Will this h-bridge design actually work in practice? - Page 1
Forum section: electronics / beginners

author: jdraughn
author_rank: Regular Contributor
timestamp: April 01, 2017, 01:22:45 pm
content: I have been wanting to build some motor drivers for a few different projects ranging from dual 24v motors from electric wheel chairs to a 36v motor in a golf cart.I want the h-bridges to be capable of continuous gate drive while also being able to drive the golf cart motor which has a stall current of hundreds of amps. In an effort to really learn I wanted to build the h-bridge driver with n-channel mosfets on the high side using discrete parts with continuous gate drive. I built this based on the circuits that were posted herehttp://electronics.stackexchange.com/questions/196203/is-it-possible-to-build-an-h-bridge-with-only-n-mosfets-and-these-other-compone/196207.There is still more work to be done on it, much of it has just been playing around with the logic, and experimenting with the charge pumps frequency and capacitor values. I am also not sure what value to use for the pull-up resistors. Anyway, just wanted to know if I am on the right track with what I have so far.I designed it using falstad.com (sorry for the long link), and there are two logic inputs on the right, one changes the h-bridge direction, the other enables/disables the drivers.Thanks for your help.Short URL link:http://tinyurl.com/kkoo7jtActual 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author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2017, 02:09:18 pm
content: Seems like an awful lot of logic gates... I'd think that can be optimized down.Tim

author: jdraughn
author_rank: Regular Contributor
timestamp: April 01, 2017, 08:12:19 pm
content: Quote from: T3sl4co1l on April 01, 2017, 02:09:18 pmThanks Tim for the feedback on that logic. I'm mostly worried about how well the other parts should work, and some recommended part numbers for the transistors and other parts in the gate driver.

author: EPTech
author_rank: Regular Contributor
timestamp: April 02, 2017, 02:06:08 am
content: I agree with Tim,There is a wide selection of integrated H-bridges out there, or H bridge controllers if you want to use beefier external FETs.IGBTs are also nice but require a little bit more drive circuitry, especially when you need fast switching times.

author: jdraughn
author_rank: Regular Contributor
timestamp: April 04, 2017, 11:34:42 am
content: I don't know if the question I asked was just really dumb, or what, but I was really hoping for some more feedback on if the circuit will work very well while driving real 24v to 36v motors at high currents. I don't really care about the logic section of it, the logic is mainly just for driving the h-bridge part of the circuit so I can see it working, I planned on actually using a microcontroller to drive the 4 bridge inputs, but was thinking of using 74 series logic to ensure I can't accidentally fry something while testing and uploading code.I am trying to stay away from the boot strapped driver chips because:1. I want to LEARN how to do it with discrete parts so I can get a better understanding of whats required for driving high side n-channel mosfets with continuous drive.2. Hardly any driver chips support continuous gate drive.3. Many driver chips don't support #2 while still being capable of driving very large mosfets.Mainly it's for reason number 1, I want to learn.If it's because I didn't post a simple image and instead posted a link to that simulator, here is a screenshot of the circuit. I would like to build the circuit in Eagle and start ordering parts, but I'm not 100% sure what transistors and other parts would be suitable until I get some feedback.

author: capt bullshot
author_rank: Super Contributor
timestamp: April 04, 2017, 11:59:38 am
content: Quote from: jdraughn on April 04, 2017, 11:34:42 amFor 1.: The pull-up resistors (10k and 33k) look quite too large. As a result, the leading edge may be slow. Otherwise, the concept may work in theory, but might blow up in practice, especially when handling high currents. You'll need some prototyping and iteration.For 2 and 3: Look at the HIP4081A, add some external transistors if the driver isn't strong enough. This design will work in theory and blow up in practice (I've some experience with that chip).The critical points: layout and ringing (both depending on each other) - bad layout causes excessive ringing, ringing kills the MOSFETs and/or driver (no matter if discrete or IC). Large MOSFETs have high parasitic capacitance, together with their (and layout) parasitic inductance form LC networks that ring. Ringing increases with load current.dead time: too short dead time leads to bridge cross conduction which shorts the source, too long dead time causes the motor current commutating into the body diodes, which in turn causes even more ringing due to bad recovery of the body diodes ...At least the dead time is taken care of by many integrated drivers (like the HIP4081), your circuit doesn't

