In many power supplies and circuits it is advantageous and even necessary to monitor the currents flowing in, out, or through a power supply circuit. For example, monitoring for transient overcurrent conditions in the output a DC-DC converter to trigger protection circuitry or to monitor the resonant tank current for control of a Resonant LLC circuit.
While many AC current sense solutions exist on the open market, the current sense transformer can be an appealing solution in many applications. A combination of low primary DCR, low core losses (due to typically low core flux densities), and small secondary currents give rise to a simple, effective, and low-cost method of sensing AC current.
The operation of a current sense transformer is relatively straight-forward but often misunderstood. A current sense transformer typically has two windings each of which passes through a ferromagnetic core. The primary winding (typically one turn) is designed with low resistance and is in series with the current that is being measured. The secondary winding (typically ranging from twenty to two hundred turns) has higher DCR and carries the much lower current which will be sensed via a burden resistor ( ) that is in series with the secondary winding. The sensed voltage () can then be fed to a comparator or other IC to take the appropriate action. A typical application circuit can be seen below, although it is possible for the rectifying diode to be omitted depending on the sensing requirements:Read More