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'[OT]: How to heatsink a lot of power electronics?'
2001\08\28@091751 by Byron A Jeff

face picon face
After years of development ;-) I've finally have my prototype robotic lawnmower
platform to a usable state. The most valuable lesson I learned is that
everything in the drive path must be metal, clamped, and use lock washers.

Next on the agenda is motor control for the drive motors. Bob Blick's
discrete h-bridge is a natural for the project. But of course you end up
with 8 power transistors, plus another one for the cutting motor.

I wanted to get some ideas of the best way to heatsink a bunch of power
transistors.

TIA

BAJ

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2001\08\28@092301 by Jim

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face
Can you somehow couple this heat-load to the
metal mower deck? (If indeed the mower deck is
made of metal.)

Jim

{Original Message removed}

2001\08\28@093354 by John Walshe

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face
Why don't you make use of all of that metal in the drive path? Assuming the
FETs/Transistors are isolated devices the metal framework should provide a
reasonable heatsink, without too much trouble. If the above are not isolated
devices then you will have to put a mica strip(or modern equivalent) between
the device and the metal.
John

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2001\08\28@095850 by Eisermann, Phil [Ridg/CO]

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       That would be the way to go. Lots of distributors sell mounting kits
for power transistors, which generally include the insulator. Check for
example DigiKey, Mouser, Jameco.

       If you can't use the existing metal, another way to go is an
aluminum channel underneath the board (assuming TO-220 or somesuch). Mount
the transistors on the bottom of the board, at a 90 degree angle so the
metal portion of the case makes contact with the aluminum. Crude ascii art
to illustrate concept:

         (board) ----------------
    (transistor)    *****
     (heatsink)  ================

In other words, you're sandwitching the transistor between the board and
heatsink. By mounting the transistors on the bottom, you don't have to worry
about interference with large caps and connectors. You can also buy finned
heatsink material and cut that to size; but I find plain aluminum channel
stock is cheaper, has a reasonably smooth surface finish, and works well in
most cases. You can probably get the channel stock through Grainger or
McMaster. You can make the heatsink as big/long as your board without adding
a lot of height.


> {Original Message removed}

2001\08\28@101936 by Byron A Jeff

face picon face
On Tue, Aug 28, 2001 at 08:22:27AM -0500, Jim wrote:
> Can you somehow couple this heat-load to the
> metal mower deck? (If indeed the mower deck is
> made of metal.)

It's not. Believe it or not it's a $5 round particleboard tabletop.

But to beg the question. Since historically heatsink tabs are connected to
one or the other of the transistor pins, how does one electrically isolate
yet thermally couple. I know that the mica washers are supposed to work, but
do they really?

BAJ

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2001\08\28@103319 by Thomas C. Sefranek

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Byron A Jeff wrote:

>On Tue, Aug 28, 2001 at 08:22:27AM -0500, Jim wrote:
>
>>Can you somehow couple this heat-load to the
>>metal mower deck? (If indeed the mower deck is
>>made of metal.)
>>
>
>It's not. Believe it or not it's a $5 round particleboard tabletop.
>
>But to beg the question. Since historically heatsink tabs are connected to
>one or the other of the transistor pins, how does one electrically isolate
>yet thermally couple. I know that the mica washers are supposed to work, but
>do they really?
>
Mica is the insulator, heat grease is the thermal conductor.
The FAR better solution is a sil-pad which does both.

>
>BAJ
>
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>
>

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2001\08\28@103326 by Byron A Jeff

face picon face
On Tue, Aug 28, 2001 at 02:28:24PM +0100, John Walshe wrote:
> Why don't you make use of all of that metal in the drive path?

Because it's spinning. The motors are direct mounted to the wheels and
are outboard of the platform.

>Assuming the FETs/Transistors are isolated devices...

Any way to make that assumption? In almost every case that tab on a TO220
case is connected to something. I was thinking that each half of the HBridge
could share a sink because the tabs are connected to the collectors and the
collectors are shared.

> the metal framework should provide a
> reasonable heatsink, without too much trouble. If the above are not isolated
> devices then you will have to put a mica strip(or modern equivalent) between
> the device and the metal.

Now that's an idea. Would anyone care to explain how mica gives good heat
coupling while maintaining electrical isolation?

Thanks for the info.

