HV9921/22
switching currents of the converter. As a rule of
thumb, this capacitor should be approximately 0.1-
0.2
µF/W
of LED output power. A recommended
input filter is shown in Figure 2 for the following
design example.
Design Example 1
Let us design an HV9921 LED lamp driver meeting
the following specifications:
Input:
Universal AC, 85-264VAC
Output Current: 20mA
Load:
String of 10 LED (LW541C by
OSRAM
V
F
= 4.1V max. each)
Step 1.
Calculating L1.
The output voltage V
O
= 10
⋅
V
F
≈
41V (max.). Use
equation (1) assuming a 30% peak-to-peak ripple.
L
1
=
41
V
⋅
10.5
µ
s
=
72
mH
0.3
⋅
20
mA
Switching power loss:
P
SWITCH
≈
1
(
264
V
⋅
31
pF
+
2
⋅
100
mA
⋅
20
ns
)
⎛
264
V
−
41
V
⎞
⎟
⎜
2
⋅
10.5
µ
s
0.7
⎠
⎝
P
SWITCH
≈
120
mW
Minimum duty ratio:
D
m
=
41
V
/(0.7
⋅
264
V
⋅
2 )
≈
0.16
Conduction power loss:
P
COND
=
0.25
⋅
(
20
mA
)
⋅
210
Ω +
0.63
⋅
200
µ
A
⋅
264
V
≈
55
mW
2
Total power dissipation in HV9921:
P
TOTAL
=
120
mW
+
55
mW
=
175
mW
Step 6.
Selecting input capacitor C
IN
Output Power
=
41
V
⋅
20
mA
=
820
mW
Select L1 68mH, I=30mA. Typical SRF=170KHz.
Calculate the coil capacitance.
1
1
C
L
=
=
≈
13
pF
2
L
1
⋅
( 2
π
⋅
SRF
)
68
mH
⋅
( 2
π
⋅
170
KHz
)
2
Select C
IN
ECQ-E4104KF by Panasonic (0.1µF,
400V, Metalized Polyester Film).
Design Example 2
Let us design an LED lamp driver using the HV9922
that would meet the following specifications:
Input:
Universal AC, 85-135VAC
Output Current: 50mA
Load:
String of 12 LED (Power TOPLED®
by OSRAM, V
F
= 2.5V max. each)
Step 1.
Calculating L1.
The output voltage V
O
= 12
⋅
V
F
= 30V (max.). Use
equation (1) assuming a 30% peak-to-peak ripple.
L
1
=
30
V
⋅
10.5
µ
s
=
21
mH
0.3
⋅
50
mA
Step 2.
Selecting D1
Usually, the reverse recovery characteristics of ultra-
fast rectifiers at I
F
=20~50mA are not provided in the
manufacturer’s data books. The designer may want
to experiment with different diodes to achieve the
best result.
Select D1 MUR160 with V
R
= 600V,
t
rr
≈
20ns
(I
F
=20mA, I
RR
=100mA) and C
J
≈
8pF (V
F
>50V).
Step 3.
Calculating total parasitic capacitance using
(3)
C
P
=
5
pF
+
5
pF
+
13
pF
+
8
pF
=
31
pF
Select L1 22mH, I=60mA. Typical SRF=270KHz.
Calculate the coil capacitance.
C
L
=
1
1
=
≈
15
pF
2
L
1
⋅
(2
π
⋅
SRF
)
22
mH
⋅
( 2
π
⋅
270
KHz
)
2
Step 4.
Calculating the leading edge spike duration
using (4), (5)
T
SPIKE
=
264
V
⋅
2
⋅
31
pF
+
20
ns
≈
136
ns
<
T
BLANK
(
MIN
)
100
mA
Step 2.
Selecting D1
Select D1 MUR160 with V
R
= 600V,
t
rr
≈
50ns and C
J
≈
8pF (V
F
>50V).
Step 5.
Estimating power dissipation in HV9921 at
264VAC using (8) and (10)
Let us assume that the overall efficiency
η
= 0.7.
7
NR040805
SUPERTEX [ Supertex, Inc ]
