Isolated Transformer Driver

for PCMCIA Applications

______________________________________________________________________________________

13

2) Use a test winding to measure ET product (if using

3) Determine the number of turns required for the pri-

mary winding. For an ungapped toroid, ET product

from center-tap to D1 must be at least 5V-µs. Other

core types must have sufficient inductance to limit

D1 and D2 output current under minimum load con-

ditions, and must not be allowed to saturate.

4) Select a rectifier topology based on performance

requirements (ripple vs. loss, and space required

for secondary winding). Refer to Table 2, Rectifier

Topology Trade-Offs.

mine the secondary to primary turns ratio. Include

losses in the rectifier diodes, and estimate resistive

losses in the windings. For load currents exceed-

ing 150mA, use a voltage step-down transformer to

step up the output current from the MAX845. Do

not exceed the MAX845’s absolute maximum out-

put current rating (200mA).

6) Wind the transformer with the largest diameter wire

that will fit the winding area. Select a wire gauge to

fill the winding aperture as much as possible.

Larger diameter wire has lower resistance per unit

length. Doubling the wire diameter reduces resis-

tive losses by a factor of four.

Bobbin or drum cores suffer from low coupling between

windings. This usually requires bifilar winding for the

two halves of the primary.

Due to the inherent complexity of magnetic circuit

design, it will be necessary to build a prototype and re-

iterate the design. If necessary, adjust the design by

altering the number of primary or secondary turns, or the

wire gauge. If using a different core material or geome-

Rectifier Topology

Figure 11 shows various rectifier topologies. Refer to

Table 2 for selection criteria. The turns ratio of the trans-

former must be set to provide the minimum required out-

put voltage at the maximum anticipated load, with the

minimum expected input voltage. In addition, the calcu-

lations should allow for worst-case losses in the recti-

fiers. Since the turns ratio determined in this manner will

ordinarily produce a much higher voltage at the sec-

ondary under conditions of high input voltage and/or

light loading, be careful to prevent an overvoltage con-

dition from occurring (see the Output Voltage vs. Load

Current graph in the

Typical Operating Characteristics).

Diodes

Use fast-switching diode rectifiers. Ordinary silicon sig-

nal diodes like the 1N914 or 1N4148 may be used for

low output current levels (less than 50mA), but Schottky

diodes have a lower forward voltage drop and should

be used for higher-current applications. Central

Semiconductor has low-current Schottky diodes as

duals in SOT-23 packages (CMPSH-3 series). The

Nihon SB05W05C is a common-cathode dual in a SOT-

23; it works well in the two-diode full-wave configura-

tion. The Motorola MBR0520 is an excellent choice for

all configurations.

Figure 11c. Voltage Doubler

Figure 11a. 2-Diode Push-Pull

Figure 11b. 4-Diode Bridge

1

8

MAX845

GND1 GND2

6

2

7

D1

D2

1

8

MAX845

GND1 GND2

6

2

7

D1

D2

MAX845

GND1 GND2

1

8

6

2

7

D1

D2