H-Bridge Driver Circuit Critique Request - Page 1
Forum section: electronics / projects

author: Pack34
author_rank: Frequent Contributor
timestamp: January 20, 2014, 11:14:32 pm
content: I'm working on an NFET H-Bridge motor driver circuit.Chosen MOSFETS:Part Number: IRLZ34NSTRLPBFCTLink:http://www.digikey.com/product-detail/en/IRLZ34NSTRLPBF/IRLZ34NSTRLPBFCT-ND/2441092Why? I'm running the bridge over 5V PWM logic so I need FETS that can properly switch at that voltage level. Also, keeping it lower at 2V should take care of any issues with the charge pumps getting up high enough. The package is larger and beefier than needed but that's intended. The larger FET will make heat sinking easier. The low Rds(on) and input capacitanc should also keep the temperature down.Chosen Driver:Part Number: IR2302Link:http://www.digikey.com/product-detail/en/IR2302SPBF/IR2302SPBF-ND/812495Why? The driver only requires 5V which will eliminate a required additional voltage level. The driver also takes care of some dead-timing for me.Schematic (see attachment)The schematic is simple. I have two of the driver chips that will actuate easch side of the bridge independently. The protection diode for the boost is a standard 1N4001 diode and for the charge pumps I'm using some beefy 10uF ceramics to be sure that the high side gets to where it needs to go.The input signal for the bridge is generated by an enhanced PWM signal from a micro-controller that takes care of dead-timing and signal inverting. The PWM frequency is set to 20kHz.My main question, is if anyone can see something I'm missing with this design? Is there a fundamental characteristic that I'm missing? How's my component selection?

author: Pack34
author_rank: Frequent Contributor
timestamp: January 20, 2014, 11:41:26 pm
content: I was originally using this circuit but the high side signal just wasn't as clean as I would like it to be. Using the drivers in the previous post solved that.

author: zapta
author_rank: Super Contributor
timestamp: January 21, 2014, 08:26:14 am
content: I have hard time to read the fine labels. Are you using N chan on the high side? If so, how do you drive it? The gate need to be high enough above the source. Have you considered to have P chan on the high side?

author: fed0r
author_rank: Newbie
timestamp: January 21, 2014, 12:45:20 pm
content: Quote from: zapta on January 21, 2014, 08:26:14 amThat's the reason he uses a high side driver in the first place, to be able to use fast and powerful N-channel fetsAs for the circuit, isn't 100 ohm for gate resistor a bit too much for a 5v supply? I'm not used to low voltage stuff yet, but 100 ohm is about right for charging at 15v. And, personally, i think 10uf is a total overkill, but who knows. Maybe this appnote will clear this out.http://www.irf.com/technical-info/designtp/dt98-2.pdfAnd also, AN-6076 seems to be relevant.I probably would stick a protection diode across every fet, since motors can indeed cause large voltage spikes which aren't good for fets.Anyway, the gate waveform should tell whether it's done right, and we still need experts in this thread.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 21, 2014, 01:31:38 pm
content: Quote from: zapta on January 21, 2014, 08:26:14 amIn the updated schematic in my first post it's being handled by a half bridge driver. In the second I'm achieving the VBATT + 5V using the charge pump capactior attached to the motor channel and a diode protecting the 5V rail.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 21, 2014, 02:15:23 pm
content: Quote from: fed0r on January 21, 2014, 12:45:20 pmI can probably drop the resistor value. For the charge pump, I've tried 0.1u and 1.0u caps and it wasn't up until I went to 10u that the waveforms started to look right. Thank you for the app-notes. I'm giving them a read now.Both schematics work and I can control a motor with them. I just can't shake the feeling that I'm missing something with them that could affect it under higher loads. I'm a bit of a novice with these and I appreciate input.

