Maker Pro
Maker Pro

PWM drive of DC contactor

P

P E Schoen

Jan 1, 1970
0
I have a DC contactor with a coil that draws about 4 amps at 24 VDC and
normally uses an economizer resistor of about 75 ohms to reduce the current
to about 400 mA when pulled in (controlled by a delayed contact). But I want
to use a PWM drive for another similar contactor (actually a 120 VAC unit)
so it will work on DC and thus be more tolerant of AC line brownouts.

Previously I used a combination of a 75 ohm resistor in parallel with a 3300
uF capacitor to provide a full current pull-in pulse and then 400 mA to hold
it in. This worked OK but the resistors get hot and it is not always
reliable. Thus the desire to use PWM.

I made a simple version of this using a PIC12F675 and it worked well with a
PWM frequency of about 1 kHz. It applied full voltage for about 100 mSec and
then 10% PWM which drew only about 300 mA. Now I am trying a PIC12F1822
which has a real PWM module and a clock frequency of 16 MHz, so I tried a 20
kHz PWM which started with 95% for 100 mSec and then 10%. But the
FQD13N06LTM MOSFET (60V 11A 110mOhm) shorted. I thought it was due to a
wiring error on the prototype so I replaced it, and it seemed to work about
right, but the PWM occurred before the armature pulled in. So I pushed it in
manually, and the MOSFET once again exuded magic smoke. The coil inductance
changes from about 15 to 40 mH when pulled in.

So, I think it may be the higher frequency PWM, and I plan to try 5 kHz or
even 1250 Hz, but first I thought I would run a simulation (at about 96%
duty cycle). I am driving the logic level gate directly from the PIC so I
figure that it may be somewhat slow and weak, so I used both a 5 ohm and 50
ohm resistor in series. Also I tried it with a standard diode and a
Schottky. The Schottky produced only about 90 watts peak in the MOSFET for
about 1.5 uSec, and the 50 ohm gate resistor showed about 3.5 watts average
compared to 1.5 watts for 5 ohms. The standard rectifier produced about 325W
peak for 30 nSec and then about 80 watts for an additional 1.2 uSec. The
average power was not much different for the two diodes.

The power dissipation with DC on the gate is about 1.25 W. The SOA seems to
allow a 100 uSec pulse of 40 amps at 20 volts or 800 watts, but continuous
power is about 2.5 watts without heatsinking other than the PCB. The MOSFET
I used for the original prototype was a "self-protected" VNP14NV04 (40V 12A
35 mOhm).

I think the lower frequency will help, and also perhaps using DC (100% PWM)
until it pulls in, should help greatly. But I wanted to see if anyone had
experience with this and perhaps might offer some advice.

Thanks,

Paul

LTSpice:
==========================================================
Version 4
SHEET 1 1380 680
WIRE 144 0 -144 0
WIRE 368 0 144 0
WIRE 144 96 144 64
WIRE 368 96 368 80
WIRE 368 96 144 96
WIRE 448 96 368 96
WIRE -144 144 -144 0
WIRE 128 176 80 176
WIRE 256 176 208 176
WIRE 320 176 256 176
WIRE 80 208 80 176
WIRE -144 288 -144 224
WIRE 80 288 -144 288
WIRE 368 288 368 192
WIRE 368 288 80 288
FLAG 368 288 0
FLAG 448 96 coil
FLAG 256 176 gate
SYMBOL voltage -144 128 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 24
SYMBOL voltage 80 192 R0
WINDOW 3 -207 116 Left 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value PULSE(0 4.5 10u 50n 50n 600u 500u 2000)
SYMATTR InstName V2
SYMBOL nmos 320 96 R0
SYMATTR InstName M1
SYMATTR Value IRL530NS_L
SYMBOL ind 352 -16 R0
SYMATTR InstName L1
SYMATTR Value 40m
SYMATTR SpiceLine Rser=6
SYMBOL res 224 160 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R1
SYMATTR Value 50
SYMBOL diode 160 64 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value MURS120
TEXT 136 264 Left 2 !.tran 300m startup
TEXT -128 336 Left 2 ;R1=50 2 kHz 96% MBRS140 IRL530NS_L 87W peak 1.66 uSec
1.32W Avg
TEXT -128 368 Left 2 ;R1=50 20 kHz 96% MBRS140 IRL530NS_L 87W peak 1.42 uSec
3.55W Avg
TEXT -128 400 Left 2 ;R1=5 20 kHz 96% MBRS140 IRL530NS_L 92W peak 261 nSec
1.49W Avg
TEXT -128 432 Left 2 ;R1=5 20 kHz 96% MURS120 IRL530NS_L 325W peak 30 nSec
1.57W Avg
TEXT -128 464 Left 2 ;R1=50 2 kHz 96% MURS120 IRL530NS_L 325W peak 30 nSec
1.15W Avg
TEXT -128 496 Left 2 ;R1=50 DC MURS120 IRL530NS_L 1.25W Avg
 
