otherwise use a heat sink with bigger surface area
The metal plate is more usually called a mounting tab. It's meant for mounting the transistor on a separate heatsink but it does act as a small heatsink.
One way of estimating the temperature in the absence of a thermometer is to grasp the tab firmly between thumb and forefinger. If it's not too hot to hold for as long as you want, then it's quite safe. Silicon devices are capable of operating safely at considerably higher temperatures. If you want to mount it on a heatsink anyway, yes, there are many heatsinks types for such transistors (technically, the physical shape and size of the IRF9540 is called a TO-220). One of the types I have in stock is shown in the composite photo below.
You can see the threaded hole in the left-hand picture and a threaded groove along the length of the heatsink in the right-hand picture. The first one is for mounting the transistor and the second is for securing the heatsink to a PCB or chassis. Sometimes I also use a couple of square inches of 1.5mm aluminium sheet.
Such heatsinks are suitable for power dissipations up to a very few watts. Higher powers need larger heatsinks.
I submitted post #7 shortly before going out for some time. Now that I've had more time to think about your problem, I see that you may need more serious cooling.
I started to type out a long post with detailed calculations and explanations, but decided that it would be better if you supplied some more information about your circuit first.
1) When you said "it is 5V and up to 10A", did you mean that you have 5V between the drain and source and 10A drain current at the same time? Or did you mean that Vdd = 5V and 10A is the maximum current when the transistor is switched into full 'on' condition with the transistor in saturation?
2) Is the 10A current constant or pulsed? If pulsed, what is the frequency and duty cycle?
If you're not sure about the answer to some of these questions, please explain what you want to do with your circuit in as much detail as possible.
Maybe i am kicking in an already closed door.
But if You Already have 15 Volt to drive the mosfet why not use a simple voltage doubler and drive a n-mosfet with opto and totempole.
This would drasticly reduce your need for a heatsink and it would improve your switching time of the mosfets (by reducing your switching losses)
Maybe reducing your losses might be a better option then adding the heatsink.
(tomorrow i'll be on the road but i will check in tomorrow night)
The heatsink I showed as an example is a no-name product that I bought over the counter, so I'm afraid it's not possible to give you a model number. I've had a look at the heatsinks offered by RS Malaysia, but there's a problem in choosing specific models to suggest because some things in your account don't quite add up correctly.
The specs for the IRF9540 give the on-state drain-source resistance as 0.15Ω. At 10A it should dissipate 10²*0.15W = 15W. Even at a 50% duty cycle, it's still 7.5W. The thermal resistance is given as 80°C/W which means that, even at 50% duty, the transistor temperature should rise by 600°C above ambient if it's left running for an appreciable length of time. Yet you said that it's not hot enough to burn your fingers. The only conclusion I can make is that you're running it at quite a low duty factor.
With these in mind, the only suggestion I can make is to use this **broken link removed** or a similar model. The 9°C/W thermal resistance is a dramatic reduction from the 80°C/W of the bare transistor and should lower the temperture considerably. An alternative is to use one of the wider heatsink models and let several transistors share it, taking care to electrically insulate each transistor from the heatsink with mica or a synthetic thermally conductive material.
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