EE 136 Project CURRENT TRANSFORMERS By Pramod Haridass

ADVANTAGES OF CURRENT TRANSFORMERS Indicates trends, changes or peak values in most switchmode power supplies. Indicates trends, changes or peak values in most switchmode power supplies. Give good signal-to-noise ratio (remember EE 164 !) Give good signal-to-noise ratio (remember EE 164 !) Provides isolation between the control circuit and the monitered line. Provides isolation between the control circuit and the monitered line. Provides good Common Mode Rejection Ratio Provides good Common Mode Rejection Ratio Less power loss for high current applications. Less power loss for high current applications.

Four types of Current Transformers Unidirectional Current Transformer (discussed) Unidirectional Current Transformer (discussed) Current transformers for AC Current (push-pull) applications. (discussed) Current transformers for AC Current (push-pull) applications. (discussed) Flyback-type current Transformers (not discussed) Flyback-type current Transformers (not discussed) DC Current Transformers (not discussed) DC Current Transformers (not discussed)

Diagram showing the schematic for a unidirectional current transformer TYPE 1: UNIDIRECTIONAL CURRENT TRANSFORMER

For above diagram diode D1 blocks reverse recovery voltage (thus the name unidirectional current transformer).

Diagram showing the effect of magnetization current on the secondary current output of the current transformer

Factors affecting secondary current Number of turns on secondary coil Number of turns on secondary coil Inductance of core Inductance of core Permeability of core(opposite to remanance) Permeability of core(opposite to remanance) Core Size Core Size

Solution for the problem of magnetizing current Reduce the secondary voltage to as small as possible so that a good signal to noise ratio would be obtained. The reason for this is that when we have larger values for Vo (secondary voltage), larger magnetizing currents would be obtained Notice that there only ONE TURN in the primary region of the coil (turns ratio would be larger)

Diagram of a single ended forward converter, incorporating a UNIDIRECTIONAL current transformer)

HYSTERESIS LOOP Mr - remanance flux density at the end of one forward cycle, and at the start of the next one.

TYPE 2: Current transformers (For AC current) push-pull applications

The above diagram shows an AC current transformer in series with the primary of a half bridge converter transformer. The same current flows for forward and return pulses, as the series cap C3 ensures that there will not be a DC component. Hence, the core will not saturate.

Conclusion Current transformers are used to monitor the discontinuous current pulses in the output rectifiers of a high current forward converter or the primary current of a single ended forward converter. Current transformers are used to monitor the discontinuous current pulses in the output rectifiers of a high current forward converter or the primary current of a single ended forward converter. Only the unidirectional current transformer and the AC current Transformer are widely used in industry today. Only the unidirectional current transformer and the AC current Transformer are widely used in industry today.