Flyback Converter Calculator

A flyback converter operates by storing energy in a coupled inductor (often called a flyback transformer) during the primary switch conduction phase and releasing it to the output during the off phase. The mathematical model relies on several core relationships:

1. Windings Ratio (n): Defined as the ratio of primary winding turns to secondary winding turns (n = N_p / N_s).

2. Duty Cycle (D): The proportion of the switching period during which the primary switch is closed, calculated as: D = (V_out + V_rect) * n / (((V_out + V_rect) * n) + V_in).

3. Secondary Peak Current (I_spk): Derived from the output current and duty cycle: I_spk = (2 * I_out) / (1 - D).

4. Primary Peak Current (I_ppk): Scaled by the windings ratio: I_ppk = I_spk / n.

5. Secondary Inductance (L_s) & Primary Inductance (L_p): L_s is determined by the output voltage, diode drop, duty cycle, output current, and switching frequency (f_s): L_s = ((V_out + V_rect) * (1 - D)^2) / (2 * I_out * f_s). Primary inductance is then found via L_p = L_s * n^2.

Let us walk through a practical design scenario. Suppose we need to configure a flyback converter with the following specifications:

• Input Voltage (V_in): 24 V
• Output Voltage (V_out): 12 V
• Output Current (I_out): 2 A
• Rectifier Diode Drop (V_rect): 0.7 V
• Switching Frequency (f_s): 100 kHz (100,000 Hz)
• Windings Ratio (n): 1.5 (Primary: 45 turns, Secondary: 30 turns)

Step 1: Calculate the Duty Cycle (D)
D = (12 + 0.7) * 1.5 / (((12 + 0.7) * 1.5) + 24) = 19.05 / (19.05 + 24) = 19.05 / 43.05 = 0.4425 (or 44.25%)

Step 2: Calculate Secondary Peak Current (I_spk)
I_spk = (2 * 2) / (1 - 0.4425) = 4 / 0.5575 = 7.17 A

Step 3: Calculate Primary Peak Current (I_ppk)
I_ppk = 7.17 / 1.5 = 4.78 A

Step 4: Calculate Secondary Inductance (L_s)
L_s = ((12 + 0.7) * (1 - 0.4425)^2) / (2 * 2 * 100,000) = (12.7 * 0.3108) / 400,000 = 3.94 µH

Step 5: Calculate Primary Inductance (L_p)
L_p = 3.94 µH * (1.5^2) = 3.94 µH * 2.25 = 8.87 µH

Account for Parasitic Inductance: Real transformers feature leakage inductance that can cause voltage spikes across the switching transistor. Always incorporate a proper snubber circuit to protect your hardware.

Thermal Management: Peak currents dictate thermal stress on semiconductors. Ensure your chosen MOSFET and output diode have adequate current ratings and heat sinking to handle the calculated peak loads safely.