I have a DC contactor with a coil that draws about 4 amps at 24 VDC and
allow a 100 uSec pulse of 40 amps at 20 volts or 800 watts, but continuous
power is about 2.5 watts without heatsinking other than the PCB. The MOSFET
I used for the original prototype was a "self-protected" VNP14NV04 (40V 12A
35 mOhm).

Is it possible that your coil kickback higher than 40V and damaging the MOSFET? I use a 400V MOSFET for 12V relay.
 
P

P E Schoen

Jan 1, 1970
0
"Tim Wescott" wrote in message

I don't have an impedance analyzer, so that would be difficult. And I don't
have the exact contactor (120 VAC) here, so I am using a similar size with
24 VDC coil.

You are correct. The pins are rated for about 25 mA. So I tried the
simulation with 200 ohms. Now at 2 kHz it shows only 100 watts peak but 2.1
watts average for the 98% duty cycle. For 10% it shows 11 watts peak and
only 41 mW average.

For 20 kHz and 90% duty cycle (which I now see is actually what I used), it
gives 9.9 watts average and 96 watts peak, so that would fully explain the
failure. Even at 2 kHz it is 1.8 watts average. And at 20 kHz with 96% duty
cycle it is 2.3 watts.

I have a 60 MHz storage scope and I was able to observe the waveforms with a
resistive load, but with the intended load the MOSFET popped before I could
get any waveform information.
I just noticed your final question -- yes, a 100% duty cycle in the "on"
phase will help greatly -- with your wimpy PIC pins the FET dissipates
more when its switching because of slow gate drive, so that will help.

That may be best, and it is easily accomplished.
Then your biggest worry will be the 10ms or so when you drop to low
duty cycle but the coil current is dropping -- the FET will experience
a bunch of little heat pulses unless you want to get fancy and turn
the PWM off for three or four ms, then turn it on again.

That is also easily done, and a good idea.
If you really want to save on the gate driver I'd suggest doing two
things: first, see if you can find a FET with lower gate charge --
that'll help your wimpy "gate drive" charge it up; second, parallel
up every available spare pin on the device to increase the oomph.
That means that you can't use the software PWM, and it means
that you have to pay attention to the chip's overall current limit --
but it'll give you more drive capability.

It would be difficult to use more pins on this 8 pin PIC, and I would like
to use the built-in PWM module.
Some of the little single-channel gate drive chips are pretty cheap,
you may want to take a gander at what's available.

I might also be able to use a smaller version of the protected MOSFET (some
of which are actually listed as a gate driver). Here is a 60V 10A "OmniFET"
for about a dollar:
http://www.mouser.com/ProductDetail...0N06-E/?qs=sGAEpiMZZMu0dYp3dYbBlUps1mx%2bPR3J

The least expensive gate driver I found on a quick search is about $0.35,
but is limited to 25V. My raw supply is 24V, and I will have a 5V regulator
for the PIC. However, 5V would be OK for the logic level MOSFETs.
http://www.mouser.com/ProductDetail...=sGAEpiMZZMvQcoNRkxSQkkC4NeUaTUuMSZpeTA86SA0=

Thanks,

Paul
 
P

P E Schoen

Jan 1, 1970
0
wrote in message
Is it possible that your coil kickback higher than 40V and damaging the
MOSFET? I use a 400V MOSFET for 12V relay.

The commutating diode across the coil should take care of any kickback. But
it might need to be faster than the 1N4004 presently used for this
application.

