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A8426

High Performance Photoflash Capacitor Charger with IGBT Driver

Allegro MicroSystems 

A8426

High Performance Photoflash Capacitor Charger

with IGBT Driver

following illustration, when the battery voltage drops

below 2.5 V, the signal at BL (battery-low) should go

high. The resistor RBL, connecting BL to the ISET

pin, then injects 20 μA into RSET. This effectively

reduces ISET current to 30 μA (for ISWLIM = 1.65 A).

The disadvantage of this method is that 20 μA flows

continuously while BL is high.

BL RBL

ISET

RSET

Selection of Transformer

1. The primary inductance, LP , determines the on-time

of the switch, as follows:

ton = –LP / R × ln (1 – ISWlim × R / VBAT) , (2)

where R is the total resistance in the primary current

path (including RSWDS(on) and the DC resistance of the

transformer).

If VBAT is much larger than ISWlim × R, then ton can be

approximated using the following formula:

In another example of a possible application, we can

make use of a PTC thermistor to decrease the switch

current limit when the board temperature exceeds

65°C. Refering to the following figure, R3 is a

PTC type thermistor such as the Murata

PRF18BG471QB1RB.

RSET

ISET

R1

54.9 kΩ

+t°

R2

45.3 kΩ

R3

470 Ω

In this configuration, the peak currents at various PCB

temperatures are as follows:

TPCB

(°C)

25

65

80

R3

(kΩ)

0.470

4.7

47.0

RSET

(kΩ)

25.0

26.2

34.4

ISWpeak

(A)

3.2

3.0

2.3

ton = ISWlim × LP / VBAT .

(3)

2. The secondary inductance, LS, determines the off-

time of the switch, as follows:

toff = (ISWlim / N ) × LS / VOUT .

(4)

Because LS / LP = N × N:

toff = (ISWlim × LP × N ) / VOUT .

(5)

The minimum pulse width for toff determines the

minimum primary inductance required for the trans-

former. For example, if ISWlim = 1.0 A, N = 10, and

VOUT = 315 V, then LP must be at least 6.3 μH in order

to keep toff at 200 ns or longer. In general, choosing a

transformer with larger LP results in higher efficiency

(because the higher the value of LP, the lower the

switch frequency, and hence the lower the switching

loss). But transformers with higher LP ratings also

require more windings and larger magnetic cores.

Therefore a trade-off must be made between trans-

former size and efficiency.

Allegro MicroSystems, Inc.

10

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

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