Power MOSFETS

Thread Starter

Mogman

Joined Jan 20, 2022
10
Hello all, I am a newbie here but have been around..
I am looking at solid state switching of a 24V restive load which has an inrush of about 125A and then a momentary, 10-15sec load of around 80A
So I am looking at P channel MOSFETS because it must be a high side switch. I see a Vishay device
SQP100P06-9M3L_GE3
That is rated at 100A continuous, and in a TO220 package, what am I missing hear, I can in no way see how a TO220 device can handle/conduct 100A....
 

Audioguru again

Joined Oct 21, 2019
4,537
The datasheet says it can continuously pass 100A but only if you cool its case to 25 degrees C.
But then it will try to heat with 93W and you must douse its heatsink with liquid nitrogen or something to keep its case from produce more heat than the ambient.
 

Thread Starter

Mogman

Joined Jan 20, 2022
10
The datasheet says it can continuously pass 100A but only if you cool its case to 25 degrees C.
But then it will try to heat with 93W and you must douse its heatsink with liquid nitrogen or something to keep its case from produce more heat than the ambient.
Thanks, but to begin with I cannot see how a lead from a TO220 device can pass 100A, I was planning to use multiple devices but still just cannot understand how it could possibly conduct 100A period.
The duty cycle is so low that even just a fair amount of copper would do, this event only happens once a day
 

Thread Starter

Mogman

Joined Jan 20, 2022
10
Nope, it is a continuous max allowed 100A current with no duty cycle. The key is the almost impossible amount of cooling.
The duty cycle is only 10-15 sec at most, usually only 4 sec on and a DAY off, the heat sink would only have to absorb a relatively small about of heat, just because it is rated at continuous duty it does not mean it will be OPERATED continuous duty.
 

Ian0

Joined Aug 7, 2020
4,863
Thanks for the input but I certainly did not want a relay
Let’s see if you are still of the same opinion once you have added up the price of the MOSFETs, and the heatsink, and the PCB with enough copper to carry the current, and the drive circuitry level-shifted to the +24V rail (and the MOSFET is probably out of stock until mid2023)
 

Thread Starter

Mogman

Joined Jan 20, 2022
10
Let’s see if you are still of the same opinion once you have added up the price of the MOSFETs, and the heatsink, and the PCB with enough copper to carry the current, and the drive circuitry level-shifted to the +24V rail (and the MOSFET is probably out of stock until mid2023)
I have not found the price on the 200A relay but I doubt it is less than $44 and Mouser has 656 in stock, Digi-Key also has stock. With this device I do not need a PCB to carry the current only for the drive circuitry and as stated before the duty cycle is so low that a modest heat sink should do. I am unclear why a simple voltage divider would not work as a drive circuit
 

LowQCab

Joined Nov 6, 2012
1,927
Now we start getting into the juicy complexities.

1)
Does it absolutely have to be a "High-Side" Switch for some particular reason ?,
or, do You simply not really understand how a "Low-Side" Switch could work in your circumstances ?
A Low-Side-Switch will have better specs, cost half as much, and provide 10X the choices.

2)
FETs don't like being "part-way" On.
Specifically, how do You intend to Drive this Gate, and why ?

3)
A detailed overview of exactly what You are trying to accomplish will get You more useful answers,
and possibly multiple different approaches to solving the problem.
.
.
.
 

Thread Starter

Mogman

Joined Jan 20, 2022
10
Now we start getting into the juicy complexities.

1)
Does it absolutely have to be a "High-Side" Switch for some particular reason ?,
or, do You simply not really understand how a "Low-Side" Switch could work in your circumstances ?
A Low-Side-Switch will have better specs, cost half as much, and provide 10X the choices.

2)
FETs don't like being "part-way" On.
Specifically, how do You intend to Drive this Gate, and why ?

