How to protect H-bridge from short?

shame the whole shebang isnt running off a psu volt reg with inbuilt i limit. Very simple.

NT

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

No, the power supply is limited to 12A, and my amp's surge current is

Thanks for the response.

Good day!

Because a short from one leg to ground shunts current away from the low side switches. It's the high-side switches that will see the fault current.

Also a single sense resistor wouldn't allow implementation of bipolar current sensing, rather returning the absolute value of the load current.

Thanks for input.

Good day!

How so ?

Btw - why not position a single sense R so that's it's common to both of the low side devices ?

Graham

I see you're concerned about a short to ground rather than across the load.

In which case you can detect the high side current and level-shift the signal.

Indeed but why would you need to do that ?

Graham

why does it need to be earthed ? if its just safety could you get away with earthing the high side V instead of the low side ? that would apear to be the simplest solution.

Colin =^.^=

One relatively easy way is a faster current limit. Something like a 12A FET, with sufficient voltage rating, and Vgs set to the right level to get it to intrinsically current limit.

Add a feedback loop to short the Vgs capacitor out, if the voltage across the FET exceeds 20V, and you're done.

(Of course, you need to be sure this FET is either on, or off)

If you monitor anything other than the current out of the reservoir cap, you need multipoint monitoring to cover all possible problems. If you monitor only i out from the reservoir, all overcurrent fault modes are covered.

NT

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 s*it" command. I know a guy who patented this in the early '80s. A DCCT makes this a doddle,

Cheers Terry

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)

DC servo amp, not AC coupled audio. Need 4-quadrant control.

Yes, I had reached that conclusion as well when I should have been sleeping last night.

Thanks for the input.

signal.

I don't see the issue. Consider audio as slowly moving DC if you like.

Either there's an overcurrent issue or there's not. Since it's PWM you'll see the overcurrent anyway,

Graham