The three devices in Table 1
are applied to DCM PFC and LLC resonant converter and switching losses are measured with oscilloscope. Because switch current varies by each switching cycle in DCM PFC, the switching loss at the peak of AC line input voltage is measured and overall switching loss data are calculated by assuming the loss as sinusoidal as shown in Figure 1
The average switching loss is calculated by making product of operating frequency and energy loss per switching cycle. In DCM PFC, usually on-time is fixed and off-time is changing according to line input voltage. From design variables, on/off time and therefore equivalent switching frequency at every switching cycle are derived. Conduction losses are easy to evaluate when rms current of power switch is available. In PFC, the switch current is sinusoidal so rms current can be calculated simply. In LLC resonant converter, resonant current and magnetizing current can be assumed as sinusoidal and triangular respectively. Then, rms current can be calculated with the measurement of peak current as shown in Figure 2
Design article archive Table 2
shows summary of power losses. It is confirmed that switching loss is dominant at light load in both PFC and LLC.
At heavy load, conduction loss is important as expected. The latest planar outperforms the super-junction at light load condition. It shows less power losses in both DCM PFC and LLC resonant converter. At 200W load, it has little bit smaller loss in LLC but higher loss in DCM PFC. Overall loss is slightly higher by 0.4W at 200W load.
The performance of the latest planar, UniFET II has evaluated in both DCM PFC and LLC resonant converter. It showed quite good results against the super-junction device up to 200W load condition. Considering price merit and performance, UniFET II technology is a primary choice when designing switching power supply for mid-power LED lighting application.
Courtesy of EETimes Europe
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