Understanding MOSFET characteristics

Question

I am trying to select an n-channel MOSFET to switch a 12 V power rail. The use case here is to switch on/off a couple of LEDs which are wired in series. The LEDs (datasheet) can draw a maximum of 700 mA. I want to control the MOSFET via a microcontroller (3.3 V logic level). I am now struggling to choose a suitable MOSFET, mostly because I do not understand some of their characteristics.

What I know so far is:

• MOSFET needs an SMD package for my application
• V_cgs(th) should be smaller than my 3.3 V logic level
• FET type should be n-channel

What I don’t know:

• What does Continuous Drain Current (I_d) mean? What is important here?

• What does Drain to Source Resistance mean? What is important here?

• How important is the Maximum Power Dissipation?

For more information on the circuit, you can look at my original question here.

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An example circuit.

This MOSFET may be suitable for the use case.

Answer 1

What does Continuous Drain Current (Id) mean? What is important here?

It means continuous current (as apposed to pulses of current) between the Drain and Source. Generally it is specified in the 'maximum' or 'absolute maximum' ratings section, which means it's the absolute maximum current the FET can pass continuously (at the specified case temperature) without getting so hot that it burns out. You need to keep the actual Drain current well below this for reliability and efficiency.

What does Drain to Source Resistance mean? What is important here?

It's the ratio of voltage drop across the Drain-Source junction to current passing through it, when the FET is fully turned on. A Drain current should be specified for this because at some higher current the resistance will increase dramatically as the FET 'saturates'. Temperature should also be specified because the 'ON' resistance also increases as the FET gets hotter. Several Gate voltages may also be specified because at lower Gate voltage the FET will not turn on as hard (ie. it will have higher 'ON' resistance) and it will reach saturation at lower current.

R_DSon is the most important characteristic you need to calculate voltage drop, power loss, and temperature rise.

How important is the Maximum Power Dissipation?

Not very important, because it is not a realistic number. What is actually important is how hot the FET gets (ie. its junction temperature), which is dependent on heat sink performance and ambient temperature as well as power dissipation. In practice these numbers are poorly defined, so temperature cannot be accurately predicted and you cannot run the FET at its 'maximum' power dissipation rating without severe risk of overheating it - which makes the figure almost useless.

For reliability and safety the FET should be operated well below its maximum junction temperature, and for efficiency you want it to dissipate negligible power compared to the load. Therefore in practice the actual power dissipation generally needs to be much lower than the 'optimistic' datasheet spec.

What I know so far is:... Vgs(th) should be smaller than my 3,3V logic level

Not just smaller, much smaller. 'Threshold' voltage is the Gate voltage at which the FET just begins to turn on. It is typically specified at a Drain current of only 0.25 mA, and varies over a wide range between individual units. If the threshold voltage is 3.0 V the FET will not turn on fully with 3.3 V. The minimum voltage you should use is the one specified in the R_DSon spec.

Answer 2

These parameters are related to each other and are important because they determine how hot the MOSFET gets during operation. Excessive junction temperature will destroy the device quickly. It is important to note that these parameters are specified under a certain set of conditions, such as junction temperature, gate voltage, drain current. Datasheets have graphs describing how the parameters change as these conditions change.

A MOSFET is not an ideal switch; it has a small amount of resistance when it is on. This is the drain-source resistance parameter. Note that this is normally spec'd at 25 deg C junction temperature; its value can double at max temperatures.

The drain current is the maximum continuous current the device can conduct. Note that this value is usually spec'd at 25 deg C case temperature, which is normally hard to maintain. It should be regarded more as a figure of merit rather than a realistic maximum current for the device.

The drain current flowing through the drain-source resistance causes a power loss in the device, which is manifested by heating at the junction. This heat has to dissipate into the environment; heat flow is limited by the thermal resistance of the package, which is spec'd on the data sheet. The maximum power dissipation is the maximum amount of power that the package can dissipate without exceeding the maximum junction temperature. Again, this value for the condition that the case is maintained at a certain temperature. As in the case of the drain current, this figure is only a starting point.

So, how do you know if the chosen MOSFET is suitable, from a heating point of view? Most MOSFETS have a max junction temperature of 175 deg C, but you really don't want to be operating that high. 100 to 125 deg max is more reasonable. So, take your device, look up its drain-source resistance at 100 degrees. You know what the current is, so calculate the power dissipation: power = amps squared times resistance. Multiply this power by the thermal resistance, add this temperature rise to the ambient temperature, and see how close the junction temperature is to 100 degrees. If it's under 100, then you're good. If it's way under, you could maybe choose a smaller MOSFET. But if it's over by more than a little, you should choose a device having lower resistance, or in a package having a lower thermal resistance.

Answer 3

The two things I would look at first are drain to source resistance which is abbreviated "R_DS(on)" and the V_GS at which R_DS(on) is specified.

It is a mistake to focus too much on V_GS(th) because this is the point where the transistor starts to turn on. You want to make sure that when you turn it on, it is "on like Donkey Kong" at 3.3V. It cannot be just kind of sort of starting to turn on.

Practically speaking you are going to be looking for a transistor where R_DS(on) is very low and is specified at 3V or 2.7V.

There may be transistors out there that do not specify R_DS(on) at 2.7V but would work for you. But it is more complicated to try to figure that out so I would start off looking for something that is fully specified at the voltage you want to use.

In your case you probably want the R_DS(on) to be less than 0.2 ohms (you want the maximum less than this) to keep heat and voltage drop reasonable. That will give you 0.14 V of drop, and around 100 mW of power dissipation in the transistor. You can probably find a transistor with R_DS(on) much lower than 0.2 Ohms, and in that case the dissipation and voltage drop will be negligible, which could be a good thing for peace of mind.

Answer 4

What does Continuous Drain Current (Id) mean? What is important here?

A: This is continuous DC drain current the transistor can handle. Normally you want this about 1.5x your target current.

What does Drain to Source Resistance mean? What is important here? A: Allows you to calculate the IR drop in the transistor itself. Normally this is specified for a certain Vgs.

How important is the Maximum Power Dissipation? A: Maximum power going through the transistor that the device can handle. Again you want this to be about 2x the power going into your load.

Answer 5

A FET acts more or less as a current-limiting device between drain and source, with the magnitude of the current being set by the gate-source voltage. Graphs of current vs voltage are typically published in datasheets so it’s fairly easy to predict how a particular device will behave in-circuit. In your application you’ll want to find a device that can easily pass 700mA with a gate voltage of 3V or somewhat less. As a FET nears saturation (I.e. switched fully on) it behaves increasingly like a resistor. The exact behaviour varies somewhat with temperature and can vary from one device to another, so it’s wise to allow a good safety margin, but most unwise to rely on the device as a current-limiter - for example the datasheet may show that the FET will allow 700mA to flow when 3.3V is applied to the gate, but this should certainly not be relied upon- use a discrete resistor instead. You can, however, use the FET’s resistance value to estimate the power dissipation for the device. Without significant heat sinking you could estimate 300mW for a SOT-23, 1W for a SOT-223, 3W for a DPAK. The headline power figures in the datasheets often call for significant heat sinking.