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

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