______________Component Selection

Transformer

The MAX845 drives any transformer that has a center-

tapped primary and a saturation rating of at least 5V-µs

(ET product) per side. The oscillator frequency varies

saturation at the minimum frequency, because the

the transformer must withstand at least:

1

1

4.5V x ———–——— x — = 5V-µs

450kHz min

2

at least:

1

1

5.5V x ———–——— x — = 5V-µs

550kHz min

2

Thus, the required ET product is constant over the

entire 5V ±10% range.

Select either a toroid or a gapped core. Although some

applications will require custom transformers, many

can use standard transformer designs, such as those

listed in Table 1. Some of these manufacturers have

standard products designed for the MAX845, while

some have standard products that can be adapted for

specific customer requirements. Table 1 also lists some

suppliers of suitable magnetic cores.

An ungapped toroid core must never be allowed to sat-

urate. An empirical way to measure a toroid’s ET prod-

uct is to wind 20 turns on the bare core and observe

the current waveform on an oscilloscope while driving

the winding with a function generator. Generate a 50%

duty-cycle square wave at a test frequency of 500kHz,

with no DC offset. Gradually increase the driving volt-

age until the waveform suddenly begins to draw more

current. At this point, the core is saturating, so reduce

the driving voltage until the core just barely stops satu-

rating. The ET product indicated is simply the maxi-

mum voltage that can be applied without saturation,

multiplied by 1µs (the time of half of the period of the

input signal). Because the ET product varies linearly

with the number of turns, this test winding can be

scaled up or down to act as a suitable primary for that

particular core.

A gapped core, such as a bobbin or drum core, is not

limited by ET product, but rather by inductance and

winding resistance. The primary inductance must be

high enough to prevent excessive current flow under

light-load conditions, yet low enough that it can be

wound on the core. Good results can be achieved by

using a primary inductance between 50µH and 200µH.

Calculate the number of turns required by using the

or measure a test winding with an inductance meter.

Inductance varies with the square of the number of turns.

While most MAX845 applications will use a toroid trans-

former for highest efficiency and lowest EMI, there may

be applications that can utilize less expensive trans-

formers, such as E, I, or U-shaped cores, magnetic

bobbins, or etched windings on a printed circuit board.

Table 1 lists some transformer and core suppliers who

can assist with your magnetics design.

The secondary or secondaries can be scaled to produce

whatever output is required for the application at hand,

taking into account the rectifier topology to be used and

the forward voltage loss of the diodes selected.

Step-by-Step Transformer

Design Procedure

Before starting the design, determine the minimum and

maximum output voltage requirement, the minimum

and maximum load current, the physical size con-

straints, and the cost budget.

1) Select an appropriate core shape and material from

core vendors’ data sheets; trade-off EMI vs. space

and cost. Since the MAX845’s output waveform is a

square wave, it is rich in harmonics, so choose a

material with low losses at up to several MHz.

Isolated Transformer Driver

for PCMCIA Applications

12

______________________________________________________________________________________

Table 1. Transformer and Transformer-Core

Suppliers

TRANSFORMERS

TRANSFORMER CORES

Halo Electronics

Phone: (415) 969-7313

FAX: (415) 367-7158

Ask for MAX845 Transformer

Magnetics Inc.

Phone: (412) 282-8282

FAX: (412) 282-6955

Coilcraft

Phone: (708) 639-6400

FAX: (708) 639-1469

Ask for MAX845 Transformer

Fair-Rite Products

Phone: (914) 895-2055

FAX: (914) 895-2629

BH Electronics

Phone: (612) 894-9590

FAX: (612) 894-9380

Ask for MAX845 Transformer

Philips Components

Phone: (401) 762-3800

FAX: (401) 762-3805, ext. 324

MMG (Magnetic Materials Group)

Phone: (201) 345-8900

FAX: (201) 345-1172

Amidon Associates

Phone: (714) 850-4660

FAX: (714) 850-1163

Sumida USA

Phone: (708) 956-0666

FAX: (708) 956-0702