BAJ

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2001\08\28@103558 by Byron A Jeff

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On Tue, Aug 28, 2001 at 08:54:46AM -0500, Eisermann, Phil [Ridg/CO] wrote:
>         That would be the way to go. Lots of distributors sell mounting kits
> for power transistors, which generally include the insulator. Check for
> example DigiKey, Mouser, Jameco.
>
>         If you can't use the existing metal, another way to go is an
> aluminum channel underneath the board (assuming TO-220 or somesuch). Mount
> the transistors on the bottom of the board, at a 90 degree angle so the
> metal portion of the case makes contact with the aluminum. Crude ascii art
> to illustrate concept:
>
>           (board) ----------------
>      (transistor)    *****
>       (heatsink)  ================

Outstanding idea. I couple even mount the sink perpendicular to the board
and have the transistors out on the edge.

Thanks for the info...

BAJ

{Quote hidden}

> > {Original Message removed}

2001\08\28@104206 by Jim

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face
Byron:
     "I know that the mica washers are supposed
      to work, but do they really?"

They have been used as far back as I can remember - on
'gear' I was into at the time (the 70's, Delco car radios as a
kid).

My Heathkit AR-1500A (in near mint condition) uses mica
TO-3 insulators on the PA transistors to couple them thermally
to the heatsink.

Mica insulators do introduce *some* thermal resistance - and I've
seen other material used in place of mica, such as black-oxide-
coated aluminum.

Jim

{Original Message removed}

2001\08\28@104748 by Eisermann, Phil [Ridg/CO]

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face
> >
> >           (board) ----------------
> >      (transistor)    *****
> >       (heatsink)  ================
>
> Outstanding idea. I couple even mount the sink perpendicular
> to the board
> and have the transistors out on the edge.
>

Yes, whatever arrangement works for you. Let me just add that it is
*significantly* easier to secure the heatsink to the board if they are
sandwitched; all it takes is some screws and spacers.

Another assembly hint for anyone wanting to use this: attach the transistors
to the heatsink first, bend leads if necessary, then mount the heatsink to
the board using whatever arrangement you have come up with. Only then do you
solder the transistors in place. This makes sure there is minimal stress on
the transistor leads.

If you solder the transistors to the board first, and then attach the
heatsink, i can guarrantee that they will fail faster over time than the
method described above.

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2001\08\28@111454 by Bob Barr

picon face
Byron A Jeff wrote:
>
>I know that the mica washers are supposed to work, but
>do they really?
>

With a bit of heatsink goop, they work like a champ. Thermally conductive,
electrically insulating - just like they're supposed to be.

Regards, Bob




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2001\08\28@111931 by Dan Michaels

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At 09:39 AM 8/28/01 -0500, you wrote:
>Byron:
>      "I know that the mica washers are supposed
>       to work, but do they really?"
>
>They have been used as far back as I can remember - on
>'gear' I was into at the time (the 70's, Delco car radios as a
>kid).
>


Mica is somewhat flimsy in the robustness category. I wonder
whether it will flake apart under the constant vibration
from the motor.

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2001\08\28@112118 by Dan Michaels

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At 09:42 AM 8/28/01 -0500, you wrote:
>> >
>> >           (board) ----------------
>> >      (transistor)    *****
>> >       (heatsink)  ================
>>
>> Outstanding idea. I couple even mount the sink perpendicular
>> to the board
>> and have the transistors out on the edge.
>>
>
>Yes, whatever arrangement works for you. Let me just add that it is
>*significantly* easier to secure the heatsink to the board if they are
>sandwitched; all it takes is some screws and spacers.
>
..........


I'll bet if any sort of R-A vertical mount heat sink is used, the
motor vibrations will tear the whole thing apart - unless the heat
sink is "very" firmly attached. The flat pack/sandwich arrangement
sounds more robust for this app.

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2001\08\28@114824 by Jim

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Dan:
    "Mica is somewhat flimsy in the robustness
     category. I wonder whether it will flake
     apart under the constant vibration from the
     motor."

How would this ocurr if the insulator is coated
in HS compound/thermal grease and securely
held between the heatsink and transistor body?

I have *never* seen a Mica insulator 'flake' apart
in use - perhaps you have a scenrio where this has
ocurred? Perhaps the extra-severe application
on a lawn mower would contribute to this?

