A 300-kHz 3-Level Flyback Converter Achieving 93% Peak Efficiency and 50% Reduction in Transformer Size

Flyback converters are widely used in smartphone fast chargers and industrial power supplies. A conventional flyback converter (Fig. 1) consists of a transformer, a primary-side switch <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$Q_{\mathrm{P}}$</tex> and a secondary-side synchronous rectifier <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{Q}_{\text{SR}}$</tex> (for high efficiency). For 220-V<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RMS</inf> applications, the input voltage <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(V_{\text{IN}})$</tex>, of the converter is around 300V. The voltage across <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{Q}_{\mathrm{P}}, V_{\text{QP}}=V_{\text{I N}}+N_{\text{PS }} \times V_{\text{OUT }}$</tex> (eq. (1) in Fig. 1, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$N_{\text{PS}}$</tex> is the turn ratio of primary and secondary windings), can exceed 400V. This value may increase further due to the high-amplitude, high-frequency ringing caused by the leakage inductance. Therefore, a high- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$V_{\text{DS}}$</tex> (typically 650V) device is commonly used for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$Q_{\mathrm{P}}$</tex>. However, such a high- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$V_{\text{DS}}$</tex> device has large parasitic capacitance, leading to high V-I overlap loss <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(P_{\text{sw}})$</tex> and output capacitance loss <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(P_{\text{coss}})$</tex>. The max frequency of the inductor <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(f_{\mathrm{L}}=f_{\text{SW}}$</tex>, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{\text{SW}}$</tex> s the switching frequency) is thus limited to 100kHz. This low <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{\text{SW}}$</tex> significantly increases the size of the transformer, as seen in Apple A2164 [1] (Fig. 1). The size of its transformer is 20×15×13 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup>, occupying around 30% of the entire charger. Meanwhile, the high peak current <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$I_{\text{PK}}$</tex> in the transformer results in high core loss <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$P_{\text{CORE}}$</tex>.