Small EV Speed Controller - Page 1
Forum section: electronics / projects

author: unit998x
author_rank: Contributor
timestamp: September 06, 2014, 06:12:55 pm
content: Hi again,I have designed and built a buck mode speed controller for use in a very small EV, it runs at 24V with a 240w Motor. The stall current of the motor is 130A, and there is a MCB of 70A fitted, so the rating of the controller is about 130A ~10 seconds, 50A ~ 2 mins, 15A ~ Continous. However those figures are just estimates. The current built version is just a prototype atm, but appears to work so far but has not had any extensive testing.The uC runs an 18kHz PWM to the THROTTLE_PWM net, at a duty cycle depending on the voltage from the THROT_IN net.Some errors were made on the PCB layout (footprint sizes not matching component, argh!), and whilst fixing them I would be interested if there were any improvements that people think could be made to the design. Please remember that this was designed and budgeted as a one-off hobbyist/educational design, so synchronous rectifying might be out of our ballpark.Attached is a (sorry, bad) picture and the schematic, If you need any more info, just ask!So go ahead, and critique!Thanks.

author: Pedram
author_rank: Regular Contributor
timestamp: September 06, 2014, 07:16:26 pm
content: LM2575 should smoke

author: nuno
author_rank: Frequent Contributor
timestamp: September 07, 2014, 12:16:00 am
content: Nice... A few years ago I started developing an h-bridge controller for a similar motor mostly as a learning exercise, for friend of mine's project. Hw & sw available here for free:https://github.com/vnevoa/DiffTrike/tree/MarkIV_RasPi_NJAY/Electronics/PowerBridge.130A seems way overkill for a 240W motor on 24V. For a typical 250W 24V chinese standup scooter DC motor 1 FET should be enough. The diodes will heat up a lot... switch over to synchronous rectificaton when you can, the difference is *huge*. I designed mine to use assynch, but ended up using synch (at great sw cost, since the hw wasn't designed for that; the PWM is now done in sw at a low freq, of ~7.5KHz, but still room to and planning to bring it up to 15KHz).I did FET drivers in discreet to, similar to yours. Make sure your driver is OFF when not activelly driven by the microcontroller. Try a slow power supply voltage raise/fall with an LED at the driver's output or similar.I don't have the extra (Q7) bipolar in my drivers. It takes quite some time for the bipolars to leave saturation; we're talking several us, 7-8us in my case if I remember correctly. That was reduced to basically zero with addition of anti-saturation schottky diodes (2us with PN diodes). At least when doing synch rectification it is important, since minimizing this latency also minimizes dead time, leading to less power dissipation.I desgined the drivers to take the 24V supply to avoid 2 voltage regulators.My favorite tactic to prevent power stage caused control module voltage supply dips is to use a diode in series with the supply (replacing of L3). Do you know the "double pulse test" method? Easy way to check the voltage transients on the power stage. See attached scope pic of bottom FET switching behaviour in my bridge.In my tests I have not seen any need for the snubber, but I have not yet checked carefully measured in all relevant nodes. Low inductance layout is critical for these power stages (don't look at mine as a reference, yet). By ignorance I did a long trace from GND to the bottom FET's source, but it actually ended up saving my day my preventing the FET from switching too fast (and generating transients too high). But it still switches in some 300ns; driving just 1 FET from a FET driver similar to yours but with 100 Ohm gate resistor.A 100nF on the PIC's reset line to GND helps to ensure it stays stable.I do my tests with a constantly locked rotor.You don't seem to have much room for a heatsink, but can't see clearly.Styrofoam is notorious in creating static electricity...

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 08, 2014, 08:09:35 am
content: Hi I'd use something like a FAN73832 or similar so you can synch switch a top NMOS or three. It'd be cheaper than all those parts in your "MOSFET Driver"PS, can't wait to read page X of the manual