I *have* had a Mica insulator fail before - it was due
to a small machining burr in the HS to which a TO-3
power device was mounted. Over the years the burr
must have worn it's way through the mica - one day on
a sound peak (from the Heathkit AR1500A)  the burr
shorted through to the TO-3 transistor body and took
out that transistor, another transistor and the PS bridge
rectifier ...

Granted, in a vibe prone app the mechanical securing
of all fasteners has got to be a prime consideration
towards longevity of product ...

Mica, like concrete, would seem to thrive under compression
(forces) and fail under sheer or tensile stress. Watch those
burrs though ...

Jim

PS. All the old Motorola Motrac 2-way radios (built from
1960 - 1972) used TO-3 power transistors attached to
HS's - must have used mica insulators there and I've never
had any of my 30 year old plus Motracs fail due to a failed
mica insulator - I've had PS switching transistors fail/short -
this blows the 30 Amp 12 V mains fuse, I've had the tube
PAs fail - but no mica insulators.

Also - the GE Mastr Pro series of UHF transmitter strips
used a large square piece of mica as C143 - this "stood
off" upwards of  +650 VDC plate potential. Of the several
strips I've has in my GE Mastr repeater I've never had one
of those fail either (other things, again, *have* failed though).


{Original Message removed}

2001\08\28@130416 by Douglas Butler

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face
How about "metal, clamped, and use lock washers."
More seriously, anything you can do to provide airflow is good.  Of
course you have to keep the grass clippings from collecting on the
TO220s.  If your motor has a fan and you can duct the intake over the
transistors it will help cooling a lot.

Sherpa Doug

> {Original Message removed}

2001\08\28@131426 by Mike Kendall

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face
Howdy,
   I had an application where I wanted to purchase cheap heatsinks.  I
ended up buying surplus pentium II heatsinks that are not in demand dirt
cheap on Ebay.
Regards,
Mike Kendall
----- Original Message -----
From: Douglas Butler <RemoveMEdbutlerspamTakeThisOuTIMETRIX.COM>
Sent: Tuesday, August 28, 2001 11:51:34 AM
To: <PICLISTEraseMEspam.....MITVMA.MIT.EDU>
Subject: Re: [OT]: How to heatsink a lot of power electronics?

> How about "metal, clamped, and use lock washers."
> More seriously, anything you can do to provide airflow is good.  Of
> course you have to keep the grass clippings from collecting on the
> TO220s.  If your motor has a fan and you can duct the intake over the
> transistors it will help cooling a lot.
>
> Sherpa Doug
>
> > {Original Message removed}

2001\08\28@174353 by Jinx

face picon face
> TO220s.  If your motor has a fan and you can duct the intake
> over the transistors it will help cooling a lot

If it hasn't got a fan you could make one out of just about
any sheet material, as long as it gets the air moving. Even
eg high power audio amps with huge heatsinks (and room
to add more) employ at least one fan

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2001\08\28@223057 by Tom Handley

picon face
  Byron, if you're building Bob's bipolar H-Bridge with TIP120 and
TIP125 power Darlingtons, the tab is connected to the collector in
both devices. You can easily use two heat sinks, one for each side of
the bridge since their collectors are connected. Just make sure the
sinks do not contact other circuitry. If you go the isolated route,
silicon thermal pads are generally preferred to the older mica/grease
method. I recently built myself an AMD Athalon 1.33GHz system the heat
sink is crucial to the survival of the chip. Mine is a large aluminum
assembly approved by AMD with a fan and silicon pad. AMD specifically
states to not use thermal grease.

  - Tom

At 10:31 28-08-01 -0400, Byron A Jeff wrote:
{Quote hidden}

isolated
>> devices then you will have to put a mica strip(or modern equivalent)
between
>> the device and the metal.
>
>Now that's an idea. Would anyone care to explain how mica gives good heat
>coupling while maintaining electrical isolation?
>
>Thanks for the info.
>
>BAJ


------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

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2001\08\28@233820 by Dan Michaels

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At 10:48 AM 8/28/01 -0500, you wrote:
>Dan:
>     "Mica is somewhat flimsy in the robustness
>      category. I wonder whether it will flake
>      apart under the constant vibration from the
>      motor."
>
>How would this ocurr if the insulator is coated
>in HS compound/thermal grease and securely
>held between the heatsink and transistor body?
>
>I have *never* seen a Mica insulator 'flake' apart
>in use - perhaps you have a scenrio where this has
>ocurred? Perhaps the extra-severe application
>on a lawn mower would contribute to this?
.............
>


Jim, my "conjecture" was based upon what I said in my
last sentence above, and what you said in your last
sentence here below ..... I could think of any number
of scenarios on how excessive, continuous hi-speed
vibrations could lead to mechanical failure, etc.

