A8837 查看數據表（PDF） - Allegro MicroSystems

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A8837

Photoflash Capacitor Charger with IGBT Driver

Allegro MicroSystems 

A8837

Photoflash Capacitor Charger with IGBT Driver

Applications Information

Transformer Design

Turns Ratio. The minimum transformer turns ratio, N, (Second-

ary : Primary) should be chosen based on the following formula:

N ≥ VOUT + VD_Drop

(1)

40 − VBATT

where:

VOUT (V) is the required output voltage level,

VD_Drop (V) is the forward voltage drop of the output diode(s),

VBATT (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 VBATT is 3.5 V and VD_Drop is 1.7 V (which could

be the case when two high voltage diodes were in series), and the

desired VOUT is 320 V, then the turns ratio should be at least 8.9.

In a worst case, when VBATT is highest and VD_Drop and VOUT are

at their maximum tolerance limit, N will be higher. Taking VBATT

= 5.5 V, VD_Drop = 2 V, and VOUT = 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. As a loose guideline when choosing the

primary inductance, LPrimary (μH), use the following formula:

LPrimary

≥

300 ×10−9 ×VOUT

N × ISWLIM

.

(2)

Ideally, the charging time is not affected by transformer pri-

mary inductance. In practice, however, it is recommended that a

primary inductance be chosen between 10 μH and 20 μH. When

LPrimary is lower than 10 μH, the converter operates at higher fre-

quency, which increases switching loss proportionally. This leads

to lower efficiency and longer charging time. When LPrimary is

greater than 20 μH, the rating of the transformer must be dra-

matically 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 LPrimary of 12 to

14 μH, with minimized leakage inductance and secondary capaci-

tance, and minimized primary and secondary series resistance.

See the table Recommended Components for more information.

Recommended Components

Component

Rating

Part Number

Source

Remarks

C1, Input Capacitor

C2, Input Capacitor

0.1 μF, ±10%, 16 V X7R

ceramic capacitor (0603)

4.7 μF, ±10%, 10 V, X5R

ceramic capacitor (0805)

GRM188R71C104KA01D Murata

LMK212BJ475KG

Taiyo Yuden

10 V minimum rating can

be used

COUT, Photoflash

Capacitor

D1, Output Diode

330 V 100 μF (or 19 to 180 μF) EPH-331ELL101B131S

2 x 250 V, 225 mA, 5 pF

BAV23S

150 kΩ each 1/4 W, ± 1%;

R1, R2, FB Resistors 1206, 0805, or 0603 resistors

rated for 150 V

R3, FB Resistor

1.2 kΩ 1/10 W ±1% (0603 or

0402)

Chemi-Con

Philips Semiconductor,

Fairchild Semiconductor

Instead of two resistors,

a single 300 kΩ resistor

with 350 V rating can be

used

LP = 14.2 μH, IP = 2 A,

N = 10

T-15-154M

Tokyo Coil

Suitable for ILIM from

0.7 to 2.0 A

T1, Transformer

LP = 7.4 μH, IP = 2 A,

N = 10

T-16-103A

Tokyo Coil

Suitable for ILIM from

1.2 to 2.0 A only

LP = 14 μH, IP = 1.5 A,

N=10

ST-652956A

Asatech

Suitable for ILIM from

0.7 to 1.4 A only

Allegro MicroSystems, Inc.

9

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

www.allegromicro.com

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