Buck Converters and LTSpice

There is no doubt that switch mode power supply designs are becoming more and more common. The demands on the battery to last longer to provide better talk time means that an efficient way is needed to convert one voltage to another.

While the battery voltage remains fairly constant, the processor voltage gets lower and consumes more current. This puts pressure on power supply engineers to design more efficient money converters.

Once the design is complete, a valid simulation tool (such as LTSpice) is needed to cross-check the finer points of the design to ensure optimal efficiency has been achieved.

The buck converter relies on the properties of the inductor to efficiently convert a high voltage to a lower voltage. The inductor has a minimum dc resistance and if a low resistance transistor is used to switch voltage to the inductor then the heat dissipation (and hence power loss) of the circuit can be kept to a minimum.

The buck converter switches the voltage to the inductor, waits for the current in the inductor to rise to a certain level then disconnects the voltage. The flyback nature of the inductor means the activated lead flies negative, causing the inductor energy to be passed to the output capacitor to power the load.

All current and voltage waveforms of the boost converter can be simulated in LTSpice, giving the design basic knowledge of how the circuit performs.

The current in the inductor slopes linearly with time and this path is proportional to the voltage across the inductor and inversely proportional to the value of the inductance. So by selecting the correct inductor value, the operating speed of the circuit can be determined. Again, current and voltage can be checked in LTSpice.

Keeping the inductor ripple current to about 40% of the output current ensures a good trade-off between switching losses and inductor size.

Optimal efficiency is obtained by careful selection of the upper and lower MOSFETs as well. The top MOSFET must have a low gate charge (Qg) as well as a low ON resistance. The bottom MOSFET requires only low ON resistance.

The duty cycle also comes into play. The duty cycle of the buck converter is Vout/Vin. If the input voltage is high or the output voltage is low, the duty cycle will be low, so the upper MOSFET will only turn on for a short time. The upper MOSFET therefore requires a low gate charge, almost irrelevant to the ON resistance.