Snubber circuit for MOSFET

Hello Sumangala,

Thanks for the question. Can you be a little more specific about your BMS application? In TI speak, BMS is battery management solutions which includes chargers, gas gauges and protection for batteries. There are many technical articles available for snubbers as shown below. If you can share more detailed information about your specific application, I may be able to better help you.

http://www.ti.com/lit/an/snva744/snva744.pdf

http://www.ti.com/lit/an/slpa010/slpa010.pdf

http://www.ti.com/lit/an/slyt740/slyt740.pdf

Hello John,

Let me explain my question. In a battery management system, we will have charge, discharge and pre-charge MOSFETs. In a typical EV application, the load is inductive. When I turn off the battery supply to my load, a back emf will be generated and this may damage the MOSFETs in the power path. So I want to add snubber circuits near these MOSFETs to protect from back EMF. Please let me know how this can be done and correct me if I am wrong anywhere.

Thank you!

Hello Sumangala,

I'm not all that familiar with this type of application where the back EMF could cause damage to the FETs. I don't think a snubber is the right answer and you may require some sort of TVSS to clamp the energy and prevent damage to the FETs. I am going to send this to our automotive team to see if they can better help you.

Hi Sumangala,

As you note the purpose of a snubber circuit is to protect the VDS of MOSFET not too high to destroy it.

In fact, your circuit looks similar to what we have been working on - see below.

If the load inductive, like a motor, there will be a high voltage at the output while Q2 is turned off (Assume no freewheeling or clamping path). At this situation, this high voltage will go to Q1 through the parasitic diode of Q2. That is to say, Q2 is safe but Q1 might be damaged due to the high voltage across the Drain and Source. Hence, we add some protection components like below D1, D2, D3 and C1. Here, D3 is the freewheeling path which will make the load point to about -0.7V referenced to GND. And D1 and D3 represent the alternative protection mechanisms to ensure the voltage not larger than the maximum Vds. Together, the Al-Cap can also absorb the energy to prevent the voltage from changing a lot.

This circuit is quite different from a classic snubber as used in a DC/DC converter -  A snubber for a converter is somewhat easier to design as the inductance is fixed.  In the BMS case, howeer, the inductance will depend heavily on the load and therefore it becomes quite difficult to use a snubber as its too difficult to decide the RC value that should be used in that snubber

Thanks,

Mark

Hello Mark,

Thank you for the detailed explanation. That really helped to understand the conventional way to avoid back emf from damaging the MOSFETs. Although, most of the engineers say the back emf must be discharged through the source (in BMS, the battery itself). Is this correct? If yes, is there a way to discharge in a controlled manner? Won't it affect the source?

Thank you!

Sumangala, I have reached out to the designer on this one to ensure that they get a follow up response to you. We apologize for the delay in response here. 

Best Regards,

J. Fullilove

Texas Instruments

Hi Sumangala,

Sorry for the delay.  The above circuit is what we advise to use.

However, I am not necessarily in a position to advise on the specifics of your question as you may want to ask the battery manufacturer.

Thanks,
Mark