author: granz
author_rank: Regular Contributor
timestamp: January 21, 2014, 06:32:23 pm
content: What are the specs on the motors you are driving?Also, what is your battery voltage?Schematically, it looks like what you have is good, but you may need to play with some values.For best performance, you'll want your gate drive voltage (GS) up in the 10-15V range, not lower at 2V (if I understand your intentions correctly). The gate threshold voltage is really just when the FET *begins* to conduct, but for this type of application you want a much higher voltage. Take a look at figure 3 in the FET datasheet: at 2V VGS the FET is just barely on.Also, Increasing your switching frequency to something like 40kHz will reduce the requirement for the bootstrap cap. Of course, higher switching frequency means more switching losses, so it's a trade off.Hope that helps.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 21, 2014, 07:54:09 pm
content: I'm using a single 24V motor with an expected output current of 6-12A.For the FET thresholds, is there a rule of thumb for the recommended delta? With these drivers (and in the circuit on my second post) I'm achieveing the VBAT + 5V which elevates me to twice the threshold voltage. I'd like to try to eliminate the need for a third voltage level, but if I'm forced into it to improve the reliability, then I am.For deadbanding, is there a range that's just too much? I'm implementing it in the PWM signal before the bridge driver rather significantly and then the drivers implement their own. This provides me with everything shutting off and then the lower FET opens and then the appropriate high side FET.EDIT:// My main issue right now is that they seem to work the majority of the time and then sporadically the bridge fails. I can drive the thing all day and then under the same conditions it'll blow. There's no way that I'm pushing the FETs past their limit. The maximum voltage is 55V and the maximum current is 30A, so I should be opperating at 50% max. Which should make the weakest point the motor and not the motor controller. This is making me look at how I'm driving the bridge itself.The few key points that I'm thinking are the issue are dead-timing and the circuit driving it. For dead-timing, the microcontroller is taking care of it and the two PWM channels will never be high at the same time. My only concern with this is if there could be an issue driving the bridge low for too long.I had thought for sure that using these newer drivers would have solved all of my issues. The waveforms on the scope look fantastic. Right now I'm thinking the cause of the failure would be related to the physical layout of the board. I'm moving from my schematic to the FET driver on the same board which means that I have the drivers on breakout boards that are wired into the motor controller. Being that the wires are rather long and I'm operating at 20kHz I'm thinking that this could be an issue. However, I don't want to spin another board with the hunch that I just need to shorten up all of my traces.I was told that when it blew, the FET driver itself got really hot. My exact setup is that I have the driver on a SOIC8 breakout with a 10uF ceramic soldered to the floating supply voltage (Vs) and floating absolute supply voltage (VB). I also have a 0.1uF ceramic wired across Vcc a ground. Then I have the appropriate pins wired to their destinations with one driver attached to one side of the bridge and the other to other side.EDIT2:// Ah, one thing I noticed was that the side of the bridge that failed was actually missing the gate resistor on the high side FET. This is the one that ended up blowing. I had used the footprint for the signal FET on the working side to hack in the gate resistor. I think I ended up flicking off the resistor on this side when troubleshooting why the drivers were not functioning properly. That issue was caused by insufficient capacitance on the bootstrap capacitor. This could very well be a cause of this board's failure.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2014, 11:28:48 pm
content: I never did like "logic level" FETs. Yeah, they turn on at 5V, but not very hard, and switching takes forever (you need a solid driver to compensate, and you still only get so much due to internal limits). That may or may not be a problem.The only thing that jumps out at me is the lack of protective circuitry, and what your physical layout is. Ideally, you'd control the motor by instantaneous current first and foremost, then adjust for average PWM or output voltage based on that. That way, if the motor suddenly decides to be a short circuit or some approximation thereof, your FETs aren't left alone to chomp on it.Protective circuitry includes something to clamp voltage spikes. An H-bridge with MOSFETs is naturally constrained by the supply voltage (if the motor tries to drive power back into your circuit, the worst it can do is get dumped into the supply by the MOSFET body diodes). But if the supply voltage isn't stable (say, there's a lot of stray inductance in the layout, or the bypass capacitor and wiring have high ESR and ESL), nasty awful things can happen.What frequency are you running this at? Are you at liberty to share your layout, and do you have any waveforms of this thing in operation, at a known load (not just like spinning the motor, but actually delivering real power)? How hot do the transistors get normally, and what's happening when they fail, has it been running under heavy load for a while so they were getting really hot? Or what?Probably, you're at risk of two failure mechanisms: either just dumb thermal overload (the transistors get too hot, magic smoke is released), or avalanche failure (switching too much current, too suddenly, with too loose of a layout), where voltage spikes cause local failure inside a transistor, leading to further destruction.Tim