A 400V MOSFET with sufficiently low ON resistance may be hard to find and/or
expensive. Among 400V MOSFETs rated 5 to 10 amps continuous the lowest ON
resistance is about 550 mOhms and cost is over $1 while the MOSFET I plan to
use is only $0.50 and is smaller. But more importantly, if the 12V relay
coil is generating inductive spikes requiring a 400V device, there may be
other problems such as insulation breakdown and flashover on the PCB or
wiring.

Paul
 
K

kevin93

Jan 1, 1970
0
.
to use a PWM drive for another similar contactor (actually a 120 VAC unit)
....
The contactor intended to be driven from AC will probably have a shading ring (a shorted turn on part of the magnetic circuit) to avoid chattering. It will not act like a pure inductance but like a transformer with high-leakage inductance and a shorted secondary (or a secondary loaded with a low resistor).

kevin
 
if the 12V relay
coil is generating inductive spikes requiring a 400V device, there may be
other problems such as insulation breakdown and flashover on the PCB or
wiring.

Oh no, i don't expect much more than 50V. But a 400V 13001 (TO-92) is only nickle or dime in volume. Anyway, i am dealing with much less current than you are.
 
P

Phil Allison

Jan 1, 1970
0
"P E Schoen"
Is it possible that your coil kickback higher than 40V and damaging the
MOSFET? I use a 400V MOSFET for 12V relay.

The commutating diode across the coil should take care of any kickback. But
it might need to be faster than the 1N4004 presently used for this
application.

** Betcha your MOSFET is dying from overvoltage spikes.

A Schottky across the coil is good but you also need zener protection across
the FET.

The wiring to and from the coil and FET may have enough inductance to do the
job.

I have some experience with DC motors and PWM powered by a 12 cell NiCd
pack.

Despite a nice Schottky across the motor, BUZ11 FETs failed until I added a
24V, 5W zener across them.


..... Phil
 
P

P E Schoen

Jan 1, 1970
0
"kevin93" wrote in message
....
The contactor intended to be driven from AC will probably have a shading
ring (a shorted turn on part of the magnetic circuit) to avoid chattering.
It will not act like a pure inductance but like a transformer with
high-leakage inductance and a shorted secondary (or a secondary loaded
with a low resistor).

So in that case I think it should act like a very small inductor on DC and
the current should quickly peak to the value determined by the coil
resistance. That might actually work better as far as the inductive energy
causing high power during switching. I'll try simulating it by adding a
second small coil with a coupling factor K between 1 and 0.95 to see how it
looks.

Thanks,

Paul
 
On Friday, October 18, 2013 5:19:00 AM UTC-4, P E Schoen wrote:

If you use a high side current monitor in combination with a hysteretic controller, you may find you're switching at sub-kHz with those time constants(L/R), and you will also know exactly when the contactor is pulled in as well as by how much margin you have it pulled in when you back off to the low power mode. The hysteretic controller can be your PIC if you're determined to use it.
 
P

P E Schoen

Jan 1, 1970
0
"Tim Wescott" wrote in message
I didn't know about them thar shading rings.
It could very well make it lossy -- try it out.

I simulated it with a 1 mH coil and a 1 mOhm series resistance with K=0.99.
Here are the results:

R1=200 20 kHz 90% MURS120 IRL530NS_L 95W peak 6 uSec 9.8W Avg
R1=200 20 kHz 10% MURS120 IRL530NS_L 6W peak 8 uSec 419 mW Avg

R1=200 2 kHz 90% MURS120 IRL530NS_L 94W peak 6 uSec 1.70 W Avg
R1=200 2 kHz 10% MURS120 IRL530NS_L 30W peak 3.8 uSec 128 mW Avg

I don't know if this is really any better. But the most apparent result is
that there is no significant build-up of current as there was previously, so
that seems to indicate much lower inductance and a much faster response. And
~2 kHz seems like the way to go. It may make more audible noise but these
are inside a steel box along with big transformers and powerful fans and
used in a typically noisy shop or factory environment, so a little more
industrial music won't be an issue. :)

But also I finally found a contactor with a 120 VAC coil, and it measures
575 mH open and 3.72 H closed, with 136 ohms resistance. Also, it will not
even actuate with 24 VDC, and that;s barely enough to hold it in (with about
170 mA). So now I am thinking that perhaps the contactors in the unit are 24
VAC. I may get a chance to test this circuit on the actual machine on
Monday. It's been a while since I worked on it, and my memory is foggy, but
that makes more sense. Here is the relay we use:
http://www.alliedelec.com/search/productdetail.aspx?SKU=70060220

The relay I found is not this exact part but a similar 3 pole HVAC type. It
seems that 24 VAC and 120 VAC coils are most common. We decided to use these
because they are inexpensive and available from most electrical supply
houses such as Grainger. Otherwise, for 24 VDC devices, we have used
American Solenoid Karaus & Naihmer as well as ABB, but they are more
expensive and harder to obtain.

