Theory & Background

Flyback converter: energy stored in transformer (acting as inductor) during MOSFET ON time, transferred to secondary during OFF time. Not a true transformer — gap in ferrite core allows energy storage. When primary MOSFET turns ON: energy stored in transformer magnetizing inductance. When MOSFET turns OFF: energy released to secondary through diode. Voltage conversion: V_out = V_in × (1-D)/D × Ns/Np (DCM). Isolation: primary and secondary are galvanically isolated. Power range: 5W–200W efficient range. Applications: phone charger, laptop PSU, SMPS adapters.

Components & Requirements

10 components required for this project.

#	Component	Purpose	Qty
1	UCC28180 PFC controller + UCC28740 LLC controller	PFC and flyback control ICs	x1
2	Custom EE/ETD ferrite core transformer	Isolated power conversion	x1
3	SiC MOSFET (C3M0075120K, 1200V, 20A)	Primary switch	x1
4	Schottky diode (80V, 10A) or sync rect MOSFET	Secondary rectification	x1
5	Electrolytic capacitors (400V, 100µF)	PFC output / flyback input bulk	x2
6	EMI filter (common mode choke + X/Y caps)	Conducted EMI suppression	x1
7	Optocoupler + TL431 (feedback)	Isolated voltage feedback	x1
8	Litz wire (0.1mm × 100 strands, AWG7 equivalent)	Transformer winding (reduce AC losses)	x1
9	Oscilloscope (200MHz) + current probe	Waveform analysis	x1
10	Dimmer (variac 0–250V, 5A)	Safe AC voltage variation testing	x1

Step-by-Step Implementation

Follow these 4 steps carefully.

1

Flyback Converter Theory

Flyback converter: energy stored in transformer (acting as inductor) during MOSFET ON time, transferred to secondary during OFF time. Not a true transformer — gap in ferrite core allows energy storage. When primary MOSFET turns ON: energy stored in transformer magnetizing inductance. When MOSFET turns OFF: energy released to secondary through diode. Voltage conversion: V_out = V_in × (1-D)/D × Ns/Np (DCM). Isolation: primary and secondary are galvanically isolated. Power range: 5W–200W efficient range. Applications: phone charger, laptop PSU, SMPS adapters.

2

Transformer Design for Flyback

Core selection: ETD34 ferrite (N87 material, low loss at 100 kHz). Primary inductance: Lp = V_in_min² × D_max² / (2 × Fsw × Pout / η) → determines stored energy. Primary turns: Np = V_in × D_max / (ΔB × Ae × Fsw) → prevent core saturation. Secondary turns: Ns = Np × V_out / (V_in × D_max). Secondary winding: sandwiched between primary layers reduces leakage inductance (critical for snubber sizing). Litz wire: at 100 kHz, skin depth = 210 µm in copper → use stranded wire (0.1mm strands) to minimize AC resistance.

3

EMI Compliance Requirements

Switching power supplies generate conducted and radiated EMI (electromagnetic interference). Regulatory: CISPR 32 (consumer), FCC Part 15 (US), CE marking (Europe). Conducted EMI test: connect to LISN (Line Impedance Stabilization Network), measure noise on mains. Limits: CISPR class B (residential): 56 dBµV from 150 kHz to 30 MHz. EMI reduction: common-mode choke on mains input (rejects common-mode noise), X-capacitors across mains (differential mode), Y-capacitors mains-to-chassis (common mode). Spread spectrum: modulate switching frequency ±10% reduces peak EMI at fundamental by 10dB.

4

Feedback and Control Loop

Output voltage feedback: TL431 precision shunt regulator compares output voltage with internal 2.5V reference. Error current drives optocoupler LED. Optocoupler transmits error signal across isolation barrier to primary-side control IC. Primary IC adjusts duty cycle to maintain constant output voltage. Control loop stability: measure open-loop gain and phase using injection transformer + analyzer. Target: phase margin > 45°, gain margin > 10dB at unity gain crossover. Compensator design: Type 2 or Type 3 op-amp compensator in TL431 feedback network.

Code & Implementation

Core code for flyback_design.py:

flyback_design.py
  Python

# Flyback Transformer Design Calculator  def flyback_design(V_in_min, V_in_max, V_out, I_out, Fsw, eta=0.85):     """     Design a flyback converter transformer.     V_in: Input voltage range (V)     V_out: Output voltage (V)     I_out: Output current (A)     Fsw: Switching frequency (Hz)     eta: Efficiency estimate     """     P_out = V_out * I_out     P_in  = P_out / eta      # Maximum duty cycle at minimum input (worst case)     D_max = 0.45  # Limit to 0.45 for DCM margin      # Primary inductance for DCM at full load     Lp = V_in_min**2 * D_max**2 / (2 * Fsw * P_in)      # Turns ratio     V_f_diode = 0.5  # Secondary diode forward drop (Schottky)     V_clamp   = 80   # Reflected clamp voltage     n = (V_in_min * D_max) / ((V_out + V_f_diode) * (1 - D_max))      # Core selection (ETD34): Ae = 97mm², Ve = 7700mm³     Ae = 97e-6   # m²     dB = 0.2     # Tesla swing (keep below 0.3T for N87)     Np = (V_in_min * D_max) / (dB * Ae * Fsw)     Ns = round(Np / n)     Np = round(Np)      # Peak primary current     Ipk = 2 * P_in / (V_in_min * D_max)      print(f"=== Flyback Design: {P_out}W, {V_out}V/{I_out}A ===")     print(f"Primary inductance: {Lp*1e6:.1f} µH")     print(f"Turns ratio (n):    {n:.2f}")     print(f"Primary turns:      {Np}")     print(f"Secondary turns:    {Ns}")     print(f"Peak primary I:     {Ipk:.1f} A")     print(f"Max duty cycle:     {D_max*100:.0f}%")     print(f"MOSFET voltage stress: {V_in_max + V_out*n:.0f} V (use 600V+ FET)")  flyback_design(V_in_min=85, V_in_max=265, V_out=20, I_out=3.25, Fsw=100e3)

Testing & Troubleshooting

Test Flyback SMPS Design by verifying each subsystem individually before full integration.

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Troubleshooting Tips

Verify power voltages, check ground connections, use serial monitor for debug.

Real-World Applications

*Laptop and phone charger design

*Industrial power supply

*LED driver power supply

*Telecom rectifier power supply

*Medical isolated power supply

*Electric vehicle on-board charger

*UPS power supply module

*Renewable energy micro-inverter

Extensions & Next Steps

Add active power factor correction (PFC) pre-stage
Implement LLC resonant converter for higher efficiency
Design a synchronous rectifier secondary for lower losses
Add digital control with DSP for adaptive operation
Build an automatic test bench for power supply characterization

Interactive Playground

Coming Soon

An interactive simulator will be available here — simulate circuits and run code in-browser without hardware.

Frequently Asked Questions

What are the safety considerations for mains-connected electronics projects?

CRITICAL SAFETY RULES: (1) Always use an isolation transformer (variac + isolation transformer) between mains and your circuit during development — prevents electrocution when touching circuit ground. (2) Use earthed workbench and anti-static mat. (3) Never work alone on mains circuits. (4) Add safety interlock: primary-side capacitors hold lethal charge (400V, 100µF = 8 joules) long after power removed — add discharge resistor (100kΩ, 5W) or bleed resistor permanently, verify voltage before touching. (5) Enclosure: all mains-connected parts must be covered, inaccessible without tools. (6) Fusing: always fuse on both L and N.

Introduction