Switch-mode power supplies for LED lighting applications are increasingly being designed with an active power factor correction (PFC) at the input stage to meet international regulations for harmonics. The boost topology in discontinuous current mode (DCM) is most suitable PFC method for converters less than 300W power rating.
In this topology, the switching-on power loss of boost switch is negligible, and the major power losses are the switching-off losses and conduction losses. After the super-junction devices have been introduced, they are often considered as an optimized switch for active PFC because of extremely low on-resistance and highly non-linear capacitance curves.
For downstream converter, LLC resonant converters are proven to be very effective for higher efficiency. Because of zero voltage switching-on capability of LLC resonant converters, conduction losses are primary power losses and therefore, low on-resistance is key requirement for switching power devices. Thanks to the advance in device technology, the planar MOSFET can be competitive to super-junction devices in both PFC and LLC stages at low to mid power range. Considering the price of the devices, the planar MOSFET is more attractive up to 200W.
Power losses in switch-mode power supply When designing switching power converters, estimation of power losses is critical work for power device selection. Major power losses of power device in PFC pre-regulator and LLC resonant converter are conduction loss and switching loss. Generally, switching loss becomes dominant at light load condition and conduction loss takes up as load increases. The gate charge (Qg) parameter is widely used as benchmark of switching speed and therefore switching loss. The super-junction MOSFET tends to have smaller Qg than similar rating conventional planar MOSFET because of smaller chip size that comes from ultra low specific on-resistance. For an example, the Qg of 0.95Ohm/600V super-junction MOSFET is even smaller than 1.6Ohm/600V planar MOSFET as shown in Table 1.
It can be expected that the super-junction MOSFET will have smaller switching loss and conduction loss together in PFC or LLC converter. This is the main reason why the super-junction technology is preferred as power switch in switch-mode power supply. The latest planar MOSFET, however, has very small Qg as power device technology advances. Its on-resistance is still higher but it should be competitive at light load area. Because there is about 30 percent of price merit with the planar technology, it is worth evaluating it in switching power supply for LED lighting. It is also important to check which power level it can remain competitive in PFC and LLC converter.
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