stress on the switch. A FET with the lowest total charge

expected (plus some safety factor) is the best choice.

The choice of package is a function of the application,

the total power, and the cooling methods available.

Transformer

Transformer parameters, once calculated in the design

process, can be used to find standard parts whenever

possible. The most important factors are the saturation

current, primary inductance, leakage inductance turns

ratio, and losses. Packaging and EMI generation and

susceptibility are closely connected, and must be con-

sidered. In general, parts with exposed air gaps (not

contained inside the magnetic structure) will generate

the most radiated EMI, and might need external shield-

ing. If the design is in high-voltage power supplies, the

insulation specifications are also important. Pay close

attention if the circuitry is galvanically connected to the

mains at any point, since serious safety and regulatory

issues might exist.

Capacitors

As in any high-frequency power circuit, the capacitors

used for filtering must meet very low ESR and ESL

requirements. At the 300kHz frequency (of which the

MAX5003 is capable), the most favorable technologies

are ceramic capacitors and organic semiconductor (OS

CON) capacitors. The temperature dependence of the

capacitance value and the ESR specification is important,

particularly if the ESR is used as part of the compensation

network for the feedback loop. If using through-hole-

mounted parts, keep lead length as short as practical.

Components with specifications for switching power con-

verters are preferred. Decoupling capacitors must be

mounted close to the IC.

Diodes

The choice of rectifier diodes depends on the output

voltage range of the particular application. For low-volt-

age converters, the diode drop is a significant portion

of the total loss, and must be kept to a minimum. In

those cases, Schottky diodes are the preferred compo-

nent for the design. At higher voltages, ultra-fast recov-

ery diodes must be used, since Schottky components

will not satisfy the reverse voltage specification.

For all cases, the specifications to be determined

before choosing a diode are the peak current, the aver-

age current, the maximum reverse voltage, and the

maximum acceptable rectification losses. Once a type

is identified, a thermal analysis of the diode losses vs.

total thermal resistance (from junction to ambient) must

be carried out if the total power involved is significant.

Industrial-frequency (60Hz) rectifiers are not recom-

mended for any function in these converters, due to

their high capacitance and recovery losses. If using

overdimensioned rectifiers, the junction capacitance

influence must be reviewed.

High-Voltage PWM

Power-Supply Controller

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are

implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600

© 1999 Maxim Integrated Products

Printed USA

is a registered trademark of Maxim Integrated Products.

___________________Chip Information

TRANSISTOR COUNT: 1050

SUBSTRATE CONNECTED TO GND

MANUFACTURER

PHONE

DEVICE TYPE

Current-Sense Resistors

Dale-Vishay

402-564-3131

Power FETs

Fairchild

408-822-2000

International Rectifier

310-322-3333

Motorola

303-675-2140

Diodes

Central Semiconductor

516-435-1110

Transistors

Central Semiconductor

516-435-1110

FAX

402-563-6418

408-822-2102

310-322-3332

303-675-2150

516-435-1824

516-435-1824

Capacitors

Sanyo

619-661-6835

619-661-1055

Taiyo Yuden

408-573-4150

408-573-4159

AVX

803-946-0690

803-626-3123

Coils

Coiltronics

561-241-7876

561-241-9339

Table 1. Component Manufacturers