repetitive avalanche currents

IanR

Single-die CMF FETs are rated for repetitive-avalanche currents, limited by the junction temperature. 

C3M FETs are not listed for this. Would they similarly operate with repetitive-avalanche currents?

Thanks, Ian

TBhatia

Hello, thank you for your message. A member of our technical team is looking into your questions and we will respond shortly. 

IanR

To correct my post: this is about FETs, not diodes, and the limit is energy, not current.

jshao

Hello Ian,

C3M can operate under repetitive avalanche conditions. Usually avalanche energy is proportional to die size. So for same Rdson, there is significant difference in die size for different generation. If you have question for particular device, please let us know.

Thanks,

Jianwen

IanR

Hello Jianwen,

The specific device is the C3M0075120D. Might you also provide the single-pulse avalanche energy limit? And are the avalanche-energy limits thermal, or are there other limitations that affect this?

Many thanks, Ian

jshao

Hi Ian,

The limit is due to thermal. The energy level is highly depended on current, temperature conditions. Do you have peak current and temperature info? What is your application?

Thanks,

Jianwen

IanR

Hello Jianwen,

The FETs are in a hard-switched bridge, discontinuous operation.

The switching conditions are 21 A peak, 20 kHz, stray inductance 63 nH. So repetitive avalanche power is

P = 0.5 L I^2 f = 0.28 W.

Thermal resistances are 1.1 junction-case, 0 case-sink, 1.6 sink-ambient: 2.7 deg C/W total.

Hence the avalanche temperature rise would be just

dT = 0.28 W / 2.7 deg C/W = 0.1 deg C.

This contribution is negligible. So would it be correct that repetitive avalanche energy is not a problem?

Thanks, Ian

IanR

Ah, dT = 0.28 W * 2.7 deg C/W = 0.76 deg C. Still not a problem?

jshao

The energy is quite small. I don't see huge risk hear. What is estimated junction temperature during operation? I can check if we have some testing data on this part.

For you, you need to run the system under the condition and see if it is reliable.

Jianwen

IanR

Estimated junction temperature is 50 degrees C. You needn't check on testing data.

Thanks for you help!

IanR

The part worked fine in an hour of testing. Do you have an opinion as to whether this is sufficient to establish long-term reliability?

jshao

Hi Ian,

For long term reliability, usually you would need to use the accelerated life time test to evaluate the long term reliability. It will be a good conversation with your reliability department.

From design point of view, can you avoid the operation in avalanche condition? Do you have to go into this mode?

Jianwen

IanR

Hello Jianwen,

Unfortunately, we can't avoid the avalanche condition, due to the inductances of the series FET connection and the snubbers. But since long as the temperature is the limiting factor on reliability, and the temperature is low, the FETs should be OK. Thank you again for your help.

Ian