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Selection of PFC type, progression and working mode

Selection of PFC type, progression and working mode
1.PFC type selection
Power adapters can be divided into three types: PFC with no PFC, PFC with passive and PFC with source. According to the certification standard of "energy star", power adapter above 5W must add PFC. In addition, the ic10003-2 standard of the international electrotechnical commission imposes strict limits on the input harmonics of power supplies, which cover class D devices such as personal computers, televisions and monitors. PFC type should be selected according to the power, power factor and total harmonic distortion of the power adapter.
The actual measurement of a 250WPC power supply without PC, with passive PFC and with source PFC is shown in figure 4-8-1. It is not difficult to see that the harmonic amplitude of 3~15 in the absence of PFC is higher than the limit level of en1000-32. When using passive or active PFC, each harmonic amplitude is lower than the limit level of en10003-2, and the 3-20 harmonic amplitude of active PFC is significantly lower than that of passive PFC.
2.PFC power series selection
(1) single-stage PFC power supply. It combines PFC function and DC/DC function into one level, and makes the monolithic integrated circuit of PFC+LED driver, which is suitable for medium and large power adapter of 40-100w
(2) two-stage (also known as two-stage)PFC power supply. Two-stage PFC is the commonly used PFC power supply type at present. The first stage is PFC, and the second stage is DC/DC converter. This power supply can suppress harmonics well and achieve a higher power factor. With independent PFC level, the DC input voltage of DC/DC level can be pre-regulated, so the output voltage of PFC is relatively accurate. With strong load capacity, suitable for large and medium power power adapter. The disadvantage is that more components are needed and the cost is higher. Low power density, high loss.
(3) three-stage (also known as three-stage)PFC power supply. It includes PFC, IIC resonant converter and DC/DC converter, which are three-stage unit circuit, suitable for high-power power adapter over 100W. If PFC and LLC are integrated in a chip (such as PFC+LLC controller PIC810PG), it belongs to a single-stage PFC power supply.
3. Selection of PFC working mode
The topological structure of PFC has two working modes. One is the booster converter operating in the continuous conduction mode (CCM). Its characteristic is that the current on the inductance of PFC is in a continuous state, so the output power is large, and it is suitable for the power adapter above 200W. The other is the critical conduction mode (CRM) booster converter, which is characterized by the current of the PFC inductor at the boundary between continuous conduction and discontinuous conduction. It can adopt low-cost chip, simple circuit and convenient design, and there is no conduction loss of the power switch tube. It is suitable for power adapter under 100W.
For 100~200W power adapter, the continuous conduction mode or critical conduction mode can be determined according to the comprehensive index of the whole power system.
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