Evaluation of Efficiency

As part of our discussion of conversion efficiency which is the main subject of this section, we present the results of evaluations using actual power supply circuits. Specifically, we compare results using five types of fast-recovery type SJ MOSFETs, with on-resistances of approximately 0.2 Ω, as the MOSFETs Q1 to Q4. Measurement conditions were input voltage V_in = 390 V, output voltage V_out = 12 V, output current I_out = 10 to 50 A, switching frequency f_sw = 100 kHz.

As shown in the figure, the results indicate that efficiency was highest for the R6020JNX (solid red line) over all load ranges. The R6020JNX is a latest-generation PrestoMOS™, with one of the fastest t_rr values in the industry, and with parasitic capacitances optimized as well.

Moreover, the gate threshold voltage V_GS(th) is high compared with the R6020FNX (corresponding to the previous generation) and the other MOSFETs used in the comparison, and consequently shoot-through currents do not flow so readily due to erroneous turn-on. In general, when V_GS(th) is high, turn-on losses are increased; but in a PSFB circuit, the dead time can be adjusted under light loading and ZVS operation is possible under heavy loading to reduce turn-on losses in those respective cases, thus minimizing the disadvantage of a high V_GS(th).

From the above, we can say that in order to improve the efficiency of a PSFB circuit that adopts SJ MOSFETs, it is important to select SJ MOSFETs having as small a t_rr value as possible and an excellent switching characteristic.