author: Pack34
author_rank: Frequent Contributor
timestamp: January 22, 2014, 12:17:44 am
content: Quote from: T3sl4co1l on January 21, 2014, 11:28:48 pmTim,Thank you for your reply. I feel comfortable posting a cropped image of the layouts that I'm working with. Layout_a is the previous version that I'm hacking to work with the new driver circuit. Layout_b is what I'm moving to but haven't had produced yet. I'm currently using Layout_a but have the drivers connected in using some SOIC8 breakout boards. The general layout is that the FETs are on the bottom and located as close to the motor and power connector as I could. The lone pad near the buck circuit on the top and the square pad at the bottom is the beefy battery filter cap that I'm using (330uF). You should be able to follow the circuit using the schematics found at the top of the thread.The PWM feeding the drivers is at 20kHz. I can try to grab some waveforms before I head home. Under standard load it gets warm but not scorching hot. I did have some issues with the motor really heating up when it's run for periods of time but that was resolved when adding an inductor in series with the motor. I wasn't around when the bridge failed but I was told that the drivers were exceptionally hot. This had occurred after extended use.For protection circuity on the bridge, do you have any suggestions or reference designs I can look at?

author: Pack34
author_rank: Frequent Contributor
timestamp: January 22, 2014, 02:02:09 am
content: Here's some waveforms of the board with no load. I disabled the main power going to the board and wired in logic power in order to get the waveforms. I wanted to be able to look at what's going on before applying main power. If there was a glitch somewhere then it would have immediately blown.Bridge Waveforms-----scope_2: Motor Forwardscope_3: Motor BackwardYellow is low side channel BGreen is high side channel BRed is low side channel ABlue is high side Channel APwm Input waveforms.-----scope_4: Motor Forwardscope_5: Motor BackwardGreen should be channel Ayellow should be channel BTiming measurement----scope_7:For deadbanding, I measured 3.4us

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2014, 02:26:12 am
content: This is all with zero volts 'main battery', right? So in scope_2 and scope_3, the high side gate drives are *just* the gate drive?Are the green and yellow both supposed to be overlapping positive like that? The red and blue appear to be pleasantly apart.Or... is it actually, Ch2 = HS A, Ch3 = LS A, Ch1 = HS B, Ch4 = LS B..?It shouldn't be possible to convince an IR2302 to produce the waveform pairs green-yellow, and red-blue, respectively. (If that's your problem, it's certainly an interesting one!)Also, on closer inspection, I see "Filtering capacitors between motar channels" 2 x 0.1uF, is that really between the output pins?? That's death right there. You need a series choke first, then some cap to take the RFI out (if it matters).Tim

author: kxenos
author_rank: Frequent Contributor
timestamp: January 22, 2014, 02:59:35 am
content: I think I would add 4 fast diodes to protect the mosfets as tesla coil suggested

author: Pack34
author_rank: Frequent Contributor
timestamp: January 22, 2014, 03:00:24 am
content: Quote from: T3sl4co1l on January 22, 2014, 02:26:12 amYes, in this test setup the VBATT for the circuit is 0 volts. I wouldn't put it out of the possibility that I mixed up some of the channels in the waveforms. I was at the office 14 hours at that point. I know green and yellow are low side (channels 1 and 2 of the scope) and red and blue are the high side gates. The ending waveforms with just the two channels are the inputs to the feta drivers.As for the caps across the motor channels, the choke is in the motor line. I am working with a much higher quality motor so I can flick those off. On cheaper Chinese motors noise can mean death for a lot of my electronics. A small cap across the motor terminals and ferrite beading next to the motor tends to help significantly.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 22, 2014, 03:08:48 am
content: Quote from: kxenos on January 22, 2014, 02:59:35 amI completely overlooked that. I'll see if I have any in stock that I can slap in there in the morning. I wish I had read those app notes earlier.

