PWM drive of DC contactor

Started by October 18, 2013
```I have a DC contactor with a coil that draws about 4 amps at 24 VDC and=20
normally uses an economizer resistor of about 75 ohms to reduce the =
current=20
to about 400 mA when pulled in (controlled by a delayed contact). But I =
want=20
to use a PWM drive for another similar contactor (actually a 120 VAC =
unit)=20
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=20
uF capacitor to provide a full current pull-in pulse and then 400 mA to =
hold=20
it in. This worked OK but the resistors get hot and it is not always=20
reliable. Thus the desire to use PWM.

I made a simple version of this using a PIC12F675 and it worked well =
with a=20
PWM frequency of about 1 kHz. It applied full voltage for about 100 mSec =
and=20
then 10% PWM which drew only about 300 mA. Now I am trying a PIC12F1822=20
which has a real PWM module and a clock frequency of 16 MHz, so I tried =
a 20=20
kHz PWM which started with 95% for 100 mSec and then 10%. But the=20
FQD13N06LTM MOSFET (60V 11A 110mOhm) shorted. I thought it was due to a=20
wiring error on the prototype so I replaced it, and it seemed to work =
right, but the PWM occurred before the armature pulled in. So I pushed =
it in=20
manually, and the MOSFET once again exuded magic smoke. The coil =
inductance=20
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=20
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=20
figure that it may be somewhat slow and weak, so I used both a 5 ohm and =
50=20
ohm resistor in series. Also I tried it with a standard diode and a=20
Schottky. The Schottky produced only about 90 watts peak in the MOSFET =
for=20
about 1.5 uSec, and the 50 ohm gate resistor showed about 3.5 watts =
average=20
compared to 1.5 watts for 5 ohms. The standard rectifier produced about =
325W=20
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=20
allow a 100 uSec pulse of 40 amps at 20 volts or 800 watts, but =
continuous=20
power is about 2.5 watts without heatsinking other than the PCB. The =
MOSFET=20
I used for the original prototype was a "self-protected" VNP14NV04 (40V =
12A=20
35 mOhm).

I think the lower frequency will help, and also perhaps using DC (100% =
PWM)=20
until it pulls in, should help greatly. But I wanted to see if anyone =
experience with this and perhaps might offer some advice.

Thanks,

Paul

LTSpice:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D
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=3D6
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=3D50 2 kHz 96% MBRS140 IRL530NS_L 87W peak 1.66 =
uSec=20
1.32W Avg
TEXT -128 368 Left 2 ;R1=3D50 20 kHz 96% MBRS140 IRL530NS_L 87W peak =
1.42 uSec=20
3.55W Avg
TEXT -128 400 Left 2 ;R1=3D5 20 kHz 96% MBRS140 IRL530NS_L 92W peak 261 =
nSec=20
1.49W Avg
TEXT -128 432 Left 2 ;R1=3D5 20 kHz 96% MURS120 IRL530NS_L 325W peak 30 =
nSec=20
1.57W Avg
TEXT -128 464 Left 2 ;R1=3D50 2 kHz 96% MURS120 IRL530NS_L 325W peak 30 =
nSec=20
1.15W Avg
TEXT -128 496 Left 2 ;R1=3D50 DC MURS120 IRL530NS_L 1.25W Avg=20

