Flyback

Question 1

Lp

Answer 1

Primary Magnetizing Inductance

Question 2

Ip

Answer 2

Peak primary current

Question 3

FS

Answer 3

Switching Frequency

Question 4

Ton

Answer 4

MOSFET on-time per cycle (D / fs)

Question 5

D

Answer 5

Duty Cycle
(Vout+Vdiode)N/(Vin +(Vout+Vdiode)N)

Question 6

T

Answer 6

Period (1/fs)

Question 7

Np

Answer 7

Number of Turns on the primary side

Question 8

Ns

Answer 8

Number of turns on the secondary side

Question 9

toff

Answer 9

MOSFET off-time per cycle ((1 − D) / fs)

Question 10

N

Answer 10

Np/Ns

Question 11

Draw the two energy-storage phases of a flyback

Answer 11

The two energy-storage phases of an ideal flyback converter are the charging phase (primary is active, energy stored in the magnetizing inductance) and the discharging phase (secondary is active, energy transferred to the load).

Phase 1: Charging (Magnetizing) - MOSFET is ON
A representation showing (V_{in}) connected across the primary, the MOSFET closed, and the secondary diode open.

Phase 2: Discharging (Demagnetizing) - MOSFET is OFF
A representation showing the MOSFET open, the primary open circuit, the secondary winding driving current through the forward-biased diode to the load/capacitor.

Question 12

write the volt-second balance equation

Answer 12

The principle of volt-second balance for an ideal inductor states that over one complete switching cycle in a steady state, the net change in magnetic flux must be zero. The total volt-seconds applied across the magnetizing inductance (Lm) during the on-time must equal the total volt-seconds applied during the off-time.

VinDT=(VoutN)((1-D)T)
Vin*D = Vout * N *(1-D)

Question 13

Label the three current waveforms you see on the primary side

Answer 13

Ip(t) : ramp on top of DC (CCM) or triangle (DCM)
Imag : pure triangle (magnetizing)
Ileak : narrow spike at turn-off

Question 14

Give two ways to measure leakage inductance

Answer 14

one with a lab LCR meter, one with a scope
LCR: Short the secondary coil terminals. Solder a wire across the secondary winding terminals. This effectively cancels out the magnetizing inductance when measured from the primary side.
Connect the LCR meter to the primary coil terminals.
Set the LCR meter to measure inductance (L mode) at a suitable frequency (often 1 kHz, or the converter’s operating frequency if possible).
Read the value. The value displayed on the LCR meter is the leakage inductance of the primary winding

SCOPE: Using a Current Probe (Easiest and non-invasive)
Connect a current probe around the primary winding wire or the MOSFET drain lead.
Connect the current probe’s output to an input channel of the oscilloscope.
Set the oscilloscope channel’s scale according to the probe’s sensitivity (e.g., 1A/V).
Power the flyback circuit and observe the current waveform.
Analyze the waveform: The current waveform during the MOSFET’s ON time will show a rising slope. Any sharp peaks or ringing at the turn-off transition are often due to the energy stored in the leakage inductance interacting with parasitic capacitances. The magnitude of this current can be measured from the screen.

Question 15

Write the single-line energy equation that links Pin, Lp, Ip_pk and fs

Answer 15

Pin = ½ Lp Ip² fs

Answer 16

Ip_new = Ip_old / √2 (ramp slope doubles, but peak drops 29 %)

Question 17

List three design variables you can trade off to reduce peak primary current.

Answer 17

Increase Lp
Increase fs
Add parallel secondaries (share energy)

Question 18

Why does a flyback operated in DCM give better cross-regulation between multiple outputs?

Answer 18

Each secondary conducts its own energy packet; load variations do not steal volt-seconds from other windings.

Question 19

Sketch the MOSFET Vds waveform and mark the three voltage levels: Vin, Vor, and Vspike.

Answer 19

Vds

Vin+VOR+Vspike
| ┌───┐
| │ │
|—— ┘ └ ——– Vin
|__________|__________ t
ton toff

Question 20

Derive the reflected voltage Vor in terms of Vout, Vdiode and turns ratio n = Np/Ns.
Professional answer:

Answer 20

Vor = (Vout + Vdiode) · n

Question 21

Pick a random 650 V MOSFET—prove its SOA at 2 A for 200 µs at 125 °C.

Answer 21

Device: STP65N65M5
SOA graph: 400 V, 2 A → 100 µs @ 25 °C; derate 2× @ 125 °C → 50 µs safe.
200 µs exceeds limit ⇒ add soft-start to keep linear time < 50 µs.

Question 22

Write the RCD snubber power-loss formula and state the two worst-case corners you must evaluate.

Answer 22

P_R = ½ Lleak Ip² fs
Corners: max temp (125 °C) & max Vin (gives highest Vclamp).

Question 23

Choose a secondary diode: give the three electrical parameters that matter and the derating rule.

Answer 23

Maximum Repetitive Reverse Voltage > 1.3 × (Vout + Vor/Ns)
Average Current > Iout
Reverse-Recovery Time < .1 * switching period
Derate: 50 % voltage, 50 % current per NASA-STD-5012.

Question 24

Explain why the opto-coupler CTR drop at –55 °C can collapse your loop gain and how you prevent it.

Answer 24

CTR –40 % @ –55 °C → add 20 % overhead in pull-up resistor; or use digital isolator.

Question 25

Draw the Type-II error-amp around a TL431 and label the pole, zero, and mid-band gain.

Answer 25

+----RLED-----+---- C1 ---+ | | | TL431 cathode R2 | | | | +----R1------+---- C2 --- GND

Question 26

Give two PCB layout tricks that cut common-mode EMI from the drain node.

Answer 26

Shield layer tied to primary ground under drain copper. Y-cap primary GND ↔ chassis right at transformer center-pad.

Question 27

State the IPC-2221 creepage requirement for 100 V secondary-to-primary at sea-level vs. 50 000 ft.

Answer 27

100 V: 0.5 mm sea-level, 1.0 mm @ 50 000 ft (barometric).

Question 28

How do you guarantee the transformer saturates nowhere over –55 °C to +150 °C?

Answer 28

Bmax = (Lp · Ip_pk) / (Np · Ae) < 0.25 T @ 100 °C (PC40 ferrite).

Question 29

What lab test proves that the core loss number you calculated is actually true?

Answer 29

Wrap two turns of wire around core, drive with sine, measure P with power analyser, scale by ΔB².

Question 30

MIL-STD-461 CE102 fail at 500 kHz—flyback contributor: CM or DM and why?

Answer 30

CM dominant: drain 200 V pk, 15 pF to heatsink → 20 mA CM current; add 2 mH CM choke, 4 nF Y-cap.

Question 31

Your controller dies and the MOSFET stays on—how much energy is stored in Lp and where does it go?

Answer 31

Energy = ½ Lp Ip² (at that instant). Primary fuse blows; if fuse is too slow, avalanche rating of MOSFET must absorb it → verify single-pulse avalanche energy EAS ≥ ½ Lp Ip².