Question about Diode Sizing for DC Speed controller

[chapmjw]

I'm designing a speed controller for a mid size dc motor. I've got the
Mosfet(s) selected and power generation worked out. The flyback diode
I'm trying to size will be on the same heat sink as the mosfet.

I need some help with the following:
How can I choose the right size diode? (current rating)
Power dissipated by the diode?

Details:
Low side drive (mosfet sorce to ground)
24 Volt
40 amp max current
Details for the motor or unknown at this time.

thanks
jim

 

[Tom Bruhns]

I'm designing a speed controller for a mid size dc motor. I've got the
Mosfet(s) selected and power generation worked out. The flyback diode
I'm trying to size will be on the same heat sink as the mosfet.

I need some help with the following:
How can I choose the right size diode? (current rating)
Power dissipated by the diode?

Details:
Low side drive (mosfet sorce to ground)
24 Volt
40 amp max current
Details for the motor or unknown at this time.

thanks
jim

So...I suppose the "right" answer will depend not only on the motor
but on the load on the motor as well. Assuming an electrical model
with a moderate amount of inductance, whatever the inductor peak
current is, is what the diode peak current will be, not accounting for
high dv/dt and charge being swept out of the diode as it reverses.
With an infinite inductance, the current is constant at whatever the
motor draws, and the average diode current will just be that current
times the percentage of time the diode is conducting. If you're
supplying 24V to the motor, the diode conducts zero percent of the
time, but if you are driving a heavy load at low speed, so the motor
current is 40 amps when the motor voltage is 1 volt, then the diode
will be conducting most of the time. Diode dissipation is just the
average over one cycle of the instantaneous diode current times the
instantaneous diode voltage drop. LTSpice can simulate this sort of
stuff quite nicely for you. And you may find that you can save
considerable power if you use a half-H-bridge, with a mosfet to the +
side, too, though you need a very low on-resistance mosfet to make
that work at 40 amps. Oh, and beware of conditions of regenerative
braking, where the motor delivers power back to the controller.

Cheers,
Tom

 

[Phil Allison]

"chapmjw" <

I'm designing a speed controller for a mid size dc motor. I've got the
Mosfet(s) selected and power generation worked out. The flyback diode
I'm trying to size will be on the same heat sink as the mosfet.

I need some help with the following:
How can I choose the right size diode? (current rating)
Power dissipated by the diode?

Details:
Low side drive (mosfet sorce to ground)
24 Volt
40 amp max current
Details for the motor or unknown at this time.

** So what does that 40 amp figure mean if you have no motor info ????

Your Q is basically silly, since the answer depends on info YOU say you have
not got and must have.

How long is a piece of string ??



.......... Phil

 

[chapmjw]

"chapmjw" <




** So what does that 40 amp figure mean if you have no motor info ????

Your Q is basically silly, since the answer depends on info YOU say you have
not got and must have.

How long is a piece of string ??

......... Phil

Tom,

Thanks for the reply, that's the info I was looking for.

jim

 

[Phil Allison]

"John Popelish"

If the motor is run very near stall, at full current, the diode will carry
almost all the motor current.

** Huh ???

At full PWM width setting, the flyback diode carries no current.

At a low PWM settings, the diode carries nearly the same ( average) current
as the motor - but this will be a small value as the effective DC voltage
seen by the motor is low.

Worst case dissipation for the diode is about 50 % duty cycle with the motor
stalled or near so.

All this assumes the PWM frequency is reasonably high - ie well above 50
Hz.




.......... Phil

 

[John Popelish]

I'm designing a speed controller for a mid size dc motor. I've got the
Mosfet(s) selected and power generation worked out. The flyback diode
I'm trying to size will be on the same heat sink as the mosfet.

I need some help with the following:
How can I choose the right size diode? (current rating)
Power dissipated by the diode?

Details:
Low side drive (mosfet sorce to ground)
24 Volt
40 amp max current
Details for the motor or unknown at this time.

thanks
jim

If the motor is run very near stall, at full current, the
diode will carry almost all the motor current. I would use
a diode essentially rated for full motor current, and heat
sink it for that current. With a 24 volt supply, you can
cut the diode losses almost in half by using a Schottky
diode, but it will probably cost more than a fast silicon
junction diode. Doal diodes are very easy to find, and
because they are well coupled, thermally, you can parallel
the two sections.

Here is an example that costs less than $6 from Digikey,
quantity 1:
http://www.irf.com/product-info/datasheets/data/60ctq045pbf.pdf

 

[Tom Bruhns]

"John Popelish"




** Huh ???

At full PWM width setting, the flyback diode carries no current.

At a low PWM settings, the diode carries nearly the same ( average) current
as the motor - but this will be a small value as the effective DC voltage
seen by the motor is low.

Try that again, Phil. If it's locked-rotor, you get lots of current
at low voltage. The motor doesn't mind, as long as you keep it cool:
zero RPM means it's not turning its own fan, but nothing says it can't
be cooled with a fan driven by a different motor.

