In PC and server applications, one might choose bridgeless power factor correction circuit (PFC), such as the one shown in Figure 1, to meet the 80 Plus Platinum or Titanium requirement.
Figure 1. Power stages of bridgeless PFC circuit.
When compared to the traditional boost PFC in Figure 2, the bridgeless PFC eliminates the bridge rectifier and the power losses on the bridge rectifier. For a 400-W power supply, up to 6-W power losses are on the bridge rectifier at 120VAC/60Hz input. That is a 1.5% efficiency reduction due to the power dissipation on the bridge rectifier, making it clear why people consider bridgeless PFC when they have high efficiency requirement.
Figure 2. Power stages of a convential Boost PFC circuit.
The switches S1 and S2 of bridgeless PFC in Figure 1 cannot be conducted at the same time, making a complex driving strategy often required for bridgeless PFC. Digital controllers such as C2000 Piccolo MCU or UCD3138 are generally applied as the controller for bridgeless PFC.
Digital controllers are capable on almost any control strategy that you want to apply to your power supply. They also provide the possibility to read the power supply operational information in real-time. However, using digital controller means a higher BOM cost. Those who want high converter efficiency and don’t need communication interface might still want to find a low cost analog controller for bridgeless PFC. Semi-bridgeless PFC in Figure 3 is a choice of a high efficiency low cost PFC circuit.
Figure 3. Power stages of semi-bridgeless PFC circuit.
Semi-bridgeless PFC combines two boost converters with return diodes to perform power factor correction. The current flow paths in positive and negative half AC cycles are illustrated in Figure 4. With the favor of return diodes DR1 and DR2, the switches S1 and S2 can now be turned on/off at the same time. We are now able to apply any of the following low cost standard analog boost PFC controllers to Semi-bridgeless PFC:
Figure 4. Bridgeless PFC with return diodes operations: at the positive half of the AC cycle (a) ; and at the negative half of the AC cycle (b).
PMP9640 is an AC-DC power supply design incorprate [KW1] with Semi-bridgeless PFC. Transition mode PFC controller UCC28051 is applied in this design and provides over 98.5% peak efficiency on the Semi-bridgeless PFC.
In my next blog post, I will provide more detail about what are the keys to implement a standard transition mode PFC controller to Semi-bridgeless PFC circuit.
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