Boosting Efficiency in Off-Line Power Converters

May 1, 2008 12:00 PM
By Michael O'Loughlin, Applications Engineer, Texas Instruments, Dallas

Over the past few years there has been a big push by some environmental agencies to have off-line ac-dc power converters include power-factor correction (PFC). It started with the European Union's EN61000-3-2 input-current harmonic content specifications for power converters. Even though this was not a power factor specification per se, power-supply designers found it easier to meet this requirement with PFC preregulators. So this specification was a driving force for off-line power converters to have PFC.

In the United States, electric utilities have been sponsoring the 80 PLUS incentive program, which offers a cash rebate for power converters that are greater than 80% efficient with a power factor of 0.9 at full load. Having PFC preregulators in off-line power converters helps the power company greatly by reducing losses in the transmission lines and also by making better use of the available line power.

However, this benefit does not come for free and has created many challenges for power-supply designers. The PFC preregulator makes the off-line power converter less efficient, which makes it more difficult for some designs to meet the higher efficiency requirements of 80 PLUS. Fortunately, there are several innovative control techniques that make PFC preregulator designs more efficient.

The Efficiency Penalty

It is no secret to the utilities that PFC reduces transmission line losses by reducing line root-mean-square (rms) currents caused by ac-dc power converters. However, off-line power converters with PFC are generally less efficient than a well-designed power converter without PFC.

Typically, off-line converters with PFC are done with two power stages. Stage 1 is generally a boost converter that uses average current-mode control techniques to shape the input current. Stage 2 is a stepdown converter that steps the boost voltage down to a more usable voltage (Fig. 1). The overall total system efficiency (η_TOTAL) is the product of Stage 1's efficiency (η_STAGE1) and Stage 2's efficiency (η_STAGE2), which makes for an overall less-efficient power supply:

η_TOTAL = η_STAGE1 × η_STAGE2.

A large contributor to losses in traditional PFC preregulators is the reverse-recovery current in the boost diode. The following equation describes the switching losses in the boost diode at a given switching cycle due to reverse-recovery current:

where I_D is the peak diode current, t_RR is the boost diode's reverse-recovery time and f_S is the switching frequency at which the boost converter is operating.

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