Component Selection

Transformer Selection

Transformer selection for the MAX256 can be simplified

by the use of a design metric, the ET product. The ET

product relates the maximum allowable magnetic flux

density in a transformer core to the voltage across a

winding and switching period. Inductor current in the

primary linearly increases with time in the operating

region of the MAX256. Transformer manufacturers

specify a minimum ET product for each transformer. For

the MAX256, the requirement on ET product is calculat-

ed as:

By choosing a transformer with sufficient ET product in

the primary winding, it is ensured that the transformer

will not saturate during operation. Saturation of the

magnetic core results in significantly reduced induc-

tance of the primary, and therefore a large increase in

current flow. Excessive transformer current results in a

temperature rise and possible damage to the trans-

former and/or the MAX256.

When CK_RS is unconnected, the internal oscillator is

programmed for the minimum frequency. The default

required ET product for the MAX256 is 42.3Vµs, (assum-

ing +5.5V maximum VCC), or 27.7Vµs for +3.3V opera-

tion (assuming +3.6V maximum VCC). Both of these ET

products assume the minimum oscillator frequency of

65kHz. See the Typical Operating Characteristics plot,

In addition to the constraint on ET product, choose a

transformer with a low DC-winding resistance. Power

dissipation of the transformer due to the copper loss is

approximated as:

ondary. In most cases, an optimum is reached when:

For this condition, the power dissipation is equal for the

primary and secondary windings.

RN R

SEC

PRI

=

2

PI

N R

R

D TX

LOAD

PRI

SEC

_

=×

+

⎛

⎝

⎞

⎠

2

2

ET

V

f

CC

SW

=×

×

1

2

3W Primary-Side Transformer H-Bridge Driver

for Isolated Supplies

10

______________________________________________________________________________________

GND

MAX256

MODE

CK_RS

+5V

47kΩ

ST2

ST1

7815

10µF

0.1µF

0.1µF

-15V

COMMON

4.7µF

470nF

7815

10µF

0.1µF

MBRS140

TGM-280NS

TGM-280NS

MBRS140

MBRS140

MBRS140

0.1µF

+15V

Figure 5. +5V to Isolated Regulated ±15V