Buck Converters and LTSpice – Easier Than You Think
There is no doubt that switched mode power supply design is getting more prevalent. Demands on batteries to last longer to give improved talk time mean an efficient way is needed to converter one voltage to another.
While battery voltages remain fairly constant, processor voltages are getting ever lower as well as consuming more current. This puts pressure on the power supply engineer to design more efficient buck converters.
Once the design has been completed, a valid simulation tool (such as LTSpice) is needed to cross check the finer points of the design to ensure optimum efficiency has been achieved.
Buck converters rely on the properties on inductors to efficiently convert a high voltage to a lower voltage. An inductor has minimum dc resistance and if a low resistance transistor is used to switch the voltage to an inductor, then the heat dissipation (and hence power loss) of the circuit can be kept to a minimum.
A buck converter switches a voltage to an inductor, waits for the current in the inductor to ramp to a certain level then disconnects the voltage. The flyback properties of the inductor mean the switched end flies negative, causing the inductor energy to be passed to an output capacitor to power the load.
All of the current and voltage waveforms of the boost converter can be simulated in LTSpice, giving the design a fundamental knowledge of how the circuit is performing.
The current in an inductor ramps linearly with time and this ramp is proportional to the voltage across the inductor and inversely proportional to the inductance value. Thus by choosing the correct value of inductor, the speed of operation of the circuit can be determined. Again, the currents and voltages can be probed 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.
Optimum efficiency is obtained by careful selection of the top and bottom MOSFETs too. The top MOSFET needs to have low gate charge (Qg) as well as low ON resistance. The bottom MOSFET only needs low ON resistance.
The duty cycle also comes into play. The duty cycle of a 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 top MOSFET will only be on for a short period of time. Therefore the top MOSFET needs low gate charge, almost at the irrelevance of ON resistance.