Flyback Converter¶

Overview¶

The flyback converter is an isolated DC-DC topology derived from the buck-boost converter. It uses a coupled inductor (flyback transformer) to provide galvanic isolation while enabling step-up or step-down voltage conversion. Flyback converters are widely used in low-power applications (5-150W) such as phone chargers, LED drivers, and auxiliary power supplies.

Difficulty: Intermediate

Estimated Time: 30-45 minutes

Learning Objectives¶

After completing this example, you will be able to: - Understand flyback converter operation in CCM and DCM - Design the coupled inductor (flyback transformer) - Analyze voltage stresses on switch and diode - Calculate output voltage as a function of duty cycle and turns ratio

Prerequisites¶

• Basic understanding of DC-DC converter operation
• Familiarity with Buck-Boost Converter
• Understanding of transformer/coupled inductor behavior

Circuit Description¶

Topology¶

                   Flyback Transformer
                         n:1
    +Vin ──┬──[S]──┬────●  ●────[D]──┬── +Vout
           │       │    ║  ║         │
           │      ┌┴┐   ║  ║        ┌┴┐
           │      │ │ Lm ║  ║        │ │ C
          ┌┴┐     │ │   ║  ║        │ │
          │ │ Cin └┬┘   ║  ║        └┬┘
          │ │      │    ●  ●         │
          └┬┘      │               R │ Load
           │       │                 │
    GND ───┴───────┴─────────────────┴── GND (isolated)

    Primary Side         Secondary Side

Key Components: - S: Primary-side switch (MOSFET) - Lm: Magnetizing inductance (energy storage) - n:1: Turns ratio (primary:secondary) - D: Secondary-side rectifier diode - C: Output capacitor

Operating Principle¶

Switch ON (Energy Storage Phase): 1. Primary current ramps up through magnetizing inductance Lm 2. Energy stored in magnetic field: E = ½ Lm Ip² 3. Secondary diode is reverse-biased (due to transformer polarity) 4. Output capacitor supplies load current

Switch OFF (Energy Transfer Phase): 1. Primary current interrupted 2. Transformer polarity reverses (flyback action) 3. Secondary diode conducts, transferring energy to output 4. Magnetizing current decreases, reflected to secondary: Is = Ip × n

Key Parameters¶

Voltage Conversion Ratio¶

CCM (Continuous Conduction Mode):

Vout/Vin = D/(n(1-D))

DCM (Discontinuous Conduction Mode):

Vout/Vin = D/(n × √(2Lm fs/RL))  (for D < Dcrit)

Where: - D = duty cycle - n = turns ratio (Np/Ns) - Lm = magnetizing inductance (primary referred) - RL = load resistance

Design Equations¶

Turns Ratio Selection¶

For Vin = 150V (typical rectified line), Vout = 12V, D = 0.4:

n = (Vin × D) / (Vout × (1-D))
n = (150 × 0.4) / (12 × 0.6) = 8.33 ≈ 8:1

Magnetizing Inductance¶

For CCM operation at minimum load:

Lm > (Vin,min × Dmax)² / (2 × Pout,min × fs)

For boundary/DCM operation (common in low-power):

Lm = (Vin × D × (1-D)) / (2 × Iout × n × fs)

Switch Voltage Stress¶

The switch must withstand input voltage plus reflected output voltage plus leakage spike:

Vds,max = Vin + n × Vout + Vspike

Example: Vin=400V, n=8, Vout=12V, Vspike=100V

Vds,max = 400 + 8×12 + 100 = 596V → Use 800V MOSFET

Diode Voltage Stress¶

Vr,diode = Vout + Vin/n

Example: Vout=12V, Vin=400V, n=8

Vr,diode = 12 + 400/8 = 62V → Use 100V Schottky

Building in GeckoCIRCUITS¶

Step 1: Create Primary Side¶

1. Add DC voltage source (Vin = 150V for testing)
2. Add input capacitor (Cin = 100μF, optional)
3. Add switch (MOSFET or ideal switch) in series with primary winding

