negative rail for low side current sense?...

[Lasse Langwadt Christensen]

fredag den 18. marts 2022 kl. 05.08.35 UTC+1 skrev DJ Delorie:

If we could take a moment to talk about something ON topic for a change ;-)

I\'m updating an old design that uses low-side current sensing for BLDC
drive[1]. I\'m sensing both forward and reverse current. So the signal
is about +- 40 mV but could be more. I had been using an LM324 in
differential mode with a 2.5V \"ground\" to offset the sense voltages, but
the signal still went below 0V at the input pins. I\'m trying to do
better this time ;-)

My first thought is, add a negative power rail. I\'ve got +15v and +5v
so there\'s already one switcher (LM2842 based), so I could add another
one. Suggestions for a simple invert-switcher? How negative do I need
to go? I\'d been running the LM324 on +5 and GND, and need a 0..5 output
signal anyway (+- 2.5v biased to 2.5v), so -5? Then I\'m thinking a 5.1v
zener to protect the ADC input.

Alternately, opinions on an alternate quad low-side sensor that can
handle positive and negative currents...

Also, I\'d been using a resistor divider for each opamp to bias it to
2.5V. Is it worth the effort to put in a 2.5V rail and use that, so
that all the sensors have a common \"zero\" at the ADCs, vs possibly
different ones due to resistor tolerances?

Thanks!

[1] http://www.delorie.com/electronics/bldc/

if you skip the short circuit sense resistor, does it have to be differential?

three opamps with ~60x gain and a pull up to center the output at 2.5V with zero current
diode or the output and use an opamp as comperator for the short circuit detect

 

[DJ Delorie]

Sylvia Else <sylvia@email.invalid> writes:

so there\'s already one switcher (LM2842 based), so I could add another
one. Suggestions for a simple invert-switcher?

Or a negative charge pump.

I thought of just outputting a square wave from the MCU into a charge
pump inverter, but 4.6v minus two diode drops is only a few volts, and
not a lot of milliamps (you\'re limited by the pin\'s max output). It
might be enough, but if a solid 1A buck/boost negative rail is cheap and
easy enough...

 

[DJ Delorie]

Klaus Vestergaard Kragelund <klauskvik@hotmail.com> writes:

You offset both the input and the reference, they should both be
around 50% VDD, with a little difference due to the gain and the
needed 50% VDD on the ADC

That sounds like multiple op amps per leg... either both signals need an
opamp to offset them, plus a third to difference them, or if you try to
reduce the opamps, you can\'t offset them enough *and* amplify them at
the same time (which is what I\'m seeing now).

 

[Lasse Langwadt Christensen]

lørdag den 19. marts 2022 kl. 02.02.07 UTC+1 skrev DJ Delorie:

Sylvia Else <syl...@email.invalid> writes:
so there\'s already one switcher (LM2842 based), so I could add another
one. Suggestions for a simple invert-switcher?

Or a negative charge pump.
I thought of just outputting a square wave from the MCU into a charge
pump inverter, but 4.6v minus two diode drops is only a few volts, and
not a lot of milliamps (you\'re limited by the pin\'s max output). It
might be enough, but if a solid 1A buck/boost negative rail is cheap and
easy enough...

https://www.maximintegrated.com/en/design/technical-documents/tutorials/3/3740.html

 

[DJ Delorie]

Lasse Langwadt Christensen <langwadt@fonz.dk> writes:

[1] http://www.delorie.com/electronics/bldc/

if you skip the short circuit sense resistor, does it have to be differential?

The IKCM15L60GAXKMA1 power module has a design without the separate
short sense (just three resistors) but it uses diodes to \"or\" the
signals into the sense pin. The side effect of that is you need larger
sense resistors (36mR instead of 5mR, and 3W instead of 0.5W) which
creates a larger voltage swing, which means the negative swings are much
larger. This kind of design assumes the motor will always be \"running\"
which is not true in my case (a hard stop caused the fault in the last
design)

You kinda want differential anyway to avoid ground drops, with that much
current.

three opamps with ~60x gain and a pull up to center the output at 2.5V
with zero current diode or the output and use an opamp as comperator
for the short circuit detect

Yup, but that doesn\'t stop the sensed value from going negative...

