Help understanding N-Channel MOSFET body diode

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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

 

[Phil Allison]

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?

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.

** 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

 

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** 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- Hide quoted text -

- Show quoted text -

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

 

[Roger Hamlett]

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,

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

 

[John Larkin]

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

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

 

[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.

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