In the midst of playing in my spare time with a half-bridge to drive
up to 20A to a motor at up to 100V (but also 20A at ~10V when driving
a heavy load slowly),
Cheers,
Tom

 

[Phil Allison]

"Tom Bruhns"
"Phil Allison"

Try that again, Phil.

** Every word is correct.

If it's locked-rotor, you get lots of current
at low voltage.

** Hardy a precise statement.

Ohms law applies to a stalled DC motor.





......... Phil

 

[John Popelish]

"John Popelish"


** Huh ???

At full PWM width setting, the flyback diode carries no current.

Agreed. At near stall, the PWM duty cycle gets pretty low,
yet the current gets really high. This is because the motor
is producing little back EMF, so a small average voltage can
drive a large current.

At a low PWM settings, the diode carries nearly the same ( average) current
as the motor - but this will be a small value as the effective DC voltage
seen by the motor is low.

A motor delivering full rated torque at low speed draws full
rated current.

Worst case dissipation for the diode is about 50 % duty cycle with the motor
stalled or near so.

Most drives exceed current limit and won't reach 50% duty
cycle at stall.

All this assumes the PWM frequency is reasonably high - ie well above 50
Hz.

Yes.

 

[Phil Allison]

"John Popelish"

** Replacing the MISSING context line !!!!

Agreed.

** Then your words above are quite wrong.

At near stall, the PWM duty cycle gets pretty low,

** Nonsense.

Low speed operation and stopped are not equivalent to " stalled ".

yet the current gets really high.

** More non specific language ....... dear, oh dear.

A motor delivering full rated torque at low speed draws full rated
current.

** The rain in Spain stays mainly on the plain ....

( You are not even on the same topic. )

Most drives exceed current limit and won't reach 50% duty cycle at stall.

** More complete irrelevances to the actual topic.

" The rain in Spain stays mainly on the plain .... "


The issue I raise is about clear expression and not confusing folk with
wrong or ambiguous usage.

Try reading my post again and do NOT disassemble it and post silly replies
to its components.


" At full PWM width setting, the flyback diode carries no current.
At a low PWM settings, the diode carries nearly the same ( average) current
as the motor - but this will be a small value as the effective DC voltage
seen by the motor is low.
Worst case dissipation for the diode is about 50 % duty cycle with the motor
stalled or near so. "



........ Phil

 

[Robert Scott]

At full PWM width setting, the flyback diode carries no current.

At a low PWM settings, the diode carries nearly the same ( average) current
as the motor - but this will be a small value as the effective DC voltage
seen by the motor is low.

Worst case dissipation for the diode is about 50 % duty cycle with the motor
stalled or near so.

All this assumes the PWM frequency is reasonably high - ie well above 50
Hz.

What is really relevant is the relationship between the PWM frequency and the
L/R time constant of the motor inductance and circuit resistance. As others
have said, all other things being equal, the current is greatest for a stalled
motor. But as you pointed out, that current doesn't travel through the flyback
diode exact during the periods when the PWM is pulsed off. 50% duty cycle is
probably close to the worst-case situation, but in certain situations you might
make it even worse with a little different duty cycle.


Robert Scott
Ypsilanti, Michigan

 

[Tom Bruhns]

What is really relevant is the relationship between the PWM frequency and the
L/R time constant of the motor inductance and circuit resistance. As others
have said, all other things being equal, the current is greatest for a stalled
motor. But as you pointed out, that current doesn't travel through the flyback
diode exact during the periods when the PWM is pulsed off. 50% duty cycle is
probably close to the worst-case situation, but in certain situations you might
make it even worse with a little different duty cycle.

Robert Scott
Ypsilanti, Michigan

In the case of the controller I'm playing with, the output is filtered
DC to keep from spraying 100kHz all over the neighborhood. The
inductor is 100uH; the input voltage is nominally 160VDC. Current
limit is set at 20A. Locked rotor, the motor is nominally 0.4 ohms;
it's rated for 90VDC operation at full output. For this case, clearly
the diode--or rather the "other" mosfet in this case--sees highest
average current when the output voltage is lowest, and the current is
at the limit. That would normally be with locked rotor, about
20A*(160-20*0.4)/160, but might be momentarily even worse when
reversing the motor. At nominally 80V @ 20A output, the average
current in each FET is about 20A*1/2. -- And yes, the application
for this motor is such that it is asked to deliver full torque at a
wide range of speeds. It's my whole reason for replacing an AC
induction motor with the DC motor.

Perhaps Phil is considering a controller with no current limit. I'd
just as soon have both my controller and the motor last longer than it
would if there were no current limit.

Cheers,
Tom

 

[Phil Allison]

"Tom Bruhns"

Perhaps Phil is considering a controller with no current limit. I'd
just as soon have both my controller and the motor last longer than it
would if there were no current limit.

** Probably be impossible to get 50% PWM duty cycle into a stalled motor
when overcurrent limiting is included.

Anyone know what "begging the question " means ???




.......... Phil