Phase Management Raises Interleaved PFC Efficiency

Jul 1, 2007 12:00 PM
By Michael O'Loughlin, Applications Engineer, Texas Instruments, Dallas

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As the demand for higher power densities and more efficient power supplies increases, so does the popularity of interleaving power converters. It has been proven that interleaving power factor corrected preregulators can reduce the overall magnetic and electro-magnetic interference (EMI) filter volume, as well as reduce overall system conduction losses. For this reason, it has become a more popular topology for offline power converters requiring power factor correction (PFC).

However, interleaving does not come free. It increases the complexity of the design, as well as a number of diodes and FETs required. These added semiconductor devices increase switching losses, decreasing the converter's light-load efficiency and making it harder for the design to pass 80 PLUS requirements at light loads (For more on 80 PLUS initiatives, see www.80plus.org/). In these designs, it is advantageous to let just one of the phases run at light load to regulate the power and to turnoff any additional phases. With this approach, 80 PLUS requirements can be met easily. Phase-management circuitry can be implemented with discrete circuitry or integrated within a PFC controller to turn on and off phases based on overall system loading.

80 PLUS Specifications

In 2008, 80 PLUS specifications will require offline power-converter designs to have a power factor of greater than 0.9. These power systems will require PFC. Generally, an offline power converter with PFC is a two-stage system. Typically, stage one is a PFC boost pre-regulator and stage two is some type of stepdown converter. Adding the PFC pre-regulator to the system introduces additional losses, making it more difficult for the design to meet the 80 PLUS efficiency requirements from 20% to 100% load.

To help improve system efficiency, the single-stage PFC boost pre-regulator stage can be replaced with an interleaved PFC boost pre-regulator stage (Fig. 1). Interleaving increases system efficiency by reducing conduction losses. This is made visible by evaluating the simplified conduction loss equations for the single-phase PFC (P_SINGLE), and the conduction losses for an interleave PFC pre-regulator (P_INTERLEAVED). From these equations, you can see that the conduction losses of the interleaved pre-regulator could be as low as one-half that of a single-phase pre-regulator. In these equations for conduction losses, R represents the effective wiring and switch impedance of the power converter.

Even though interleaving PFC pre-regulators can improve efficiency where conduction losses dominate over switching losses, interleaving pre-regulators can decrease light-load system efficiency. When the converter is operating at lighter loads where switching losses (P_SWITCHING) dominate, one of these converters should be shut down to improve light-load efficiency.

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