Hi,

I am working on a PMOS-based reverse polarity protection circuit for a lithium thionyl chloride cell, and I have a few doubts about the necessity and sizing of certain components — specifically, a TVS diode and a resistor (R1) in the gate drive path.

From my LTspice simulations, the circuit seems to function correctly even without R1 and the TVS diode. However, I have seen designs include them, and I want to understand their purpose better.



Here are my specific questions:

1. Will the circuit function properly without R1 and the TVS diode?

• In simulation, the PMOS reverse polarity protection circuit works fine without them. Are these components necessary for real-world reliability or protection?

2. If R1 is required, how should its value be calculated?

• The resistor is often seen in series with the PMOS gate. Is it used for limiting inrush current, protecting against gate-source ESD, or something else?

3. What is the purpose of the TVS diode, and how should its breakdown voltage be chosen?

• My assumption is that the TVS diode clamps the gate-source voltage in case of an overvoltage or reverse polarity scenario.
• If we're protecting against accidental insertion of a higher voltage battery (e.g., above 3.6 V), should the TVS breakdown voltage be around 3.6 V? However in this circuit the TVS breakdown voltage is 10V.
• How should I go about selecting the appropriate TVS diode specifications (standoff voltage, clamping voltage, power rating)?

Any guidance on best practices and design considerations for these components in such a circuit would be highly appreciated.

Thank you!

 

The TVS diodes are not part of the main circuit, it can work without them, as you said. One reason to use them like this I can think of is ESD protection. This is why TVS diodes are in bipolar connection. The working voltage should be higher than battery voltage, can be 7.5V or higher to avoid any leakage and draining the battery. The higher limit is set to protect the MOSFET. For example, your MOSFET is 30V rated, so TVS breakdown voltage should be lower than that to be able to protect it in case of ESD event. The source of ESD could be the user changing the battery. If the battery is fitted in factory and is not replaceable, consider removing it.

Resistor R1 potentially can be 0R. Usually it is used if your battery voltage is higher than 20V, then you would have a Zener between Gate and Source of the MOSFET to protect the gate (usually clamping at around 18V), then this resistor would limit the current through this Zener and you would use high value (like >100k) to reduce the current from battery that will be always present in this case.

 

Thank you so much for the detailed explanation! Makes perfect sense now—ESD protection with the TVS diode. I’m now clear about the resistor’s use with your statement—can you please elaborate on it a bit more? Really appreciate you taking the time to clarify this

 

MOSFET transistor's gate voltage is limited to 20V, going over that can damage it. So if system has higher voltages (>20V) we need to protect the gate against overvoltage. This is done by placing Zener diode across gate. Your battery is 3.6V, so this problem does not apply in your case. You can put 1k resistor or 0R, up to you.

Here is an example of protection I meant:

You don't need to use it, this is just for illustration

 

MOSFET transistor's gate voltage is limited to 20V, going over that can damage it. So if system has higher voltages (>20V) we need to protect the gate against overvoltage. This is done by placing Zener diode across gate. Your battery is 3.6V, so this problem does not apply in your case. You can put 1k resistor or 0R, up to you.

Here is an example of protection I meant:
View attachment 350704
You don't need to use it, this is just for illustration

In your example. the zener will protect M1 when polarity is correct, but will act as a diode when polarity is reversed. A second zener might be needed, depends on your need. The TVS would ensure that you are not over the gate-source voltage. A fuse might be needed since it will crowbar with high enough voltage. This all depends on what you are trying to prevent and/or what is required.

 

the zener will protect M1 when polarity is correct, but will act as a diode when polarity is reversed. A second zener might be needed

The polarity would be reversed at the drain, not the source, so no additional protection is required

 

I stand corrected!