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 loss-

es produce a temperature rise that reduces the effi-

ciency of the transformer. The lower efficiency, in turn,

produces an even larger temperature rise.

To ensure that the transformer meets these require-

ments under all operating conditions, the design should

focus on the worst-case conditions. The most stringent

demands on ET product arise for minimum switching

frequency, maximum input voltage, maximum tempera-

ture, and load current. Additionally, the worst-case val-

ues for transformer and rectifier losses should be

considered.

The primary should be a single winding; however, the

secondary can be center-tapped, depending on the

desired rectifier topology. In most applications, the

phasing between primary and secondary windings is

not significant. Half-wave rectification architectures are

possible with the MAX256; however, these are discour-

aged. If a net DC current results due to an imbalanced

load, the magnetic flux in the core is increased. This

reduces the effective ET product and can lead to satu-

ration of the transformer core.

Transformers for use with the MAX256 are typically

wound on a high-permeability magnetic core. To mini-

mize radiated electromagnetic emissions, select a

toroid, pot core, E/I/U core, or equivalent.

+3.3V Operation

The MAX256 can be operated from a +3.3V supply by

increasing the turns ratio of the transformer, or by

designing a voltage-doubler or voltage-tripler circuit as

shown in Figure 1B.

Optimum performance at +3.3V is obtained with fewer

turns on the primary winding, since the ET product

is lower than for a +5V supply. However, any of the

transformers for use with a +5V supply will operate

properly with a +3.3V supply. For a given power level,

the transformer currents are higher with a +3.3V supply

than with a +5V supply. Therefore, the DC resistance

of the transformer windings has a larger impact on the

circuit efficiency.

3W Primary-Side Transformer H-Bridge Driver

for Isolated Supplies

______________________________________________________________________________________

11

MAX256

-15V

COMMON

+15V

RS485

MPU

M

U

X

DAC/ADC

OPTOISOLATORS

OPTOISOLATORS

Figure 6. Isolated Power Supply for Process Control Applications