Flyback Converter Example¶

Isolated DC-DC converter with transformer energy storage.

Overview¶

The flyback converter provides: - Galvanic isolation - Multiple outputs possible - Buck-boost functionality - Simple topology

Specifications¶

Circuit Files¶

• flyback_basic.ipes - Basic flyback operation
• flyback_ccm.ipes - Continuous conduction mode
• flyback_dcm.ipes - Discontinuous conduction mode
• flyback_snubber.ipes - With RCD snubber

Theory¶

Operating Principle¶

Switch ON: - Energy stored in transformer primary (magnetizing inductance) - Secondary diode reverse biased

Switch OFF: - Energy transfers to secondary - Output diode conducts

Key Equations¶

Voltage Conversion Ratio (CCM): $\(\frac{V_{out}}{V_{in}} = \frac{D}{1-D} \cdot \frac{N_s}{N_p}\)$

Maximum Duty Cycle: $\(D_{max} = \frac{V_{out} \cdot N_p/N_s}{V_{out} \cdot N_p/N_s + V_{in,min}}\)$

Peak Primary Current: $\(I_{pk} = \frac{2 \cdot P_{out}}{V_{in,min} \cdot D_{max} \cdot \eta}\)$

Transformer Design¶

Magnetizing Inductance¶

For CCM operation: $\(L_m > \frac{V_{in,min} \cdot D_{max}^2}{2 \cdot f_s \cdot I_{out,min}}\)$

Air Gap¶

Required to store energy: $\(l_g = \frac{\mu_0 \cdot N_p^2 \cdot A_e}{L_m}\)$

Snubber Design¶

Leakage Inductance Spike¶

Voltage spike on switch turn-off: $\(V_{spike} = L_{leak} \cdot \frac{dI}{dt}\)$

RCD Snubber¶

Clamp voltage selection: $\(V_{clamp} \approx 1.5 \times V_{reflected}\)$

Exercises¶

1. CCM vs DCM: Compare operation at different loads
2. Transformer Saturation: Increase duty cycle, observe magnetizing current
3. Snubber Effect: Measure voltage spike with/without snubber
4. Voltage Regulation: Vary input voltage, verify output

Related Resources¶

• Forward Converter
• Transformer Design Tutorial