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Another Fork in the Road

A project log for Magnetic Switches for RC Aircraft

Four versions of a simple on/off power switch activated by a magnet.

bud-bennettBud Bennett 02/21/2018 at 06:140 Comments

I've been thinking about integrating a Battery Eliminator Circuit (BEC) with the magnetic switch electronics for a few days. Yesterday, at the monthly RC club meeting I floated a Mag Switch BEC board after demonstrating two mag switch prototypes. There seemed to be more interest in a MagSwBEC than just a switch, so off in the weeds we go again.

I posted the feature set and specs in the project details. It makes a lot of sense to integrate the BEC with the switch. Most users just want to hook up a battery and get the proper voltage and current at the outputs to drive the receiver and servos. 

This device is a whole different beast than the previous circuits. Switch-mode regulators are generally a pain to get working correctly. I trolled Digikey and found a series of synchronous buck switchers in SOT23-6 packages made by Richtek. There are three parts that look promising: RT6254B, RT6255B and RT6257B. These parts have a VIN range from 5-18V, integrated high-side and low side FET switches, 500kHz switching frequency, and have an enable pin that can shutdown the converter with only 3µA of supply current.

I decided to target a 5A BEC, but a 3A and 6A circuit can be made with the same PCB since the 3 regulators ICs have identical pinout, and I'll need to find some inductors with similar footprints. The inductors should all have 6.5mm to 9mm nearly square outside dimensions. I settled on a range of inductance between 3.3µH and 6.8µH, with the higher values needed to keep the ripple current to a reasonable level with higher input voltages. It is difficult to find a reasonably priced inductor in this size that will handle 5A to 6A.

I found a ECS MPIL0630-5R6MC 5.6µH for about $0.70 in low quantities. It's rated for 5.5A, with Isat = 6A. I calculate ripple current should be around 1A, or 25% at VIN=16.8V. I have a PG0083.332 3.3µH in inventory that should handle the 6A switcher. The inductor for the 3A BEC should be the easiest to find, but I haven't looked for it yet.

Adding the switcher was just an extension to the basic mag switch. I have a pretty solid layout already. Here's the top side of the board:

All of the components are on the top side this time. It's easier to assemble and I wanted the bottom side to be just a large ground plane. All of the low current circuitry is located on the left side and the high current switcher circuit is on the right. I moved the pads for the B+ and B- leads far into the board to make the traces shorter. The leads will run across the bottom side of the board where there are no components. The big GND plane and 2 oz. copper should help dissipate heat.

I'm not too concerned about board size with this one. You're getting the BEC and the mag switch for not much more than the BEC alone. And more area means lower temperatures. This layout is 0.52" x 1.36" (13mm x 35mm).

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