JAJSRX5 データシート

JAJSRX5 November 2023 LM5185

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8.2.1.2.3 Flyback Transformer T₁ and Current-Sense Resistor (R_CS)

The turns ratio of the transformer is selected such that the maximum duty cycle is smaller than 70%. While the maximum duty cycle can approach 80% if needing a particularly wide input voltage application, the maximum duty cycle increases the peak current stress of the secondary-side components. The turns ratio for this design is 1:1.

Equation 14.

N_{P S} < \frac{D_{M A X}}{1 - D_{M A X}} \times \frac{V_{I N (m i n)}}{V_{O U T} + V_{D}} = \frac{0.7}{1 - 0.7} \times \frac{20 V}{16.4 V + 0.3 V} = 2.8

The magnetizing inductance is selected based on the switching frequency being 250 kHz for the nominal V_IN at 24 V at full load 1 A. The 250 kHz is usually a good tradeoff for flyback design in terms of transformer size and overall efficiency, and so forth. Use Equation 1, Equation 4, and Equation 3 to calculate the required inductance. A value of 12 µH is chosen for this design. Use Equation 4 to calculate the primary peak current and the peak current with the selected 12-µH 1:1 transformer is 3.7 A. By giving a 15% margin, the peak current limit is around 4.3 A. Use Equation 15 to calculate the R_CS. In this design, R_CS is set to 20 mΩ. A small RC filter (100 Ω, 100 pF) is added to overcome the leading edge noise of the current sense signal.

Equation 15.

R_{C S} \leq \frac{V_{C S - M A X}}{I_{p k}} = \frac{100 mV}{4.3 A} = 23.3 m Ω

Note that a higher magnetizing inductance provides a larger operating range for BCM and FFM, but the leakage inductance can increase based on a higher number of primary turns, N_P. Equation 16 and Equation 17 give the primary and secondary winding RMS currents, respectively.

Equation 16.

I_{P R I - R M S} = \sqrt{\frac{D}{3}} \times I_{P R I - P K}

Equation 17.

I_{S E C - R M S} = \sqrt{\frac{2 \times I_{O U T} \times I_{P R I - P K} \times N_{P S}}{3}}

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8.2.1.2.3 Flyback Transformer T1 and Current-Sense Resistor (RCS)

8.2.1.2.3 Flyback Transformer T₁ and Current-Sense Resistor (R_CS)