author: granz
author_rank: Regular Contributor
timestamp: January 22, 2014, 05:10:18 am
content: Quote from: Pack34 on January 22, 2014, 03:08:48 amYou actually don't need protection diodes when using MOSFETs because of the inherent body-diodes. Take a look at the datasheet and you'll find that the Source-to-Drain current rating is also 30A via the body diode.A quick test for your motor: Connect it directly to 24V and check the current when the motor is stalled (hold it from moving). I've found that sometimes the current ratings are very optimistic. Alternatively just measure the DC resistance. That way you'll know if it's possible with close to 100% duty cycle to be exceeding the ratings of your FETs.A few scope measurements across a small in series resistor (use differential probes, of course) would also probably prove useful.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 22, 2014, 01:11:04 pm
content: Here's the properly lined up waveforms.Motor Channel A:High Side: RedLow Side: GreenMotor Channel B:High Side: BlueLow Side: YellowThe first pair (scope_8 and scope_9) show each side of the bridge lined up. The second pair shows the low side of one channel lined up with the high side of the other channel.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2014, 01:42:42 pm
content: Quote from: Pack34 on January 22, 2014, 03:00:24 amYikes, hope you slept well!QuoteI can't quite make out the net names very well, but if those caps are across the transistors, then each time they switch, they burn 0.5 * 0.2uF * Vbatt^2 microjoules (multiply by frequency to get power loss), probably with a peak current in the range of 10-20A. I wouldn't think they would even last long, since the dielectric loss in an X7R type for example might desolder itself. Maybe not such a big deal at 20kHz after all, but I don't see an inductor indicated, so it doesn't look right. The inductor has to come first, then caps. Then more caps at the motor if it's nasty.Gate waveforms look fine then, now how about... like 5V current limited supply, and no load? Then with known load?Tim

author: Pack34
author_rank: Frequent Contributor
timestamp: January 22, 2014, 05:38:44 pm
content: Here's the same setup with a 5V power supply.

author: Odysseus
author_rank: Regular Contributor
timestamp: January 22, 2014, 05:56:44 pm
content: Why is channel A not the exact complement of channel B? I understand and can see the small amount of dead-time in each channel.The sequence should be like this:1. Channel A high-side and Channel B low side turn-on simultaneously.2. Wait first half of switching period.3. Channel A high-side and Channel B low-side turn-off simultaneously.4. Wait deadtime (all mosfets off)5. Channel A low-side and Channel B high-side turn on simultaneously.6.Wait second half of switching period.7. Channel A high-side and Channel B low-side turn-off simultaneously.8. Wait deadtime (all mosfets off)9. Repeat.This would be achieved by making the logic signal to the drivers just the complement of each other.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2014, 08:54:15 pm
content: Nothing suspicious there, bootstrap seems to be working. Not obviously ringing or exploding or anything, which is reasonable with deadtime and no load.Now, with a load? An inductive one preferably, say 10mH + resistance equivalent to full rated load.Tim

author: granz
author_rank: Regular Contributor
timestamp: January 22, 2014, 08:57:10 pm
content: I looked back at the thread and didn't see it anywhere: I assume you are driving a regular DC (brushed) motor, correct? After seeing your waveforms--why are you switching the direction of the current through the motor on a cycle-by-cycle basis? Normally you turn on one or the other high-side FET and then modulate the opposite low-side FET. Your gate drive waveforms look like how I would drive an AC motor with a variable frequency drive by changing the duty-cycle with time to produce a lower frequency sinusoidal current through the motor.(See attached from Microchip App Note).The outputs A/B/D/C from the Enhanced PWM module on PICs (shown in their datasheets) is essentially exactly what you want if you are driving a brushed DC motor, but the driver IC you have will generate complementary outputs to each side of the bridge (different).

