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Phase accurate primary current sensing

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Avi

Sat Dec 29 2018, 02:37AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

Does anyone know of a galvanically isolated technique for measuring the primary current of a DRSSTC, that is phase accurate?
It must: work up to 1MHz, support isolation voltages seen in a DRSSTC, support currents seen in a DRSSTC.
The amplitude is not very relevant, this will just be to feed into a zero current detector circuit.

hen918

Mon Dec 31 2018, 11:53AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

There is one series of IC that will do it: the allegro acs732 series. Bandwidth of 1 MHz, only problem is the current rating, which is 40 A bidirectionally; a little low.
A current transformer will do fine though, it fits every requirement and can even be made pretty easily, just don't forget a burden resistor to convert the current to a voltage!

klugesmith

Mon Dec 31 2018, 04:58PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Hen's current transformer suggestion sounds good to me. There are ferrite core materials with high permeability and low loss up to hundreds of MHz, e.g. in baluns. One might explore Rogowski coils, if no suitable ferrite core is on hand, but I think they are not trivial at high frequencies.

Since you only care about zero crossings, the burden on CT secondary could be two diodes in "antiparallel". How about a single Zener diode instead, to simplify digital logic level shifting?

That Allegro series current sensor could probably have its range extended with appropriate external shunt resistor (mind the inductances). Or use several identical devices in parallel, and combine their outputs.

I bet it's based on Hall effect. What's the time delay corresponding to its 1 MHz bandwidth rating, and does the bandwidth vary widely with process and temperature?

If both ends of DRSSTC primary are "hot", voltage wise, consider electrostatic shielding in or around the current sensor.

Why is zero current a time of interest in a DRSSTC? Just curious.

Avalanche

Mon Dec 31 2018, 08:06PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I did some work on this a while ago. I've dug up the old thread because it had some excellent info from Richie Burnett -

Uspring

Tue Jan 01 2019, 05:09PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Why is zero current a time of interest in a DRSSTC? Just curious.

Usually a transistor bridge drives the primary. Switching at zero current minimises switching losses and transients of the inductive load.

Avi

Wed Jan 02 2019, 12:16AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

wrote ...

Why is zero current a time of interest in a DRSSTC? Just curious.

as Uspring said, it is best (for the semiconductors) for the interrupt turn off to be delayed until there is 0 current going through the primary

wrote ...

current transformer

here is a comparison of a non isolated resistive bridge (top waveform), vs a current transformer (bottom waveform)
you can see the ~90 degree phase shift

wrote ...

That Allegro series current sensor could probably have its range extended with appropriate external shunt resistor

Interesting!
unfortunately without a DSO I have no idea what sort of peak currents are involved to calculate the shunt (when run from 330v rectified mains)

wrote ...

(mind the inductances)

im thinking if i use a shunt bar (similar to the one attached), it'd be ok

Weston

Wed Jan 02 2019, 05:15AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Current transformers can work reliably up to many MHz. They do use some patented distributed resistive termination scheme, but you can buy Pearson current transformers that provide accurate phase measurement to 100MHz+. Most of the DRSSTC controllers use a current transformer for phase accurate current sensing.

Unless you used a completely unsuitable core (something like powdered iron) the phase shift you are seeing is probably in the resistive shunt. Big shunt resistors like that typically have a very low resistance but a comparatively large inductance due to the large physical size, becoming predominately inductive around a few tens or hundreds of KHz.

klugesmith

Wed Jan 02 2019, 05:17AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

wrote ...
here is a comparison of a non isolated resistive bridge (top waveform), vs a current transformer (bottom waveform)
you can see the ~90 degree phase shift.

Are those nice pictures yours? What's the frequency, or the scope horizontal scale?
An ideal current transformer with resistive load has no phase shift. What if some or all of the apparent phase shift is in the signal from resistive shunt, due to parasitic inductance, and/or from induced voltage in its scope-probe-and-ground loop? One way to check the latter is to connect probe signal and ground to the same end of current shunt.

wrote ...
>>That Allegro series current sensor could probably have its range extended with appropriate external shunt resistor ...
>>(mind the inductances)
I'm thinking if i use a shunt bar (similar to the one attached), it'd be ok

To make a point about inductance in low-ohm circuits, here's a thought experiment using ACS732-ish values.
Make a current divider by connecting two 1-milliohm resistors in parallel.
Disturb the symmetry by adding, in just one branch, 1 nanohenry of equivalent series inductance.
What is the current ratio between branches A and B, at 1 MHz?
What is the phase difference between currents A and B at 100 kHz?

[edit] Written before but had to be re-typed. Nanohenries can be sneaky. Consider power supply bypass capacitors, on a multilayer circuit board with dedicated power and ground planes. Place leadless ceramic chip capacitors with vias right next to the pads. Each one comes with equivalent series inductance on the order of a nanohenry, when you include the connection loop.

Uspring

Wed Jan 02 2019, 10:45AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Current transformer phase accuracy depends on the inductance reactance to shunt resistance ratio. Ideally it becomes a 100% accurate, if the shunt resistance is zero. In practice one has to add the copper resistance of the CT to the shunt resistance, so it never is really a 100% accurate. Usually the copper resistance can be neglected. Care hast to be taken, though, not to make the shunt resistance too large.

Avi

Wed Jan 02 2019, 10:49AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

wrote ...
They do use some patented distributed resistive termination scheme, but you can buy Pearson current transformers that provide accurate phase measurement to 100MHz+.

ohhhhhhhhh!

not that those prices :(

wrote ...
Big shunt resistors like that typically have a very low resistance but a comparatively large inductance due to the large physical size, becoming predominately inductive around a few tens or hundreds of KHz.

given that everything is connected with clipleads, a shunt-bar is probably the least inductive part of the setup!

wrote ...
Are those nice pictures yours?

"nice"?
yes they are mine

wrote ...
What's the frequency

probably about 320Khz, however it must work with different coils too without manually tuning some delay parameter

if i was to use a CT, seems i have to chose between a Flame Emitting Resistor

or an out of phase signal

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