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Source-drain diode.

Greetings. I am building a PWM bidirectional variable speed motor drive using 4 x FQP85N06 MOSFETs. My question is whether it is kosher to use the internal source-drain diodes to handle the turn off pulses. I realize that external shottky diodes would give less heat and higher eficiency, but it is only a 1A 12V motor and I expect heat will be only a few watts. Are there other reasons to use external diodes?

Here is the spec for the S-D diode. Drain-Source Diode Characteristics and Maximum Ratings Maximum Continuous Drain-Source Diode Forward Current -- -- 85 A Maximum Pulsed Drain-Source Diode Forward Current -- -- 300 A VSD Drain-Source Diode Forward Voltage VGS = 0 V, IS = 85 A -- 1.5 V Reverse Recovery Time VGS = 0V, IS=85 A, dIF/dt = 100 A/uS -- 70ns Reverse Recovery Charge VGS = 0V, IS=85 A, dIF/dt = 100 A/uS --135nC

Advice appreciated.

Reply to
edson
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For this kind of power it's ok to use the internal diode. I even did it on a linear 3phase motor running at 48V, 6A, 20kHz. No sweat.

Meindert

Reply to
Meindert Sprang

Are most substrate diodes, nowadays, soft-recovery types? I once did a motor controller wherein the substrate diodes got the idea that they were step-recovery diodes, and generated such wild dv/dt that they blew their own fet's gate.

John

Reply to
John Larkin

I notice some devices spec the diode accodring to some development type.

Graham

Reply to
Pooh Bear

Thanking you.

Reply to
edson

It's well worth trying before complicating the circuit for nothing. Far as I know it's the recovery time of the body diode you need to be concerned with relative to the slew rate of the switching node, if it recovers too slowly, you're going to have issues, and this becomes an even greater problem at higher frequencies of course. You're likely operating it fairly slow, with the addition of a snubber across the mosfets and proper tweaking of the gate-source shape, they ought to run cool, quiet, and last forever.

Reply to
c_bielek

This deals with parts of the datasheet that I don't usually read so bear with me.

if it

Is that because a slow recovery diode would allow voltage buildup from load inductance before the diode conduction kicks in?

You're likely

The output of a MSP430 uC drives the gate via a 100R series resistor.

with the addition of a snubber across the

Do you mean an RC snubber or an external diode?

This info may be useful in some future project.

Regards

Reply to
edson

This deals with parts of the datasheet that I don't usually read so bear with me.

Is that because a slow recovery diode would allow voltage buildup from load inductance before the diode conduction kicks in?

Do you mean an RC snubber or an external diode?

This info may be useful in some future project.

Regards

Reply to
edson

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