A8740 Просмотр технического описания (PDF) - Allegro MicroSystems

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A8740

Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger with IGBT Driver

Allegro MicroSystems 

A8740

Ultra Small Mobile Phone Xenon Photoflash

Capacitor Charger with IGBT Driver

Applications Information

Transformer Design

1. The transformer turns ratio, N, determines the output voltage:

N = NS / NP

VOUT = 31.5 × N – Vd ,

where 31.5 is the typical value of VOUTTRIP , and Vd is the for-

ward drop of the output diode.

2. The primary inductance, LPRIMARY, determines the on-time of

the switch:

ton = (–LPRIMARY / R ) × ln (1 – ISWlim × R /VIN) ,

where R is the total resistance in the primary current path (includ-

ing RSWDS(on) and the DC resistance of the transformer).

If VIN is much larger than ISWlim × R, then ton can be approxi-

mated by:

ton = ISWlim × LPRIMARY /VIN .

3. The secondary inductance, LSECONDARY, determines the off-

time of the switch. Given:

LSECONDARY / LPRIMARY = N × N , then

toff = (ISWlim / N) × LSECONDARY /VOUT

= (ISWlim × LPRIMARY × N) /VOUT .

The minimum pulse width for toff determines what is the mini-

mum LPRIMARY required for the transformer. For example, if

ILIM = 1.5 A, N = 10, and VOUT = 315 V, then LPRIMARY must

be at least 4.2 μH in order to keep toff at 200 ns or longer. These

relationships are illustrated in figure 6.

In general, choosing a transformer with a larger LPRIMARY results

in higher efficiency (because a larger LPRIMARY corresponds to

a lower switch frequency and hence lower switching loss). But

transformers with a larger LPRIMARY also require more windings

and larger magnetic cores. Therefore, a trade-off must be made

between transformer size and efficiency.

Leakage Inductance and Secondary Capacitance

The transformer design should minimize the leakage induc-

tance to ensure the turn-off voltage spike at the SW node does

not exceed the absolute maximum specification on the SW pin

(refer to the Absolute Maximum Ratings table). 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 N2 when

reflected to the primary, leading to high initial current swings

when the switch turns on, and to reduced efficiency.

VSW

VIN

ISW

ton

toff

Vr

Figure 6. Transformer Selection Relationships

tf

tneg

ISW

VIN

VSW

Allegro MicroSystems, Inc.

12

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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