 

[Lasse Langwadt Christensen]

lørdag den 19. marts 2022 kl. 02.13.16 UTC+1 skrev DJ Delorie:

Lasse Langwadt Christensen <lang...@fonz.dk> writes:
[1] http://www.delorie.com/electronics/bldc/

if you skip the short circuit sense resistor, does it have to be differential?
The IKCM15L60GAXKMA1 power module has a design without the separate
short sense (just three resistors) but it uses diodes to \"or\" the
signals into the sense pin. The side effect of that is you need larger
sense resistors (36mR instead of 5mR, and 3W instead of 0.5W) which
creates a larger voltage swing, which means the negative swings are much
larger. This kind of design assumes the motor will always be \"running\"
which is not true in my case (a hard stop caused the fault in the last
design)

You kinda want differential anyway to avoid ground drops, with that much
current.
three opamps with ~60x gain and a pull up to center the output at 2.5V
with zero current diode or the output and use an opamp as comperator
for the short circuit detect
Yup, but that doesn\'t stop the sensed value from going negative...

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[DJ Delorie]

Lasse Langwadt Christensen <langwadt@fonz.dk> writes:
> https://www.maximintegrated.com/en/design/technical-documents/tutorials/3/3740.html

Interesting! Hadn\'t thought of that... Thanks!

 

[whit3rd]

On Friday, March 18, 2022 at 2:40:10 PM UTC-7, DJ Delorie wrote:

whit3rd <whi...@gmail.com> writes:
Just use a +12 supply (and a 2.5V source/sink for \"ground\") . The symmetric-dual-supply
language is merely a convention, not a requirement.

It needs that \'2.5V\' point to be wired to its \'ground\' pin, is the hard-to-communicate part.

For a unity gain amp, yeah, that would bring it up. I need to also
amplify it up to +- 2.5v swing...

Yeah, the choice of a gain-of-one current sense amp isn\'t great. There\'s gain-of-20 and gain-of-60
available, ST.com shows TSC2010, TSC2011, and for gain-of-100 TSC2012. They don\'t claim accuracy
for big sense voltages, though.

 

[Mikko OH2HVJ]

DJ Delorie <dj@delorie.com> writes:

If we could take a moment to talk about something ON topic for a change ;-)

I\'m updating an old design that uses low-side current sensing for BLDC
drive[1]. I\'m sensing both forward and reverse current. So the signal
is about +- 40 mV but could be more. I had been using an LM324 in
differential mode with a 2.5V \"ground\" to offset the sense voltages, but
the signal still went below 0V at the input pins. I\'m trying to do
better this time ;-)

My first thought is, add a negative power rail. I\'ve got +15v and +5v
so there\'s already one switcher (LM2842 based), so I could add another
one. Suggestions for a simple invert-switcher? How negative do I need
to go? I\'d been running the LM324 on +5 and GND, and need a 0..5 output
signal anyway (+- 2.5v biased to 2.5v), so -5? Then I\'m thinking a 5.1v
zener to protect the ADC input.

Things I\'ve used for similar setup depending on accuracy need. BLDC
driver does not sound like metrology application ;-)

- Large negative rail
- LM7705 to generate -0.23V rail
- Resistor divider (one or two sided) from sense resistor to 3.3V (say,
40 ohms and 3.3V cause the -40mV to show up at 0mV, increase 40ohms to
\'lift\' the signal).
- Instrumentation amplifier
- Hall current sensor
- Opamp with negative input CM (TL27L4 and successors, TS921...)

-- mikko

 

[server]

On Fri, 18 Mar 2022 00:08:23 -0400, DJ Delorie <dj@delorie.com> wrote:

If we could take a moment to talk about something ON topic for a change ;-)

I\'m updating an old design that uses low-side current sensing for BLDC
drive[1]. I\'m sensing both forward and reverse current. So the signal
is about +- 40 mV but could be more. I had been using an LM324 in
differential mode with a 2.5V \"ground\" to offset the sense voltages, but
the signal still went below 0V at the input pins. I\'m trying to do
better this time ;-)

My first thought is, add a negative power rail. I\'ve got +15v and +5v
so there\'s already one switcher (LM2842 based), so I could add another
one. Suggestions for a simple invert-switcher? How negative do I need
to go? I\'d been running the LM324 on +5 and GND, and need a 0..5 output
signal anyway (+- 2.5v biased to 2.5v), so -5? Then I\'m thinking a 5.1v
zener to protect the ADC input.

Alternately, opinions on an alternate quad low-side sensor that can
handle positive and negative currents...

Also, I\'d been using a resistor divider for each opamp to bias it to
2.5V. Is it worth the effort to put in a 2.5V rail and use that, so
that all the sensors have a common \"zero\" at the ADCs, vs possibly
different ones due to resistor tolerances?

Thanks!

[1] http://www.delorie.com/electronics/bldc/

Something like this should work.

https://www.dropbox.com/s/kcvi27lqv60nhdi/BLDC_lowside_shunt.jpg?raw=1

The opamp inputs run above ground.

