The MOSFET switch makes up one of the major sources of efficiency loss in a switch mode power supply circuit. Often times, as in the synchronous boost there are both high and low side switches. Although the synchronous FET has lower power loss compared to the Schottky diode, it still brings with it a fair amount of loss in efficiency.
To demonstrate the contribution to efficiency loss from both the high and the low side MOSFETs, a thorough analysis of the synchronous buck topology using external MOSFETs is helpful. See circuit of Figure 1.
Figure 1. Synchronous Buck Topology
To begin with, the high side switch (Q1) has typical loss components of:
- RMS inductor current through Q2's on resistance.
- The gate driver supplying current to charge the gate capacitances every switching cycle.
- The overlap of the inductor current and the drain to source voltage during turn on and turn off.
- The charging of the drain to source capacitance every switching cycle.
The low side switch (Q2) has major loss components of:
- RMS inductor current through Q2's on resistance.
- The gate driver supplying current to charge the gate capacitances every switching cycle.
- The body diode conducting during the converters dead time and while Q1 and Q2 are off.
- The charging Q2's drain to source capacitance every switching cycle.
To estimate these efficiency losses, and compare them with the measured values it's important to develop an understanding of the loss components.
High side switch ON
The RMS current through Q1's on resistance is fairly straightforward. Assuming continuous conduction, during every on cycle Q1 connects VIN to the inductor and conducts the output current plus the inductor current ripple. The resulting power loss
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. IRMS is,
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