```
```On Fri, 18 Oct 2013 05:19:00 -0400, P E Schoen wrote:

> 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.

If you have any way of measuring the capacitance of the contactor coil,
do so -- that would cause increased (possibly vastly increased)
dissipation in the FET.

I agree that if you're driving the gate directly from the PIC that it may
not be very fast.  Look at the current rating for the pins on the PIC --
your 50 ohm resistor in the simulation may be barely accurate.

Do you have an oscilloscope, that you can measure actual circuit
behavior?  When one of the most important components in your circuit is a
box labeled "unknown -- dragons inside" you want to rely even less on
simulation than usual.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
```
```On Fri, 18 Oct 2013 05:19:00 -0400, "P E Schoen" <paul@peschoen.com> wrote:

>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

V2 in your sim, the gate drive pulse, is zero impedance. The PIC will be much
wimpier, so the mosfet will switch slowly and get hot. You need a gate driver
chip, and may as well go for more gate drive voltage.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
```
```> 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.
```
```On Fri, 18 Oct 2013 08:53:03 -0700, John Larkin wrote:

> On Fri, 18 Oct 2013 05:19:00 -0400, "P E Schoen" <paul@peschoen.com>
> wrote:
>
>>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
>>
>>Thanks,
>>
>>Paul
>
>
> V2 in your sim, the gate drive pulse, is zero impedance. The PIC will be
> much wimpier, so the mosfet will switch slowly and get hot. You need a
> gate driver chip, and may as well go for more gate drive voltage.

John, what do you think the 50 ohm resistor between V2 and the gate is
simulating?  Particularly given what the OP says in the text?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

```
```On Fri, 18 Oct 2013 09:37:27 -0500, Tim Wescott wrote:

> On Fri, 18 Oct 2013 05:19:00 -0400, P E Schoen wrote:
>
>> 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.
>
> If you have any way of measuring the capacitance of the contactor coil,
> do so -- that would cause increased (possibly vastly increased)
> dissipation in the FET.
>
> I agree that if you're driving the gate directly from the PIC that it
> may not be very fast.  Look at the current rating for the pins on the
> PIC -- your 50 ohm resistor in the simulation may be barely accurate.
>
> Do you have an oscilloscope, that you can measure actual circuit
> behavior?  When one of the most important components in your circuit is
> a box labeled "unknown -- dragons inside" you want to rely even less on
> simulation than usual.

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.

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.

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 --
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.

Some of the little single-channel gate drive chips are pretty cheap, you
may want to take a gander at what's available.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

```
```On Fri, 18 Oct 2013 11:23:20 -0500, Tim Wescott <tim@seemywebsite.really> wrote:

>On Fri, 18 Oct 2013 08:53:03 -0700, John Larkin wrote:
>
>> On Fri, 18 Oct 2013 05:19:00 -0400, "P E Schoen" <paul@peschoen.com>
>> wrote:
>>
>>>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
>>>
>>>Thanks,
>>>
>>>Paul
>>
>>
>> V2 in your sim, the gate drive pulse, is zero impedance. The PIC will be
>> much wimpier, so the mosfet will switch slowly and get hot. You need a
>> gate driver chip, and may as well go for more gate drive voltage.
>
>John, what do you think the 50 ohm resistor between V2 and the gate is
>simulating?  Particularly given what the OP says in the text?

50 ohms simulates 50 ohms. And I really doubt that a PIC port can source and
sink 100 mA. But all he needs is an oscilloscope on the gate to find out what
the slew rate is, and then estimate power dissipation.

Clearly something happens between 1 KHz and 20 KHz PWM rates. If it's not
switching loss, what is it? (Well, aside from a bad breadboard layout.)

There's a reason that people make and sell mosfet gate driver chips. In his
case, at 20 KHz, he could probably parallel all the sections of an HC04 and fix
things.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
```
```"Tim Wescott"  wrote in message=20
news:N6WdnSA8DZMw__zPnZ2dnUVZ5h4AAAAA@giganews.com...

> On Fri, 18 Oct 2013 09:37:27 -0500, Tim Wescott wrote:

>> If you have any way of measuring the capacitance of the contactor
>> coil, > do so -- that would cause increased (possibly vastly
>> increased) dissipation in the FET.

I don't have an impedance analyzer, so that would be difficult. And I =
don't=20
have the exact contactor (120 VAC) here, so I am using a similar size =
with=20
24 VDC coil.

>> I agree that if you're driving the gate directly from the PIC that
>> it may not be very fast.  Look at the current rating for the pins
>> on the PIC -- your 50 ohm resistor in the simulation may be
>> barely accurate.

You are correct. The pins are rated for about 25 mA. So I tried the=20
simulation with 200 ohms. Now at 2 kHz it shows only 100 watts peak but =
2.1=20
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=20
gives 9.9 watts average and 96 watts peak, so that would fully explain =
the=20
failure. Even at 2 kHz it is 1.8 watts average. And at 20 kHz with 96% =
duty=20
cycle it is 2.3 watts.

>> Do you have an oscilloscope, that you can measure actual circuit
>> behavior?  When one of the most important components in your
>> circuit is a box labeled "unknown -- dragons inside" you want to
>> rely even less on simulation than usual.

I have a 60 MHz storage scope and I was able to observe the waveforms =
with a=20
resistive load, but with the intended load the MOSFET popped before I =
could=20
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 --=20
> 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=20
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=20
of which are actually listed as a gate driver). Here is a 60V 10A =
"OmniFET"=20
http://www.mouser.com/ProductDetail/STMicroelectronics/VNP10N06-E/?qs=3Ds=
GAEpiMZZMu0dYp3dYbBlUps1mx%252bPR3J

The least expensive gate driver I found on a quick search is about =
\$0.35,=20
but is limited to 25V. My raw supply is 24V, and I will have a 5V =
regulator=20
for the PIC. However, 5V would be OK for the logic level MOSFETs.
http://www.mouser.com/ProductDetail/Diodes-Inc/ZXGD3005E6TA/?qs=3DsGAEpiM=
ZZMvQcoNRkxSQkkC4NeUaTUuMSZpeTA86SA0%3d

Thanks,

Paul=20

