To continue this Easy Electrons series, this time I will go a little bit into MOSFETs.

To me, MOSFETs rank high up there, next to operational amplifiers as one of the foundation components in electronic circuits which are extremely useful and practical. The most amazing detail is that MOSFETs only exist since a few decades – newbies when you consider the time scale of most electrical components.

In a way, a MOSFET is like a BJT transistor. Even the symbol for it looks similar (from Wikipedia):

This “N-channel enhanced MOSFET” type is the most common one, and like the NPN transistor, current flows into the top (D = drain) and comes out the bottom (S = source), all controlled by a third pin (G = gate).

One key difference between a BJT transistor and a MOSFET, is that a BJT is driven by current, whereas a MOSFET is driven by voltage. You feed a (small) current into a transistor to make it conduct, whereas you apply a (small) voltage potential to make a MOSFET conduct. The gate of the MOSFET doesn’t conduct current – it just senses the voltage. Bit like a capacitor.

You could say that a transistor is like a water wheel: you have to keep churning the crank to keep the water flowing through it. Whereas a MOSFET is more like a flexible tube: you pinch (well, “un-pinch”) to control the flow, but that pinch doesn’t consume energy. You could use a small mechanical clamp to maintain the pressure and keep the flow going.

This also explains why a transistor won’t conduct if the base is left unconnected (no current coming in), whereas a MOSFET could be doing anything when its gate is left unconnected, depending on how much charge was left when last connected. Early MOSFETs were in fact incredibly sensitive to static electricity – just touching the gate with a finger would often destroy a MOSFET. Nowadays, they are ESD protected.

MOSFETs are perfect for controlling large currents via a microcontroller. Even the weakest output pins can drive them, as long as the voltage is high enough. In the past, MOSFET’s needed at leat 4.5 to 5V to make them conduct, but nowadays voltages in the 2.5..3V range are sufficient in these so-called “logic-level” MOSFETs.

I’ll take the MOSFET Plug as example. It has two MOSFETs tied directly to two ATmega output pins:

Let’s look at a manufacturer’s datasheet for that “IRLZ34N” MOSFET, because there’s a lot of useful information in there.

The IRLZ34N datasheet is a great example. Seven pages, full of details, facts, graphs, circuits, pinouts, drawings, etc. It’s worth getting used to reading datasheets. They are loaded with info. To me, datasheets are the user interface of electronics. Give me a part number, and I’ll grab the datasheet to understand what it can do.

Here’s the first part:

• logic level – aha! it can probably work with 3.3V
• V_DSS – that must be the max switching voltage, 55V .. plenty!
• R_DS(on) – will come to that in a minute
• I_D – max current through the drain, a whopping 30 amps
• 175°C – looks like it can witstand scorching hot temperatures

Ok, there’s your helicopter view of this component. What I’m interested in is: will it be able to switch my <insert-some-high-power-device-here> ?

Well, we’ve seen the max voltage and current specs. But what really matters is power consumption. Because that’s the heat that gets generated, and that’s usually what breaks things.

Power is voltage x current (E x I). The voltage in this case is the voltage across the MOSFET. But we don’t know that – not directly. What we do know is its “RDS(on)” – this is the resistance between drain and source when turned on. Heh, how obvious. And exactly the value we want. It’s a mere 0.035 Ω.

Ohm’s law says V = I x R, so the voltage across the device is the current through it times its resitance.

Combine these two and we get P = E x I = (I x R) x I = I x I x R. Power consumption is proportional to the square of the current. Aha – that explains why large currents can be so destructive!

Let’s try this. Let’s go all out and push 30 amps of current through the MOSFET. Its power consumption will be 30 x 30 x 0.035 = 31.5 Watt. That’s a fair bit of heat (small lightbulb).

But will it work?

To find out, we need to do a thermal calculation. What’s going to happen to those 31.5 Watt of power? Well, they will come out as heat, but how much heat?

Time to look at another bit of info on the datasheet:

Let’s take the last value first: R(theta)JA, or Junction-to-Ambient thermal resistance = 62°C/W. In other words, each watt of power at the junction (i.e. inside the MOSFET package) will leed to a whopping 62°C temperature rise when the component is suspended in “ambient”, i.e. free, air.

Hmmm: 31.5 x 62 = nearly 2000°C. Yikes, our MOSFET is going to evaporate!

What we need to do is mount this part on a massive heat sink to make sure those temperatures are never reached, by drawing the heat away and keeping the MOSFET (relatively) cool.

Fortunately, there are two other values. The way these work, is that they tell you how much “heat resistance” there is when it flows away from the junction where all the heat is being generated. And it’s really easy to work with:

1. draw a picture of the MOSFET and how it’s mounted
2. find out the heat resistance in each step
3. add them up to get a combined °C/W value
4. re-calculate 31.5 x <that-value>
5. make sure it stays under the max temp you want stay under (175°C would be too hot for a plastic case, for example – or even for a printed circuit board)

I’ll use a quick example, just to see how far we can push our MOSFET. Let’s assume the MOSFET is mounted on a (very good) heat sink which has only 5°C/W. Then we add up: 2.2 (junction to case) + 0.5 (case to heat sink) + 5 (heat sink to air) = 8.7°C/w.

With 30A current, we get 30 x 8.7 = 261°C. Whoops, can’t be sustained without damage.

Ok, let’s aim a bit lower: 15 amps. Now the power consumption becomes: 15 x 15 x 0.035 = 7.9 Watt. Without heat sink: 7.9 x 62 = 490°C – still way too hot, but with heat sink we get 7.9 x 8.7 = 69°C.

This value is a relative value. It means 69°C above the ambient temparature. So in a 25°C room, the whole thing would become 94°C. Still very hot, but not a problem for the MOSFET!

In other words: take a MOSFET, mount it on a big heat sink, and you can see how a tiny little microcontroller could control a 15A light or a motor which has a 15A peak current. That’s what makes MOSFETS so magical…

Careful with heat sinks, though. To get it right, you really have to include all the paths to “ambient”. If you mount the heat sink in a big box (which can withstand 94°C), then the temperature inside will rise. And those 69°C we calculated will make the whole setup rise accordingly! – it doesn’t take much to get a “thermal runaway”: core heats up, ambient heats up, core heats up further, etc. Until disaster strikes. Not quite Tchernobyl, but hey… be careful.

Soooo… what started out as a MOSFET introduction, has turned into a power and heat calculation. As you can see, it’s not very complex. It’s not a bad idea to find out up front whether a power circuit will self-destruct or not. Now just add a 2x safety margin, and you should be OK. Or better still: build the circuit, and confirm that the results match these predictions, especially under stress and near design limits.

Ehm… I’ve swept a little detail under the rug:

These calculations assume that we’re driving the gate to 10V, but we’ll only be applying a feeble 3.3V or so. Whoopsy daisy. Let’s go into that tomorrow.