- dan
===========

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2001\08\29@021950 by Roman Black

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Jim wrote:
>
> Dan:
>      "Mica is somewhat flimsy in the robustness
>       category. I wonder whether it will flake
>       apart under the constant vibration from the
>       motor."
>
> How would this ocurr if the insulator is coated
> in HS compound/thermal grease and securely
> held between the heatsink and transistor body?
>
> I have *never* seen a Mica insulator 'flake' apart
> in use -

I agree Jim, and a lot of DC motors used mica as
the main insulator between the copper commutator
segments on the armature. :o)
-Roman

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2001\08\29@090028 by Jim

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Roman:
     "... and a lot of DC motors used mica
     as the main insulator between the copper
     commutator segments on the armature."

Now that you mention it, I *do* recall seeing Mica
used in applications like that on DC motors.

Jim

{Original Message removed}

2001\08\29@112531 by Jim

flavicon
face
Roman - I distinctly remember what I believe to
be Mica insulators (best described appearance-wise
as simply 'clear and glistening' ) used in such common
household products as toaters ... Have you seen that
as well?

Jim

{Original Message removed}

2001\08\29@113851 by Spehro Pefhany

picon face
At 10:26 AM 8/29/01 -0500, you wrote:
>Roman - I distinctly remember what I believe to
>be Mica insulators (best described appearance-wise
>as simply 'clear and glistening' ) used in such common
>household products as toaters ... Have you seen that
>as well?

There's a kind of mica that is (was?) used in lamp shades
and I think it meets that description. The stuff used in
toasters these days is brown and flakes, IIRC.

It's used in large quantities in heaters (mica band
heaters) and it is probably still used in China and
Russia in their electron tube production lines .

Best regards,
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2001\08\29@115131 by Jim

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Doh!

How about 'dipped silver-mica' capacitors!

Also the 'open' style micas - the higher power
low series-inductance xmit styles!

Mica compression-trimmers!

Doh!

(It's all coming back now ...)

Jim

{Original Message removed}

2001\08\29@121837 by John Ferrell

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face
It is a very common mineral that is available in several forms. I believe
that Vermiculite that is used by gardeners and sometimes for thermal
insulation by builders is one form.

I THINK it is the material that used to be used for windows in wood stoves
and sometimes lab furnaces.  Called Isinglass(?)

It is used as a component in paint because of its reflective qualities.

It is fragile in thin sheets, but otherwise nearly indestructible.
John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2001\08\29@123017 by Dan Michaels

flavicon
face
Oh, it might work.

I figure there is a difference between sealing mica
inside a dipped epoxy [whatever] casing and compressing
a thin mica sheet "mechanically" between 2 pieces of
metal using a screw & lock washer, and then subjecting
it to a summer's long worth of vibration on a lawnmower.
Hit a rock, bend the blade a tad, even more vibration.

Newer lawnmowers are of course "laser-balanced" for
minimal vibration at the factory [ ;-) ]. I remember an
old one my father had where my arms would be vibrating
after spending an hour cutting the grass.


At 10:50 AM 8/29/01 -0500, you wrote:
{Quote hidden}

>{Original Message removed}

2001\08\29@163448 by Russell McMahon

picon face
> After years of development ;-) I've finally have my prototype robotic
lawnmower
> platform to a usable state. The most valuable lesson I learned is that
> everything in the drive path must be metal, clamped, and use lock washers.
>
> Next on the agenda is motor control for the drive motors. Bob Blick's
> discrete h-bridge is a natural for the project. But of course you end up
> with 8 power transistors, plus another one for the cutting motor.
>
> I wanted to get some ideas of the best way to heatsink a bunch of power
> transistors.

If power dissipation is significant you want to really DESIGN your heat
sinking system. How hard this is depends on how close to their limits you
want to run your transistors. What power dissipation are you expecting? What
devices are you using? (part numbers or required ratings).

There are 3 thermal sections to the heat path.

1 - die to case
2 - case to heat sink
3 - heat sink to air

If you have lots of metal for heatsink you will be able to get the HS down
under 1 C/W (1 degree Celsius rise per watt dissipated).