Thanks,

Paul
 
P

P E Schoen

Jan 1, 1970
0
"Phil Allison" wrote in message
"P E Schoen"
** Betcha your MOSFET is dying from overvoltage spikes.
A Schottky across the coil is good but you also need zener protection
across the FET.
The wiring to and from the coil and FET may have enough inductance
to do the job.
I have some experience with DC motors and PWM powered by a 12
cell NiCd pack.
Despite a nice Schottky across the motor, BUZ11 FETs failed until
I added a 24V, 5W zener across them.

There very well could be damaging spikes. I was having strange problems
where the relay would pull in and then drop out before the PWM kicked in,
and I found that the PIC was resetting. I had replaced the 24 VDC contactor
(6 ohm coil) with a smaller relay (280 ohm coil) that draws only about 100
mA, and even that had problems, but it cleared up when I put a 0.047 uF
capacitor across the coil and diode. I had to use 20%-40% PWM to keep it
pulled in, however.

But when I reconnected the larger contactor, even with 10% PWM, it drew
about 2.5-3 amps. That was too much for the MOSFET so I could not leave it
on long enough to look at the waveform, but with the smaller relay the
voltage took about 100 uSec to rise after the PWM turned off. The smaller
relay coil has an inductance of about 1.1 H and a resistance of 280 ohms.
Oddly, the inductance only increased slightly when the armature is pulled
in.

I think the larger contactor may have taken even longer to stop conducting
and thus caused this effect. When I redid the simulation with a 100 mSec
full-on pulse, followed by 200 mSec off, and then the PWM, the relay current
was still 85 mA after 90 mSec. However, the simulation showed the coil
current should drop to about 320 mA at 10% PWM after about 100 mSec so it
may be something else.

I may still add a 30V or so TVS diode across the MOSFET. And I should
probably add like a one ohm resistor in series with the coil capacitor to
limit the turn-on current.

Thanks for the tip.

Paul
 
P

P E Schoen

Jan 1, 1970
0
"P E Schoen" wrote in message
There very well could be damaging spikes. I was having strange problems
where the relay would pull in and then drop out before the PWM kicked
in, and I found that the PIC was resetting. I had replaced the 24 VDC
contactor (6 ohm coil) with a smaller relay (280 ohm coil) that draws
only about 100 mA, and even that had problems, but it cleared up when
I put a 0.047 uF capacitor across the coil and diode. I had to use
20%-40% PWM to keep it pulled in, however.
But when I reconnected the larger contactor, even with 10% PWM, it
drew about 2.5-3 amps. That was too much for the MOSFET so I could
not leave it on long enough to look at the waveform, but with the
smaller relay the voltage took about 100 uSec to rise after the PWM
turned off. The smaller relay coil has an inductance of about 1.1 H
and a resistance of 280 ohms. Oddly, the inductance only increased
slightly when the armature is pulled in.
I think the larger contactor may have taken even longer to stop
conducting and thus caused this effect. When I redid the simulation
with a 100 mSec full-on pulse, followed by 200 mSec off, and then
the PWM, the relay current was still 85 mA after 90 mSec. However,
the simulation showed the coil current should drop to about 320 mA
at 10% PWM after about 100 mSec so it may be something else.
I may still add a 30V or so TVS diode across the MOSFET. And I should
probably add like a one ohm resistor in series with the coil capacitor
to limit the turn-on current.

Well, I tried the circuit in the unit where it is to be used, and I had the
same problem with the contactors drawing too much current. I think the
circuit was also resetting. So I got one of the actual contactors and
brought it home to do better testing.

The 24VAC coil is 4.3 ohms and has an inductance of 9.05 mH, which increases
to 20.4 mH when pulled in. I tried it here and it acted the same, and then
the MOSFET blew. So, I replaced it with a VNP14NV04-E protected OmniFET, and
it works fine. It draws 5 or 6 amps for 100 mSec to pull in, and then
throttles back to 90 mA with 10% PWM.

