A feedback loop monitors the power supply output and varies the boost converter voltage, which in turn varies the voltage at the input to the main converters. The primary purpose of the boost converter is to boost the PFC voltage of approximately 380 Vdc to 420 Vdc. This enables the design of the main converters to be optimized around tightly defined voltage parameters, another factor that helps to achieve high efficiency.
The final stage uses synchronous rectification instead of normal diodes as this greatly reduces power loss.
Timing for the boost converter, main converters and synchronous rectifiers needs to be precisely controlled to achieve accurate ZCS. A crystal-controlled clock is used as the timing reference and a divider network is employed to get the desired switching frequency. Using this approach is crucial for the efficient operation of synchronous rectifiers, especially for higher output voltages.
This power supply architecture results in high efficiency across a wide range of loads and input voltages, as Figure 4 demonstrates.
(Click on Image to Enlarge)
Figure 4: ZCS switching and the use of resonant converters delivers high efficiency over a wide range of loads and input voltages, not just at full load
A further benefit of ZCS is the relatively low levels of both conducted and radiated emissions as well as the output ripple and noise. The power supply referred to above exhibits less than 90 mV peak-to-peak ripple and noise at 20 MHz bandwidth and is below the level B limit line for EN55011 for conducted and radiated emissions. As noted earlier, this is another important consideration for medical power supplies as there are more demanding EMC requirements for equipment used outside of hospitals.
