Source-drain diode.

[edson]

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.

 

[Meindert Sprang]

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?

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

 

[John Larkin]

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.

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

 

[Pooh Bear]

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.

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

Graham

 

[edson]

Thanking you.

 

[[email protected]]

Thanking you.

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.

 

[edson]

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

if it

recovers too slowly, you're going to have issues, and this becomes an
even greater problem at higher frequencies of course.

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

You're likely

operating it fairly slow,

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

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.

Do you mean an RC snubber or an external diode?

This info may be useful in some future project.

Regards

 

[edson]

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

if it recovers too slowly, you're going to have issues, and this
becomes an even greater problem at higher frequencies of course.

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

You're likely operating it fairly slow,

The output of a IR2304 drives the gate via a 100R series resistor.

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.

Do you mean an RC snubber or an external diode?

This info may be useful in some future project.

Regards