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Design an AC/DC switching power supply for a notebook computer.
The circuit diagram shown below is the design of an AC/DC switching power supply for a notebook computer. The circuit was designed using a two-‐stage approach. In the first stage a buck-‐boost PFC converter was implemented, and in the second stage a DC/DC boost converter was designed.
Figure 1: Circuit diagram of an AC/DC switching power supply for a notebook computer. Design Procedure A buck-‐boost PFC converter was used in the first stage in order to minimize the displacement and the distortion factor, which determine the value of power factor (p.f.). This converter can achieve a power factor of 1 if the switching frequency and the duty cycle can be kept constant. As we know that the equation of the equivalent resistance, Re is
𝑅! = 𝑣!"(𝑡)𝚤!"(𝑡)
=2𝐿!
𝑇!𝑑!(𝑡) (𝑒𝑞. 1)
Since for this configuration we kept L1 and d(t) constant so p.f. is assumed to remain close to 1. We calculated the value of L1 by assuming that the circuit is working at 100% efficiency. So 𝑃! = 𝑃!" =
!!!!!!!
! !
As we know that Po = 100W and we are using DCM mode so D = 0.32, therefore L=64μH.o The output of this circuit is approximately -‐3V. To supply the positive voltage to the DC/DC boost converter the connection was reversed as shown in figure 1. For the second part, a DC/DC boost converter in CCM mode was used to boost the output voltage at 19.5V. The duty cycle was calculated using the formula shown below:
𝐷 = 1−𝑣!"𝑣!
So D = 0.85. The output wave shapes are shown below
Figure 2: The output of the Buck-‐Boost PFC converter.
Figure 3: The output of the DC/DC boost converter.
Figure4: Current in the inductor of the buck boost PFC converter