stress on the switch. A FET with the lowest total charge
and the lowest R
DS(ON
) for the maximum drain voltage
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.
MAX5003
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
© 2002 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