Step 2: Add Coupled Inductor/Transformer¶

1. Option A - Ideal Transformer + Inductor:
2. Add ideal transformer with turns ratio n:1

3. Add inductor Lm in parallel with primary winding

4. Option B - Coupled Inductors (if available):

5. Use coupled inductor component
6. Set primary inductance Lp = Lm
7. Set coupling coefficient k ≈ 0.95-0.99

8. Secondary inductance Ls = Lm/n²

9. Note transformer dot convention (primary and secondary dots on opposite sides for flyback operation)

Step 3: Create Secondary Side¶

1. Add rectifier diode (cathode to output positive)
2. Add output capacitor
3. Add load resistor (R = Vout²/Pout)

Step 4: Add PWM Control¶

1. Add PWM signal generator (fs = 100kHz)
2. Set duty cycle D = 0.4 (for initial test)
3. Connect to switch gate

Step 5: Configure Simulation¶

• Simulation time: 5-10 ms (settling time)
• Time step: 10-50 ns (1/100 of switching period)
• Solver: Trapezoidal

Expected Results¶

Steady-State Waveforms¶

Design Verification¶

For Vin=150V, D=0.4, n=8:1:

Vout = Vin × D / (n × (1-D)) = 150 × 0.4 / (8 × 0.6) = 12.5V

Output Voltage Ripple¶

ΔVout = (Iout × D) / (fs × C)

For Iout=2.5A, D=0.4, fs=100kHz, C=470μF:

ΔVout = (2.5 × 0.4) / (100k × 470μ) = 21mV (0.17%)

Exercises¶

Exercise 1: CCM vs DCM Operation¶

1. Set Lm = 500μH, load R = 10Ω (heavy load)
2. Run simulation, observe secondary current waveform
3. Increase R to 100Ω (light load)
4. Question: Does the converter enter DCM? How can you tell?

Exercise 2: Input Voltage Variation¶

1. Fix D = 0.4, vary Vin from 100V to 200V
2. Record Vout for each Vin
3. Plot Vout vs Vin
4. Question: Why does a fixed duty cycle not regulate output?

Exercise 3: Turns Ratio Trade-off¶

1. Test n = 5:1, 10:1, and 15:1 with fixed Vin = 150V
2. Adjust D to achieve Vout = 12V in each case
3. Measure: switch voltage stress, primary current magnitude
4. Question: What are the trade-offs in selecting turns ratio?

Exercise 4: Leakage Inductance Effects¶

1. If using coupled inductors, reduce k from 0.99 to 0.90
2. Observe switch voltage during turn-off
3. Advanced: Add an RCD snubber to clamp the voltage spike
4. Question: How much energy is lost in the snubber?

Practical Considerations¶

Transformer Design¶

• Core selection: Ferrite (EE, ETD, PQ cores common)
• Air gap: Required for energy storage (Lm), typically 0.5-2mm
• Wire gauge: Primary handles high peak current, secondary handles DC

Common Issues¶

Related Examples¶

• Buck Converter - Non-isolated step-down
• Forward Converter - Alternative isolated topology
• Full Bridge - Higher power isolated

References¶

1. Pressman, A., Billings, K., Morey, T. "Switching Power Supply Design" - Chapter on Flyback Converters
2. Basso, C. "Switch-Mode Power Supplies" - SPICE Simulation
3. Texas Instruments SLUP127: "Flyback Transformer Design"
4. ON Semiconductor AND9124: "Flyback Design Guidelines"

Circuit Files¶

Note: Example circuits to be added: - flyback_basic.ipes - Basic flyback without feedback - flyback_dcm.ipes - DCM operation example - flyback_ccm.ipes - CCM operation with larger Lm - flyback_snubber.ipes - With RCD snubber for leakage

Example Version: 1.0 Last updated: 2026-02 GeckoCIRCUITS v1.0