author: Pack34
author_rank: Frequent Contributor
timestamp: January 23, 2014, 01:50:28 pm
content: Quote from: granz on January 22, 2014, 08:57:10 pmYes, I'm using a DC brushed motor. I'm using halfbridge mode on the ECCP module to drive a full bridge circuit. Essentially what is seen on this microcontroller's datasheet. PDF page 147, document page 145http://ww1.microchip.com/downloads/en/DeviceDoc/39599g.pdfIs there a detriment to this? I had originally planned to use just the two in order to minimize the amount of pins use on the PIC. I'm guessing this is why most of the bridge drivers I've seen have individual high side and low side inputs.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 23, 2014, 03:53:09 pm
content: waveforms running at voltage with no load. The bridge heats up slightly to 89 degrees F

author: Pack34
author_rank: Frequent Contributor
timestamp: January 23, 2014, 05:01:29 pm
content: Some updated waveforms under load. The bridge heats up to about 102F and the motor eventually heats up to 120F when run for long periods of time.I do seem to be seeing some glitches when moving in one direction but I'm not entirely certain if it's some sort of ghost on the scope or if it's a cause of concern.

author: granz
author_rank: Regular Contributor
timestamp: January 23, 2014, 05:43:37 pm
content: Yes, that is your problem. Although it may be physically rotating, you are actually trying to drive current through the brushed DC motor in one direction for some amount of time and then the opposite direction for the remainder of the period. This is why everything is heating up so much. As far as the gates of the FETs are concerned, they should be seeing this:Forward:A high-side: ONA low-side: OFFB high-side: OFFB low-side: PWM signalBackward:A high-side: OFFA low-side: PWM signalB high-side: ONB low-side: OFFYou then simply change the duty-cycle of the single PWM signal to control the motor speed. The brushes in a DC motor are there to very much simplify the drive circuitry.The way you currently have it is what you would do if you were trying to drive an AC motor without brushes (with the addition of a varying PWM duty-cycle). Unfortunately the IRF2302 isn't intended for what you are doing, it's meant to switch the high and low-side pair complementarily (as you've seen) for driving something like a transformer in a power supply. Considering the integrated charge-pump for the high-side gate you wouldn't be able to keep the high-side on anyhow if it even gave separate H/L inputs.You could go to P-Ch FETs for the high-sides or keep the N-Ch FETs and use a separate charge-pump circuit for the high-side gate drive voltages. Either way, the IRF2302 isn't the driver you want for what you are doing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 23, 2014, 07:51:44 pm
content: That faint ringing signal is very strange. I don't suppose you can convince the scope to trigger on those events alone?Tim

author: Pack34
author_rank: Frequent Contributor
timestamp: January 23, 2014, 08:09:19 pm
content: Quote from: T3sl4co1l on January 23, 2014, 07:51:44 pmI've been trying. I tried a range of trigger levels and there's probably a way of setting the trigger on this Agilent scope but I'm not sure how. I also attempted a bunch of single captures to no luck.I'm going to try Granz' suggestion with moving to full H-Bridge control instead of half and then specing out some new drivers.

author: Pack34
author_rank: Frequent Contributor
timestamp: January 27, 2014, 08:47:06 pm
content: Here's an update on the components.New Half-Bridge driver with VDD set to 12Vhttp://www.ti.com/lit/ds/symlink/lm5109b.pdfFor the FETs, the previous ones I was using,http://www.irf.com/product-info/datasheets/data/irlz34nspbf.pdf,had a threshold voltage of 2V. Since the Drive will be 12V, I'll need to pick a similar one with a higher threshold.New FEThttp://www.irf.com/product-info/datasheets/data/irfs3806pbf.pdfVgs(th): 4VRds(on) 12.6m-Ohm (lower than the existing ones)VDSS(max): 60V (more than sufficient)Id(max): 43A (more than sufficient)Input gate capacitance: 1150pF (previous was 880pF)I think the new FETs should work well, but I'm concerned with the 50% larger gate capacitance.Thoughts?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2014, 11:51:56 pm
content: Capacitance is meaningless (see Fig.5).IRLZ34NS: Qg tot. max. 25nC at 5V. The typical graph (Fig.6) shows about 15nC at this voltage, and about 30nC at 10V, so figure double or 50nC max.IRFS3806: Qg tot. max. 30nC at 10V. A clear winner!Those old parts, like anything IRFnnn, IRFZnn or IRFPnnn, are really bad in modern terms. The only thing they have going for them is universal availability and low cost.Modern devices have about 1/3 the Qg/Rds(on) ratio. I forget what improvements lower voltage (<100V) devices have made, but the most important for high voltage devices has been superjunction technology. Older VDMOS scales as Rds(on) ~~ Vds^2, while SJ scales as Rds(on) ~~ Vds -- high voltage transistors used to be embarassingly bad, but they are quite good now, even above 600V.Tim

