Parallel Mosfets and Heat Sink
Parallel Mosfets and Heat Sink
(OP)
I am using an NTD70N03R mosfet from On Semiconductor in a half bridge configuration and would like to put several of them in parallel to handle more current (i.e. instead of 2 mosfets, I will have 6) Assuming I can generate the drive current to do this (I'm using a FAN7382 to drive it) my concern is dissipating the heat.
The circuit could draw about 100A which should divide about evenly between the three parallel mosfets and be within their ratings. Running 100A through a PC board trace seems excessive, so I'm looking at using 1/8inch by 1/2inch by about 2inches copper busbar. I am planning to solder the mosfets to the busbar so in addition to being a conductor it will also act as a heat sink. The gate pins will be soldered to a trace on the board, and the source pins will be soldered to another copper busbar.
My questions are one: is this a reasonable way to parallel mosfets and two: will the busbar provide enough of a heat sink?
Thanks for your help.
Ed
The circuit could draw about 100A which should divide about evenly between the three parallel mosfets and be within their ratings. Running 100A through a PC board trace seems excessive, so I'm looking at using 1/8inch by 1/2inch by about 2inches copper busbar. I am planning to solder the mosfets to the busbar so in addition to being a conductor it will also act as a heat sink. The gate pins will be soldered to a trace on the board, and the source pins will be soldered to another copper busbar.
My questions are one: is this a reasonable way to parallel mosfets and two: will the busbar provide enough of a heat sink?
Thanks for your help.
Ed





RE: Parallel Mosfets and Heat Sink
Glenn
RE: Parallel Mosfets and Heat Sink
Nonetheless, the datasheet max current >30 A ONLY applies if you have a gigantic heat sink AND forced air cooling on the heatsink. Otherwise, the rather limpy 1.87 W max power limit is more likely the case. And, even then, you'd need a REAL heatsink and some amount of air flow.
TTFN
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RE: Parallel Mosfets and Heat Sink
Glenn
RE: Parallel Mosfets and Heat Sink
Various manufacturers make assemblies that are combinations of heat sink and components that mount as part of the mechanical design of your system. Depending on the heat dissipation and temperature requirements you could also be looking at using fans for cooling. One place that I can think of to start looking for this sort of thing would be Darrah Electric (assuming your in the USA).
As far as soldering the PCB component to bus bar, well, I can think of all sorts of mechanical problems with this approach, not the least of which is getting everything up to soldering temperature.
I would also suggest taking a look at IGBTs instead of mosfets given your power requirements. IGBTs are readilly available in "brick" packages which are designed for use with heat sink assemblies.
RE: Parallel Mosfets and Heat Sink
I would worry a bit about your copper verse board expansion coefficients.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Parallel Mosfets and Heat Sink
Since you are using 25 Volt MOSFETS, I don't think you need a half-bridge like the FAN7382 which is rated up to 600V and is generally used for florescent lights, motor controls, and inverters (don't be fooled by the ratings of the logic supply). There are other MOSFET drivers out there for lower voltage applications.
RE: Parallel Mosfets and Heat Sink
I'm reviving my EE education from 30 years ago (worked as a computer scientist instead). In this project, I''m building a multilevel converter. This is one module. It switches a battery (eventually in parallel with a solar panel) into a circuit with a bunch of other modules to generate a 120Vrms 60Hz power source. The maximum continuous operation will be more like 30A, but once in a while I might have 100A load for a minute or two. I may eventually make a duplicate and operate it 180 degrees out of phase to make a 240Vrms system with both A and B phases.
So far, I have this working using single mosfets with 12 modules driving small loads like light bulbs and portable fans. The logic is basically correct, although I haven't yet implemented synchronizing with another 120Vrms 60Hz source to charge the batteries.
