SLUAAA5 June 2021 TPS54218 , TPS628501-Q1 , TPS628502-Q1 , TPS628510 , TPS628511 , TPS628512

2 Main Sources of Power Losses in Synchronous Buck Converters

There are three main sources of power loss in synchronous buck converters: quiescent losses (static losses), switching losses and conduction losses. In today’s buck converters the quiescent losses generally represent only a small percentage of the total losses when the load at the output of the converter is above a few tens of milliamps. This application note looks at relatively higher currents, in which case, the switching and conduction losses play a much bigger factor than the quiescent losses – which can be neglected.

The following two sections will briefly go through the causes of switching losses and conduction losses in synchronous buck converters.

Switching Losses

Switching losses in synchronous buck converters are the result of charging and discharging the gate to drain and gate to source capacitances of the power MOSFETs when they are turned on and off. These losses can be calculated with the following equations:

Equation 1.

Equation 2.

Where:

P_{SW-TURN ON} and P_{SW-TURN OFF} are the switching losses caused by turning-on and turning-off the MOSFET respectively
V_IN is the input voltage
I_OUT is the output current
f_SW is the switching frequency
ΔT_ON is the turn-on time of the MOSFET
ΔT_OFF is the turn-off time of the MOSFET

Important to note is that the switching losses are linearly proportional with the switching frequency and with the output current.

Figure 2-1 MOSFET Turn-on and Turn-off

Conduction Losses

One part of the conduction losses (Joule losses) in a buck converter are due to the drain-source on-state resistance of the power MOSFETs:

Equation 3.

Equation 4.

and the other part is due to the DCR of the inductor:

Equation 5.

Where:

P_ON-HIGH and P_ON-LOW are the conduction losses in the high-side and low-side MOSFETs respectively
P_DCR are the conduction losses due to the resistance of the inductor
R_DS(ON)-HIGH and R_DS(ON)-LOW are the drain-source on-state resistances of the high-side and low-side MOSFETs respectively
I_HIGH-RMS and I_LOW-RMS are the RMS values of the current flowing through the high-side and low-side MOSFETs respectively
V_OUT is the ouptut voltage
I_P is the peak of the inductor current
I_V is the valley of the inductor current

What is important to notice from these equations is that the conduction losses are dependent on the square of the RMS value of the current flowing through the particular MOSFET and through the inductor. And unlike the switching losses they are independent of switching frequency, as long as the inductor current ripple is held constant.

Figure 2-2 shows a calculated example efficiency curve of a synchronous buck converter plotted on the left y-axis along with the corresponding switching and conduction losses plotted on the right y-axis with respect to output current. This figure clearly displays how, due to the dependency on the square of the output current, at a certain point the conduction losses become the dominating factor and at higher currents their effect on efficiency is much more significant than that of the switching losses. In general, the switching losses and conduction losses are equal near the peak of the efficiency curve.

Figure 2-2 How Conduction and Switching Losses Vary with Output Current