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.
current sensors for high frequency converters
Mar 22, 2007
2 Replies
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 ;)
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
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