I was planning on blowing air over the copper busbar and I have a thermistor there so I can shut things down if it gets too hot. I was also thinking of dipping the copper busbar in a solder bath to tin it, then with some surface mount solder paste, stick the mosfets to the busbar and use a heat gun to solder them.
I guess I'll go back to the drawing board. It is interesting that I was unable to find an app note by on-semiconductor on how to use this part in a high current application. I did find one with surface mount parameters using the board as the heat sink. I don't think that could possible carry the current without melting the traces.
Comcokid, what would you suggest for the mosfet driver? The FAN7382 is quite easy to use. I need something with a floating supply that can handle double the voltage of my battery safely. I have a voltage doubler circuit that turns on when the high side mosfet turns on so the mosfet can be held on for a long enough time.
Thanks to all for the feedback.
Ed
RE: Parallel Mosfets and Heat Sink
http://www.aavidthermalloy.com/
TTFN
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RE: Parallel Mosfets and Heat Sink
I've seen DC motor packages using surface mounted devices that could handle much more than 100A. However, I didn't pay enough attention to how they were transferring the heat. It's possible though to use a whole bunch of vias (no thermal fingers) to a bottom layer that is bolted to a heatsink. It's also possible to use a big copper pad area on one side of the board which has a bare copper strip at the edge that is then clamped to a heatsink. There are manufacturers that will make boards with thicker copper layers.
You could also just go more conventional and use tab devices and hang them off the edge and clamp them to a heatsink.
RE: Parallel Mosfets and Heat Sink
Sounds like you're in a similar situation to my current design project. 28 years of analog small-signal and rf, and my employer assigned me to design a 1/2 kW pure sine inverter with features specific to our market. Neither my employer or me had experience in this area. Ive done plenty of buck-boost regulators, but never more than 15 watts before.
Some of the consumer-grade inverters I took apart for this project beef-up their input traces by soldering 14 gauge buss-bar wire along the traces, or with a tooled 1/8" thick x 1/4" tall w/thru-hole legs, standing buss-bar to handle the 20 to 40 amp input currents. All of these design parallel 2 to three MOSFETS on each leg of the first push-pull switch-mode step-up stage.
All good new designs are always a little "off-base". If they weren't, no progress would ever be made.
RE: Parallel Mosfets and Heat Sink
Here's one I just did after an extensive heat sink search. You won't find more cooling bang for the buck than this particular extrusion. And it is a stocked model at that!
The clips are totally the way to go too. Never, ever, consider screws and nuts torture. Let me know if you want me to hunt up the part number.
If you're interested there are more pictures at the bottom of this page:
http://www.flaminsystems.com/page/page/2456297.htm
Keith Cress
kcress - http://www.flaminsystems.com
RE: Parallel Mosfets and Heat Sink
It looks like you have four devices attached to each one. How much power are you sinking with this? What is the temperature difference?
RE: Parallel Mosfets and Heat Sink
See here:
ht
I used item 619-0068 as the heatsink.
I have some you can have. Make sure it will fit your design before I go to the effort though.
Power? I can't remember off hand. It's a 280W battery charger that had to run with no fan in a hot enclosure. About 56W I'd guess.
Got to Aavid and watch the video on the Max Clips. You will never consider screws again.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Parallel Mosfets and Heat Sink
It also looks like you're using something like the In-Sil-8 pads described on the bottom of the catalog page?
TTFN
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RE: Parallel Mosfets and Heat Sink
Using just the tab hole can actually make a gap under the device in some instances.
Yeah SIL pads go hand in hand with the max-clip scheme.
I was majorly impressed when I put those devices onto the sink. Without a jig it took me about 50 seconds to do them all. I would guess screws and hardware would take about 4 minutes. Two a minute with no fumbles.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Parallel Mosfets and Heat Sink
That heat sink application looks strangely familiar. Where did you come up with that one?
RE: Parallel Mosfets and Heat Sink
Hey! Maybe you remember the power on the hottest device? But likely, probably just the rise.
