current sensors for high frequency converters

[aravind]

hi all,
I am working on a project which involves, sensing the current
through the switch (connected in series with the flyback transformer)
of a flyback converter (max current is abt 15 amp).Operating frequency
is 50KHz. I have tried to use LEM Hall Effect sensor LA 55-P, but the
sensor is not able to sense the currents at this frequency faithfully,
though its bandwidth is about 200KHz (at -1 dB), can anyone please
suggest anyother sensor, or anyother way of sensing the current
faithfully.

 

[[email protected]]

hi all,
I am working on a project which involves, sensing the current
through the switch (connected in series with the flyback transformer)
of a flyback converter (max current is abt 15 amp).Operating frequency
is 50KHz. I have tried to use LEM Hall Effect sensor LA 55-P, but the
sensor is not able to sense the currents at this frequency faithfully,
though its bandwidth is about 200KHz (at -1 dB), can anyone please
suggest anyother sensor, or anyother way of sensing the current
faithfully.

You need a current transformer. Do you want to design one or measure
current once?

If all you want to do is measure, take a look at something like a
Tektronix P6042.
It is a clip-on DC-50MHz current probe. I'm sure there are modern
instruments like this.

If you are in-circuit, you can add a low value resistor and sense the
current through that. Bandwidth of a resistor is quite high

 

[scada]

hi all,
I am working on a project which involves, sensing the current
through the switch (connected in series with the flyback transformer)
of a flyback converter (max current is abt 15 amp).Operating frequency
is 50KHz. I have tried to use LEM Hall Effect sensor LA 55-P, but the
sensor is not able to sense the currents at this frequency faithfully,
though its bandwidth is about 200KHz (at -1 dB), can anyone please
suggest anyother sensor, or anyother way of sensing the current
faithfully.

try www.ohiosemitronics.com

 

[Tom Bruhns]

hi all,
I am working on a project which involves, sensing the current
through the switch (connected in series with the flyback transformer)
of a flyback converter (max current is abt 15 amp).Operating frequency
is 50KHz. I have tried to use LEM Hall Effect sensor LA 55-P, but the
sensor is not able to sense the currents at this frequency faithfully,
though its bandwidth is about 200KHz (at -1 dB), can anyone please
suggest anyother sensor, or anyother way of sensing the current
faithfully.

It's common to use a current transformer, though you should be careful
about saturation of the core, especially since the switch current must
have a large DC component in your flyback design. Of course, that
won't give you the DC value. It's also common to put in a resistor
across which you can measure the voltage drop to determine the
current. If you can put it in the source/emitter lead of the switch,
it will probably be easier to measure the drop across it, since it
will be referred (presumably) to ground. You are operating at a low
enough frequency that you should be able to find an appropriate
resistor with low enough inductance. Assuming the current is 15A for
50% of the time, the power dissipation in an 0.01 ohm resistor would
be 1.125 watts. You'd want the inductance of such a low-value
resistor to be less than about 5 nanohenries, preferably quite a bit
less, to make it useful out to 200kHz and beyond. One form such
resistors take is a wide strap of metal with three terminals at each
end, allowing a Kelvin connection to measure the drop across the
resistance.

You can find current transformers (really just small ferrite toroid
cores with a secondary already wound on them) designed specifically
for monitoring currents in switching power supplies. They have a
"flat" frequency response out to several MHz, usually. They add a
small amount of inductance and resistance to the lead around which
they are placed, but it's not a lot if you load the secondary properly
with a low resistance.

Cheers,
Tom