Designing for 90 percent + efficiency
The best of today's 250 W, convection-cooled power supplies operate at over 90 percent efficiency across an input voltage range of 90 to 240 VAC. This level of efficiency is essential in order to keep within an industry-standard 6 x 4 inch footprint whilst still ensuring adequate heat dissipation without a cooling fan or large external heatsinks.
Over 90 percent efficiency can only be achieved with near lossless switching in the active power factor correction circuit, the main converter(s) and the rectifiers. A diagram for a 250 W AC/DC power supply that achieves up to 95 percent efficiency at 240 VAC input and 92 percent efficiency at 90 VAC input is shown in Figure 2.
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Figure 2: This 250W AC/DC power supply is up to 95 percent efficient
From the outset achieving high efficiency was the primary design goal for this power supply. Consequently, for each stage the power loss budget was determined and this drove the choice of circuit topology. Power losses were minimized in each stage, striving to save every mW of unnecessary dissipation. For example the input filter for the power supply shown above uses very low resistance winding wire that virtually eliminates I2R losses in the chokes.
The EMI filter employed in this design is a 2-stage filter with a high permeability magnetic core. This was carefully selected to attenuate switching noise and to minimize power loss. The other components in the filter are X and Y capacitors with the Y capacitor values being chosen so as not to exceed 300 uA of earth leakage current, as set out in UL60601-1, the most widely referenced medical standard.
A quasi-resonant, lossless power factor correction circuit operates in a discontinuous mode. Its operating frequency changes between 30 kHz and 500 kHz to achieve zero current switching (ZCS) throughout the specified range of loads and input voltages. This is important because it ensures that the voltage switches when the current is truly at zero, thereby eliminating switching losses.
The main converters are of fixed frequency, resonant, half-bridge design - again with lossless ZCS. Two transformers are employed; the combination has lower I2R switching loss than if one larger transformer had been utilized. The two converters operate at 51.2 kHz and one of them has its output phase-shifted by 90-degrees. Combining the outputs reduces ripple and doubles the ripple frequency, as illustrated in Figure 3. In turn, this halves the value, and size, of the output filter capacitors.
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Figure 3: Combining the outputs of two converters that are 90-degrees out of phase reduces ripple level and doubles ripple frequency
