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Author Topic: H-Bridge Help  (Read 2636 times)

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Offline Builder1Topic starter

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H-Bridge Help
« on: November 22, 2008, 09:57:51 PM »
Hello Everyone,
I am trying to understand the H-Bridge schematic shown on this website. Please correct me if I am wrong, according to the schematic the semiconductor ISL9N310AD3ST can handle 30V and 35A. Does that mean that if I want something that can handle a higher current like 60A, I could use two ISL9N310AD3ST semiconductors in series? Also exactly where do I use these semiconductors, are they the circled A and B? I've tried to find out how much one of these costs but it seems there is no site out there that will tell you what it is. Does anyone know how much they cost? Thank you in advance for your reply.
« Last Edit: November 22, 2008, 09:59:41 PM by Builder1 »

Offline szhang

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Re: H-Bridge Help
« Reply #1 on: November 22, 2008, 10:10:15 PM »
No you put them in parallel, or get a bigger mosfet

Do you REALLY need 60A?

(30V@60A is 1.8kW, that's more heat than my space heater, and times 4... 7.2kilowatt!  :o)

EDIT:

And yes, the circle with A/B are the mosfets, you need four of them for an h bridge.

If you're just starting out I suggest getting a discrete mosfet chip like the LMD18200.  They can't handle 35A, but unless you got super heatsinks on the mosfets the 9N310 can't either.

EDIT2:

Mouser had them, but they are obsolete/out of stock.  About $.50 if you buy in 1000, so probably around a dollar or two if you buy one at a time.
http://mouser.com/Search/Refine.aspx?Keyword=ISL9N310AD3ST

Also, they have a thermal dissapation of 70Watts, no way you can get to 35Amp without melting something.
« Last Edit: November 22, 2008, 10:17:52 PM by szhang »

Offline Builder1Topic starter

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Re: H-Bridge Help
« Reply #2 on: November 22, 2008, 10:53:58 PM »
Hello Szhang,
Thanks for responding, yes I do need 60A but at lower voltage. I'm in the planning phase of my project, I am planning to build a security robot using two electric motors each with an output of 350W, rated current of 22A, and voltage of 24VDC. It will be powered by a battery weighing at least 50lb. I figure the total weight of the robot will exceed 70lbs so the motors will each require a lot more juice than 22A. Please give your opinion on what I can use to build the H-Bridge for this situation. Thanks again for your response.

Offline szhang

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Re: H-Bridge Help
« Reply #3 on: November 22, 2008, 11:18:56 PM »
Since you have two motors, you need two H-bridges, so you don't need 60A mosfets.  (I am assuming you are doing differential drive for the robot).  Any power mosfet (with ratings that match what you need) should do, though some are more optimized for specific applications.  Read the datasheet.

Two motors at 350W will be almost a horse power, so i'm pretty sure you're fine with each motor @ 22A.

Maybe you want something like this: http://www.dimensionengineering.com/Sabertooth2X25.htm, it is expensive, but if you make one yourself it might cost just as much to get it not to melt.

Offline ArcMan

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Re: H-Bridge Help
« Reply #4 on: November 24, 2008, 12:20:05 PM »
Maybe you want something like this: http://www.dimensionengineering.com/Sabertooth2X25.htm, it is expensive, but if you make one yourself it might cost just as much to get it not to melt.


I can vouch for that!
Plus the intangible costs.  I can never get to sleep after blowing up a couple of MOSFETS - jangly nerves.

Offline Builder1Topic starter

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Re: H-Bridge Help
« Reply #5 on: November 25, 2008, 09:59:49 AM »
Hi Guys,
I have just two questions, according to the H-Bridge schematic on this website the MOSFETs are supposed to have a diode built in in each of them. Then how come the diodes are drawn on the outside of the MOSFET symbols? Someone asked if there is a cost advantage to building your own electronic components such as an H-Bridge, a motor control, etc. and the answer was no? However, I did some calculations and found that one could build a motor control, for less than the commercially available ones. Thanks in advance for your response.
« Last Edit: November 29, 2008, 06:33:34 PM by Builder1 »

Offline szhang

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Re: H-Bridge Help
« Reply #6 on: November 25, 2008, 10:45:13 AM »
Most MOSFETs have built-in diodes, though they might not be able to switch fast enough or sink enough current, so for high current applications you might want to put external diodes to help protect the driver.

