Flyback

Part 19 · Voltage Regulation

Key point: energy goes into the core while Q1 is on, and comes out the other winding while Q1 is off. Both cables are pulses. The switch node is ground, then \(V_{\mathrm{IN}}+N V_{\mathrm{OUT}}\).

\(V_{\mathrm{out}}/V_{\mathrm{in}} = (N_s/N_p)\,D/(1-D)\). The knob \(N\) is \(N_p:N_s\). Axes stay at −4…40 V and 0…8 A. \(f_{\mathrm{sw}}\) is 250 kHz.

1
Primary stores. Secondary dumps.
Preset 12 → 5 V, \(N_p:N_s = 2\), 1.5 A, 22 µH. D is 45.5 %. SW is 0, then 22 V. Δi_L is 0.99 A. I_L,pk is 1.87 A. I_IN average is 0.63 A. i_IN flows only while Q1 is on. i_OUT flows only while the secondary is dumping.
2
N is 4. D is 62.5 %. SW reaches 32 V.
Preset 12 → 5 V, \(N_p:N_s = 4\), 1.5 A, 22 µH. D is 62.5 %. SW is 0, then 32 V. Δi_L is 1.36 A. I_L,pk is 1.68 A. I_IN average stays 0.63 A. More primary turns raises duty and the off-state switch node. The secondary pulse is taller.
3
N is 1. D is 29.4 %. SW reaches 17 V.
Preset 12 → 5 V, \(N_p:N_s = 1\), 1.5 A, 22 µH. D is 29.4 %. SW is 0, then 17 V. Δi_L is 0.64 A. I_L,pk is 2.45 A. I_IN average stays 0.63 A. The input pulse and the output pulse reach the same height.
4
Δi_L falls to 0.46 A.
Preset 12 → 5 V, \(N_p:N_s = 2\), 1.5 A, 47 µH. D stays 45.5 %. Δi_L is 0.46 A. I_L,pk is 1.61 A. I_IN average stays 0.63 A. The magnetizing ramp is shorter. Both cables are still pulses. SW is still 0 and 22 V.
5
Δi_L is clamped at 0.68 A.
Preset 12 → 5 V, \(N_p:N_s = 2\), 0.4 A, 22 µH. D is 45.5 %. The sketch clamps Δi_L to 0.68 A. I_L,pk is 0.71 A. I_IN average is 0.17 A. The core would go discontinuous. The drawing keeps the pulses above zero.

Isolation does not quiet the pins. Leakage and the clamp spike are not in this sketch. A forward converter transfers while both windings conduct, and its output current is a triangle. The gold dash on i_IN is peak magnetizing current. The Y axes do not rescale.

Switch node, then the two currents

SW is fixed at −4 to 40 V. Currents are fixed at 0 to 8 A. Two cycles. 250 kHz.

\(V_{\mathrm{IN}}\) (V) \(V_{\mathrm{OUT}}\) (V) \(I_{\mathrm{OUT}}\) (A) \(L_p\) (µH) \(N_p:N_s\)

Q1 on

Flyback. Amber i_IN is the primary pulse. Green i_OUT is the secondary dump. They do not overlap.

0.00 µs

Switch node · −4 to 40 V

i_IN · amber · rectified · gold dash is I_L,pk · 0 to 8 A

i_OUT · green · rectified · 0 to 8 A

Duty D—

Δi_L—

I_L,pk—

I_IN avg—

N_p:N_s—