AN1059 datasheet(5/20 Pages) STMICROELECTRONICS

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the peak of the sinusoid of the primary voltage.

The following relationships relate IPKp to the input power Pin and allow both to explicate the timing rela-

tionships and to calculate all the currents circulating in the circuit.

The primary current Ip(t) is triangular-shaped and flows only during the switch ON-time, as illustrated by

the shaded triangles shown in fig. 3. As earlier stated by equation (1), during each half-cycle the height

of these triangles varies with the instantaneous line voltage:

Ipkp(

θ) = IPKp ⋅ |sin (θ)|,

their width is constant but they are spaced out by a variable amount given by (3).

Looking at the primary on a "fL" time scale, the current Iin(

θ) downstream the bridge rectifier is the aver-

age value of each triangle over a switching cycle (the thick black curve of fig. 3):

Iin

(θ) =

⋅ Ipkp(θ) ⋅ D =

⋅ IPKp ⋅

|sin

(θ)|

+ Kv ⋅ |sin (θ)|

Primary current

peak envelope

Secondary current

peak envelope

Average

primary current

OFF

SWITCH

Figure 3. High-PF Flyback current waveforms

Iin(

θ)

after the

bridge

Kv=4

Kv=2

Kv=1

Kv=0.5

1.57

3.14

4.71

6.28

0.25

0.5

0.75

Figure 4a. Primary Current (@ fL time scale)

Iin(

θ)

before

the

bridge

Kv=0.5

Kv=1

Kv=2

Kv=4

1.57

3.14

4.71

6.28

0.5

Figure 4b. Line Current (@ fL time scale)

AN1059 APPLICATION NOTE

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