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Part 22 · Voltage Regulation

Key point: power crosses while Q1 is on. Both windings conduct together. The output current is the Lout triangle. Off, the drain sits at the reset voltage until the core flux is back to zero, then at \(V_{\mathrm{IN}}\).

\(V_{\mathrm{out}} = V_{\mathrm{in}}(N_s/N_p)D\). The knob is \(N_p/N_s\). Axes stay at −8…112 V. Input current is 0…10 A. Output current is 0…2 A. \(f_{\mathrm{sw}}\) is 250 kHz. Every step below is a 0.5 A load, except the last.

1
Step down. The output is the triangle.
Preset 48 → 5 V, \(N_p:N_s = 4:1\), 0.5 A, 22 µH. D is 41.7 %. SW is 0, then 96 V while the core resets, then 48 V. Δi_L is 0.53 A. I_L,pk is 0.77 A. I_IN average is 0.05 A. i_IN is a small pulse while Q1 is on. i_OUT is the Lout triangle the whole cycle.
2
Step up. 12 V to 48 V.
Preset 12 → 48 V, \(N_p:N_s = 1:8\), 0.5 A, 150 µH. D is 50.0 %. SW is 0 while Q1 is on, and 24 V for the whole off time. Δi_L is 0.64 A. I_L,pk is 0.82 A. I_IN average is 2.00 A. Eight secondary turns make the primary pulse several amps. The output is still the Lout triangle.
3
1:10. D is 40.0 %. The primary pulse is taller.
Preset 12 → 48 V, \(N_p:N_s = 1:10\), 0.5 A, 150 µH. D is 40.0 %. SW is 0, then 24 V while the core resets, then 12 V. Δi_L is 0.77 A. I_L,pk is 0.88 A. I_IN average stays 2.00 A. More secondary turns narrow the duty and raise the primary pulse. The output is still a triangle.
4
2:1 still steps down. The input pulse stays small.
Preset 48 → 5 V, \(N_p:N_s = 2:1\), 0.5 A, 22 µH. D is 20.8 %. SW is 0, then 96 V while the core resets, then 48 V. Δi_L is 0.72 A. I_L,pk is 0.86 A. I_IN average is 0.05 A. Fewer primary turns raise the pulse a little. It is still far below the 1:10 step-up pulse. The output is still a triangle.
5
Δi_L is clamped at 0.37 A.
Preset 48 → 5 V, \(N_p:N_s = 4:1\), 0.2 A, 22 µH. D is 41.7 %. The sketch clamps Δi_L to 0.37 A. I_L,pk is 0.39 A. I_IN average is 0.02 A. Lout would go discontinuous. The drawing keeps the triangle above zero.

The reset level is the flat voltage that balances the core: \(2V_{\mathrm{IN}}\) when that pulse fits in the off time, and \(V_{\mathrm{IN}}/(1-D)\) when duty is above 50 %. A real capacitor ring is rounded and carries a leakage spike. Those are not drawn. Magnetizing current is not on the input pulse. A flyback stores, then dumps, and has no Lout. The gold dash on i_OUT is peak inductor current. The Y axes do not rescale.

Switch node, then the two currents

SW is fixed at −8 to 112 V. i_IN is 0 to 10 A. i_OUT is 0 to 2 A. Two cycles. 250 kHz.

\(V_{\mathrm{IN}}\) (V) \(V_{\mathrm{OUT}}\) (V) \(I_{\mathrm{OUT}}\) (A) \(L_{\mathrm{out}}\) (µH) \(N_p/N_s\)

Q1 on

Forward. Amber i_IN is the primary pulse. Green i_OUT is the Lout triangle.

0.00 µs

Switch node · −8 to 112 V

i_IN · amber · rectified · 0 to 10 A

i_OUT · green · continuous · gold dash is I_L,pk · 0 to 2 A

Duty D—

Δi_L—

I_L,pk—

I_IN avg—

N_p:N_s—