3)
A detailed overview of exactly what You are trying to accomplish will get You more useful answers,
and possibly multiple different approaches to solving the problem.
.
.
.
Yes it has to be a high side switch as it will be powering glow plugs that are already referenced to ground, the control will be a simple momentary switch as the glow plugs will be manually controlled, of course it would be much simpler to use a relay but I am also thinking of using this same type of device to also switch the 24V run power for the vehicle which would also have to be a high side switch, both of these duties are being carried out by relays now and fail quite often.
Not sure why this simple circuit would not work for both applications
example.pngThe run power switch would only need to be rated at 60A and in practice would only handle 20A or so.
 
Last edited:

LowQCab

Joined Nov 6, 2012
1,927
OK, the picture is getting more clear now.
The FET I recommended will work just fine,
but don't go thinking that You don't need a Heat-Sink.
It should preferably be mounted inside the Cab as well,
or at least in an area that receives good Air-Flow.
Moisture / High-Humidity can also sometimes create problems with Corrosion.

I would prefer to use separate FETs, one for each Glow-Plug,
but You probably are not interested in that much extra work,
never the less, a single Switch will work.

Battery-Cables are REQUIRED, ( 4-Gauge ). ( Fine-Stranded Welding Cable works best ).
You will need to change to a more compact/thinner Lug on the Cables to fit the
smaller "M-4" Screws on the FET Package.
There must be total strain-relief/clamping before the Cables attach to the FET.

The Computer-supplied power to operate the original Relay needs to be specified
as to whether it is a Switched-Ground, or a Switched-Hot.
You will ultimately need a Switched-Ground to operate the FET.
If the control-power is a Switched-Hot,
a much smaller N-Channel-FET can be used to swap the polarity for the Gate of the big FET,
or, a 24-Volt Relay can be added to provide a Switched-Ground.

The Gate of the FET must be protected with a 5-Watt, 15-Volt Zener-Diode, not "just" a voltage-divider.
A 1,000-Ohm, and a 500-Ohm, 1-Watt Resistors should be used to insure
complete, very FAST, and reliable, Turn-On/Turn-Off of the Gate.
The Gate must never be less than minus~10-Volts when
it is expected to be "On", under any circumstances, ever.
If the Gate-Voltage ever gets to less than minus ~8-volts,
and stays there for more than a few seconds,
the FET will very likely Smoke, even with a Huge-Heat-Sink.
The Gate-Voltage must ALWAYS either be at
Battery-Voltage, or Battery-Voltage minus 15-Volts, never anywhere in between.
.
.
.
 

Thread Starter

Mogman

Joined Jan 20, 2022
10
Thanks, I am planning on converting the original box, it has a single lead for power in and a single lead for power to the glow plugs out (8ga), why I wanted to use one device or at least multiple devices in parallel.
The glow plugs will be manually controlled with a momentary switch, the run voltage switch would have +24V to switch to on.
I will of course fit the largest heat sink I can to the box and try to find one in copper or at least use a copper plate between the devices and heat sink.
 

LowQCab

Joined Nov 6, 2012
1,927
Copper is not necessary, actually 99.99% of the time the FET will run stone-cold.
It's that "fluke-moment" when something bizarre happened, like a loose-connection or something,
that will send the Heat to the Moon in seconds.
The Heat-Sink gives You a ~20-second buffer against instant smoke.
.
Here's how to hook them up ...........
.
.
Glow-Pugs Power  .png
 

Thread Starter

Mogman

Joined Jan 20, 2022
10
Copper is not necessary, actually 99.99% of the time the FET will run stone-cold.
It's that "fluke-moment" when something bizarre happened, like a loose-connection or something,
that will send the Heat to the Moon in seconds.
The Heat-Sink gives You a ~20-second buffer against instant smoke.
.
Here's how to hook them up ...........
.
.
View attachment 258385
AWESOME!! THANKS!!!!!
 

Thread Starter

Mogman

Joined Jan 20, 2022
10
Doesn't every diesel engine on the planet switch the glowplugs with either a manual switch or a relay?
No actually in this application many boxes use multiple FETs they usually die in spectacular fashion
The reason they used FETs is because after the initial timeout they pulse the glow plugs, they call that afterglow. nice but not really necessary actually makes life tough on the alt
 
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