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In choosing a MOSFETs for an Electronic Speed Controller, be aware that you need two different types.  One or more 'N' channel MOSFET is needed to power the motor (depending on the amperage of the output desired) and a 'P' channel is required for a brake.

As a general rule, to increase the amperage load to drive the motor, simply replicate the 'N' channel MOSFET circuit in parallel the number of times required.

 As background, before knowing the detail below, the neophyte Ironsides obtained an IRF5390 'N' Channel MOSFET for the motor driver and an IRF953 'P' Channel MOSFET for the brake.  These were chosen simply because they were easy to find and were for testing purposes rather than production.

The following is a compilation of the notes subsequently provided by CatsEyes and TugBoat in tutoring Ironsides. [Nicknamed the Three MOSFETeers]

See the sidebar IRF530 as a sample datasheet.

Terminology:

CatsEyes explains that, if you have never run across the terms source, gate and drain, you can equate them, more-or-less to bipolar transistors as follows: An N-Channel MOSFET is similar to an NPN Bipolar and a P-Channel MOSFET is similar to a PNP. The Drain = Collector, Source = Emitter, Gate = Base. The main difference is that the MOSFET is voltage-sensing, where the bipolar is current-sensing. To turn a bipolar transistor "on" you make some current flow from the Base to the Emitter, whereas to turn a MOSFET on, you apply a voltage to the Gate, relative to the Source.

Source:  International Rectifier

It turns out that the IRF530 is good for testing purposes, in that it is cheap and readily available.  The main problem is that the RDS(on) of 0.16 ohm is too high for a production model.   It comes in the familiar TO-220 package that has a partial heatsink (the drain flange) but can only dissipate about 2 watts without an external heatsink. 

Dissipating only 2 watts, a formula is used to calculate how much amperage that would allow.  The formula is  P=IČR [P=Power(watts), I=Amperage and R=resistance].   For the IRF530, P=2 and R=0.16    Solving this equation for the amperage, gives the current at about  3.5Amps.

So, it will overheat if you try it with a motor that draws much more than 3.5 amps. Equally, a lot of precious power is being wasted heating up the FET.

From the datasheet, we find that the RDS(on) is quoted as 0.16 ohms at 10Volts.  But, the nominal voltage powering most of our radio control PIC projects is only 4.8 volts full on.  The Pd (power Dissipation) for this FET is 88 watts.  Unfortunately,  the IRF530 FET can't dissipate the heat generated for extended periods without an external heatsink.  So, to use the IRF530 in an ESC buried inside a foam aircraft would be a bit silly.

The IRL530 would probably work fine as a low RPM glow plug driver

The Solution for the 'N' Channel Motor

What we need is a device they call "logic level," which will generally give us a rating at about 4.3 volts. We want something with an RDS(on) of at most 10mOhm (0.01 ohm) at 4.3V. 

We can look up MOSFETs on the manufacturer's web site.  For MOSFETs with product codes starting with the trigram IRF/IRL/IRN - made by International Rectifier - look at http://www.irf.com

One choice would be the IRL1404, in a TO-220 package. Its RDS(on) is 4 mOhms (i.e. 4 thousandths of an ohm) at 10V and 5.9 mOhm at 4.3V.  One device is quite sufficient for a Speed 600 application (25 or 30 amps) without any heatsink, and probably it can handle much more than that with a heatsink.

Another possibility is the IRL2203N. This is also a TO-220 format, but has an RDS(on) of 0.01 ohms at 4.8V. This gives a power dissipation of 1watt at 10A which would be fine without a heatsink. The data for the IRL2203N indicates that, while being a lower voltage device (30V as opposed to the 100 volts of the IRF530), it does have higher current capability (116A in lieu of the 8 amps of the IRF530).

The usual setup for throttle control looks like this, and uses an N-Channel MOSFET (not a P-Channel).

Diagram by CatsEyesNote that you can connect the signal source directly to the Gate, since it's voltage-sense. If you're using more than one MOSFET in parallel to increase power handling, you should use 100 ohm resistors on each Gate, apparently to prevent some sort of oscillations - this might not be needed if you're just using the one MOSFET.

G=Gate  D=Drain  S=Source
 

Source:  International Rectifier Source:  International Rectifier

Note: An IRF3706 can also be used for the 'N' Channel MOSFET, but notice that in the series of three MOSFETs the RDS(on) has crept up from 4.0 mOhms to 8.5mOhms, while the VDSS has dropped from 40Volts to 20Volts and the ID has shifted from 160 Amps to 77 Amps.

Source: International Rectifier

The Brake - 'P' Channel MOSFET


The and the IRF9530 IRF9540 are both cheap, easy to find 'P' Channel MOSFETs and are good choices for the brake FET.

Source:  International Rectifier Source:  International Rectifier

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Last modified: March 18, 2004