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參數(shù)資料
| 型號： | A8439EEJTR-T |
| 廠商： | Allegro MicroSystems, Inc. |
| 英文描述： | Photoflash Capacitor Charger with IGBT Driver and Refresh |
| 中文描述： | 照相閃光燈電容充電器與IGBT驅動器和刷新 |
| 文件頁數(shù)： | 10/12頁 |
| 文件大?。?/td> | 537K |
| 代理商： | A8439EEJTR-T |
A8439-DS, Rev. 1
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Photoflash Capacitor Charger with IGBT Driver
A8439
10
Transformer Design
Turns Ratio.
The minimum transformer turns ratio, N,
(Secondary:Primary) should be chosen based on the following
formula:
V
BATT
V
D_Drop
V
OUT
N
+
≥
40
(1)
where:
V
OUT
(V) is the required output voltage level,
V
D_Drop
(V) is the forward voltage drop of the output diode(s),
V
BATT
(V) is the transformer battery supply, and
40 (V) is the rated voltage for the internal MOSFET switch,
representing the maximum allowable reflected voltage from the
output to the SW pin.
For example, if V
BATT
is 3.5 V and V
D_Drop
is 1.7 V (which could
be the case when two high voltage diodes were in series), and the
desired V
OUT
is 320 V, then the turns ratio should be at least 8.9.
In a worst case, when V
BATT
is highest and V
D_Drop
and V
OUT
are
at their maximum tolerance limit, N will be higher. Taking V
BATT
= 5.5 V, V
D_Drop
= 2 V, and V
OUT
= 320 V ×
102 % = 326.4 V as
the worst case condition, N can be determined to be 9.5.
In practice, always choose a turns ratio that is higher than the
calculated value to give some safety margin. In the worst case
example, a minimum turns ratio of N = 10 is recommended.
Primary Inductance
.
The A8439 has a minimum switch off-time,
t
OFF(min)
, of 300 ns, to ensure correct SW node voltage sensing.
As a loose guideline when choosing the primary inductance,
L
Primary
(
μ
H), use the following formula:
SWLIM
I
N
V
OUT
Primary
L
9
10
300
×
×
×
≥
.
(2)
Ideally, the charging time is not affected by transformer primary
inductance. In practice, however, it is recommended that a
primary inductance be chosen between 10
μ
H and 20
μ
H. When
L
Primary
is less than 10
μ
H, parasitic elements associated with
flyback from the transformer lead to lower efficiency and longer
charging time. When L
Primary
is greater than 20
μ
H, the rating
of the transformer must be dramatically increased to handle the
required power density, and the series resistances are usually
higher. A design that is optimized to achieve a small footprint
solution would have an L
Primary
of 12 to 14
μ
H, with minimized
leakage inductance and secondary capacitance, and minimized
primary and secondary series resistance. Please refer to the table
Recommended Components for more information.
Leakage Inductance and Secondary Capacitance.
The trans-
former design should minimize the leakage inductance to ensure
the turn-off voltage spike at the SW node does not exceed the
40 V limit. An achievable minimum leakage inductance for this
application, however, is usually compromised by an increase in
parasitic capacitance. Furthermore, the transformer secondary
capacitance should be minimized. Any secondary capacitance is
multiplied by
N
2
when reflected to the primary, leading to high
initial current swings when the switch turns on, and to reduced
efficiency.
Applications Information
Symbol
Rating
C1
0.1
μ
F, X5R or X7R, 10 V
C2
4.7
μ
F, X5R or X7R, 10 V
C3
1 nF, X5R or X7R, 10 V
D1
Fairchild Semiconductor BAV23S
(dual diode connected in series)
T1
Tokyo Coil Engineering T-16-024A,
L
Primary
= 12
μ
H, N = 10.2
1206 Resistor, 1 %
R1, R2
R3
0603 Resistor, 1 %
R4
Pull-up resistor
+
To IGBT Gate
COUT
100 μF
V
OUT
D1
T1
1:10.2
R1
4.99 M
Ω
R2
4.99 M
Ω
39 k
Ω
100 k
Ω
R4
V
BATT
1.5 to 5.5 V
Two Alkaline/NiMH/NiCAD or one Li +
V
BIAS
3.0 to 5.5 V
C2
4.7 μF
C3
1 nF
303 V
C1
0.1 μF
A8439
CHARGE
GND
FB
SW
TRIGGER
IGBTDRV
DONE
VIN
Figure 8. Typical circuit for photoflash capacitor charging application.
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