```
```wrote in message=20

> Is it possible that your coil kickback higher than 40V and damaging =
the=20
> MOSFET?  I use a 400V MOSFET for 12V relay.

The commutating diode across the coil should take care of any kickback. =
But=20
it might need to be faster than the 1N4004 presently used for this=20
application.

A 400V MOSFET with sufficiently low ON resistance may be hard to find =
and/or=20
expensive. Among 400V MOSFETs rated 5 to 10 amps continuous the lowest =
ON=20
resistance is about 550 mOhms and cost is over \$1 while the MOSFET I =
plan to=20
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=20
other problems such as insulation breakdown and flashover on the PCB or=20
wiring.

Paul=20

```
```On Fri, 18 Oct 2013 15:15:48 -0400, P E Schoen wrote:

> "Tim Wescott"  wrote in message
> news:N6WdnSA8DZMw__zPnZ2dnUVZ5h4AAAAA@giganews.com...
>
>> On Fri, 18 Oct 2013 09:37:27 -0500, Tim Wescott wrote:
>
>>> If you have any way of measuring the capacitance of the contactor
>>> coil, > do so -- that would cause increased (possibly vastly
>>> increased) dissipation in the FET.
>
> 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.
>
>>> I agree that if you're driving the gate directly from the PIC that it
>>> may not be very fast.  Look at the current rating for the pins on the
>>> PIC -- your 50 ohm resistor in the simulation may be barely accurate.
>
> 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.
>
>>> Do you have an oscilloscope, that you can measure actual circuit
>>> behavior?  When one of the most important components in your circuit
>>> is a box labeled "unknown -- dragons inside" you want to rely even
>>> less on simulation than usual.
>
> 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 --
>> 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"
> http://www.mouser.com/ProductDetail/STMicroelectronics/VNP10N06-E/?
qs=sGAEpiMZZMu0dYp3dYbBlUps1mx%252bPR3J
>
> 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/Diodes-Inc/ZXGD3005E6TA/?
qs=sGAEpiMZZMvQcoNRkxSQkkC4NeUaTUuMSZpeTA86SA0%3d

Make sure the gate driver can work at 5V!  Most of them expect 12-ish.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

```