Need comments on a H-Bridge design - Page 1
Forum section: electronics / projects

author: RigorM
author_rank: Contributor
timestamp: May 22, 2013, 06:07:01 pm
content: Hi all,First time poster, longtime viewerI'm working on a project that needs a H-Bridge to drive a 12V / 4A motor. (the motor assembly is a car's door lock module).This will be controlled by a arduino/attiny AVR controller. with some tests I made, the power only needs to be on for 500ms max.I designed a H-Bride, here is the schematic : (see attachement)I would like to know what you think.Thanks :-)Jeff.

author: kfitch42
author_rank: Frequent Contributor
timestamp: May 22, 2013, 06:30:05 pm
content: Uh, your web server is a little funked up.QuoteOh, and why not just use something like this for the H-Bridge:https://www.sparkfun.com/products/9479

author: RigorM
author_rank: Contributor
timestamp: May 22, 2013, 06:46:49 pm
content: Quote from: kfitch42 on May 22, 2013, 06:30:05 pmSorry about that, my ssl certificate is not installed yetI attached the schematic in my original postLooks good but a bit short on the amp side.. 4A is a bit border line because this is a motor and can go over.Thanks.

author: Paul Price
author_rank: Super Contributor
timestamp: May 22, 2013, 06:51:25 pm
content: I don't like the power wasted in junction transistors and the input resistors from the Arduino seem to go to ground without any current limiting to the bases. If you use N-Channel power MOSFETs on the bottom of the bridge and P-Chan's on the top you could greatly improve the efficiency of the circuit. I don't like the idea of ordering a special part to do this job then waiting for the part to arrive and then having the special part fail to repeat this cycle.In my circuits, I create a small LM555 timer circuit to give a very short pulse to drive a small N-chan MOSFET driving an inductor and a diode and a small cap creates a 20V supply.(A boost convertor to generate a 20V zener reg'd supply from the +12V.) I use this supply to have the high voltage to control all N-Chan MOSFETs in the H-brdige. (I can get as many of the small N-chan MOSFET as I want from discarded PC CPU power circuits in old discarded motherboards and they are rated at 20V-30V and 20-30Amps) and I use a complementary pnp-npn pair whose common-connected bases are driven from the collector of a common base configured npn transistor as a level shifting interface to control each MOSFET, so the MCU toggles emitters to control the H-Bridge at 98% or better efficiency. If I add a 74LS06, I can eliminate the complementary pairs.

author: RigorM
author_rank: Contributor
timestamp: May 22, 2013, 06:58:58 pm
content: Quote from: Paul Price on May 22, 2013, 06:51:25 pmI agree, I should probably use mosfets and I forget the limiting resitors on the 2N2222s ;-)any type on N/P Mosfet or do they need to be paired ? (Like I used TIP41C and its conterpart TIP42C)Thanks

author: Paul Price
author_rank: Super Contributor
timestamp: May 22, 2013, 07:09:26 pm
content: The N and P chan mosfets need not match at all, they just have to handle 12V(they all do) and (>12 amp max Idrain...easy to find those with N-chan MOSFETs.) You can use all N-Chan MOSFETS (which you get for free) though it is necessary to add additional npn-pnp complementary pairs or use a 74LS06 (Hex open collector 30V inverted outputs) to control the N-chan MOSFETs, although a 5V zener diode, and a small bypass VCC cap is required to power the 74LS06.

author: RigorM
author_rank: Contributor
timestamp: May 22, 2013, 07:33:25 pm
content: Quote from: Paul Price on May 22, 2013, 07:09:26 pmThe 5V wont be a problem as I have a 7805 in the circuit (not shown)The pnp/npn or hex inverter is needed when using all N-Mosfet or its needed in a N/P mosfet setup ?I've seen some designs with a all N-Channel but required a special control to make sure (i dont remember what side) would not shutdown.Do you have a example of the circuit your talking about ?Thanks

author: Paul Price
author_rank: Super Contributor
timestamp: May 22, 2013, 07:42:06 pm
content: The Hex Inverter is only needed if you want to simplify a H-bridge using all n-chan MOSFETS.Ok, if you use P-chan --- N-chan complementary pairs, you connect the gates together and you turn one (pair) side on, after the other comp. pair has been turned off with a simple inverter(npn transistor) to provide the communtating.With all N-chans, you need to have separate drive signals for each gate. The top N-chan must be off when it's bottom brother is on, on one side of the bridge, and we have the opposite control signals on the other side of the bridge....can't you figure out how to do this? It means that that only one upper MOSFET gate of each side of the bridge is turned on, and the lower MOSFET is turned-on on the other side of the bridge. And so it alternates back and forth.