If you use electrically isolated packages they can be bolted directly to the
HS thereby eliminating path 2 above BUT the inbuilt insulation has extra
drop. look carefully at data sheets. This extra drop is typically around 1
C/W. Use grease.

Mica washers require thermal grease because the washer is relatively rigid
and does not make good content with transistor or heatsink. If the grease
dries out with time you can get an increase in thermal resistance. AFAIR
without looking it up you get around 1 C/W for the washer in a TO220
package. .

Modern silicon rubber washers do not need grease and also run at about 1 C/W
for a TO220 package. Mechanically larger packages (eg TO247) have a better
C/W rating all things being equal.

The transistor's internal thermal drop (1 above) varies markedly with
devices, even in the same package. This can be well under 1 C/W up to
several C/W so be careful. Also note that SOME manufacturers spec thermal
resistance as W/C (inverse) so don't be fooled.

You can get proprietary thermally conductive gasket materials with much
better thermal resistances. These are often dear but may be worthwhile in
extreme cases.

Mounting to heatsinks with silicon rubber or similar is typically by spring
clips. Ensure that adequate tension will ALWAYS be present across lifetime.
Loss of tension with age can lead to higher thermal drop. Maximum clamping
are specified by manufacturers but too low a force is liable to be more of a
problem. Avoid point contact of clamping clips as this can lead to internal
failures.

If you can design the system so that the heatsink is at the same potential
as the transistors tabs then no insulation is needed and direct mounting
will give better thermal results. Use grease here!

If the heatsink has a thinner portion between the transistor contact points
and the main HS body or the heat path is mechanically long beware
temperature rise across this section.

You could consider fluid heat transfer or heat pipes but that is probably
getting excessively complicated.



       Russell McMahon

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2001\08\29@170737 by Peter L. Peres

picon face
With non- complementary bridges you have a problem. Therefore it is nice
to either buy insulated VFETS (modified TO220 case - several types), or
modify the scheme (if necessary) to use a single power supply for
everything in a complementary bridge and then you have two big heatsinks,
one for transistors with grounded case and one for those with case at Vcc,
the latter being well insulated on heat resistant insulators. The
transistors themselves need no insulation in this case, assuming that you
know where ground currents go.

As to sizes etc, it depends on what you do with the drivers. Know that a
simple fan will reduce the heatsink requirements two to three times.

I find a thermistor watchdog that latches a relay using a thyristor
invaluable during smoke let-out phases. It is set to trip the thyristor at
100 degrees C (roughly), uses a 358 opamp, and is powered by the circuit
it is to keep the smoke in. I leave it in the circuit until I am sure that
the smoke does not try to escape.

One last item: I have learned the hard way that with power electronics you
*have* to make eyelets on each component and each and every wire (even low
power) connected 'in air'. I am always pleasently surprised that the
circuit can reach the melting temperature of solder and not fall apart or
drop high voltage/high current wires onto something. Form tight closed
eyelets and put them over the pins (f.ex. of TO3 cases), and make sure
that they are not under mechanical tension so they cannot spring off and
cause solder to fly about if it comes to it.

Peter

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2001\08\29@211446 by Byron A Jeff

face picon face
On Wed, Aug 29, 2001 at 08:41:10PM +1200, Russell McMahon wrote:
> > After years of development ;-) I've finally have my prototype robotic
> lawnmower
> > platform to a usable state. The most valuable lesson I learned is that
> > everything in the drive path must be metal, clamped, and use lock washers.
> >
> > Next on the agenda is motor control for the drive motors. Bob Blick's
> > discrete h-bridge is a natural for the project. But of course you end up
> > with 8 power transistors, plus another one for the cutting motor.
> >
> > I wanted to get some ideas of the best way to heatsink a bunch of power
> > transistors.
>
> If power dissipation is significant you want to really DESIGN your heat
> sinking system. How hard this is depends on how close to their limits you
> want to run your transistors.

> What power dissipation are you expecting?

On the order of 10-12W each with 1/2 of the transistors on and 1/2 off at any
one time.  I'm basing this on Bob Blick's comments that the parts will drop
about 2V each under load and the motors draw 3.5 to 4A on level ground. It
rained today So I didn't get a chance to test on grass.

