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A8439

Photoflash Capacitor Charger with IGBT Driver and Refresh

Allegro MicroSystems 

A8439

Photoflash Capacitor Charger with IGBT Driver and Refresh

Adjusting Output Voltage

The peak current of the rectifying diode, ID_Peak, is calculated as:

The A8439 senses output voltage continuously in order to provide

auto-refresh function. The output voltage can be adjusted by

/ ID_Peak = IPrimary_Peak N .

(5)

selecting proper values of the voltage divider resistors. Use the

following equation to calculate values for Rx (Ω):

Input Capacitor Selection

R1 + R2 = VOUT − 1 .

(3)

R3

VFB

R1 and R2 together need to have a breakdown voltage of at

least 300 V. A typical 1206 surface mount resistor has a 150 V

breakdown voltage rating. It is recommended that R1 and R2

have similar values to ensure an even voltage stress between

them. Recommended values are:

R1 = R2 = 4.99 MΩ (1206)

R3 = 39.4 kΩ (0603)

which together yield a stop voltage of 305 V.

Output Diode Selection

Choose the rectifying diode(s), D1, to have small parasitic

capacitance (short reverse recovery time) while satisfying the

reverse voltage and forward current requirements.

The peak reverse voltage of the diode, VD_Peak , occurs when the

internal MOSFET switch is closed, and the primary-side current

starts to ramp-up. It can be calculated as:

VD_ = Peak VOUT + N ×VBATT .

(4)

Ceramic capacitors with X5R or X7R dielectrics are recom-

mended for the input capacitor, C2. It should be rated at least

4.7 μF / 6.3 V to decouple the battery input, VBATT , at the primary

of the transformer. When using a separate bias, VBIAS , for the

A8439 VIN supply, connect at least a 0.1 μF / 6.3 V bypass

capacitor to the VIN pin.

Layout Guidelines

Key to a good layout for the photoflash capacitor charger circuit

is to keep the parasitics minimized on the power switch loop

(transformer primary side) and the rectifier loop (secondary side).

Use short, thick traces for connections to the transformer primary

and SW pin.

Output voltage sensing circuit elements must be kept away from

switching nodes such as SW pin. It is important that the ¯D¯¯O¯¯N¯¯E¯

signal trace and other signal traces be routed away from the

transformer and other switching traces, in order to minimize noise

pickup. In addition, high voltage isolation rules must be followed

carefully to avoid breakdown failure of the circuit board.

Recommended Components Table

Component

Rating

C1, Input Capacitor

0.1 μF, ±10%, 16 V X7R ceramic

capacitor (0603)

C2, Input Capacitor

4.7 μF, ±10%, 10 V, X5R ceramic

capacitor (0805)

COUT, Photoflash Capacitor 330 V 100 μF (or 19 to 180 μF)

D1, Output Diode

2 x 250 V, 225 mA, 5 pF

R1, R2, FB Resistors

R3, FB Resistor

T1, Transformer

4.99 MΩ, 1/4 W ±1% (1206)

39.0 kΩ 1/10 W ±1% (0603)

1:10.2, LPrimary = 14.5 μH

1:10.2, LPrimary = 12 μH

1:10, LPrimary = 10.8 μH

Part Number

GRM188R71C104KA01D

LMK212BJ475KG

EPH-331ELL101B131S

BAV23S

9C12063A4994FKHFT

9C06031A3902FKHFT

LDT565630T-002

T-16-024A

ST-532517A

Source

Murata

Taiyo Yuden

Chemi-Con

Philips Semiconductor,

Fairchild Semiconductor

Yageo

Yageo

TDK

Tokyo Coil Engineering

Asatech

Allegro MicroSystems, Inc.

13

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

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