Board description

AN2590

8/17

●

Stage 1: [t0... t1] charge to L1. At t0, S1 is already turned on and S2 is off. Assume the

current through L1 (iL1) is zero, then iL1 increases linearly by the voltage across Cf1,

and the current flows through Cf1, Db1, S1 and L1.

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Stage 2: [t1… t2] discharging by L1. At t1, S1 is turned off and S2 is turned on. At the

moment iL1 reaches the positive peak that forces Ds2 to turn on, negative voltage

(Vc1-Vcf1) is applied to L1, causing iL1 to decrease linearly. The current flows through

Cf1, Db1, C1, Ds2 and L1.

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Stage 3: [t2… t3] charge to L1. At t2, iL1 reaches zero, and iL1 linearly increases to the

negative peak by the voltage across Cf2. The current flows through Cf2, L1, S2 and

Db2.

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Stage 4: [t3… t4] discharging by L1. At t3, S1 is turned on and S2 is turned off. When

iL1 reaches the negative peak that forces Ds1 to turn on, voltage (Vc1-Vcf2) is applied

to L1, causing iL1 to decrease linearly. The current flows through Cf2, L1, Ds1, C1 and

Db2. At t4, iL1 reaches zero, where the new switching cycle begins.

The overall current flow shape in L1 during one half AC line cycle is shown in Figure 9. It is

clear that the boost inductor current flows in two directions and the PFC circuit works at

boundary conduction mode.

2.4

Calculations

From the above analysis, the main parameters of the circuit can be calculated for the

application. To design the PFC circuit, the inductor (L1) and its peak current (Ipk) are the

most important.

For the resonant circuit, Lr and Cr are the main parameters. The calculation steps are given

as follows:

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Step 1: Ipk during one switching cycle is given by,

Equation 1

Where Vin is the input voltage after rectifier, Ts is the period of a switching time, t' is the

discharging time of L1, and Vc1 is the voltage across C1.

Figure 8.

L1 current per switching period

Figure 9.

L1 current shape

Ipk

1

2

---Vin

1

2

---Ts

t'

–

⎝⎠

⎛⎞

•

L1

--------------------------------------------

Vc1

1

2

---Vin

t'

•

–

L1

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=

=