Selecting Your Boost Converter Inductance

January 03, 2014

A battery alone may not be capable of supplying complex systems with all the voltage rails necessary to function properly. Applications such as automotive LED drivers, audio amplifiers, and telecommunications, to name a few, require boost converters to create a higher output voltage from a lower input. To the boost converter designer, it may not be obvious whether or not the converter should be designed to operate in continuous conduction mode (CCM), discontinuous conduction mode (DCM), or a combination of both.

Boost converters come in many shapes and sizes, with a wide range of power levels and boost ratios. These requirements determine whether the boost is best designed to operate in CCM or DCM. In DCM, the inductor current ramps up from zero when the FET is on and fully discharged back to zero again before the next switching period. But in a non-synchronous CCM boost, the inductor's current is always greater than zero when current is ramping up, as well as when ramping down and discharging the inductor's stored energy into the output capacitor and load.

In CCM, the duty cycle is constant with loading, but varies with input voltage. In most CCM designs, below a certain minimum load, the operating mode transitions to DCM because the inductor current eventually decreases to zero before the next switching cycle.

In most cases, high-power boost converters are designed to operate in CCM, and low-power boosts operate in DCM. This is because CCM allows lower peak currents throughout the entire circuit, which typically results in lower circuit losses. One exception may be in the output rectifier for high-voltage boosts, such as in a PFC where reverse-recovery currents can induce additional losses. Generally these losses can be handled with a high quality (fast) rectifier.

If operating in DCM, expect to see peak inductor currents that are two times larger than in CCM, but could be much higher if the inductance value is purposely reduced. These higher currents increase the rms currents in the input and output capacitors and can add to the switching losses in the FET, which results in larger (or more) components to handle the additional stresses. This disadvantage alone often outweighs the other advantages DCM offers at high power.

While the inductor's rms current is higher in DCM, its wire resistance is usually much lower, so the copper losses tend to be the same or less than CCM. But the core losses in DCM are greater at high-power levels. Sometimes this can make the often hyped benefit of reduced inductor size invalid because a larger core may be necessary to handle these added losses. But where DCM really shines is at lower power levels, where the added stresses in the capacitors and FET don't necessarily require larger components and a smaller inductor can work.

An added benefit of DCM is when operating at high boost ratios, where CCM operation requires large on-times; the inductance can be decreased in value to allow a reduced on-time (along with higher peak currents). This is desirable because controllers often run into their maximum controllable on-time (or

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