Now I will need to observe the operation more closely and see if I can
detect a high voltage spike or oscillations or other behavior that could
have overstressed the MOSFET. I think a 28V TVS diode across it will take
care of it, and I may just spec in the OmniFET. I think I will use this 60V
10A version which is only about $1.00:
http://www.mouser.com/Search/Produc...6-Evirtualkey51120000virtualkey511-VNP10N06-E

The VNP14NV04 is about $2. The VNL5050N3TR-3 is another possibility with 19A
and 41V in a SOT-223 package and about $1.
http://www.mouser.com/ProductDetail...50N3TR-E/?qs=sGAEpiMZZMslBFvnKnOhclaEsXxHCjd2

But I'd really rather not have to depend on a self-protected MOSFET, since
it is rather special and could easily become obsolete. So I do need to make
sure I understand what's going on and deal with it properly. The protected
device could then just be an extra layer of protection.

Paul
 
J

Jeroen Belleman

Jan 1, 1970
0
Rs is probably a small value, so it makes little difference.

I think this was the classical case of an unintentional Hartley
oscillator, with the tank circuit composed of the load + gate
drive inductances in parallel with Cgd.

Jeroen Belleman
 
L

Lasse Langwadt Christensen

Jan 1, 1970
0
Den torsdag den 24. oktober 2013 09.21.18 UTC+2 skrev P E Schoen:
"P E Schoen" wrote in message
























Well, I tried the circuit in the unit where it is to be used, and I had the

same problem with the contactors drawing too much current. I think the

circuit was also resetting. So I got one of the actual contactors and

brought it home to do better testing.



The 24VAC coil is 4.3 ohms and has an inductance of 9.05 mH, which increases

to 20.4 mH when pulled in. I tried it here and it acted the same, and then

the MOSFET blew. So, I replaced it with a VNP14NV04-E protected OmniFET, and

it works fine. It draws 5 or 6 amps for 100 mSec to pull in, and then

throttles back to 90 mA with 10% PWM.



Now I will need to observe the operation more closely and see if I can

detect a high voltage spike or oscillations or other behavior that could

have overstressed the MOSFET. I think a 28V TVS diode across it will take

care of it, and I may just spec in the OmniFET. I think I will use this 60V

10A version which is only about $1.00:

http://www.mouser.com/Search/Produc...6-Evirtualkey51120000virtualkey511-VNP10N06-E



The VNP14NV04 is about $2. The VNL5050N3TR-3 is another possibility with 19A

and 41V in a SOT-223 package and about $1.

http://www.mouser.com/ProductDetail...50N3TR-E/?qs=sGAEpiMZZMslBFvnKnOhclaEsXxHCjd2



But I'd really rather not have to depend on a self-protected MOSFET, since

it is rather special and could easily become obsolete. So I do need to make

sure I understand what's going on and deal with it properly. The protected

device could then just be an extra layer of protection.



Paul

Several manufacturers of protected mosfet in that voltage range,
they are used in cars so I don't think obsoletion will be a big issue

the protection is mostly for temperature and current, many standard
fets are avalanche rated for switching inductive loads

with pwm the voltage should be clamped to the supply via a flyback diode

-Lasse
 
T

Tim Williams

Jan 1, 1970
0
Tim Wescott said:
Heh. I just had a thought. The clever designer will come up with a
circuit design that uses the fewest components, and takes advantage of
the changing coil inductance to automagically cut the average current as
the relay pulls in (and hence the inductance increases).

It has to include some sort of self-oscillating feature, but I'll be
damned if I can figure out what it needs to be.

Maybe we need to have a design competition -- but I think we'd need to
bar Joerg on the grounds that he'd automatically win. Maybe he could be
the judge.

Can I take a hacksaw and soldering iron to the shunts ('free' feedback
winding)? ;-)

Tim
 
J

Jeroen

Jan 1, 1970
0
---
Maybe it's just me, but - regardless of the difference - I'd expect a
schematic showing a capacitor described as being connected from drain
to source to look like this:

. |
. load
. |-------
. d |
. -- R ------------g ===
. | s | 2n2
. c |-------
. b---R ---- |
. e Rs
. | |
. /// ///

instead of:

. |
. load
. |-------
. d |
. -- R ------------g ===
. | s | 2n2
. c | ///
. b---R ---- |
. e Rs
. | |
. /// ///

Note also that the capacitor being connected from drain to source
comprises a high-pass filter which diminishes - rather than augments -
the effectiveness of the feedback from RS.

