Re: How to protect H-bridge from short?

Mac wrote: 
On Fri, 27 May 2005 11:40:12 -0700, Chris Carlen wrote:


Greetings:

I have a H-bridge PWM amp using Apex SA-60. I have implemented an
average current mode control loop, thanks to assistance from Genome,
which makes it possible for the amplifer to safely handle a differential short circuit fault.

The current sensing is done through two resistors on the low sides of
the bridge, going to a differential amplifier.

Trouble is, this current sense will read zero and thus attempt to push
even more current if there is a short from one H-bridge output leg to
ground.

How can I protect from this? I had considered floating the whole power
amp system, which already has a differential input. Then tie it to
earth ground via a R||C combo. This would be fine except that
if one shorts an output leg, now the input diff amp saturates with excess common mode input, resulting in an indeterminate command signal to the PWM amp.

I can't have this because it is a DC amplifier driving a mechanical
system, that if the drive pegs for instance, will result in mechanical
things crashing together.

Thus, current to the load must cease if this short condition occurs.

I am inclining back toward keeping the thing connected to earth ground,
and figuring out some straightforward way to implement short protection.

My thoughts so far:

1.  simply fuse each output, after the LC filter, and putting the
voltage sense after the fuses so their resistance doesn't cause voltage
drops.

But the PWM amp might just keep chugging along if one side blows, until
it attempts to reverse the output polarity at which point it will clip.
  This isn't ideal.

2. fuse the power supply. The fuse will blow cutting power to the
whole thing if either side shorts. Trouble is, can any fuse blow fast
enough? I don't think so. The rate of current rise is slowed a bit by the output filters, but still it's on the order of 5us to rise 15A with my L and supply V.

3.  Add an additional current sense on the B+ supply to the H-bridge,
and shut down the amp via the SA-60 DISABLE input if the average current
exceeds 15A with a time constant of something appropriate.

This is probably the best solution, and also would help me if the
amplifier is configured as a straight PWM voltage amp without current
mode, which may be desirable in some cases due to better step response
performance.

But more circuitry to add too something that has already spiraled somewhat out of control.


Other ways?


Thanks for input. 


As I recall, a simple way to detect a short circuit fault is this: the
voltage across the high-side device (or low for that matter) should be
very low when that device is on. If this is NOT the case, you need to shut
the amp down very quickly.

You need to allow reasonable time for the device to turn on, of
course.

If this doesn't make sense, then maybe you could draw a simplified version
of your output stage, and indicate where you are envisioning the short?

In any event, you can always add high-side AND low-side current sensing.
Maybe you could somehow even employ a GFI unit to automatically switch off
some portion of the circuitry when the upper and lower currents don't
match.

--Mac


In the IGBT world this is called "desat" detection, IOW detecting that the IGBT has pulled out of saturation. I typically use a Vcesat threshold of 10V, but thats because IGBTs tend to desaturate completely when these sorts of faults appear (so Vcesat = Vbus). Transient thermal impedance is your friend - many IGBTs are rated for 10x rated current for 10us under these conditions. A good thermal model helps here. 30us into a desat, kaBOOM. You would need to check the desat behaviour of your FET, but the +ve Rdson tempco helps here (many hundreds of kW peak pulse power....)

other techniques:
- DCCT in DC bus, with both +Vdc and -Vdc running thru the CT (dont forget to run the -Vdc wire in the opposite direction so the DM currents dont cancel). OTTOMH I dont see why an ordinary CT wont do the trick, as we are looking for a transient (and a big one, too). Beware the extra inductance :). A company I worked for built tens of thousands of small AC motor controllers like this.

- current sense resistor in low side of DC bus cap. wont help much if short is to Earth, and a current path exists w/o passing thru cap

- current sense resistor in output lead (can float cct with upper gate drive) and, say, an opto to feed back the "oh ***" command. I know a guy who patented this in the early '80s. A DCCT makes this a doddle, *but* wont protect against a half-bridge failure (where your resistors will). Again, many tens of thousands of motor controllers exist that do this.

Cheers
Terry
.

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