1. Home
  2. Electronics Design
  3. Help understanding N-Channel MOSFET body diode

Help understanding N-Channel MOSFET body diode

I tried this question in the BASICS forum but did not receive a response. I thought I would try here next. I appologise in advance for cross-posting.

Is there a rule of thumb stating how much power an N-Channel MOSFET's internal body diode (anode at source cathod at drain) can handle vs. the normal use drain to source power rating of the device?

Does this make sense?

I am using an Nch MOSFET to switch a load to ground in a battery operated project. The design does not allow me to place a rectifier diode in series with the load to prevent current flow through the body diode in the event of reverse battery connection. I was wondering if the battery was connected in reverse, would the load limit current enough to save the device? Unfortunately I never see the power rating (or forward voltage drop of this diode) published in manufactures data sheets so I have no way to tell.

Thank you in advance,

Ge0

Reply to
Gerbermultit00l
Loading thread data ...

** The intrinsic body diode of a mosfet has a fairly high conduction voltage - from 1 to 2 volts - making heat dissipation in the device much higher than is typically the case for saturated switching in the normal direction.

Work out the load amps and estimate the dissipation in watts to see if your case is OK.

If not - add a parallel ( Schottky low forward drop ) diode with sufficient ratings and heatsinking to do the job instead.

...... Phil

Reply to
Phil Allison

Thanks for the prompt response Phil. Great idea adding the Schottky. I guess I'll need to get/create a forward voltage drop graph showing V(f) throughout the temp span and at the worse case fault I(f) to see if I need to go to these measures.

Cheers,

Ge0

Reply to
Gerbermultit00l

You might want to look at devices like the Harris HUF75337G3, and Hitachi do similar transistors, which have got proper specifications for the diode, and are designed to allow this to be used. Generally, if the diode doesn't have a specification in the data sheet, then reckon that it may well have quite poor characteristics. Specifically, the conduction voltage may well be high, risking damage to other components. Remember also that you can generate a diode with a lower voltage drop than a Schottky diode, using a MOSFET, for series connection, if it is the drop that is the problem for a series diode.

Best Wishes

Reply to
Roger Hamlett

I once blew up a lot of Mororola mosfets in a half-bridge motor controller application. The motor back-emf forward-biased a substrate diode, then the opposite fet turned on, and the substrate diode made a step-recovery spike so big it blew out its own gate. I wonder if their reverse recovery is better controlled these days.

John

Reply to
John Larkin

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

Yes, in some. However it is an amazingly common failure mode, with people 'routinely' relying on the internal diode, without realising how poorly suited it may be... Unfortunately, there is a tendency to 'see' the diode shown in the data sheet, and assume it is well matched for the application, without realising just how slow it is.

Best Wishes

Reply to
Roger Hamlett

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.