Floating local ground?

Greetings:

I am nearing completion of a PWM amplifier design based on the Apex SA60. It will be a highly flexible platform capable of being configured for voltage PWM amplification mode, voltage-to-current PWM amplification mode, and voltage amplification with average current mode control.

The power amp will contain it's own 4W DC-DC converter to generate local signal processing supply voltage +-15V. That freed me from concerns about ground loops resulting from having one low voltage supply, 2 high power supplies, and 4 PWM amps all tied together. So, there will be basically two banks of 2 PWM amps, each supplied by its own high power supply.

The signal input to the PWM amp PCB is via a LT1167 instrument amp. Thus, it is differential. The inputs are protected by 5k resistors, and have 1M resistors to the PWM amp's local ground.

Here's the potential problem: what bad might happen if I leave the PWM amp's power system totally floating with respect to the chassis (earth) ground? The reason I would want to do this is because of short circuit protection. I have the ability to current limit via the current mode control if the differential H-bridge output is shorted. However, is one leg or another of the bridge is shorted to the PWM amp's local power ground, then BANG!

Can I simply expose the differential input to the outside world of a completely floating power system without trouble?

Thanks for input.

Good day!

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Christopher R. Carlen
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Chris Carlen
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Chris,

For safety reasons, you want to have any exposed leads (inputs and outputs) held to some reasonable voltage level with respect to earth/chassis.

You could tie your circuit common to the chassis through some (relatively) high value resistor. That way, if an output is accidentally shorted to chassis or earth it will not draw tons of current, and the inputs and outputs will 'hover' around earth/chassis potential -- and thus not have the ability to build up to some high common mode level (which would be a safety hazard).

Bob

Reply to
Bob

Yes.

What do you think about using a high value resistor of say 10k or so from the PWM amp ground to earth to shunt DC leakage and keep the DC common mode near zero, and bypass high frequency common mode with a capacitor as well, perhaps 1uF or so?

Thanks for input.

Good day!

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Christopher R. Carlen
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Chris Carlen

Yes, common mode junk was what concerned me.

In addition, if it is also not capacitor, who knows what DC level the

Well, it is a DC amplifier with bandwidth up to about 12kHz, and switching frequency of about 125kHz. The differential inputs have bias current pathways via the 1M resistors to the differential amplifier's ground. Thus, a completely isolated input can operate properly. A battery would for instance work just fine, and the 1M resistors would pull the battery terminals to +0.5 and -0.5 times the battery voltage with respect to the PWM amp's ground, resulting in a zero common mode input.

The problem arises if there is significant leakage at DC, or significant AC common mode junk, as you mention.

What do you think about using a high value resistor of say 10k or so from the PWM amp ground to earth to shunt DC leakage and keep the DC common mode close to zero, and bypass high frequency common mode with a capacitor as well, perhaps 1uF or so?

Thanks for the input.

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Christopher R. Carlen
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Chris Carlen

Chris, I think you will run into trouble. Your amplifier has capacitance to ground which means it is not isolated at AC, but significantly connected. All that current flowing throught the speaker circuit and who knows what at the PWM and DC-DC converter frequencies mean your diff amp may have to reject tens of volts of audio and hash which may easily exceed its input range. In addition, if it is also not capacitor, who knows what DC level the various leakages will set up between the input and amplifier circuits. A differential amp is not magic - it requires the input circuit has some kind of common connection to the diff amp *other* than the two differential inputs.

A similar situation exists when you lift the ground of a switched mode power supply - a gross amount of signal exists from supply output to ground.

I think an audio transformer or an opto isolator, including opto isolated digital audio are the only genuine no common connection methods.

Roger

Reply to
Roger Lascelles

I'd go for something lower - maybe 1k ? Make sure its power rating will allow it to survive any fault condition too.

As Terry says you should also include a clamp to avoid excessive drift from chassis ground potential.

Graham

Reply to
Pooh Bear

BTDT, many thousands of times. And a clamp across the parallel RC, to ensure maximum voltage is never exceeded.

Cheers Terry

Reply to
Terry Given

How is a "floating differential source" implemented? Transformer output?

My interest is simply protecting an H-bridge from shorts to one of the output legs. My current mode control doesn't solve this.

I will start a thread specifically about this problem.

Thanks again for your input. I welcome your interest on the new thread:

"How to protect H-bridge from short?"

Good day!

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Christopher R. Carlen
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Chris Carlen

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Why not put an XLR connector on the amp? The differential inputs go to the two XLR lines and the amp ground goes to the XLR ground. This lets you finish the project and later on you can connect however you like to the inputs.

Professional audio uses XLR with differential inputs driven by a floating differential source and the XLR ground connecting the two equipment grounds. Sometimes the ground lead is cut to handle difficult cases, for example where there is an alternative unavoidable ground causing heavy circulating currents. For a single ended source, one of the differential inputs goes to ground at the source end of the cable.

In the professional audio scenario I am describing, 10K input resistance and

10K input to ground resistance is quite OK. Going to 1M brings no benefit.

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

Reply to
Roger Lascelles

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