Datasheet

MAX1856
Wide Input Range, Synchronizable,
PWM SLIC Power Supply
12 ______________________________________________________________________________________
gives 1.11A for 80% efficiency. With a duty cycle of
52.5%, the average switch current (I
SW(AVG)
) is 2.114A.
Choosing a primary inductance ripple current ∆I
L
to be
40% of the average switch current, the primary induc-
tance is given by:
Selecting f
OSC
= 250kHz and ∆I
L
= 0.4 x I
SW(AVG)
=
0.846A, the primary inductance value is 27µH, and the
peak primary current for this example is therefore 2.5A.
Core Selection
The transformer in a flyback converter is a coupled
inductor with multiple windings on the same magnetic
core. Flyback topologies operate by storing energy in
the transformer magnetics during the on-time and
transferring this energy to the output during the off-
time.
Core selection depends on the core’s power-handling
capability. The required output power is first consid-
ered. For example, the standard application circuit
requires 9.6W. Assuming a typical 80% efficiency, the
transformer must support 12W of power. The core
material’s properties, the core’s shape, and the size of
the air gap determine the core’s power rating. Since the
equations relating these properties to the power capa-
bility are involved, manufacturers simply provide charts
giving “Power vs. Frequency” for different core sizes.
For the standard application circuit (f
OSC
= 250kHz),
the EFD15 core from Coiltronics meets the criteria.
Once the core is chosen, the number of turns in the pri-
mary is given by:
where A
L
is the inductance factor. Ensure that the num-
ber of ampere-turns (N
P
I
SAT
) is below the saturation
limit. A significant portion of the total energy is stored in
the air gap. Therefore, the larger the air gap, the lower
the A
L
value and the larger the number of ampere-turns
at which saturation starts. Some manufacturers define
saturation as the current at which the inductance
decreases by 30%.
Current-Sense Resistor Selection
Once the peak inductor current is determined, the cur-
rent-sense resistor (R
CS
) is determined by:
Kelvin-sensing should be used to connect CS and
PGND to R
CS
. Connect PGND and GND together at the
ground side of R
CS
.
Due to inductive ringing after the MOSFET turns on, a
lowpass filter may be required between R
CS
and CS to
prevent the noise from tripping the current-sense com-
parator. Connect a 100Ω resistor between CS and the
R
V
I
mV I
CS
CS MIN
LPEAK
LPEAK
==
()
/85
N
L
A
P
P
L
=
L
VD
I
P
IN
L OSC
=
∆ƒ
INPUT VOLTS (V) OUTPUT VOLTS (V) OUTPUT CURRENT (mA) TRANSFORMER (VENDOR)
5 -48 100 VP3-0055 (Coiltronics)
12 -48 100
CTX01-14853 (Coiltronics), or
ICA-0635 (ICE Components)
12 -24 and -72 400 or 100
CTX01-14853 (Coiltronics), or
ICA-0635 (ICE Components)
12 -95 and -30 320 and 150 CTX03-15220 (Coiltronics)
Table 1. Transformer Selection for Standard Applications
VENDOR USA PHONE USA FAX INTERNET
Coilcraft 847-639-6400 847-639-1469 www.coilcraft.com
Coiltronics 888-414-2645 561-241-9339 www.coiltronics.com
ICE Components 800-729-2099 703-257-7547 www.icecomponents.com
Pulse Engineering 858-674-8100 858-674-8262 www.pulseeng.com
TDK 847-390-4461 847-390-4405 www.tdk.com
Table 2. Transformer Suppliers