BIPOLAR ALTERNATIVE TO H BRIDGE

[moodz]

... Microsoft onedrive corrupts my shared libraries.. delete V10 and v11 and enter your own.. should work then.
should be called voltage not voltage_dc
moodz

[Altra]

Looks very interesting so thought I'd have a play with the sim file you posted but LTSPICE can't seem to find the symbol for V10 & V11. (VOLTAGE_DC)
May be my copy of LTSPICE though as I've just updated it to 17.0.42 (mac) - what version are you using ?

Regards,
Olly

Hi Olly, I'm getting the same message. Probably the file got corrupted?

[Olly]

Thanks Mark ...

attached is the sim file to play with.

The TX frequency is 200 microseconds ( 5 Khz ) ... with 5 microsecond sampling at start and end of current pulse to measure "tilt or ramp error".

The target switches in and out at a 10 millisecond rate to test loop recovery.

With correct timing adjustment will support other TX frequencies.

moodz / Paul.

Looks very interesting so thought I'd have a play with the sim file you posted but LTSPICE can't seem to find the symbol for V10 & V11. (VOLTAGE_DC)
May be my copy of LTSPICE though as I've just updated it to 17.0.42 (mac) - what version are you using ?

Regards,
Olly

[eclipse]

Pair this with the zero point active damping and you have very nice combo right there.

[Altra]

Thanks for the sim. I'll study it!

[moodz]

Thanks Mark ...

attached is the sim file to play with.

The TX frequency is 200 microseconds ( 5 Khz ) ... with 5 microsecond sampling at start and end of current pulse to measure "tilt or ramp error".

The target switches in and out at a 10 millisecond rate to test loop recovery.

With correct timing adjustment will support other TX frequencies.

moodz / Paul.

[Altra]

.... a proposed solution to the ramp problem.

1.2 amp swing / 750 volt flyback / 2.6 watt consumption from 1.5 volts

moodz

Wow that looks interesting. Can't wait to see a test circuit in action.

What is the Tx frequency?

Also following the Awesome PI thread.

Thanks for posting your work.

Mark

[moodz]

.... a proposed solution to the ramp problem.

1.2 amp swing / 750 volt flyback / 2.6 watt consumption from 1.5 volts

moodz

[Tinkerer]

With real circuits I found that there is a certain amount of tolerance in the TX square wave slope.
However, the slope needs to be compensated.
The way we compensate it is measuring the losses at every cycle and adjust the TX current through microprocessor control.

More and more the PI becomes digital.

[Teleno]

After some time your circuit starts working , but the current will not be constant - a horizontal line . If a computer simulation helped you with this decision - this program is for update . I used a thought experiment , if I'm wrong - I'll update myself by hitting myself on the head …

Nothing is constant in the Universe, there are no absolutely horizontal lines but I challenge you to get a closer approximation especially when the Tx coil is coupled to a target.

[Riss]

Here's my attempt to solve the ramp problem. I've added a current limiter on each side of the bridge. When M3, M4, M5 and M8 are selected for both low Cgs and Rdson the system doesn't ring and the variation in the top current is about 100uA for a 1A pulse.

[ATTACH]temp_50921_1673892536251_942[/ATTACH]

Ingeniously simple solution , but immediately after the current is reversed , the current starts to rise , cause - losses in the coil during the reversal of the current . After some time your circuit starts working , but the current will not be constant - a horizontal line . If a computer simulation helped you with this decision - this program is for update . I used a thought experiment , if I'm wrong - I'll update myself by hitting myself on the head …

[Teleno]

Here's my attempt to solve the ramp problem. I've added a current limiter on each side of the bridge. When M3, M4, M5 and M8 are selected for both low Cgs and Rdson the system doesn't ring and the variation in the top current is about 100uA for a 1A pulse.

[lucifer]

I had a look at your references ... The h bridge is fine for non inductive loads but I think you are not going to get CC pulses in an inductive load ??.

Well.. it's been quite a while since I played briefly with those circuits. I've tested the H-bridge circuit with a capacitor across the load (coil) and it works but pulses are not quite flat-topped.

The second reference PDF is interesting for allowing the pulses to be spaced by off periods however the pulse tops show a clear tilt ( ie not constant ) so this scheme is not better than what I proposed. ( and the pulse rise times will be impacted by storage of energy in the switched cap )

Yes, I suppose this circuit won't produce perfect flat-top current pulses too. The capacitor however is charged by the coil flyback voltage and then this voltage is applied to the coil for a faster rise time. Authors claim they got faster current rise times this way.

The main aim of my proposal is to generate pulses at relatively high current int the TX coil with minimal supply requirements ( eg 1 or 2 volts ). The main criteria is that the rise times of the pulses ( di/dt ) is much greater than the dwell time ( di/dt).
DSP in the RX coil takes care of resolving target information. You can achieve CC operation with a compensation network ( in 100us the current changes by just over 1 ma ) However because the coil of a metal detector is in random motion over the target a fixed field ( ie true CC ) is a moot point.
A well known company spent alot of time and money fine tuning the CC control of their high end CC pulse metal detector product ... and guess what ... it made no difference to the performance when they did effectively achieve that goal. ( short answer = no improvement ).

Agree, I suppose you never get a perfect waveform, even with compensation circuits. If you refer to ML, I've seen their patents. Even a small tilt in current shouldn't be much of a problem. To me the coil itself and RX circuit are more important.

..just my opinion ... I respect your view there is no right or wrong ... whatever works :-)

I just want to share information and my humble experience, don't mean to offend anybody or tell what's right or wrong

Have a look at this patent ....

https://patents.google.com/patent/US4157579A/en

The guy uses a DC current source ... not to be confused with a CC source.

Thanks, I'll look at that.

[moodz]

Here is a patent to consider :-)

https://patents.google.com/patent/US20140232408A1/en

[moodz]

Some refer to this as a current source inverter. For example see attached extract from an old book on power electronics. Another similar concept with some improvements is presented in the second PDF paper attached.

I had a look at your references ... The h bridge is fine for non inductive loads but I think you are not going to get CC pulses in an inductive load ??.
The second reference PDF is interesting for allowing the pulses to be spaced by off periods however the pulse tops show a clear tilt ( ie not constant ) so this scheme is not better than what I proposed. ( and the pulse rise times will be impacted by storage of energy in the switched cap )

The main aim of my proposal is to generate pulses at relatively high current int the TX coil with minimal supply requirements ( eg 1 or 2 volts ). The main criteria is that the rise times of the pulses ( di/dt ) is much greater than the dwell time ( di/dt).
DSP in the RX coil takes care of resolving target information. You can achieve CC operation with a compensation network ( in 100us the current changes by just over 1 ma ) However because the coil of a metal detector is in random motion over the target a fixed field ( ie true CC ) is a moot point.
A well known company spent alot of time and money fine tuning the CC control of their high end CC pulse metal detector product ... and guess what ... it made no difference to the performance when they did effectively achieve that goal. ( short answer = no improvement ).

..just my opinion ... I respect your view there is no right or wrong ... whatever works :-)

Have a look at this patent ....

https://patents.google.com/patent/US4157579A/en

The guy uses a DC current source ... not to be confused with a CC source.