Explained: Snubber Circuit for MOSFET H-Bridge

Here we are going to talk deeply about snubber circuits and why we must use them with MOSFET H-bridge circuits. Because if we do not put snubber in H-bridge then that MOSFETs might blow off during switching. Specially when we drive inductive loads like transformer, motor, relay, solenoid, etc. then this thing becomes very serious.

So let us start from the beginning.

What is a Snubber Circuit?

Snubber is a simple extra small circuit which we connect across MOSFETs or IGBTs or any switching device. We do this to absorb or kill the high-voltage spikes which come suddenly during switching OFF.

These spikes come mainly because of inductive kickback which happens when we switch OFF current in inductive load. That current wants to continue flowing but suddenly MOSFET turns OFF, so high voltage is generated across drain-source pins of MOSFET. This high voltage can be even more than 500V or even 1000V if not stopped. So this huge spike can punch through the MOSFET and kill it instantly.

So snubber is like a safety device. It eats up that spike and protects the MOSFET.

Why Snubber is Very Important in MOSFET H-Bridge?

Now in H-bridge we are using 4 MOSFETs or 4 IGBTs, right? Two high side and two low side. And we are switching them ON/OFF alternately to create AC from DC or to drive motor back and forth.

But now the problem is, when we switch OFF any MOSFET and we are driving inductive load like motor or transformer primary then the load tries to push back high voltage spike to the MOSFET. Now if no protection is there, then that spike will break the MOSFET.

Also during dead time, when all MOSFETs are OFF for a tiny time to prevent shoot-through, that time also spike can hit hard.

So if we do not use snubber, then we will keep replacing MOSFETs every day.

What Type of Snubber We Can Use?

There are mainly 3 types of snubbers:

1. RC Snubber – Resistor + Capacitor in series.
2. RCD Snubber – Resistor + Capacitor + Diode.
3. TVS Diode Snubber – Only Transient Voltage Suppressor diode.
4. Zener or MOV – Not proper but sometimes used.

For most MOSFET H-bridge we use simple RC snubber or if we want better efficiency then we use RCD snubber.

RC Snubber: Most Common and Easy One

So this one is super simple. Just connect one resistor and capacitor in series and then put that series combo across the MOSFET drain-source.

Example:

• Capacitor = 100nF to 1uF (rated for 400V or more)
• Resistor = 10 Ohm to 100 Ohm (1W to 2W)

That RC combo absorbs the spike and converts it to heat in resistor.

Connection Diagram:

We put one such RC snubber across each MOSFET.

But problem is that this will always eat some power and get hot. So efficiency reduces. Also value of R and C must be selected carefully.

RCD Snubber: Little More Efficient

This one is better for high power H-bridges.

Here we add a diode in series with capacitor, so that the capacitor does not get charged continuously. It gets charged only when spike comes.

Then that capacitor discharges through resistor slowly. So less power loss.

Here the diode must be fast recovery diode like UF4007 or FR107.

Connection Diagram:

TVS Diode: Compact and Precise

If we do not want RC or RCD then we can also put a TVS diode directly across drain and source.

TVS will instantly break down and absorb the spike if voltage goes above safe limit.

But cost is more and we must select exact voltage rating.

Example: For 100V MOSFET, we can use 120V TVS diode like 1.5KE120A.

Real Life H-Bridge Example

Suppose we are making a 12V to 220V sine wave inverter with IRF540N MOSFETs in H-bridge. Then the transformer primary is like an inductor, right?

So when MOSFET turns OFF, then that primary sends back a sharp spike to MOSFET drain pin.

So we must connect RC snubber across each MOSFET.

Let us use:

• R = 47 Ohms 1W
• C = 0.1uF (100nF), 400V polyester

Connect one R+C combo directly across drain and source of each MOSFET.

This will catch spike and save our IRF540N.

How to Calculate Snubber RC Values?

Here is one crude method to calculate RC snubber to suppress any form of high voltage spike in an electronic circuit:

Step 1: Calculate load inductance (L) – optional, approx.

If you do not know exact L, no problem, we can guess.

Step 2: Use below formula to select C:

C = Ispike * tr / Vspike

Where:

• Ispike = expected spike current (A).
• tr = rise time of spike (seconds), like 100ns to 1us.
• Vspike = spike voltage we want to limit.

Example: Calculations:

Ispike = 5A
tr = 100ns = 100 × 10^-9 s
Vspike = 100V

C = 5 × 100n / 100 = 5nF
Use 10nF to 100nF for margin.

Or roughly 10 Ohm to 100 Ohm. We use trial-error method if L is unknown.

Start with 47 Ohm + 100nF, check waveform using scope. If spike is less, then keep it. If more, then reduce R or increase C.

But What is the Link Between "L" and above Formula?

Actually Bro, L is hiding inside the tr (rise time).

Let us say:

Vspike = L × di/dt

Then:

dt = L × di / Vspike

Which means that if we dont know L, then we also don’t know the exact dt (rise time).

That is why, estimating L helps us guess tr (rise time) more correctly. If L is big, spike rises slowly, then tr is longer.

So:

We estimate L to estimate spike rise time so that we can choose C using the formula.

Series RC Snubber for Slow-Switching MOSFET Circuits

For inverter circuits or motor drivers running below 1kHz PWM or 50Hz AC, we can use a classic series RC snubber across each MOSFET.

Connection Details: Connect a small capacitor (e.g 10nF to 100nF, 400V rated) in series with a resistor (e.g 100 Ohms, 1/2 watt), and join them across Drain and Source of the MOSFET.

This helps to absorb inductive spikes during turn-off.

Example:

• For 100V DC bus, 50Hz PWM
• C = 100nF / 400V
• R = 100 Ohms / 0.5W

This is effective and simple but not efficient for fast switching.

Things You Must Remember

• Always use snubber close to MOSFET legs.
• Snubber capacitor must be non-polar like polyester or ceramic.
• Resistor must be non-inductive and 1W or 2W rated.
• Use separate snubber for each MOSFET.
• Never use electrolytic capacitor in snubber.
• If still MOSFETs heat up, use RCD or TVS diode.
• Snubber improves life and reliability of H-bridge.

Final Words

So guys, we saw that snubber is like bodyguard for our H-bridge MOSFETs. It saves them from getting destroyed due to back EMF or switching spikes. Without this small helper circuit, our inverter or motor driver will not last even one week.

We can use RC, RCD or TVS snubber depending on how much spike is coming and how much cost and space we can afford.

We must try different R and C values and check waveform using oscilloscope to find best values.

So always add snubber in your H-bridge design, specially when driving inductive load like transformer, motor, coil, etc. That will make your design super strong and long lasting.