H-bridge flyback diodes - use a rectifier IC instead of discrete diodes? - Page 1
Forum section: electronics / beginners

author: HwAoRrDk
author_rank: Super Contributor
timestamp: January 29, 2016, 05:37:38 am
content: I am using a couple of dual full-bridge motor driver ICs in my circuit, and was planning to have 1N5819 flyback diodes on the outputs. However, being that they are relatively bulky (DO-41 package), I am finding that I am running out of room on my PCB layout.It occurred to me that perhaps I could save some space by instead using some bridge rectifier ICs (one per 4x discrete diodes). After all, they're just the same arrangement of diodes, but in a handy compact package, right?Looking at what's available to me, I was considering something like aFairchild DF005M. Would this be suitable?Also, one further question: my discrete diodes are schottky (because that's what I've seen used everywhere with the driver ICs I'm using), but I couldn't find any bridge rectifier ICs featuring schottky diodes. Will it matter if they're non-schottky?

author: Connoiseur
author_rank: Regular Contributor
timestamp: January 29, 2016, 08:45:55 am
content: What are the part numbers of those motor drivers?AFAIK H-bridge IC's do include flyback diodes with every MOSFET switch.The bridge rectifier that you've quoted will be adequate for a very small, toy-car-like DC motor. It won't survive for anything over 12V and ~5-6Watt(ish) if you do a lot of braking/reversing.Quote from: HwAoRrDk on January 29, 2016, 05:37:38 amThey would matter because Schottky diodes have very low forward voltage, whereas the body diode of a MOSFET is PN type(with higher forward voltage). If they are connected in parallel most current will flow through the Schottky rather than the body diode. Thus saving the body diode from excessive current.However if you connect a normal PN diode, the current distribution between it and body diode is just unpredictable (because of similar forward voltage and the fact that current rises exponentially with voltage in a diode)Thus the body diode may end up carrying the entire current and possibly blow up the MOSFET.

author: deshmukhmalhar
author_rank: Contributor
timestamp: January 29, 2016, 09:09:44 am
content: Try some rectifier diodes in TO220 packge, used in ATX(PC) power supply secondary(like this 1---|>--2--<|---3). some supplies also have a set of 4 rectifer diodes in single packge right after the AC mains input.

author: Zero999
author_rank: Super Contributor
timestamp: January 29, 2016, 09:37:19 am
content: It's a good idea to use a bridge rectifier, rather than discrete diodes.Ordinary diodes have a higher forward voltage and are slower than Schottky diodes which could be an issue if you're doing regenerative breaking or operating at high frequencies.Schottky diode bridges do exist. My local distributors stock them.CDBHM220

author: HwAoRrDk
author_rank: Super Contributor
timestamp: January 29, 2016, 10:09:43 am
content: Quote from: Connoiseur on January 29, 2016, 08:45:55 amI'm planning to use ST Micro's L293E, which doesn't have integrated diodes. Their L293D has, but alas no current sense pins, so I'm using the E, even though it's slightly overkill. They are indeed only small DC motors, 12-14V, max. of a few hundred milliamps.Quote from: deshmukhmalhar on January 29, 2016, 09:09:44 amTO-220 wouldn't be suitable, as too tall. And of course mounting horizontally makes my layout problems worse.Quote from: Hero999 on January 29, 2016, 09:37:19 amNo, not doing regenerative braking or even PWM speed control. So, given that, would non-schottky have any appreciable effect?I see that linked example is SMD - I did find there are choices for surface-mount, but my layout is all through-hole.Since my earlier post, I have actually found a THT schottky bridge IC: TI's UC1610N/UC2610N/UC3610N. But, there's no way in hell I'm using those as they're stupidly expensive - around £5 (~7 USD) each!

author: Kleinstein
author_rank: Super Contributor
timestamp: January 29, 2016, 11:51:29 am
content: If you are not using PWM or fast breaking normal diodes would be OK. So the bridge should work. But the bridge is not much smaller than 4 diodes, its just half the number of pins to solder.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: January 29, 2016, 12:29:40 pm
content: I did a mock-up layout of both options. The difference in board space is actually quite large!

author: Srbel
author_rank: Frequent Contributor
timestamp: January 29, 2016, 12:49:38 pm
content: You can also place diodes vertically, to save space.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: January 29, 2016, 01:33:38 pm
content: True, I could try that. Any downsides to vertical mounting? Also, I am aiming to keep the overall component height around 10mm, so I'd have to measure to see if this fits with that.That said, though, I am leaning towards the rectifier ICs, given that four of them will surprisingly cost less than 16 1N5819s!

author: Kleinstein
author_rank: Super Contributor
timestamp: January 29, 2016, 01:59:59 pm
content: You can put the diodes closer than drawn, the diodes outlinealso looks larger than the 1 A diodes usually are. The bridges are actually drawn so close they start to overlap.Vertical mounted Diodes are possible - just heat dissipation may not be that good, but this is not at all a problem unless using heavy PWM.The schottky diode or alternative fast diodes (e.g. UF400x, could be cheaper than 1N5819) would permit using PWM. This might be handy to limit the starting current.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: January 29, 2016, 03:31:21 pm
content: Okay, gotcha, so if doing PWM, definitely better to stay with schottky/UF.Those diode outlines are accurate, and they overlap if I nudge them one increment closer. Those bridge IC outlines are actually slightly oversize relative to max. stated dimensions of the DF005M, so that slight overlap arrangement would be no problem.I checked the prices for UF4001s - same price as 1N5819 for me, unfortunately.

author: mariush
author_rank: Super Contributor
timestamp: January 29, 2016, 05:52:17 pm
content: With individual diodes, you could try stacking a couple of them , one on top of the other./---[ diode 1] ---\| || /-[ diode2]--\ || | | |Just make sure there's 1-2mm between the diode bodies, so they won't heat from each other. You can use heatshrink tubing to insulate the leads if needed or some plastic rings to make sure the top diode stays high enough when soldering them on pcb.I think two would fit in 10mm.You could also look into those two diodes in a package, doesn't have to be to-220, have a look at dpak packages, if you want you can hack them to stay upright on the pcb instead of having them surface mount.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2016, 02:58:26 am
content: ...Or you could just give up the crusty, ancient driver and get a proper SMT one. I've seen 10A capacities in leadless packages!Tim