author: RigorM
author_rank: Contributor
timestamp: May 22, 2013, 07:55:38 pm
content: Quote from: Paul Price on May 22, 2013, 07:42:06 pmOk, i see, the All n-channel is a bit wired like I did in my schematic (without the inverter but I get the point) and the N/P version is Like using all NPN or PNP BJT :-)Thanks for the info.I guess the gates on the mosfets can be controlled with a simple 5V signal ? (I never used mosfets)

author: oPossum
author_rank: Super Contributor
timestamp: May 22, 2013, 07:59:54 pm
content: You can use the same small transistor to drive both of the opposing H bridge transistors like this:The TIP41/42 are low cost and rugged - no need for more expensive and fragile MOSFETS.

author: Paul Price
author_rank: Super Contributor
timestamp: May 22, 2013, 08:07:25 pm
content: The top N-Chans require the gate to be >5V than the source for lowest resistance as a switch, that's why you need the boost supply. The lowest RDSon is achieved at >12V and the threshold of turn on can vary and be greater than .8 to 3.5V depending on the device. LV MOSFET threshold devices start to turn on at levels as low as .8V but don't show the best fully turned on effect until they have >3V G to S. So, if you trying to apply 12V to the top of the motor, you need at least 15V at the top H-Bridge N-chan MOSFET gate.Now you can see how easy it is to make a H-Bridge using P-Chans on top and N-chans on the bottom. Nice if you have a ready source of P-chan power MOSFETS lying around..

author: RigorM
author_rank: Contributor
timestamp: May 22, 2013, 08:08:33 pm
content: Quote from: oPossum on May 22, 2013, 07:59:54 pmI did try this version on the breadbord.... ended up with magic smoke ;-)

author: Paul Price
author_rank: Super Contributor
timestamp: May 22, 2013, 08:19:44 pm
content: If the motor stalls, the high currents will require you to have large heatsinks to keep the 4 bipolar power transistors from burning out in a few seconds, while the very low turned on resistance of a typical 30-amp MOSFETs (.01 ohm, for example)will likely not generate <.1W of heat with 10 amps during overload conditions, and thus might not even require any heatsink.The MOSFET package is also 4x smaller than the TIP transistors.30-amp MOSFET cannot be considered at all delicate if they can easily handle 30-amps and are not overloaded by a motor stall.

author: asgard
author_rank: Regular Contributor
timestamp: May 22, 2013, 08:28:02 pm
content: For something like this I like to use a particularly favorite device: a pair of IRF7343's. Each device is a SO-8 package, has a matched complementary pair of MOSFETS, and can sink up to 4A at the drains. The AUIRF7343 is rated for automotive use, and is quite cheap from Mouser.

author: Strada916
author_rank: Frequent Contributor
timestamp: May 22, 2013, 08:34:12 pm
content: Shouldn't T2 and T4 be PNP? in the original circuit?

author: Paul Price
author_rank: Super Contributor
timestamp: May 22, 2013, 08:42:55 pm
content: Strad916: Sorry, no.

author: qno
author_rank: Frequent Contributor
timestamp: May 22, 2013, 08:46:38 pm
content: Try a mosfet, you get the reverse diodes for free.

author: Rufus
author_rank: Super Contributor
timestamp: May 22, 2013, 08:47:02 pm
content: Modern cars are full of electric motors and single package chips to drive them must be made in the millions.The Freescale MC33887 is the first I found. 5A 120 mohm MOSFET H bridge, 5-28v operation, over current, over temperature protection. $6.50 1 off from digikey. I'm sure there are others.You won't get close to duplicating its performance, functionality or reliability with a discrete design and won't get close to its price if you try to.Bit of a pain sourcing and most if not all will be power surface mount packages. If you want something that works and is reliable they are the way to go.

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 10:02:13 am
content: Cheap Ho Bridge:In my newest circuits, I created a small LM555 timer circuit to convert 12V to 18V zener reg'd supply from the +12V. I use this supply to have the high voltage to control all N-Chan MOSFETs in the H-brdige. (I can get as many of the small N-chan MOSFET as I want from discarded PC CPU power circuits in old discarded motherboards and they are rated at 20V-30V and 20-60Amps) and I use a complementary pnp-npn pair whose common-connected bases are driven from the collector of a common base configured npn transistor as a level shifting interface, so the MCU toggles an emitter to control the H-Bridge at 98% or better efficiency.

author: asgard
author_rank: Regular Contributor
timestamp: May 23, 2013, 04:42:03 pm
content: Following up: Here is a way to use a pair of AUIRF7343 to control a motor or armature. One benefit of using the complementary pairs on either side of the H-bridge is that the beefiest part of the MOSFET dies are on the drains, so the junction temps can be a bit lower than would otherwise be the case. The diodes D1-D4 are beefy Schottky barrier diodes. All resistors on the motor side of the optocouplers are 2W.

author: RigorM
author_rank: Contributor
timestamp: May 23, 2013, 05:00:36 pm
content: Thanks guys for the replies... really appreciate all your inputs.Better to fin that my design is a bit on the stinky side and redo it so I donc burn the house downI've ordered some N and P mosfets from a supplier. Will try to test when the parts comes in.Thanks !Jeff.