities...
Testing done in 5 PCBs.... Circuit designed person itself testing...

Find the PCB design snapshot for reference...

#### panic mode

Joined Oct 10, 2011
2,825
the list of possibilities continues:
wrong part is used
used parts are out of spec
used parts are fake
part is not soldered correctly (cold joint)
part footprint/package is wrong
wrong series resistor value (too small) or short on pcb or hidden solder bridge. that could cause zener to fail when powered

Last edited:

#### panic mode

Joined Oct 10, 2011
2,825
and it would be much simpler if instead of us guessing possibilities, you showed actual measurements (voltage and diode test) and actual photo of the board detail with diode and resistor marking clearly visible

#### panic mode

Joined Oct 10, 2011
2,825
I don't think these are the possibilities...
Testing done in 5 PCBs.... Circuit designed person itself testing...
what else did you check besides measuring voltage? how about measuring that resistor, checking for shorts on the boards?
how did you confirm that your meter is working correctly and your probing really touches the contacts?
why would you power 5 boards if one or two did not work? how many more do you have?
if there is a design or assembly problem causing boards to fail, powering more of them will just allow for more products to get damaged by being powered up.

#### crutschow

Joined Mar 14, 2008
34,715
If you are measuring the voltage across the Zener significantly higher than it's rated breakdown voltage, then either the Zener is bad, there is a very large current through the Zener, the Zener is the wrong part, or you not measuring across the Zener.
I can think of no other possibilities, so don't tell us that none of those is the problem.