HV9921/HV9922/HV9923
as:
IDD is the internal linear regulator current.
VIN ⋅CP
ISAT
When the LED driver is powered from the full-wave
rectified AC line input, the exact equation for calculating the
conduction loss is more cumbersome. However, it can be
estimated using the following equation:
(4)
TSPIKE
=
+ trr
P
= KC ⋅ IO2 ⋅ RON + Kd ⋅ IDD ⋅VAC
(10)
In order to avoid false triggering of the current sense
comparator, CP must be minimized in accordance with the
following expression:
COND
where VAC is the input AC line voltage. The coefficients KC
(5)
ISAT ⋅ TBLANK( MIN ) − trr
(
)
and Kd can be determined from the minimum duty ratio of
CP <
VIN( MAX )
the HV9921/22/23
0.7
where TBLANK(MIN) is the minimum blanking time of 200ns, and
VIN(MAX) is the maximum instantaneous input voltage.
0.6
Estimating Power Loss
0.5
Discharging the parasitic capacitance C into the DRAIN
pin of the HV9921/22/23 is responsible fPor the bulk of the
switching power loss. It can be estimated using the following
Kd (Dm)
0.4
Kc(Dm)
equation:
0.3
0.2
0.1
2
⎛
⎜
⎝
⎞
(6)
VIN CP
2
P
=
+VIN ISAT ⋅trr ⋅ F
⎟
SWITCH
S
⎠
where Fs is the switching frequency, ISAT is the saturated
DRAIN current of the HV9921/22/23. The switching loss is
the greatest at the maximum input voltage.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Dm
Fig. 1. Conduction Loss Coefficients KC and Kd
The switching frequency is given by the following:
VIN −η−1 ⋅VO
VIN ⋅TOFF
(7)
EMI Filter
FS =
As with all off-line converters, selecting an input filter is critical
to obtaining good EMI. A switching side capacitor, albeit of
small value, is necessary in order to ensure low impedance
to the high frequency 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.
where η is the efficiency of the power converter.
When the HV9921/22/23 LED driver is powered from the
full-wave rectified AC input, the switching power loss can be
estimated as:
(8)
−η−1 ⋅VO
1
P
≈
V ⋅C + 2 ⋅ I ⋅trr
V
)
(
AC
Design Example 1
(
)
SWITCH
AC
P
SAT
2 ⋅TOFF
Let us design an HV9921 LED lamp driver meeting the
following specifications:
VAC is the input AC line voltage.
The switching power loss associated with turn-off transitions
oftheDRAINpincanbedisregarded. Duetothelargeamount
of parasitic capacitance connected to this switching node,
the turn-off transition occurs essentially at zero-voltage.
Input:
Output Current: 20mA
Load: String of 10 LED (LW541C by OSRAM
VF = 4.1V max. each)
Universal AC, 85-264VAC
Conduction power loss in the HV9921/22/23 can be
calculated as:
Step 1. Calculating L1.
P
= D ⋅ IO2 ⋅ RON + IDD ⋅V ⋅ 1− D
(
)
(9)
COND
IN
The output voltage VO = 10 ⋅ VF ≈ 41V (max.). Use equation
(1) assuming a 30% peak-to-peak ripple.
41V ⋅10.5µs
0.3⋅ 20mA
where D = VO /ηVIN is the duty ratio, RON is the ON resistance,
L1 =
= 72mH
NR092005
6
SUPERTEX [ Supertex, Inc ]
