A current transformer can provide isolated, low-loss current sensing for overcurrent protection, current-mode control, and diagnostic measurement. In SiC and GaN converters, switching edges are fast and fault energy can rise quickly, so the sensor must reproduce a useful current signal before the semiconductor leaves its safe operating area. A wide-band current transformer design is therefore an exercise in pulse fidelity, saturation margin, common-mode immunity, and physical layout, not simply a turns-ratio calculation.
Define the signal the protection circuit needs
Start with minimum detectable current, maximum normal peak, short-circuit current rise rate, required trip delay, pulse width, switching frequency, duty range, and allowable measurement error. A protection circuit may not need laboratory-grade amplitude accuracy at every harmonic, but it must distinguish normal commutation spikes from a genuine fault and remain predictable at temperature.
The complete delay budget includes transformer response, burden and rectifier, filter, comparator, logic, isolator if used, and gate-driver shutdown. A sensor with excellent bandwidth cannot compensate for a slow downstream path.
Turns ratio and burden resistance
Ideally, secondary current equals primary current divided by turns. The burden resistor converts that current into voltage. A larger burden gives more signal but requires more secondary voltage, increasing flux and reducing saturation margin. It also interacts with leakage inductance and winding capacitance to shape high-frequency response.
The burden should be located close to the current transformer secondary. Long traces add inductance and create a larger pickup loop. If rectifiers or clamps are used, their forward voltage, junction capacitance, recovery, and temperature behavior become part of the transfer function.
Low-frequency droop and pulse area
A current transformer cannot pass DC. Magnetizing inductance determines low-frequency droop during a current pulse. The relevant stress is the time integral of secondary voltage, or volt-seconds. Long on-times, duty-cycle imbalance, startup pulses, burst groups, and fault-latch delays can drive the core farther than short nominal switching pulses.
Core reset must be guaranteed for every operating sequence. A unipolar pulse train can accumulate flux if the secondary circuit does not provide a suitable reset path. Clamp polarity, burden network, diode conduction, and blanking intervals need waveform-level review.
High-frequency limits
Leakage inductance limits how quickly secondary current follows the primary. Interwinding and self-capacitance create resonances and allow common-mode displacement current into the sense circuit. More turns raise magnetizing inductance and reduce droop, but also increase resistance, leakage, capacitance, and package size. The optimum is a bandwidth range, not the maximum possible turn count.
A one-turn primary made from a busbar or PCB trace seems simple, yet its position inside the core window affects coupling. Return-current placement, nearby phases, and conductor width alter the external field and pickup. The sensor should be tested in its final mechanical orientation.
Common-mode dv/dt immunity
If the current transformer straddles a switching node or its secondary references sensitive control ground, parasitic capacitance can inject a large transient current. Electrostatic shields, winding placement, core grounding where appropriate, balanced routing, and a low-impedance secondary loop can help. A shield must not form a closed conducting turn.
Measure the sensor output with zero primary load current while applying the expected common-mode voltage edge. This reveals false-trip susceptibility that a conventional ratio test misses. The test fixture and oscilloscope probe must have adequate common-mode performance.
Core material and saturation
Ferrite and nanocrystalline materials offer different permeability, loss, saturation flux, temperature behavior, and frequency response. The core should remain linear enough over the required pulse area and temperature range. Remanence after abnormal pulses can reduce margin for the next event, so fault recovery and repetitive faults deserve testing.
Validation checklist
- Measure ratio and phase over the required current and frequency range.
- Apply maximum normal and fault pulse widths at hot and cold temperature.
- Verify core reset during startup, burst, skipped pulses, and shutdown.
- Test common-mode dv/dt with zero primary current and realistic layout.
- Check false triggering from diode recovery, gate current, and neighboring conductors.
- Measure total protection delay from primary current to switch turn-off.
BaoHui Tech needs the current waveform, conductor geometry, switching voltage and dv/dt, detection threshold, output voltage target, burden circuit, isolation level, temperature, size, and trip-time budget to design a custom current transformer.
Frequently asked questions
Can a current transformer measure DC fault current?
It measures changes and pulses, not steady DC indefinitely. Droop and saturation depend on magnetizing inductance, burden, pulse duration, and reset.
Does higher permeability always improve the sensor?
No. It can improve low-frequency response, but saturation flux, loss, capacitance, remanence, temperature, and mechanical size also matter.