 DRSSTC Current Transformers
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GeordieBoy
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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
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Ideally a CT's secondary should be terminated into a short-circuit. In practice we typically just use a very low resistance.
In trying to answer the question of "how low a resistance?" we need to look at what things go wrong if the burden resistance is too large. Significant voltage develops across the CT's secondary. This can cause two things to happen, noth of which degrade measurement accuracy. Firstly, the core can saturate at low frequencies due to excessive volt-seconds causing complete loss of measurement. And, secondly a portion of the sensed current will be diverted into magnetising current. The more voltage you develop across the secondary winding of the CT, the more magnetising current will flow. This magnetising current detracts from the current being sensed and means that the CT reads under. Power electronics engineers often refer to this behaviour as "CT droop". Excessive CT droop can cause problems in current-sensing or short-circuit protection in SMPSUs.
If you have lots of turns on the secondary of a CT you can get away with a higher burden resistor because the magnetising inductance is larger mitigating the CT droop behaviour. If you only have 10 turns or so on the secondary then the CT burden needs to be very low resistance. Any voltage developed across 10 turns will quickly saturate the CT core.
Ideally you would use an active termiantion for your CT, and terminate it's secondary into the virtual-earth of an op-amp. A trans-impedance amplifier neatly provides this function whilst also converting the current to a voltage scaled by it's feedback resistor.
Another thing that is sometimes done in SMPSU's is to bias the CT into saturation in one direction to allow twice the current sensing range in the opposite direction. This is useful for measuring pulsed unidirectional currents that might otherwise saturate a CT with passive burden.
I seem to remember that Unitrode have some good app notes covering CT design and burden choice for current sensing in SMPSU.
-Richie,
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