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Boost Converters Can Be Simulated in LTSpice

Boost converters rely on the flyback properties of inductors to generate a higher voltage on the output compared to the input. When the current path in an inductor is interrupted, a voltage is generated across the inductor in an effort to keep the current flowing. Harnessing this voltage is the key to the performance of a boost converter. Boost converters appear tricky to design to begin with, but with the help of simulation tools such as LTSpice (and a common design process) any boost converter can be designed.

A boost converter converts a low input voltage to a higher output voltage. It does so by switching the input voltage across an inductor, waiting for the current in the inductor to ramp up to a certain level, then disconnecting the voltage across the inductor. When this happens, the energy stored in the inductor causes the voltage across the inductor to ramp up. Connecting a diode and capacitor to the inductor means this energy can be stored in the capacitor and used to provide power to a 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 inductor current ramps linearly over time according the voltage across the inductor divided by the inductance value. By sensing the current (by using a current sense resistor) the peak current can be determined and hence the amount of energy stored in the inductor. The switching frequency can be controlled by careful selection of the inductor value, thus the circuit can be designed to operate away from sensitive frequency bands.

Keeping the inductor current flowing ensures a circuit with lower switching noise and lower switching losses. It is advisable to keep the ripple current in the inductor to approximately 40% of the average inductor current. LTSpice can be used to check the design equations to ensure the inductor current works out as calculated.

The choice of mosfets is important too. A mosfets needs to be chosen such that its ON resistance is as low as possible, but also its gate charge is as low as possible. The ON resistance keeps the conduction losses low and the gate charge keeps the switching losses low, thus ensuring the design has minimum losses and maximum efficiency. Maximum efficiency implies longer battery life.

The output capacitor holds up the output voltage while the inductor is charging. This needs to be sized according to the load current and the inrush current coming from the inductor. A low effective series resistance capacitor is better for boost converter design in order to ensure that the inrush current does not create excessive ripple on the output voltage.