Power mosfet question

[Bob Minchin]

I want to use mosfets in an H bridge configuration to control and reverse a 12v permanent magnet motor - obviously an inductive load. Running current will be 10amps or so.

The sort of circuit I plan to use is here

Scroll down to design number 6

I'll be using IRF5305 N channel and IRLZ34 P channel devices.

The data sheet for the Mosfets show an internal diode from drain to source and yet the above circuit (and many other on the web) show an external catch diode as well.

Is this necessary, a prudent precaution or simply redundant.

Although I've been working in electronics all my working career I was moved into management and not allowed to do real work before power mosfets came into common use.

TIA

Bob

[T i m]

Isn't the external diode there to absorb back-emf's so if the Fet has one built in similarly connected, does it offer the same level of protection?

Cheers, T i m

[Bob Minchin]

Well this is the basis of my question. I would expect the integral diode to be rated to be man enough to protect the device - otherwise why put it in? The schematic shows it as a zener diode so it should clamp at or around the maximum drain source voltage in zener mode and when forward biassed it should absorb the back emf. The H bridge circuit is only showing cooking grade 1n4004 diodes which are not fast types and so might not catch fast voltage spikes

[T i m]

Well, I'm not sure something like that could know what size of (say) back-emf the device might be exposed to but something could be better than nothing?

Makes sense.

Isn't it 'Schottky' diodes that are the fast ones (I was a support tech not a design engineer). ;-)

I used some little IC / integrated bridges with the Arduino project recently that were both very cheap and worked very well but only good for the PWM'd amp or so a OO gauge model train would pull. ;-)

Sorry I can't be of any help.

Cheers, T i m

[Bob Minchin]

Thanks for responding Tim. I've used the integrated H bridges for driving stepper motors as well as bought in drives but this is my first experience of driving a golf cart motor with discrete devices for a project. I'm not short of space and so I can easily fit external diodes - I just like to try and understand what I'm doing! Yes Schotty diode are the faster ones. In the 1N400x series the suffix UF indicates the fast ones eg 1N4004UF. They tend to get used in switch mode PSUs

[Fredxxx]

To me the 1N4004 diodes seem entirely superfluous. The internal Drain Source diode is far meatier than any 1n4001. Plus it can dissipate any transient energy with ease.

The bit that worries me is the driving of these gates. The level translator is all well and good, but the biggest concern is how to not overlap the gate drives to ensure that no n and p channels MOSFETs are conducting at the same time.

10k is considered large in comparison with gate charge of 10s of nC leading to an extended on and off times.

[Bob Minchin]

I see your point. In my application reversing will be quite infrequent so should not be an issue.

[Andrew Gabriel]

The internal diode is there by accident - the device can't be made without it. However, it can carry the full load current without any problems, although it has a higher voltage drop, so power dissipation is a consideration if you actually plan on using it (as I have in a mains switching application). Because it's a big diode, it has a poor recovery time, but that's only relevant at high frequencies. I would not rely on it's zener properties, and they aren't there to protect the device.

[Fredxxx]

How will you ensure the upper and lower MOSFETs can never be turned on simultaneously?

What voltage and current is your application?

Would something like this make your life so much easier?

Datasheet for IC:

[T i m]

Or this could be easier still. ;-)

Cheers, T i m

[Dave Plowman (News)]

Mosfets and PWM drive are very common on car injection systems. To drive things like injectors, fuel pumps, idle motors etc. Many use an IC these days which includes protection etc. Just a thought.

[Brian Gaff]

I'd imagine the external diode is going to be more beefy than the internal one, for a few pence why take the risk? I've often wondered if one could harvest this back EMF and do something useful with it. Brian

[Syd Rumpo]

It's not 'put in' as such, it's there as a sort of 'side effect' of the way these are made. It's called a 'body diode' and its characteristics are on the data sheet. The forward drop is fairly high, so beware of power dissipation if you rely on this.

BTW, isn't IRF5305 a P channel?

Cheers

[Syd Rumpo]

Just seen that Andrew Gabriel said much the same, sorry for the repetition.

Cheers

[The Natural Philosopher]

I'd be very very wary of that.

[Bob Minchin]

Quite correct Syd

I got my Ns & Ps reversed!

Bob

[Bob Minchin]

That looks quite interesting Tim Thanks Might be useful if all the smoke escapes from my discrete breadboard in an instant.

A few comments

1) I just ordered a load of £1 mosfets from CPC to play with befreo seeing your post. 2) The ebay seller says no circuit info available - yes I could draw it out. No terminal blocks either. Doh! why not add a £1 to price and do a proper job on the pcb build. 3)It says two half bridges - do 2 halves make a whole? Look more like a stepper driver? I'm not up with the jargon. Also mentions two motors WTF? 4)Only N channel mosfets I think so "top" half o/p won't got to full supply. I need the full power Igor! This motor powers an elevation platform for a 1/4 tonne machine. 5)Chinglish description only partially decoded by me so far. 6)Waiting for delivery from H-K etc Wanna play now!! 7)I might not learn as much from a bought in module.

Cheers Bob

[Syd Rumpo]

Yes, two halves make a whole - both halves swing from 0V to supply, but are arranged so one's going up while the other's coming down and vice-versa.

Also, it appears to be a 2 channel board (it says dual in the title) with two full bridges to drive two motors.

It refers to a 'bootstrap capacitor' which means it generates a higher voltage for the N channel gate drive, so output swing won't be an issue. That's a pretty common technique.

Cheers

[Bob Minchin]

Thanks Syd. Learning well here!

I also googled the ref no in the ebay ad MC33886 and it is an integrated 5amp driver device so presumably this module is based on that chip (underneath perhaps) and go faster Fets for greater current handling. The data sheet mentions bootstrap too.

Bob

[The Natural Philosopher]

These guys are REALLY good electronics guys.

Also see

for local sales.

Ok its not as much a challenge as designing your own, but that's a different choice.