Understanding the Effect of Power MOSFET Package Parasitics on VRM Circuit Efficiency at Frequencies above 1 MHz

The effect of frequency on the parasitic inductance and resistance of DPAK, D2PAK, MLP, SO-8 and the proprietary DirectFET power package are presented. Each package shows a characteristic increase in resistance with frequency in the range of 100KHz to 5MHz. The observed trend in resistance with frequency is expected to be a consequence of the skin effect phenomenon. The DirectFET package exhibits the lowest inductance and resistance at frequencies up to 5MHz. The effect of package parasitics upon in circuit VRM efficiencies is presented by comparing circuits containing near identical active area silicon housed in SO-8 and DirectFET packages. VRM circuits containing the DirectFET devices display higher efficiency than those assembled with SO8’s. The difference in VRM efficiency between the two circuits increases with frequency over the range of 800KHz to 2 MHz. This trend is expected to be a consequence of the lower parasitic impedance of the DirectFET package. In circuit switching waveforms captured from both circuit are discussed and used to validate package inductance measurements.