author: granz
author_rank: Regular Contributor
timestamp: January 28, 2014, 09:27:26 pm
content: Quote from: Pack34 on January 27, 2014, 08:47:06 pmUnfortunately, you won't be able to drive the high-side FETs in the configuration I explained before with this type of driver. Consider how the bootstrap capacitor and the diode work: they are a charge pump which requires the complementary switching of the high and the low-side FETs in the half-bridge (like you were previously doing). For driving your brushed DC motor in the way I suggested you need to keep one high-side FET on for the *entire time* you are spinning the motor forward or backward and use PWM for the opposite H-bridge leg low-side FET. You won't be able to keep the high-side gate drive up without the high-side also switching. The only way to do this with N-Ch high-side FETs is to use a separate power rail. The simpler solution is to just use P-Ch FETs on the high-side with a very simple discrete driver and individual FET drivers on the low-side.When the datasheet for the LM5109B lists "Solid State Motor Drives" as an application it is referring to (brushless) AC motor control (either single-phase or three-phase) as I described previously.A simple driver from Microchip is MCP1402 (since you are already using some Microchip parts). You could use this to drive the low-side FETs, and then use a simple discrete driver for the high-side FETs (if you go with P-Ch FETs for the high-side). Since you won't be switching the high-side FETs effectively at-all (only when changing the wheel direction) there is no need for a fancy driver to charge and discharge the gate capacitance.I believe T3sl4co1l has already discussed the FET choice a bit. Assuming cost isn't a concern, a good rule of thumb is that the date on the datasheet is a good indicator of proformance. For the best performance you'll want to choose something from within the last few years, not > 5 years old.Hope that helps.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2014, 01:16:24 am
content: BTW, use phase shift PWM for controlling output, rather than latching one (high or low side) fixed and togging the other. The latter causes uneven dissipation, reducing the overall capacity of the inverter. PSPWM is exactly the same thing except which one "stays on" rotates through the cycle (it's easy to see if you draw out the waveforms), keeping dissipation even between active devices.This also keeps all charge pumps active, even when driving the motor with 0V.This can also be designed for continuous current operation, in which case regenerative braking occurs (which may or may not be useful).Tim

author: Kremmen
author_rank: Super Contributor
timestamp: January 29, 2014, 06:53:41 am
content: Quote from: granz on January 28, 2014, 09:27:26 pmWhile that is true, you can always turn the low side fet on permanently and pwm the high side fet thus avoiding this particular issue.The motor circulating currents will flow differently but normally that should not matter.

author: granz
author_rank: Regular Contributor
timestamp: January 29, 2014, 06:34:13 pm
content: Quote from: Kremmen on January 29, 2014, 06:53:41 amNo, it can't be done that way. The high-side and low-side FETs being used at any given time are on opposite sides of the bridge. You would be charging the boot cap through the motor in that case. The charge pump relies on the complementary switching of the particular bridge leg.I guess the OP has two options essentially. My suggestion or his original technique. I suggested the simpler approach considering the requirements of the motor and the PIC microcontroller he is using, which is already set up to drive a DC motor in this way.

author: Kremmen
author_rank: Super Contributor
timestamp: January 29, 2014, 07:56:05 pm
content: Quote from: granz on January 29, 2014, 06:34:13 pmTo be sure, but the charge pump does not care. A half bridge would not even have an opposite leg so nothing there to rely on. Can you clarify your point why this would be a problem?

author: granz
author_rank: Regular Contributor
timestamp: January 29, 2014, 08:24:24 pm
content: Quote from: Kremmen on January 29, 2014, 07:56:05 pmThe charge pump does care, because if the lower side of the cap is connected through the motor that side is not at ground potential, it is up at whatever the voltage across the motor is. The result will be that the cap is not charged to the intended voltage. I think maybe it was confusing when I said "The charge pump relies on the complementary switching of the particular bridge leg". What I meant was it relies on the complementary switching of a particular "half-bridge".