Sure, the MOSFETs will cost alot less than a commercial driver, but the MOSFET will blow up if you pass 25A though them.  So then you need good heatsinks, good design, etc.  You also might need to buy alot of replacements for the MOSFETs that you do blow up.  Size is also a consideration, discrete MOSFET h-bridges are usually bigger and heavier.

Also a commercial driver will likely perform better.

You might try making a cheap discrete MOSFET first, and see if it fits your application.  If you find you keep melting the MOSFETs you might want to consider just getting a commercial driver.

Offline ArcMan

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Re: H-Bridge Help
« Reply #7 on: November 25, 2008, 11:58:34 AM »
Yes.  I can confirm szhang's comments through my own experiences.  The built-in diodes can only handle flyback voltage for smaller inductive loads.  When I tried to drive an 11A wheelchair motor, I blew up MOSFETs.  I then added some external fast-recovery diodes.  No more explosions.  Then I melted a gate resistor (which I attribute to some ebay MOSFETs I bought).  Then I decided I was in this hobby to enjoy robotics, not to be frustrated by a stupid motor drive circuit.  So I bought a Dimension Engineering drive  ;D

Offline Builder1Topic starter

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Re: H-Bridge Help
« Reply #8 on: December 02, 2008, 12:03:14 AM »
Hi Guys,
I've been thinking about what you guys have said about the MOSFETs, diodes over heating, etc.  What I would like to know is are the diodes and MOSFETs overheating and melting due to the fact that they are cheaply made or is it because you guys used the wrong type of MOSFETS or diodes?  Is there no way to successfully build a reliable H-Bridge yourself? The reasons that I would like to build my own H-Bridge are to learn and at the same time experiment as cheaply as possible. Thanks again for all of your responses.
« Last Edit: December 02, 2008, 12:10:51 AM by Builder1 »

Offline Webbot

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Re: H-Bridge Help
« Reply #9 on: December 02, 2008, 12:20:22 AM »
I guess that most of us tend to think positive. As a result we under-engineer our own designs. So the problems with MOSFETs or diodes is not a problem with their construction - we've just specified a 'kid' to do a 'man's job - to keep costs down. The result is that they've blown.

Some folk end up deciding that it actually ends up cheaper buying a pre-built one.

Of course you can build your own h-bridge if you want. But beware of any 'peak' requirements - so 'over spec' your components if in doubt.
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Offline szhang

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Re: H-Bridge Help
« Reply #10 on: December 02, 2008, 03:38:42 PM »
I guess that most of us tend to think positive. As a result we under-engineer our own designs. So the problems with MOSFETs or diodes is not a problem with their construction - we've just specified a 'kid' to do a 'man's job - to keep costs down. The result is that they've blown.

Some folk end up deciding that it actually ends up cheaper buying a pre-built one.

Of course you can build your own h-bridge if you want. But beware of any 'peak' requirements - so 'over spec' your components if in doubt.

I agree, the absolute maximum rating on MOSFETs are usually for millisecond range transients.  Just because a MOSFET can handle 30A doesn't mean it can dissipate the heat created by 30A, that is why you need to put massive heatsinks on them.  You need to make sure the heatsinks can draw enough heat away from the MOSFET so it doesn't go over it's maximum rated temperature.  That is difficult to calculate, and therefore it is easy to blow a MOSFET because you have insignificant power dissipation.

Also, inductive loads have a bad habit of creating very high voltages when the current is suddenly changed, which is why big H-bridges are always hard to design.

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Re: H-Bridge Help
« Reply #11 on: December 04, 2008, 09:59:36 PM »
I selected those MOSFETs on that circuit 7 years ago . . . I'm sure there are much better ones out there today.

Also, the two top MOSFETs should be PNP, and the two bottom ones NPN.

Offline ArcMan

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Re: H-Bridge Help
« Reply #12 on: December 06, 2008, 10:33:24 AM »
Also, the two top MOSFETs should be PNP, and the two bottom ones NPN.

Works great.
But do be aware that there is also a popular arrangement where both the low-side AND high-side MOSFETs are N-channel.  A charge-pumped high side driver is then used to turn the high-side MOSFETs on.  The reason people use this arrangement is that N-channel MOSFETs have a lower Rds On than P-channel MOSFETs, creating much less heat.  If you're driving a small load (a few amps) it's probably not worth the complication, but it pays dividends for larger loads.

 


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