MAX13253 Datasheet, PDF (12/14 Pages) Maxim Integrated Products – 1A, Spread-Spectrum, Push-Pull, Transformer Driver for Isolated Power Supplies

MAX13253
1A, Spread-Spectrum, Push-Pull, Transformer
Driver for Isolated Power Supplies
For example, when the internal oscillator is used to drive
the outputs and HICLK is low, the required transformer
ET product to the center tap for an application with V_DD
(max) = 5.5V, is 13.1V-µs. An application with V_DD (max)
= 3.3V has a transformer ET product to the center tap
requirement of 7.9V-µs.
In addition to the constraint on ET product, choose a trans-
former with low leakage inductance and low DC-winding
resistance. Power dissipation of the transformer due to
the copper loss is approximated as:
P_{D_TX} = I_LOAD2 x (R_PRI/N² + R_SEC)
where R_PRI is the DC winding resistance of the primary,
and R_SEC is the DC winding resistance of the second-
ary. In most cases, an optimum is reached when R_SEC =
R_PRI/N². For this condition, the power dissipation is equal
for the primary and secondary windings.
As with all power-supply designs, it is important to opti-
mize efficiency. In designs incorporating small trans-
formers, the possibility of thermal runaway makes low
transformer efficiencies problematic. Transformer losses
produce a temperature rise that reduces the efficiency of
the transformer. The lower efficiency, in turn, produces an
even larger temperature rise.
To ensure that the transformer meets these requirements
under all operating conditions, the design should focus on
the worst-case conditions. The most stringent demands
on ET product arise for maximum input voltage, minimum
switching frequency, and maximum temperature and load
current. Additionally, the worst-case values for transform-
er and rectifier losses should be considered.
The primary must be center-tapped; however the second-
ary winding may or may not be center-tapped, depending
on the rectifier topology used. The phasing between pri-
mary and secondary windings is not critical.
The transformer turns ratio must be set to provide the
minimum required output voltage at the maximum antici-
pated load with the minimum expected input voltage. In
addition, include in the calculations an allowance for the
worst-case losses in the rectifiers. Since the turns ratio
determined in this manner will ordinarily produce a much
higher voltage at the secondary under conditions of high
input voltage and/or light loading, be careful to prevent an
overvoltage condition from occurring.
Transformers for use with the MAX13253 are typically
wound on a high-permeability magnetic core. To minimize
radiated electromagnetic emissions, select a toroid, pot
core, E/I/U core, or equivalent.
Diode Selection
The high switching speed capability of the MAX13253
necessitates high-speed rectifiers. Ordinary silicon signal
diodes such as the 1N914 or 1N4148 can be used for low-
output current levels (less than 50mA), but at high output
current levels, their reverse recovery times might degrade
efficiency. At higher output currents, select low forward-
voltage Schottky diodes to improve efficiency. Ensure
that the average forward current rating for the rectifier
diodes exceeds the maximum load current of the circuit.
For surface-mount applications, Schottky diodes such as
the B230A, MBRS230, and MBRS320 are recommended.
Suggested External Component Manufacturers
Table 1. Component Manufacturers
MANUFACTURER
Halo Electronics
Diodes Inc.
Murata Americas
COMPONENT
Transformers
Diodes
Capacitors
WEBSITE
www.haloelectronics.com
www.diodes.com
www.murataamericas.com
www.maximintegrated.com
Maxim Integrated │  12