The caps are optional. Some tricks can be played to make this 4-wire
differential, or 3-wire sorta diff which preserves the shared divider.

Autozeroing is still a good idea.





--

I yam what I yam - Popeye

 

[Hul Tytus]

\"... went below 0v at the input pins\". adc input or lm324 input?
And while we\'re here: what gain have you set the lm324 to?

Hul

DJ Delorie <dj@delorie.com> wrote:

If we could take a moment to talk about something ON topic for a change ;-)

I\'m updating an old design that uses low-side current sensing for BLDC
drive[1]. I\'m sensing both forward and reverse current. So the signal
is about +- 40 mV but could be more. I had been using an LM324 in
differential mode with a 2.5V \"ground\" to offset the sense voltages, but
the signal still went below 0V at the input pins. I\'m trying to do
better this time ;-)

My first thought is, add a negative power rail. I\'ve got +15v and +5v
so there\'s already one switcher (LM2842 based), so I could add another
one. Suggestions for a simple invert-switcher? How negative do I need
to go? I\'d been running the LM324 on +5 and GND, and need a 0..5 output
signal anyway (+- 2.5v biased to 2.5v), so -5? Then I\'m thinking a 5.1v
zener to protect the ADC input.

Alternately, opinions on an alternate quad low-side sensor that can
handle positive and negative currents...

Also, I\'d been using a resistor divider for each opamp to bias it to
2.5V. Is it worth the effort to put in a 2.5V rail and use that, so
that all the sensors have a common \"zero\" at the ADCs, vs possibly
different ones due to resistor tolerances?

Thanks!

[1] http://www.delorie.com/electronics/bldc/

 

[Hul Tytus]

One other thought: set the processor adc input to have a slightly
positive offset and then counter that offset in the \"ad conversion
complete\" interupt handler.

Hul

DJ Delorie <dj@delorie.com> wrote:

If we could take a moment to talk about something ON topic for a change ;-)

I\'m updating an old design that uses low-side current sensing for BLDC
drive[1]. I\'m sensing both forward and reverse current. So the signal
is about +- 40 mV but could be more. I had been using an LM324 in
differential mode with a 2.5V \"ground\" to offset the sense voltages, but
the signal still went below 0V at the input pins. I\'m trying to do
better this time ;-)

My first thought is, add a negative power rail. I\'ve got +15v and +5v
so there\'s already one switcher (LM2842 based), so I could add another
one. Suggestions for a simple invert-switcher? How negative do I need
to go? I\'d been running the LM324 on +5 and GND, and need a 0..5 output
signal anyway (+- 2.5v biased to 2.5v), so -5? Then I\'m thinking a 5.1v
zener to protect the ADC input.

Alternately, opinions on an alternate quad low-side sensor that can
handle positive and negative currents...

Also, I\'d been using a resistor divider for each opamp to bias it to
2.5V. Is it worth the effort to put in a 2.5V rail and use that, so
that all the sensors have a common \"zero\" at the ADCs, vs possibly
different ones due to resistor tolerances?

Thanks!

[1] http://www.delorie.com/electronics/bldc/

 

[legg]

On Fri, 18 Mar 2022 00:08:23 -0400, DJ Delorie <dj@delorie.com> wrote:

If we could take a moment to talk about something ON topic for a change ;-)

I\'m updating an old design that uses low-side current sensing for BLDC
drive[1]. I\'m sensing both forward and reverse current. So the signal
is about +- 40 mV but could be more. I had been using an LM324 in
differential mode with a 2.5V \"ground\" to offset the sense voltages, but
the signal still went below 0V at the input pins. I\'m trying to do
better this time ;-)

-2V5 seems like a rather large source signal.

Is the sensed current that produces the \'below ground\' situation
normal or overload?

Is a linear response required under those conditions?

If not, why not just ad sensor protection/conditioning?

RL

 

[DJ Delorie]

Hul Tytus <ht@panix.com> writes:
> \"... went below 0v at the input pins\". adc input or lm324 input?

The lm324\'s inputs

> And while we\'re here: what gain have you set the lm324 to?

34x (2k input, 68k feedback)

One other thought: set the processor adc input to have a slightly
positive offset and then counter that offset in the \"ad conversion
complete\" interupt handler.

Interesting idea, but I\'d lose some precision in the adc. I saw some
solutions that biased the sense the *other* way too - more adc range on
the useful side.

 

[DJ Delorie]

legg <legg@nospam.magma.ca> writes:
> -2V5 seems like a rather large source signal.

