An active clamp flyback transformer is part of a resonant switching system, not simply an energy-storage transformer with a lower-loss clamp. Leakage inductance, magnetizing inductance, switch capacitance, clamp capacitance, winding capacitance, dead time, and synchronous rectifier timing interact to determine voltage stress, soft-switching margin, efficiency, and EMI.
The topology can recover leakage energy and enable zero-voltage switching over part of the operating range. Those benefits depend on controlled parasitics and timing. A transformer copied directly from a conventional RCD-clamped flyback may not produce the intended waveforms.
Start with magnetizing inductance and peak current
Magnetizing inductance sets current ramp, stored energy, peak current, and circulating current. A lower value can increase available resonant energy for transitions but raises conduction loss and core excitation. A higher value reduces current but may narrow soft-switching range. Specify inductance at a defined frequency, voltage, and temperature, including tolerance and any DC-bias requirement.
Leakage inductance is recovered but not free
The active clamp can recycle leakage energy, yet excessive leakage still increases circulating energy, clamp current, RMS loss, and waveform sensitivity. Very low leakage may also change the energy available to charge and discharge switch capacitances during dead time. The useful target comes from the actual controller, switch capacitance, input range, and clamp design.
Winding capacitance affects common-mode current
Interleaving reduces leakage but often increases primary-secondary capacitance. With fast switching edges, that capacitance carries common-mode current into the secondary. Winding sequence, margins, shields, and split windings should be evaluated against both leakage and capacitance. Record the measurement methods for each parasitic so prototype and production data remain comparable.
Core reset and volt-second balance still matter
Clamp action changes the reset waveform, but the core still needs adequate flux margin during startup, transients, current limit, and abnormal timing. Verify primary voltage and current over the full controller operating range. Clamp-capacitor tolerance and dead-time variation can shift the reset interval and peak flux.
Coordinate synchronous rectifier timing
Secondary leakage and ringing can create false synchronous-rectifier turn-on or excessive reverse current. The transformer turns ratio, leakage distribution, diode body behavior, and controller blanking should be reviewed together. Probe placement matters because a long ground lead can make ringing appear worse than it is.
Prototype verification checklist
- Magnetizing and leakage inductance with documented connections
- Primary-secondary capacitance and common-mode current
- Main and clamp switch voltage at startup, steady state, and overload
- ZVS behavior across input voltage and load
- Clamp current and capacitor voltage
- Core and winding temperature rise
- Synchronous rectifier voltage, reverse current, and timing
- Conducted and radiated EMI at final switching speed
When requesting a high frequency transformer for an active clamp flyback, provide the controller, MOSFET capacitance data, clamp network, frequency range, dead-time strategy, and measured waveforms. BaoHui Tech can then design the magnetic component around the actual resonant transition requirements.
Frequently asked questions
Does an active clamp eliminate all drain overshoot?
No. It controls and recycles much of the leakage energy, but layout inductance, timing, capacitance, and secondary commutation can still produce overshoot and ringing.
Should leakage inductance be minimized as much as possible?
Not blindly. It should be controlled to a range compatible with clamp loss, transition energy, voltage stress, and EMI.
Why can ZVS disappear at light load?
There may be insufficient resonant current to charge and discharge switch capacitances during the available dead time.