Re: How to protect H-bridge from short?
- From: Terry Given <my_name@xxxxxxxx>
- Date: Sat, 28 May 2005 12:48:08 +1200
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.
even semiconductor fuses wont protect your silicon. What they will do, though, is prevent the serious damage that occurs when you short the national grid thru your (now dead) box of tricks. minimises fire risk, explosive potential etc.
poor mans ultra-fast 400A fuse: use a 10L plastic bucket & lid. Fill it with de-ionised water. On the lid, mount a couple of 100mm long ceramic standoffs, so they are underwater. use ordinary 5A fuse wire. The water sucks the heat out of the fuse wire, allowing it to carry much, much higher currents than otherwise. But, if current is too high (adjust empirically) the water turns to steam around the wire, thermal resistance skyrockets and the fuse vaporises. A buddy of mine taught me this trick, which he used when developing an SCR V/Hz controller for his PhD (and had no budget for $300 fuses, three at a time)
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.
.
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- From: Chris Carlen
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