FC9921

2012. 10. 31

6/8

Revision No : 0

Step 1. Calculating L1.

The output voltage V

(1) assuming a 30% peak-to-peak ripple.

L1 = (41V • 10.5µs) / (0.3 • 20mA) = 72mH

Select L1 68mH, I = 30mA. Typical SRF = 170KHz. Calculate

the coil capacitance.

C

1

=

1

68mH

≈ 13pF

Step 2. Selecting D1

Usually, the reverse recovery characteristics of ultra-

manufacturer’s data books. The designer may want to

experiment with different diodes to achieve the best result.

Select D1 MUR160 with V

= 100mA) and C

Step 3. Calculating total parasitic capacitance using (3)

C

Step 4. Calculating the leading edge spike duration using

(4), (5)

T

265V • √2 • 31pF

+ 20ns

100mA

≈ 136ns < T

BLANK(MIN)

Step 5. Estimating power dissipation in FC9921 at 265VAC

using (8) and (10)

Switching power loss:

P

1

(265V • 31pF + 2 • 100mA• 20ns)(265V - 41V)

2 • 10.5µs

0.7

P

Minimum duty ratio:

D

Conduction power loss:

P

2

• 210Ω + 0.63 • 200µA• 265V

P

Total power dissipation in FC9921:

P

Step 6. Selecting input capacitor C

N

Output Power = 41V • 20mA = 820mW

Select C

Metalized Polyester Film).

Figure 2. Universal 85-265VAC LED Lamp Driver

C

IN

AC Line

85-265V

C

DD

3

FC9921

D1

L1

LED

D2

U1

D3

D4

D5

C

IN2

L

IN

VRD1

F1

C

O

VDD

DRAIN

GND

1

2