I'm trying to remember if the reverse protection diode was the hottest device as that would allow a straight Vdrop x I power number.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Parallel Mosfets and Heat Sink
The unit with the TO-220 packages and the massive heat sinks ran extremely cool, even under full capacity. How I remember the difficulties, though, in turning the board inside out so that the transistors were on the outside edges.
If I were to go through the old email logs, I might be able to tell you which one it was and what the temperature it ran at.
I do know that I pushed that little charger up close to the limits in test. I had the thing putting out about 300W at an 80C ambient. When I told you this, you started calling me the Dr of Death.
Too bad that they ultimately decided to not go with the product. They are still messing around and scrapped the whole design including the charger and are now focusing on an entirely different processor platform (this is the 2nd time that was changed).
That company takes the concept of the bumbling method of engineering to an entirely new level. One like I have never seen.
RE: Parallel Mosfets and Heat Sink
Yep the output diode was the biggest heater.
So there you have it veryuniqueid. That diode has a VF of 0.54V
P = V x I
P = 0.54 x 11A
P = 5.94W (convection only 80C ambient)
I believe we saw numbers like 130C
Keith Cress
kcress - http://www.flaminsystems.com
RE: Parallel Mosfets and Heat Sink
Thanks, This has been very helpful to me.
Isn't the diode power dissipation only a percentage of the 5.94W depending on your PWM duty cycle? Was the problem with the dual diode because they weren't quite matched and thus one started hogging the current, got hot and started hogging more current and got hotter ...?
Also, it looks like the power attributed to the switching of your MOSFETs is probably greater than the I*I*Rdson losses because of the PWM frequency.
Is the following analysis correct?
A three inch piece of that heat sink has a thermal resistance of about 3 degrees C per watt (per the data sheet) and it looks like you have two six inch pieces. So you have 1.5 degree C per watt for each heat sink But you have four devices on them, so that should be 6 degrees C per watt per device. Keeping junction temperatures below 175C you get 95 degrees C for each sink. The FDP8441 junction to case is 0.5 degree C per watt and the insulating pads look like they are 1.5 degrees C per watt. It looks like you have four of these on the one heat sink, so that is 0.5 (case) + 1.5 (pad) + 6 (sink) = 8 degrees C per watt total junction to ambient for each device. Using the 95 degree C temperature difference, it can handle 11.9W for each device attached to it. You seem to be comfortably below this especially since you aren't using boost mode and buck mode at the same time.
Ed
RE: Parallel Mosfets and Heat Sink
Your diode assumption is off a bit as it was strictly there as a blocking diode and hence had up to about 11A of DC put across it.
It was also a single not a dual. Duals are very common and not all that useful for blocking due to their lousy sharing.
http://ww
The MOSFETs have a far far lower on resistance than the diodes. If you want to get carried away you use MOSFETs instead of diodes for reverse blocking. But that adds parts.
You're most welcome.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Parallel Mosfets and Heat Sink
Is it a good assumption that current really divides equally to three parallel low resistance devices? Note Comcokid's finding of beefed up input leads. The connecting wires would need to be low in resistance compared to the device, or else the connecting wires will direct most current to one device. Considering this, perhaps a single IGBT instead of each parallel assembly is safer to use.
Eric Ratliff
http://www.icpdas-usa.com
RE: Parallel Mosfets and Heat Sink
TTFN
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RE: Parallel Mosfets and Heat Sink
RE: Parallel Mosfets and Heat Sink
LT4355, 57, 51, 54
Keith Cress
kcress - http://www.flaminsystems.com
RE: Parallel Mosfets and Heat Sink
I saw an app-note here several years ago:
www.microsemi.com/micnotes/APT0402.pdf
I've found that a bead which is about 50ohms at 100MHz has just the right damping at the gate-drive range of freq's that you need.
RE: Parallel Mosfets and Heat Sink
Keith Cress
kcress - http://www.flaminsystems.com