EMRFD Message Archive 6149

Message Date From Subject
6149 2011-04-19 03:12:11 d29602960 Power MOSFET protection diode (Source DRain).
Hi to everyone on the list,

I wonder if the collective knowledge on this list could help to answer a question for me.

I recall a thread (last year) on this list regarding the internal protection diode in 2N7000 FET's between the source and drain. I seem to recall that the general opinion was that it is unwise to rely on the internal diode alone for protection and to fit an additional external diode in cases where a possible inductive load might be encountered.

I was wondering, does the above advice extend to the more general case of power MOSFET's used in say power converters in which the drain load might be an inductive transformer primary.

Or to put it another way, would it be "good practice" to add an additional zenor diode between the drain and source if the power mosfet is to be used in a power converter/inverter or would I be simply adding redundant components?

I would be interested to hear the groups thoughts/opinions on this subject. I dont have a specific project (yet) but I would propose to use power MOSFETS in any future inverter projects in preference to power transistors.


Des (M0AYF)
6150 2011-04-19 04:28:04 drmail377 Re: Power MOSFET protection diode (Source DRain).
Hi Des,

I'll presume you're referring to the reverse "protection" diode intentionally built into some MOSFET devices and not the intrinsic (or parasitic) body diode (which you never rely on for protection). I'll assume you're driving an inductive load. Finally I'll assume your application, which you describe is a converter/inverter, is using relatively low-frequency power switching, not RF as in a class-E HF amplifier.

A short answer is to always use an external protection Schottky diode. The Schottky diode is faster and heat dissipation occurs away from the MOSFET body. Also, it is often preferable to "smoke" an external protection diode instead of the MOSFET - if you're lucky.

The long answer is, well... long. Things get even more complex in push-pull or H-bridge arrangements. Here's a decent app note:


If you're still having problems protecting the MOSFET with an external diode, ask Google about "Snubber Circuits".

Some newer IGBT parts have internal diode structures that are fast, but you may still have the issue of dissipating heat in the package.

I highly recommend you simulate your switching MOSFET power circuit in SPICE, LT-Spice in-particular, which is freely available from www.linear.com.

73's, David WB4ONA

P.S. Regarding RF MOSFET switching amps (e.g., Class-E): For RF applications, using a free wheeling protection diode in parallel with the MOSFET may introduce unwanted parasitic capacitance. I think this is why you do not see them in use with the likes of some Class-E RF amplifier designs.

I'm not 100% sure this is the reas
6151 2011-04-19 05:16:23 kb1gmx Re: Power MOSFET protection diode (Source DRain).
6152 2011-04-19 18:20:57 Ed C Re: Power MOSFET protection diode (Source DRain).
I have been 'designing???' sorta power mosfet circuit also. And along the threads I have found searching is a resounding 'YES" you need a high power (higher capable) diode for the fly back effect of any inductive load.

From what I understand and that is very limited, you need a 'fast recovery diode' to be able to respond and dampen the reverse voltage.

The project isn't ham related at all, so offering the part number I am using isn't what you want. 40 amps!! with peak of 4o0 amps! hihi.

It is for a 10 amp motor to protect the logic on a switcher.

Also, consider a dedicated 'fet driver'. The 8 pin dip I am going to use can 'slam' the gate with a max of 2 amps for a split second. That is to overcome the huge capacitance and switch ON the gate/transistor in a square wave form instead of a ramp type. Both cost over $11 as a pair, but I reasonably expect lighter capable fets and drivers would be somewhat cheaper.

Playing on the 'wrong' computer distrobution rignt now. So the 'bookmark/favorite' isn't
6165 2011-04-22 12:46:02 d29602960 Re: Power MOSFET protection diode (Source DRain).
Hi again,

Just to say "thanks" to all those who responded (both on-and-off this list) to my question about the internal diode fitted to Power MOSFET,s between the Source and Drain.

The comments and pointers proved to be most helpful.


Des (M0AYF)