AN75

Issue 1 – January 2011

3

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© Diodes Incorporated 2010

is

V

186

2

=

. Often a 20% difference in capacitance could be observed between like

capacitors. Therefore a voltage rating margin of 25% should be considered.

Once the line drops below 50% of its peak voltage, the two capacitors are essentially placed in

parallel. The bus voltage V

at the minimum AC line voltage V

V

60

2

Vac

85

2

2

V

2

V

ac(min)

IN(min)

=

×

=

×

=

At 60Hz, the total time of a half AC line cycle is 8.33ms. The power to the buck converter is derived

from the valley-fill capacitors when the AC line voltage is equal to or less than 50% of its peak voltage.

The hold up time for the capacitors equates to

ms

77

.

2

ms

33

.

8

3

1

=

×

=

. The valley-fill capacitor

value can then be calculated,

F

μ

30

V

20

ms

77

.

2

V

60

W

96

.

12

V

t

V

P

C

DROOP

HOLD

(min)

IN

out

TOTAL

=

×

=

×

=

Therefore,

F

μ

15

2

C

1

C

=

=

. V

(max)

LED

(min)

IN

DROOP

V

V

V

−

=

in

order to ensure continuous LED conduction at low line voltage. Nevertheless, V

in the design example to avoid the need for very large valley-fill electrolytic capacitor.

A 20V V

part of the AC line cycle. As the buck regulator requires V

(V

reducing the actual output LED current at low AC input voltage. In the design example, the LED

current drops by approximately 20% from its nominal value at 85Vac (see Figure 4).

Setting the fixed off-time and switching frequency range

For fixed off-time operation, the switching frequency will vary subjected to the actual input voltage and

output LED conditions.

A nominal switching frequency f

result in smaller inductor size, but could lead to increased switching losses in the circuit. A good

design practice is to choose a nominal switching frequency knowing that the switching frequency will

decrease as the line voltage drops and increases as the line voltage increases.

The fixed off-time t

s

μ

9

.

13

55kHz

230V

54V

-

1

f

V

V

-

1

t

swi(nom)

ac(nom)

LED(nom)

off

=

=

=

The off-time is programmed by timing resistor R

()

( )

Ω

=

−

×

=

−

×

=

Ω

k

326

22

25

9

.

13

22

25

s

μ

t

k

R

OFF

T

A 330kΩ is selected for R

approximated as,

kHz

10

s

μ

9

.

13

V

69

V

59

1

t

V

V

1

f

OFF

(min)

IN

(max)

LED

swi(min)

=

−

=

−

=

kHz

8

.

63

s

μ

9

.

13

V

373

V

42

1

t

V

V

1

f

OFF

(max)

IN

(min)

LED

swi(max)

=

−

=

−

=