I decided that the ESC-BEC caused a problem in my 2M Elegance glider and had to do something about it. A minor crash due to the BEC getting hot and shutting off. The BEC powers a 8-channel 2.4G receiver, 4 small E-Sky digital servos and a couple of LEDs on the fuselage.

The Problem

The Elegance has the motor and a couple of ventilation intakes at the nose. The ESC is just behind the motor next to a fishing weight. The front compartment has a foam battery tray that will reduce air-flow. There are exit vents behind the wing. With the ESC attached to the fuselage air can only pass down one side.

The ESC generates heat as does the BEC. This is Dynam 25A ESC with a 2A BEC With the 4 servos moving, indicated current draw sits around 0.8A. This is average draw; peak could be 2A or more but for very short pulses and don’t add to the average power draw. As long as the regulator and capacitors can cope with the pulses it will be OK. This model has a toroidal inductor in the BEC lead and a large capacitor plugged directly into the receiver.

The real problem is that heat builds up and if there is not enough air-flow to keep the temperature within reason, the BEC which consists of two paralleled 5V 1A regulator chips will shut off to protect itself. This is a feature of the regulator chips. and cannot be changed. To make things worse, the ESC is wrapped in heatshrink plastic which insulates it, keeping the heat in. This is typical of all low cost linear ESC-BECs. Some ESCs now use switching BECs for a higher current rating and less heat. Switching BECs do still produce some heat, need some ventilation and generate interference; but that’s another story.

Power Loss calculation

With a battery voltage of 12V and a load of 0.8A at 5V the power loss in the BEC is 7*0.8 or 5.6 watts. This is far too much for these packages in free-air; there’s no way they can survive this wrapped in heatshrink.

I’m not sure how efficient the ESC is but being generous and assuming 85%, at half throttle which is about 6 Amps and 12V battery, the losses will be about 10 watts. Add 5 watts for the BEC and it’s no wonder these things shut down or go up in smoke.

The solution

Power system

To anyone else I’d recommend putting in a separate UBEC/SBEC regulator to power the radio and servos.

I didn’t have one on hand, and since I only need a reliable 1 Amp, I decided to improve the heatsinking on the existing ESC. Thin brass sheet will conduct the heat out of the ESC and BEC and into the open airflow within the fuselage. The ESC heats the brass and the air cools it. End result is a lower temperature. inside the ESC/BEC.

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I have the motor soldered directly to the ESC board and you can see the toroidal code in the BEC line on the right. The BEC regulators are the two black packages on the board. The package tabs are at 0V (gnd or black) potential so I formed the brass to span the board and solder directly to the tabs.

The other piece of brass sits on the existing ESC heatsink with some heat transfer paste between them and provides a small cooling tab outside of the heatshrink. It’s all held together by the black heatshrink on the right of the image.

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The end result

Finished

The exposed brass is bent to fit into the fuselage and sit neatly between the foam and the ESC.