Parallel converter modules increase power and offer modularity, but small voltage differences drive circulating current through low-impedance DC links. A current sharing inductor can add controlled impedance between modules, limit high-frequency exchange current, and improve fault isolation. It does not replace balanced buswork or a suitable active sharing strategy.
Sources of imbalance
Device voltage drop, gate timing, PWM phase, current-sensor offset, control-loop gain, capacitor ESR, cable resistance, connector contact, bus geometry, and cooling all affect module current. A hotter module may have higher semiconductor and copper resistance, changing its share over time.
Measure both average and switching-frequency circulating current. Equal average current can hide large ripple between modules.
Inductor location and function
Inductors can be placed at each module output, in split DC-link branches, or as a coupled multiwinding structure. The location determines which current component is limited and which faults are isolated. A differential sharing inductor may see little average voltage but substantial switching mismatch pulses.
Inductance and DCR
Higher inductance reduces exchange-current slope but increases size, voltage drop, and control interaction. DCR provides passive damping and average sharing but adds loss. Tolerance should be considered as a set: unequal inductance or resistance can shift current to one branch.
Specify minimum incremental inductance at operating DC current and temperature. A small-signal zero-bias value is not enough.
Coupled sharing inductors
Coupling branch inductors can present high impedance to differential circulating current while allowing common load current with lower stored energy. Winding polarity is critical. Leakage determines residual independent branch ripple. A module fault or phase loss can drive the core in a mode not present during balanced operation.
Switching strategy
Synchronizing modules in phase can concentrate ripple, while interleaving can cancel bus ripple but creates defined differential voltage between branches. Clock skew, pulse dropping, burst operation, and current limit alter the pattern. The magnetic design should use the actual switching-state sequence.
Fault duty
A shorted semiconductor, open fuse, disconnected module, precharge mismatch, hot-plug event, or DC-bus fault can place high voltage across a sharing inductor. Saturation changes current rise rate when protection needs impedance most. Define clearing time, peak current, repetition, and insulation.
Validation checklist
- Measure each module current and circulating current with synchronized probes.
- Test component and control tolerances at hot and cold conditions.
- Verify inductance under branch DC bias and differential current.
- Measure core, winding, and terminal hot spots.
- Test startup, precharge, enable skew, phase loss, overload, and module faults.
- Confirm control stability and bus-capacitor ripple with production layout.
BaoHui Tech needs module topology, voltage, current, switching sequence, bus geometry, sharing target, control method, fault cases, cooling, insulation, package, and allowable loss.
Frequently asked questions
Will equal DCR guarantee current sharing?
No. Control, semiconductor drop, bus layout, capacitance, timing, and temperature also matter.
Can one coupled core serve all modules?
Yes in suitable designs, but polarity, flux modes, fault operation, insulation, and tolerance require explicit analysis.