measuring floating inductor current in a truesine inverter application

CT. Paul Mathews

current transformer, Ya I guess that gets rid of the high commom mode voltage, and then it is just a differential measurement that is required?

Thanks I will look into this more.

cheers, Jamie

Simpler: if that's possible, just add a few turns winding to your inductor. It'll give you dPhi/dt that you then integrate (can be as simple as a lowpass RC).

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Thanks,
Fred.

That sounds like it would be the cheapest method. Since that is integrated/filtered, will it be too slow to detect cycle by cycle current for comparator based current limiting?

I found a high frequency current transformer, would this do the job?

cheers, Jamie

Slow? You just integrate di/dt.

The only missing info is the DC current, but you the same pb with a CT. and under some conditions you can restore it.

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Thanks,
Fred.

Hi,

since current transformers only work for AC, and the only AC here is the

60Hz output, I don't think current transformers can work to detect the high frequency current.

cheers, Jamie

So are there any other options to measure the ripple of a 200kHz current?

cheers, Jamie

Here's the circuit I am working with and the 200kHz current waveform for one of the output inductors:

The current waveform is DC at this frequency, but is AC at 60Hz.

I don't think a current transformer can work due to the fact that this is effectively a DC current, haven't found a fast enough hall sensor for detecting over-current, and a shunt amplifier would have to be floating at high voltage.

cheers, Jamie

You can't use a clamp-on probe?

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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LEM DCCTs. hall-effect devices. design an AC CT that wont saturate with the DC current.

Cheers Terry

you have some sort of floating gate-drive for the upper FETs. a current sense resistor in the output lead(s) sits at the Source of (one or both of) the upper FET, so there is where you can sit some circuitry to then transfer that signal to the control circuitry. there are some current sensing optos, but they tend to want large voltage drops (around 0.5V) which might not be fun, depending on your power levels. of course a small Rs, and amp and some circuitry can "fool" the optos...

Cheers Terry

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Provided that the DC doesn't saturate its core, a CT will couple the ac component of the current to its loaded secondary. This is done all the time. The appropriate core size and material will handle the dc offset. -Paul Mathews

I need to measure the DC offset though for cycle by cycle current limiting at 200kHz, the AC current is 60Hz, the DC current is 200kHz.

What about using a shunt resistor and dual rail opamp powered by a local isolated bipolar powersupply like this one:

(thats the same kind of supply I'm using to power the floating fets, except not bipolar)

Could even use this supply to power a floating ADC and DAC, I need about

12bits of current and voltage accuracy so I will need to get the supply pretty smooth, which may be a problem, maybe the current transformer is better.

cheers, Jamie

Maybe the circuit inside would work, I guess a DC clamp on probe must be hall based? I'm making a PCB! :)

cheers, Jamie

(forgot the flux draining coil!)

like here:

cheers, Jamie

use diagonal half-bridge - switch at either end of primary winding, diodes to opposit supply rails. voila, no need for a "flux draining coil" - lovely terminology BTW.

Cheers Terry

Thanks :) This seems to do the job for the dual fet/diode version with no extra coil:

cheers, Jamie

yep. and for added fun, using P and N fets (or BJTs), the lower switch can provide the gate drive for the upper switch :)

Cheers Terry

That one is best of all, thanks!

cheers, Jamie