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參數(shù)資料
| 型號(hào)： | AN-995A |
| 英文描述： | Electronic Ballasts Using the Cost-Saving IR215X Drivers(153.99 k) |
| 中文描述： | 使用電子鎮(zhèn)流器的成本節(jié)約IR215X驅(qū)動(dòng)器（153.99十一） |
| 文件頁數(shù)： | 5/10頁 |
| 文件大?。?/td> | 153K |
| 代理商： | AN-995A |
Application
Notes
C
ONTROL
I
NTEGRATED
C
IRCUIT
D
ESIGNERS’
M
ANUAL
C-63
cuits have become popular for off-line power supplies
and several semiconductor manufacturers supply control
ICs for this topology.
For electronic ballasts, however, the sophistication of
these control chips may not be necessary and it is rela-
tively simple to provide power factors exceeding 0.95 by a
simple boost topology operating at a fixed 50% duty
cycle. Using the IR2151 driver it is also possible to pro-
vide dimming merely by changing the duty cycle and,
hence, the boost ratio.
Figures 6* and 7 illustrate how this can be accom-
plished.
AC Voltage and current
PF = 0.96 LAG
200 V/Div., 0.5 A/Div., 5 msec/Div.
DC Bus voltage showing 50% Vp ripple
100V/Div., 2msec/Div.
Figure 5.
Waveforms of Figure 4
Dimming Control
The IR2151 has a ‘front end’ oscillator circuit akin to
the 555 IC and is amenable to the same type of circuitry
to control the duty cycle of the output waveforms.
Dimming control to 50% of power input is easily
achieved by this control. When R
T
(lead 2) switches high
the charging path for C
T
(lead 3) is through the forward
biased diode and the left side of the duty cycle control
pot. When C
T
charges to two-thirds V
CC
, R
T
switches
low and C
T
discharges through the right side of the con-
trol pot. Until the one-third V
CC
voltage is reached, the
cycle then repeats. Note that although the charge and
discharge times of C
T
can be varied, the sum of them
remains constant and hence the oscillation frequency is
also constant. This allows sufficient lamp striking volt-
age even under dimmed conditions.
In actual operation, the ‘on’ time of the boost MOS-
FET is reduced as R
T
(CHG) becomes smaller than
RT(DISCH). Obviously, if the “on” time of the boost
MOSFET is reduced the boost voltage ratio is also re-
duced proportionately:
boost voltage ratio =
V
1
1 D
IN
×
(6)
e.g., at 50% duty cycle:
V
1
0.5
2V
IN
IN
×
=
where V
IN
= instantaneous input voltage and D is the
‘on’ time ratio of the boost MOSFET.
A variation of this circuit, shown in Figure 8, allows
dimming to be controlled remotely by a variable resistor.
The circuits of Figures 7 and 8 both suffer from a basic
flaw; namely that if the lamps are removed or broken the
open circuit DC bus voltage rises until the power
MOSFETs avalanche and fail or the filter capacitor over-
heats and fails due to overvoltage.
To prevent this, the duty cycle of the boost transistor
can be reduced so that the DC bus is regulated to a con-
stant level, as shown in Figure 9.
In operation, the duty cycle of the boost regulator is
determined by comparing a fraction of the DC bus volt-
age with a reference triangle wave appearing on the tim-
ing capacitor C
T
. The switching levels of the IR2151
Control IC timing circuit occur at one-third V
CC
and
two-thirds V
CC
. Since V
CC
is regulated by an internal
voltage regulator, the amplitude of the C
T
waveform is
also regulated.
*U.S. Patent No. 5001400 Nilssen, March 1991
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