BTW I realize that I'm going to have to have some kind of very reliable
crossbar. The motors' stall rating is 11A each! Ouch! I need to figure out
how to build a current sensor so that when the motor stalls it'll automatically
cut out.

> What devices are you using? (part numbers or required ratings).

TIP 142/147 combo in Bob Blick's H-Bridge driver:

http://www.bobblick.com/bob/projects/hbridge/index.html

These are the 10A continouous parts.

>
> There are 3 thermal sections to the heat path.
>
> 1 - die to case
> 2 - case to heat sink
> 3 - heat sink to air
>
> If you have lots of metal for heatsink you will be able to get the HS down
> under 1 C/W (1 degree Celsius rise per watt dissipated).

So we're talking about 40C above ambient? That's hot!!

>
> If you use electrically isolated packages they can be bolted directly to the
> HS thereby eliminating path 2 above BUT the inbuilt insulation has extra
> drop. look carefully at data sheets. This extra drop is typically around 1
> C/W. Use grease.

Not isolated.

[Other suggestions trimmed]
>
> You could consider fluid heat transfer or heat pipes but that is probably
> getting excessively complicated.

I'm pretty sure that's not going to be necessary for the 50W of dissipation
required. The prototype is going to be in open air. I should be able to get
the temp down with enough metal.

I realize that I can direct couple in pairs since the collectors are shared
and are connected to the tabs.

I bought some sil-pad type thermally conductive insulators from NTE to test
with.

Thanks for the great info.

BAJ

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2001\08\30@044534 by Roman Black

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Byron A Jeff wrote:
>
> After years of development ;-) I've finally have my prototype robotic lawnmower
> platform to a usable state. The most valuable lesson I learned is that
> everything in the drive path must be metal, clamped, and use lock washers.
>
> Next on the agenda is motor control for the drive motors. Bob Blick's
> discrete h-bridge is a natural for the project. But of course you end up
> with 8 power transistors, plus another one for the cutting motor.

And you end up wasting a lot of energy on the
forward losses of the bridge transistors, see below.

> I wanted to get some ideas of the best way to heatsink a bunch of power
> transistors.


Hi, great to see the robot lawnmower taking shape!
If you haven't put too much work into the drive motor
h-bridges, I would suggest using the more old fashioned
system, ONE transistor with PWM to drive the motor,
and a cheap relay to reverse the motor wires.

You don't really need h-bridge complexity or cost,
and with 5 amp motors you mentioned, the relay system
is much more efficient, relay contacts have very little
heat or waste. You can also optimise the ideal ONE
transistor or FET for the motor, with a very low on
voltage (Vce sat) and good base drive circuit.
This is harder to do with a h-bridge. With a battery
powered mower robot you probably want to keep it as
efficient as possible.

And you only need 2 large heatsinks, one for each
transistor. :o) You don't really need high speed
control of engine braking, which is one area that
the h-bridge might be better.
-Roman

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2001\08\30@044754 by Roman Black

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Byron A Jeff wrote:
>

> >Assuming the FETs/Transistors are isolated devices...
>
> Any way to make that assumption?


Yes! See if the tab is completely covered
in plastic like the transistor body. :o)

These insulated transistors only have a fraction
of the power rating that normal ones have, like
in the TO-220 packs you get around 15w for the
insulated ones, and around 60w for the normal ones.
-Roman

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2001\08\30@053101 by Roman Black

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Spehro Pefhany wrote:
>
> At 10:26 AM 8/29/01 -0500, you wrote:
> >Roman - I distinctly remember what I believe to
> >be Mica insulators (best described appearance-wise
> >as simply 'clear and glistening' ) used in such common
> >household products as toaters ... Have you seen that
> >as well?
>
> There's a kind of mica that is (was?) used in lamp shades
> and I think it meets that description. The stuff used in
> toasters these days is brown and flakes, IIRC.
>
> It's used in large quantities in heaters (mica band
> heaters) and it is probably still used in China and
> Russia in their electron tube production lines .

Yep they still use it in quality stuff, Mica is a
rock and I imagine has a VERY high melt temperature...

Most of the new appliances and light fittings use that
fibreglass sheet stuff, silver coloured. Look in
you toaster! :o) It's not as good, and yep it's more
fragile than Mica based on some appliances i've looked
inside.
-Roman

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2001\08\30@053751 by Roman Black

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John Ferrell wrote:
>
> It is a very common mineral that is available in several forms. I believe
> that Vermiculite that is used by gardeners and sometimes for thermal
> insulation by builders is one form.
>
> I THINK it is the material that used to be used for windows in wood stoves
> and sometimes lab furnaces.  Called Isinglass(?)
>
> It is used as a component in paint because of its reflective qualities.
>
> It is fragile in thin sheets, but otherwise nearly indestructible.


