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AN4314 bảng dữ liệu(PDF) 13 Page - STMicroelectronics

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Giải thích chi tiết về linh kiện  25 W wide-range high power factor buck-boost converter demonstration board using the L6564H
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AN4314 bảng dữ liệu(HTML) 13 Page - STMicroelectronics

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DocID024821 Rev 2
13/31
AN4314
Performance and considerations with LED loads
31
Figure 4. LED current vs. line voltage
Unlike boost topology, the buck-boost topology does not permit unity power factor even in
an ideal case. From AN1059 [
1], the ratio is defined as:
Equation 3
The power factor is a function of K
v
where V
pk
is the input peak mains voltage and V
out
is
the nominal 70 V
dc
LED voltage drop.
The current would normally be sinusoidal for K
v
= 0 but will be distorted from an ideal
sinusoid in proportion to the increase of K
V
. Since K
v
in this application varies according to
the mains [85-265] V
ac
in the range [1.7-5.3], a different PF versus input line voltage has
been measured.
The PF is then affected by the current phase shift caused by the CX capacitor of the input
EMI. A common-mode choke with high leakage value has been selected in order to increase
the power of the EMI filter but at the same time reducing the two CX capacitors.
The test results of the following
Figure 5 and Figure 6 show the PF and THD versus line
voltage. The power factor remains above 0.9 up to the European range even considering
the minimum tolerance value of the LED voltage drop (-10 %).
Considering the LED voltage drop variation, the PF remains above 0.9 until 250 V
ac
.
Vout
V
K
PK
V =



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