32.5 A 2W 53.2%-Peak-Efficiency Multi-Core Isolated DC-DC Converter with Embedded Magnetic-Core Transformer Achieving CISPR-32 Class-B EMI Compliance and <5mV Ripple

Integrated isolated DC-DC converters, using either on-chip [1]–[4] or package transformers [5]–[7], have been developed to minimize size and cost while achieving isolation ratings above 5kV and delivering around 1W of output power. As shown in Fig. 32.5.1, these converters typically consist of an inverter, a transformer, and a rectifier. The inverter oscillates at frequencies in the tens-of-MHz range due to the limited inductance of the size-constrained transformer to transfer power, and it is switched on and off by a PWM signal at around 1 MHz to regulate the output voltage. However, this topology faces several challenges: 1) PWM control introduces severe conducted electromagnetic interference (EMI) (in the 150kHz to 30MHz range) that is caused by the discontinuous input current, and also leads to significant output voltage ripples due to discontinuous current delivery. While an EMI filter can be added at the input [8] and additional capacitors can be placed at the output to mitigate these issues, cost and board area are greatly increased. 2) Common-mode (CM) current $(\vert _{\text{CM}})$ across the parasitic capacitance $(\mathrm{C}_{\mathrm{c}})$ of the transformer results in input-to-output dipole radiation, causing severe radiated EMI (in the 30MHz to 1 GHz range) [7]. $\mathrm{I}_{\text{CM}}$ can be suppressed by using the symmetrical inverter structure proposed in [6], [7], but this comes at the cost of increased conduction loss. Frequency hopping techniques can help disperse radiation peaks at harmonic frequencies, but this introduces larger output ripples and adds circuit complexity [1].