> The fundamental issue is that both pieces of equipment must have a common > reference - made common by wiring the chassis together. The few millivolts > of difference and pickup along the earth wire is handled by the differential > system. Otherwise, I promise you 1)audio oscillations 2)hum 3)distortion > 4)RF breakthrough, 5)Intermodulation with switching frequencies. > > If you look at the schematic of your differential amplifier, you see that > the input signals are taken in and subtracted by the common mode rejection > action of of an amplifier which operates *with reference to earth*. > Alternatively, one signal is inverted by an amplifier which operates *with > reference to earth* then the two signals are resistively added. My point is > that your amplifier operates with reference to earth right at the input, > before the differencing occurs and thus *requires* that the earth be shared > with the source device. If you don't expilicitly make that required > connection, then you must be *hoping* that it will come from somewhere. > Unfortunately, it won't. > > All this was hammered out decades ago and appears wired to XLR connectors in > broadcast, PA, recording and stage gear. > > 10K in parallel with 0.1uF is better than no connection at all, but there > has to be a reason to choose this substitute for a direct connection. This > network appears as a common mode return resistance - with interesting > possibilities for degrading common mode rejection - depending on your > differential amplifier configuration. Also depending on the resistances of > your source outputs. On a stereo amplifier, ground impedance could be > involved with channel crosstalk.
> > Roger How to protect H-bridge from short? X-Enigmail-Version: 0.90.0.0 X-Enigmail-Supports: pgp-inline, pgp-mime Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit
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 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 resistor||cap combo. This would be fine except that if one shorts an output leg, now the diff input 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:
- 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.
- fuse the power supply. The fuse will blow cutting power to the whole thing if either side shorts. Trouble is, can a fuse blow fast enough? 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.
- 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 do to better step response performance.
But it's quite a bit more circuitry to add too something that has already spiralled somewhat out of control.
Other ways?
Thanks for input.