Floating local ground?

[Chris Carlen]

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!



--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

 

[Bob]

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


"Chris Carlen" <crobc@BOGUSFIELD.sbcglobal.net> wrote in message
news:d73gui02v2i@news3.newsguy.com...

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!



--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

 

[Roger Lascelles]

"Chris Carlen" <crobc@BOGUSFIELD.sbcglobal.net> wrote in message
news:d73gui02v2i@news3.newsguy.com...

Greetings:


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

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

 

[Chris Carlen]

Bob wrote:

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.

Yes.

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).

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!

--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

 

[Chris Carlen]

Roger Lascelles wrote:

"Chris Carlen" <crobc@BOGUSFIELD.sbcglobal.net> wrote in message
news:d73gui02v2i@news3.newsguy.com...
Greetings:


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

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.

Yes, common mode junk was what concerned me.

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.

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.

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.

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.


--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

 

[Terry Given]

Chris Carlen wrote:

Bob wrote:

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.


Yes.

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).


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!

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

Cheers
Terry

 

[Pooh Bear]

Chris Carlen wrote:

Bob wrote:
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.

Yes.

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).

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?

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

 

[Roger Lascelles]

"Chris Carlen" <crcarleRemoveThis@BOGUSsandia.gov> wrote in message
news:d74qaq1268d@news3.newsguy.com...

Roger Lascelles wrote:
"Chris Carlen" <crobc@BOGUSFIELD.sbcglobal.net> wrote in message
news:d73gui02v2i@news3.newsguy.com...
Greetings:


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

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.

Yes, common mode junk was what concerned me.

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.

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.

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.


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.

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

 

[Chris Carlen]

Roger Lascelles wrote:

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.

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!


--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5