AL9901

Document number: DS37713 Rev. 1 - 2

14 of 20

www.diodes.com

March 2015

© Diodes Incorporated

AL9901

Input Bulk Capacitor

For offline lamps, an input bulk capacitor is required to ensure that the rectified AC voltage is held above twice the LED string voltage throughout

the AC line cycle. The value can be calculated from:

≥

IN

C

max

_

DC

L

min

_

Line

ch

in

V

f

2

V

2

)

D

1

(

P

∆

×

×

×

−

×

Where:

ch

D

L

max

_

DC

V

∆

min

_

Line

V

2

If the capacitor has a 15% voltage ripple, then a simplified formula for the minimum value of the bulk input capacitor approximates to:

2

IN

LEDs

LED

V

0.06

V

I

×

×

Power Factor Correction

If power factor improvement is required, then for the input power less than 25W, a simple method for improving the power factor can be

implemented by potential dividing down the rectified mains voltage (resistors R1 and R2 in Figure 4) and feeding it into the LD pin. The current

drawn from the supply voltage will follow an approximate half sine wave. A filter across the LEDs reduces the potential for flicker. This circuit also

significantly reduces the size of input capacitors.

Figure 3 Typical Application Circuit with Simple PFC

Passive power factor correction using three high voltage diodes and two identical capacitors can be implemented. For further design information,

please see AN75 from the Diodes website.

DC-DC Buck LED Driver

The design procedure for an AC input buck LED driver outlined in the previous chapters equally applies to DC input LED drivers.