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 04, 2017, 06:38:55 pm
content: Might be worth changing the "gain" FETs (the ones connected to the logic gate outputs, which produce the full gate drive voltage swing) to BJTs, using an emitter degeneration resistor to limit current. Instead of driving directly from the logic gate, you might also want a base voltage divider.This turns it from a MOSFET, which can draw huge currents -- enough to destroy itself, if the gate node voltage doesn't pull down, as might happen during regenerative braking -- into a constant current sink, which gently tugs down on the gate node, pulling it down if it is able, but not trying to force it to 0V.To increase speed, you can replace the pull-up resistors with current sources. A similar circuit would be used: PNP BJT from +V (where +V is the gate drive supply, so, the bootstrap supply for the high side cases), with series emitter resistor, and base voltage divider. (Improvement: instead of a base voltage divider, use two diodes in series to set the base voltage to 2*Vf.)I don't suggest attacking a full current circuit. Gain experience with protection mechanisms first. You must have current sensing, and current mode control, otherwise your bridge will randomly explode itself from a bad load condition (say, a wire comes loose and momentarily shorts the motor terminals).You will quickly find that, once you've implemented these features, your motor controls more smoothly too, and you can add additional controls to regulate voltage or speed, or control the rate of change (acceleration and jerk).Needless to say, the main battery must be fused, too!It is not practical to design a single stage for more than about 50A. To push more, you need to view the circuit as many individual inverter circuits in parallel, including bypass and snubbing componentson each unit. If you don't know where these things should go, make that 20A, or even 10A, instead.Tim

author: rmacintosh
author_rank: Regular Contributor
timestamp: April 04, 2017, 07:11:09 pm
content: I know its a lowly "instructable" but I came across this a while ago and saved it.http://www.instructables.com/id/Designing-a-Dual-40A-PWM-Speed-Controller-for-Brus/?ALLSTEPSBasically, someone documented a full design cycle for dual 40A h-bridge/controller. Might be able to glean some inspiration out of it.

author: jdraughn
author_rank: Regular Contributor
timestamp: April 05, 2017, 11:56:58 am
content: Thanks for the advice guys, really appreciate it. I will ensure I have dead time when driving it with my microcontroller. I will reduce the value of the pull-up resistors and look into using current sources instead, but for this first version, I will probably just stick with those resistors for now to keep the board design as minimal as possible while still providing "good enough" performance.I will probably use a high side current sensor for sensing current - I plan on adding safety features in, but I wanted to make sure that I was adding them to something that would actually work first, and that falstad simulator doesn't support everything that I would have liked to have added. It already pegs one of the CPU's at 100% in quad core processor.Now I will start creating a schematic with actual part numbers so I can start worrying about the PCB design.I will look into getting a couple of those HIP motor drivers when I order parts for this board for experimenting with later.Can anyone recommend a pair of BJT's for the mosfet drivers? What specifications should I focus on when searching for the transistors? I would also like some recommendations for a voltage regulator that can take a high voltage DC input and output 5v. I would prefer all SMD parts to try and keep the board size down.Thanks again for all your tips and advice guys.

author: capt bullshot
author_rank: Super Contributor
timestamp: April 05, 2017, 12:10:36 pm
content: The Zetex (now Diodes) ZDT6790 transistor would be useful as the gate driver booster, if you need stronger ones, look at the MJD44H11 / MJD45H11. Other may be suitable too.Edit: these may not have high enough VCBO voltage ratings for your high side circuit. I'd recommend to decouple the driver supply voltage from the charge pump output voltage in a way that the collector of the NPN is limited to ~12V...15V above the MOSFET Source potential (as it is for the low leg of the bridge). Thats the usual way bootstrapped drivers work. You need to supply the higher voltage from the charge pump only when the high side MOSFET is on to keep it on for a prolonged time. You'll need some capacitance to buffer the driver supplies either way, so I'd build a combined bootstrap and higher impedance keep alive charge pump supply for the high side driver. You might find some inspiration in the HIP4081A datasheet.The Linear Technology LTC3630 is a 65V input buck regulator, suitable for 5V / some 100mA output. They also have regulators with higher input voltages available.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 05, 2017, 04:28:15 pm
content: For the emitter followers, 2N4401/3 are fine. If you need to go super fast (which you won't with this circuit, but with some improvements, you could get into the 10ns range, and would need more current), then something in the PBSSxxx or ZTXxxx range (or a few Japanese types) would do well. These are low Vce(sat) transistors, and have excellent hFE even into the high level injection region (i.e., were hFE falls off, at high current, Vce > Vce(sat)).Tim