When you build a multi-output flyback or forward converter, only one output gets tight regulation from the feedback loop. The others droop or rise with load — often ±10% cross-regulation at best. Adding a linear post-regulator burns efficiency; adding a second buck controller adds cost and layout headaches. The elegant middle ground is a magnetic amplifier post-regulator: a saturable reactor in series with the auxiliary output rectifier that acts as a controlled switch.

The core idea: a small square-loop toroid (typically amorphous or nanocrystalline, like Metglas 2714A) sits between the transformer secondary and the output diode. During each switching cycle, the core absorbs volt-seconds until it saturates. Once saturated, its inductance collapses and current flows freely to the output. The delay time before saturation determines how much of the pulse reaches the load — effectively PWM without a transistor.

Control comes from a reset winding (or the same winding driven backward during the off-time). A small NPN transistor pulls reset current based on output voltage error. More reset current drives the core further into negative flux, so it takes longer to saturate on the next cycle, delivering less energy. Less reset current means faster saturation and more delivered power.

Real-world example: A 200 kHz forward converter has a tightly regulated +5 V main output and a loosely coupled +12 V auxiliary for fan and op-amp rails. Without post-regulation, the +12 V swings from 10.8 V (full load) to 13.6 V (light load). Adding a Metglas MP1305P4A core with 8 turns primary and 20 turns reset, plus a TL431/optocoupler-free discrete error amp, holds +12 V within ±1% across the full load range. Efficiency stays above 92% because the mag-amp dissipates almost nothing — it either blocks (high impedance, no current) or conducts (low impedance, no voltage).

Rule of thumb for core sizing: The required volt-second product is V_sec × t_blocking,max, where t_blocking is the maximum delay you need (usually 30-50% of the on-time). Pick a core where N × A_e × 2B_sat ≥ V_sec × t_blocking. For a 12 V secondary pulse at 200 kHz needing 1 μs of blocking, that's 12 μV·s — a tiny core (A_e ≈ 0.05 cm², B_sat ≈ 0.55 T) with 5-10 turns handles it easily.

Watch out for: core losses at high frequency (keep flux swing modest), reset winding leakage inductance ringing (add a small RC snubber), and the fact that mag-amps only work for bucking — they can subtract volt-seconds but never add them, so the unregulated rail must always be higher than the target.