author: granz
author_rank: Regular Contributor
timestamp: January 29, 2014, 08:34:01 pm
content: Attached is a useful App note from TI about different decay modes when using H-bridges. This is specifically in regards to stepper motors, but it's the same here. The original design posted was essentially a "fast-decay" mode, whereas what I suggested is a slow-decay mode. Also attached is a datasheet from a commercial 30A brushed DC motor driver. Note that it uses slow-decay mode only, and mentions the issues with a fast decay technique. The vast majority of BDC motor drivers use the slow-decay mode, hence why the PIC is setup for this, and why I recommended it.

author: granz
author_rank: Regular Contributor
timestamp: January 29, 2014, 08:50:54 pm
content: I discovered this driver from Linear Tech. which may be useful to you. This driver incorporates a boost converter for the high-side gate voltage, which they claim allows DC operation for the associated half-bridge:http://www.linear.com/product/LT1336Using this driver, N-ch FETs could be used all around without the charge pump issues we've been discussing. I've never used this driver, but if it were my project I would consider it. Use two of these, whatever N-ch FETs you like, and drive it all from your PIC using the four A/B/C/D outputs exactly like shown in the datasheet.

author: Kremmen
author_rank: Super Contributor
timestamp: January 29, 2014, 09:03:19 pm
content: Quote from: granz on January 29, 2014, 08:24:24 pmWell, it is not quite that simple. Assuming high side switching, when the low switch is on and the high switch turns off the motor current cannot instantaneously collapse. The body diode of the off side low switch turns on and provides a current path for the motor current to recirculate. Since the sources of the low switches are tied to ground or very nearly to it, and there can be only the forward voltage drop of the off side low switch body diode between ground potential and the motor input, the back emf will not be maintained. So the charge pump cap will recharge.

author: granz
author_rank: Regular Contributor
timestamp: January 29, 2014, 10:25:08 pm
content: Quote from: Kremmen on January 29, 2014, 09:03:19 pmAh, I see what you are saying about the body diode. I wasn't considering that. It seems like it will work then. In that case, it seems like the OP could use your suggestion of modulating the high-sides while keeping the opposite low-side on and still maintain the charge pump action.Although, I've never seen it done this way. Have you seen (or personally implemented) DC motor control with an h-bridge this way (modulating high-side only)? Then again, I don't have a huge amount of experience with motor control.

author: Pack34
author_rank: Frequent Contributor
timestamp: February 03, 2014, 03:51:51 am
content: Quote from: granz on January 28, 2014, 09:27:26 pmI'm finally able to have some time to return to this.From my understanding, the protection diode and capacitor in the bootstrap circuit is used to "pump" the drain side of the high side FET to V_BATT + VCC in order to bring the Drain-Source voltage of the high side FET to be positive so the gate can be triggered by our driver. In the situation of using a full H-Bridge control (high side held on, while low-side throttles), would the bootstrap capacitor be too weak to be able to hold it high? What would be the technical reason for why I wouldn't be able to hold the high side FET on in this situation?I'm familiar with the MCP140x line, I was originally using it to throttle the entire bridge in an earlier design.For the FETs themselves, I had hoped to keep the D2-PAK package due to the size, it would make heat sinking easier since I could mount the board to the chassis and using standard sized standoffs would allow the FETs to make good thermal contact. Unfortunately, when looking at the date stamps on these guys the package seems to have fallen out of favor years ago and they all seem to be about five years old.Moving to a different package I was able to find this guy, which seems to fit my bill nicely.Infineon's BSC059N04LShttp://www.digikey.com/product-detail/en/BSC059N04LS%20G/BSC059N04LS%20GCT-ND/3819658Vds: 40VId: 73ARds(on): 5.9 mOhmGate Charge: 40nCVgs(th) 2V @ 23uANow, I can easily go back to using the MCP140x low-side FET driver, but to drive the high side, could I re-purpose the original schematic in my second post but decouple the signal going to the high side and low side to drive them all separately using the PIC's built in full h-bridge feature? Or would I be at the mercy of the charge pump issue again?EDIT:// Added schematic attachment