Mica is GREAT for transistor heatsink washers.
I have a big collection of mica TO-3 washers from
the 70's, and you can split them with a razor
blade and make 3 or 4 thinner washers from each
standard one. You can cut them down with scissors
to make small washers for TO-220 too.

When Philips switched to plastic washers their
TVs started failing as the plastic broke down,
mica washers don't fail, they are the only type
that don't get pinhole faults on TV output stages
from the high volts punching through.
-Roman

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2001\08\30@101051 by Byron A Jeff

face picon face
On Thu, Aug 30, 2001 at 06:38:23PM +1000, Roman Black wrote:
{Quote hidden}

Hmmm. I was thinking about the relay route. I got bogged down in trying to
figure out a simple control box for the beast.

PWM isn't an issue. I plan to run full bore all the time.

The relay control problem was associated with the fact that I have two
independant drive motors. The H-Bridge had the control infrastructure that
I wanted: forward, reverse, stop for each wheel. I could then turn that into
my true interface: forward, reverse, spin left, spin right (it's a circular
base) and stop.

I will go and rethink how to do this with DPDT relays and the transistor.

In fact now that I think about it, it may be able to be done with just 3
relays.


{Quote hidden}

True.

>
> And you only need 2 large heatsinks, one for each
> transistor. :o) You don't really need high speed
> control of engine braking, which is one area that
> the h-bridge might be better.

Agreed. I will go and rethink the problem.

BAJ

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2001\08\30@102100 by Roman Black

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Byron A Jeff wrote:

> > Hi, great to see the robot lawnmower taking shape!
> > If you haven't put too much work into the drive motor
> > h-bridges, I would suggest using the more old fashioned
> > system, ONE transistor with PWM to drive the motor,
> > and a cheap relay to reverse the motor wires.
>
> Hmmm. I was thinking about the relay route. I got bogged down in trying to
> figure out a simple control box for the beast.
>
> PWM isn't an issue. I plan to run full bore all the time.


It's still worth thinking about PWM and a transistor
for each motor. This gives you soft-start and some
control over robot speed, I really think before the
robot is perfect you will be enjoying those features...
:o)
-Roman

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2001\08\30@141436 by Peter L. Peres

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There is nothing really wrong with mica if it is of good quality and
mounted right. All insulators eventually carbonize and become conductive
(even silicon rubber ones which turn into white puffy slimy dust with
nil mechanical resistance when heated enough), but mica does not, as it
contains no carbon afaik. It withstands very high temperatures (as in hot
air blowers that run at 500+degrees C etc).

Peter

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2001\08\30@184429 by Dan Michaels

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Peter Peres wrote:
>There is nothing really wrong with mica if it is of good quality and
>mounted right. All insulators eventually carbonize and become conductive
>(even silicon rubber ones which turn into white puffy slimy dust with
>nil mechanical resistance when heated enough), but mica does not, as it
>contains no carbon afaik. It withstands very high temperatures (as in hot
>air blowers that run at 500+degrees C etc).
>

20 people have now stepped forward touting the merits of mica,
but no one has yet really addressed the issue of durability
in the face of an environment with extreme vibrations - like
riding on a lawnmower.

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2001\08\31@120709 by Peter L. Peres

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I think that Roman is right, you should consider using relays for
direction change and low Rdson FETs for the ON/OFF function. This way the
relays will never spark and last a very long time. Automotive DPDT relays
with >20A contact ratings are available for little money. A 50A VFET with
0.045 ohms Rdson is also inexpensive and will not require a serious
heatsink at 5A, so you can use insulated ones (f.ex. BUK445-50 which is
what I have on the table now ;-). Since they are cheap you can use the
same kind to drive the relays too and use the same level shifter for the 5
of them (you need to shift 5V logic level to 12V gate voltage using simple
trasistor buffers for example).

You can even brake by switching the relay with the FET off. This will do
regenerative braking by feeding current back into the battery through the
reverse bulk diode of the FET. Now you can charge the battery by pushing
the mower in front of you ;-).

Peter

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