Beyond simple MOSFET selection

As the switching frequency of Synchronous Buck Converters developed for Core DC-DC converters in the Personal Computer (PC) applications moves towards the 1MHz-2MHz ranges, the MOSFET losses go higher. This is complicated by the fact that most CPUs are requiring higher currents and lower voltages. When you add to that other parameters that govern the loss mechanisms like the power source Input Voltage and Gate Drive Voltage, we have a more complicated phenomenon to deal with. But this is not all, we have secondary effects that may cause significant deterioration in losses and hence reduction of the power conversion efficiency (ξ). These secondary effects include shoot through losses and losses due to parasitic resistances like capacitor and indictor equivalent series resistance (ESR), Printed Circuit Board (PCB) resistance and inductance and finally the MOSFET package parasitic inductances. Other secondary loss mechanisms are the charging and discharging of the inter-electrode capacitances of the MOSFET like Gate-Source capacitance (Cgs), the miller Gate-Drain capacitance (Cgd), and Drain-Source capacitance (Cgs). As the frequency goes higher the losses due to the body diode reverse recovery becomes more pronounced and must be taken into account. Now it is clear that selecting a MOSFET for the Synchronous Buck converter is no more a trivial exercise and requires a solid methodology to select the best combination coupled with good understanding of all the above-mentioned issues. In this paper will discuss all of these effects in some details and will demonstrate how you can go about such a selection.

Conduction losses:
These are the Ohmic losses in the device due to current flowing in the MOSFET Rdson. The losses in MOSFETs M1 and M2 in Figure 1 may be calculated from the following two equations:

Where:

PCHS =High Side (HS) MOSFET conduction Losses
PCLS=Low Side (LS) MOSFET conduction Losses
Δ = Duty Cycle ≈ V_out/ V_in
Iload = Load Current
Rdson = MOSFET ON resistance
Vin = Power source Input Voltage
Vout = Output Voltage

Since Δ and Iload are determined by the application, Rdson must be selected as low as possible

Figure 1: Simplified Synchronous Buck converter showing MOSFET parasitic inductances