EXTEND CURRENT TRANSFORMER RANGE PHILIP TODD Transformers us

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EXTEND CURRENT TRANSFORMER RANGE PHILIP TODD Transformers us

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DN-41
EXTEND CURRENT TRANSFORMER RANGE
PHILIP TODD
Transformers used extensively current sensing because they monitor currents with very power loss they have wide bandwidth good waveform fidelity. Current transformers perform well applications with symmetrical currents such push-pull full bridge converter topologies. single-ended applications, especially boost converters, problems arise because need accurately reproduce high duty factor, unipolar, waveforms. Unipolar pulses saturate current transformer and, this happens, over current protection will lost and, current mode control, regulation will lost over voltage condition will result. transformer core must reset after each pulse that full range transformer will available next pulse. Self reset current transformer most common techniques used drawbacks. Self reset uses energy stored current transformer core reset depends open circuit impedance current transformer generate enough volt-seconds short period time reset. Current transformers operated above duty factor have enough stored energy allow complete reset time available this situation becomes worse duty factor approaches 100%. magnetizing inductance current transformer must kept high because this determines amount droop current waveform will exhibit over pulse period. higher inductance lower droop will waveform droop opposes slope compensation should kept minimum. High magnetizing inductance also means that core stores very little energy which used reset core. current transformer turns ratio generally needs high lower power loss. more turns core, however, greater leakage inductance greater parallel capacitance. leakage inductance itself generally problem will limit current rise fall times. parallel capacitance also limits bandwidth current transformer greater problem during transformer reset. transformer reset properly, energy stored core must removed. self reset this energy must transfer from magnetizing inductance parallel capacitance resonant manner. capacitance large, resonant frequency will magnetizing inductance will reset before next pulse begins.
Figure Conventional self-reset current transformer Figure shows conventional current transformer circuit which uses self reset. current flowing primary, causes current flow through generate output voltage proportional Vc=IRs/N where current transformer turns ratio. problems discussed above occur during reset interval when I=O. core have enough energy fully reset itself time available given secondary capacitance plus capacitance problems with self reset current transformers unipolar pulse applications overcome with simple forced reset techniques derived from magnetic amplifiers. Duty factors above achievable with these techniques.
DN-41
current pulse measured. This beneficial some applications doubles number volt-seconds available from transformer.
Figure Current transformer with forced reset Figure shows same circuit Figure configured forced reset. diode been moved from high side transformer secondary winding ground side. This, course, effect operation circuit during pulse this circuit behaves exactly expected. During reset, however, makes circuit operation quite different. current from through much greater than self reset current available from magnetizing current transformer. This forcing current rapidly charges parasitic capacitances reverses voltage secondary transformer. applied volt-seconds quickly reset core that high duty-factor operation possible. forced reset high enough drive current transformer into saturation this acceptable practice because core will saturated opposite direction (i.e. full reset) from Figure Negative output current sense some applications desirable generate negative voltage from current transformer. This accomplished without negative voltage source reset transformer. Figure shows configuration. this circuit there will error because reset current subtracts from sense current during pulse. Care must taken minimize this effect. There other circuit configurations which possible force reset current transformer. Switches used switch reset current off. Additional windings center tapped windings used also. Many circuits possible provide specific improvement expense complexity. circuits shown here simplest available illustrate basic concept.
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EXTEND CURRENT TRANSFORMER RANGE PHILIP TODD Transformers us

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