a8740 Allegro MicroSystems, Inc., a8740 Datasheet - Page 12

• Manufacturer Part Number: a8740
• Description: Ultra Small Mobile Phone Xenon Photoflash Capacitor Charger With Igbt Driver
• Manufacturer: Allegro MicroSystems, Inc.

A8740
Transformer Design
1. The transformer turns ratio, N, determines the output voltage:
where 31.5 is the typical value of V
ward drop of the output diode.
2. The primary inductance, L
the switch:
where R is the total resistance in the primary current path (includ-
ing R
If V
mated by:
3. The secondary inductance, L
time of the switch. Given:
IN
SWDS(on)
is much larger than I
t
on
= (–L
L
and the DC resistance of the transformer).
SECONDARY
V
OUT
PRIMARY
t
t
off
on
N = N
= 31.5 × N – V
= I
= (I
= (I
SWlim
S
SWlim
SWlim
/ R ) × ln (1 – I
/ L
SWlim
Figure 6. Transformer Selection Relationships
/ N
PRIMARY
PRIMARY
P
× L
SECONDARY
× L
/ N) × L
V
V
I
SW
× R, then t
SW
IN
PRIMARY
PRIMARY
OUTTRIP
d
, determines the on-time of
= N × N , then
,
SECONDARY
SWlim
, determines the off-
/V
on
× N) /V
, and V
IN
can be approxi-
× R /V
Ultra Small Mobile Phone Xenon Photoflash
.
Applications Information
OUT
/V
d
IN
OUT
t
is the for-
on
) ,
.
Capacitor Charger with IGBT Driver
The minimum pulse width for t
mum L
ILIM = 1.5 A, N = 10, and V
be at least 4.2 μH in order to keep t
relationships are illustrated in figure 6.
In general, choosing a transformer with a larger L
in higher efficiency (because a larger L
a lower switch frequency and hence lower switching loss). But
transformers with a larger L
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 N
reflected to the primary, leading to high initial current swings
when the switch turns on, and to reduced efficiency.
V
r
t
off
PRIMARY
t
f
required for the transformer. For example, if
t
neg
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Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
PRIMARY
OUT
V
I
SW
V
SW
off
IN
= 315 V, then L
determines what is the mini-
off
also require more windings
at 200 ns or longer. These
PRIMARY
corresponds to
PRIMARY
PRIMARY
must
2
results
when
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