Strictly speaking, you are right, but it doesn't make much difference
here. It's purpose is to swamp the drain load inductance, so that the
thing won't burst into oscillation.

Jeroen Belleman
 
P

P E Schoen

Jan 1, 1970
0
"Lasse Langwadt Christensen" wrote in message
Several manufacturers of protected mosfet in that voltage range,
they are used in cars so I don't think obsoletion will be a big issue
the protection is mostly for temperature and current, many
standard fets are avalanche rated for switching inductive loads
with pwm the voltage should be clamped to the supply via a flyback diode

I looked closely at the PWM signal at 10% and there is ringing at turn-on
with a maximum swing of -13V and a TC of about 0.6 uSec. The relay coil is
20 mH and the capacitor across it is 47 nF, so the resonant frequency is 5.2
kHz. But I am not observing this directly across the MOSFET, and I think it
is from stray capacitance and inductance in the leads and other parts of the
circuit. There is also a shorter positive spike at turn-off.

The FQD13N06L I was using is avalanche tested and rated, and I have a
flyback diode across the coil, although it is a 1N4004 and it is located
with the capacitor some distance from the coil (about 12"). I will have to
deal with that because that is how the application is wired, and this must
be a simple PCB replacement without wiring changes.

I think there will be a lot more energy and higher voltage spikes and
oscillations when the contactor first pulls in with 6 amps, and when the
current is first switched off prior to applying the PWM signal. The energy
stored is 144 mJ which can produce 144W for 1 mSec. The single pulse
avalanche energy rating of the MOSFET is 90 mJ so perhaps that is being
exceeded, although much of it should be dissipated in the diode. But perhaps
it is not fast enough?

The protected MOSFET does have an overvoltage clamp which drives the gate
when the drain voltage exceeds the limit. Perhaps that would be an effective
way to protect the standard MOSFET. But the TVS diode should do the job as
well.

Thanks,

Paul
 
J

John S

Jan 1, 1970
0
I bet you I can make a good 100 MHz oscillor with a coil made of resistor wire.

That 'Q' thing is IMNSHO very often mis-interpreted or mis-understood.

Okay. Make it with an inductor having a Q of .0001.
 
L

Lasse Langwadt Christensen

Jan 1, 1970
0
Den torsdag den 24. oktober 2013 23.26.12 UTC+2 skrev P E Schoen:
"Lasse Langwadt Christensen" wrote in message









I looked closely at the PWM signal at 10% and there is ringing at turn-on

with a maximum swing of -13V and a TC of about 0.6 uSec. The relay coil is

20 mH and the capacitor across it is 47 nF, so the resonant frequency is 5.2

kHz. But I am not observing this directly across the MOSFET, and I think it

is from stray capacitance and inductance in the leads and other parts of the

circuit. There is also a shorter positive spike at turn-off.



The FQD13N06L I was using is avalanche tested and rated, and I have a

flyback diode across the coil, although it is a 1N4004 and it is located

with the capacitor some distance from the coil (about 12"). I will have to

deal with that because that is how the application is wired, and this must

be a simple PCB replacement without wiring changes.



I think there will be a lot more energy and higher voltage spikes and

oscillations when the contactor first pulls in with 6 amps, and when the

current is first switched off prior to applying the PWM signal. The energy

stored is 144 mJ which can produce 144W for 1 mSec. The single pulse

avalanche energy rating of the MOSFET is 90 mJ so perhaps that is being

exceeded, although much of it should be dissipated in the diode. But perhaps

it is not fast enough?

only without a diode will the energy will be dumped in the FET

There shouldn't be much dissipated in FET with a fly back diode, the current will be circulating through the diode
The protected MOSFET does have an overvoltage clamp which drives the gate

when the drain voltage exceeds the limit. Perhaps that would be an effective

way to protect the standard MOSFET. But the TVS diode should do the job as

I might be wrong but I think that the active clamp it to make the clamping voltage more predictable, not that the FET can't handle the same energy when
just avalanching

-Lasse
 
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