Component Selection

Transformer Selection

Transformer selection for the MAX258 can be simplified

by the use of the ET product. The ET product relates

the maximum allowable magnetic flux density in a trans-

former core to the voltage across a winding and switch-

ing period. Inductor magnetizing current in the primary

winding changes linearly with time during the switching

period of the device. Each transformer has a minimum

ET product, though not always stated on the transformer

data sheet. Ensure that the transformer selected for use

the minimum switching frequency of the T1 and T2 ouputs.

Select a transformer with sufficient ET product for each

half of the primary winding to ensure that the transformer

does not saturate during operation. Saturation of the mag-

netic core results in significantly reduced inductance of the

primary, and therefore in a large increase in current flow.

For example, when HICLK is low, the required trans-

former ET product to the center tap for an application with

(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:

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 ordinarily produces a much

higher voltage at the secondary under conditions of high

input voltage and/or light loading, take care to prevent an

overvoltage condition from occurring.

Transformers for use with the IC 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 MAX258

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. Suggested External Component Manufacturers

MANUFACTURER

COMPONENT

WEBSITE

Halo Electronics

Transformers

www.haloelectronics.com

Diodes Inc.

Diodes

www.diodes.com

Murata Americas

Capacitors

www.murataamericas.com

MAX258

500mA, Push-Pull Transformer Driver

for Isolated Power Supplies

www.maximintegrated.com

Maxim Integrated │ 10