I'm working on a project where low cost and small BOM is just as important as optimal functionality. The goal is to drive a cheap DC motor (aka 'yellow motor'), in two directions, optionally with PWM.
We drive both a microcontroller and this motor from a regulated 3.3V source, which is able to provide more than enough current. The MCU (obviously) controls the motor direction. The motor consumes ±60mA when free running, which I think is pretty close to what it will be doing in the actual product (light load).
If I'd take the 'safe' route, I'd use the TI DRV8837 H-bridge IC and get a really small BOM and tiny footprint and be done with it. The unit price on a 3K reel for this on Digi-Key is €0,33. This is about 7% of the total product cost (± €5,-).
So IMO there is a case to look into a basic H-bridge made from transistors. For a BJT solution, I'd need the following:
- 2x NPN
- 2x PNP
- 4x back EMF Schottky diode
- 4x base R
When I choose the cheapest parts on Digi-Key for 3K reels, I get SOT-23 3904/6 transistors, some SOD-123, MINIMELF or SOT-23 Schottky diodes and a bag full of 0402 1K resistors, which would add up to only €0,16. That's less than half the price. Using GP 4148 type diodes would save me another 2 cents.
Using 2 P/N MOSFETs pairs, I could drop the base resistors, altough when I search for the optimal solution, I would ideally need gate resistors AND pull-downs. Also, there many questions on this board regarding the need for back-EMF Schottkies in parallel with the MOSFETs, when there are body-diodes in the MOSFET, too. My question is, would the body-diode inside the MOSFETs be good enough for this particular situation? Let's assume not and look for the cheapest parts again:
- 2x 2N7002 2 * €0,024
- 2x BSS84 2 * €0,038
- 4x Schottky 2 * €0,022
This adds up to €0,17 even with some resistors thrown in. So I guess using MOSFETs is just about as economically attractive as using BJTs. But again, dropping the back-EMF diodes would save a lot.
Say we would drop the (optional) requirement to PWM the drive signals, then the amount of switching would be extremely low. How much an impact would back-EMF have considering the circumstances?
Using a pair of dual Schottky common anode/cathode diodes in SOT-23-3, I can shave another 4 cents off, but adds another unique part to the BOM.