The source signal is around 40mV in the old design, closer to 0.5v in
the new design (which I\'m not happy about)

The opamp changes that to a 0-5V swing centered around 2.5v

Is the sensed current that produces the \'below ground\' situation
normal or overload?

It\'s a servo. When it\'s speeding up, it\'s a motor, and everything\'s
positive. When it\'s slowing down, it\'s a generator, and the currents
are negative.

 

[John Larkin]

On Sat, 19 Mar 2022 13:33:50 -0400, DJ Delorie <dj@delorie.com> wrote:

Hul Tytus <ht@panix.com> writes:
\"... went below 0v at the input pins\". adc input or lm324 input?

The lm324\'s inputs

Classic LM324s can go insane on all four amps if one input pin is
pulled a bit below ground.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

 

[DJ Delorie]

Mikko OH2HVJ <mikko.syrjalahti@nospam.fi> writes:

Things I\'ve used for similar setup depending on accuracy need. BLDC
driver does not sound like metrology application ;-)

When you\'re trying to hold a 0.09 degree step, it might as well be.

> - Large negative rail

One poster had a clever idea for generating this with a charge pump off
the main switcher.

> - LM7705 to generate -0.23V rail

That\'s a new one on me!

- Resistor divider (one or two sided) from sense resistor to 3.3V (say,
40 ohms and 3.3V cause the -40mV to show up at 0mV, increase 40ohms to
\'lift\' the signal).

I\'m not sure I want all the motor power potentially travelling through
the MCU\'s vcc bus.

> - Hall current sensor

One poster hinted at these, and I\'ve been looking at the TMCS1101 which
would let me move the current sense to the three BLDC wires instead of
the negative rail, so I could sense the positively-driven phases as
well. It causes some layout issues, but I think it will be the most
useful.

 

[Lasse Langwadt Christensen]

lørdag den 19. marts 2022 kl. 18.47.15 UTC+1 skrev DJ Delorie:

Mikko OH2HVJ <mikko.sy...@nospam.fi> writes:
Things I\'ve used for similar setup depending on accuracy need. BLDC
driver does not sound like metrology application ;-)
When you\'re trying to hold a 0.09 degree step, it might as well be.

- Large negative rail

One poster had a clever idea for generating this with a charge pump off
the main switcher.
- LM7705 to generate -0.23V rail
That\'s a new one on me!
- Resistor divider (one or two sided) from sense resistor to 3.3V (say,
40 ohms and 3.3V cause the -40mV to show up at 0mV, increase 40ohms to
\'lift\' the signal).
I\'m not sure I want all the motor power potentially travelling through
the MCU\'s vcc bus.

- Hall current sensor

One poster hinted at these, and I\'ve been looking at the TMCS1101 which
would let me move the current sense to the three BLDC wires instead of
the negative rail, so I could sense the positively-driven phases as
well. It causes some layout issues, but I think it will be the most
useful.

easy to try, https://www.ebay.com/itm/173284477757

 

[legg]

On Sat, 19 Mar 2022 13:38:27 -0400, DJ Delorie <dj@delorie.com> wrote:

legg <legg@nospam.magma.ca> writes:
-2V5 seems like a rather large source signal.

The source signal is around 40mV in the old design, closer to 0.5v in
the new design (which I\'m not happy about)

The opamp changes that to a 0-5V swing centered around 2.5v

I couldn\'t find any offsets in your schematic and didn\'t see the gain.

There are a number of op amps that don\'t have issues with inputs
going tens of millivolts below ground. Are you sure its an issue?

Is the sensed current that produces the \'below ground\' situation
normal or overload?

It\'s a servo. When it\'s speeding up, it\'s a motor, and everything\'s
positive. When it\'s slowing down, it\'s a generator, and the currents
are negative.

Same question - are you actually trying to control the current during
the overload, or is it simply an overloaded monitor?

RL

 

[Mikko OH2HVJ]

DJ Delorie <dj@delorie.com> writes:

Mikko OH2HVJ <mikko.syrjalahti@nospam.fi> writes:
- Resistor divider (one or two sided) from sense resistor to 3.3V (say,
40 ohms and 3.3V cause the -40mV to show up at 0mV, increase 40ohms to
\'lift\' the signal).

I\'m not sure I want all the motor power potentially travelling through
the MCU\'s vcc bus.

A typo from me - I meant 40 (or 400) ohms to shunt non-GND-end and 3k3
(or 33k) to 3.3V so you\'re running roughly constant 1mA through. This
causes a small gain/offset error, but that\'s easy to compensate.

--
mikko