ADDC02812DATV Просмотр технического описания (PDF) - Analog Devices

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ADDC02812DATV

28 V/100 W DC/DC Converters with Integral EMI Filter

Analog Devices 

ADDC02812DA/ADDC02815DA

Input Voltage Transient Protection: The converters have a

transient voltage suppressor connected across their input leads

to protect the units against high voltage pulses (both positive

and negative) of short duration. With the power supply con-

nected in the typical system setup shown in Figure 17, a tran-

sient voltage pulse is created across the converter in the

following manner. A 20 µF capacitor is first charged to 400 V.

It is then connected directly across the converter’s end of the

two meter power lead cable through a 2 Ω on-state resistance

MOSFET. The duration of this connection is 10 µs. The pulse

is repeated every second for 30 minutes. This test is repeated

with the connection of the 20 µF capacitor reversed to create a

negative pulse on the supply leads. (If continuous reverse volt-

age protection is required, a diode can be added externally in

series at the expense of lower efficiency for the power system.)

The converter responds to this input transient voltage test by

shutting down due to its input overvoltage protection feature.

Once the pulse is over, the converter initiates a soft-start, which

is completed before the next pulse. No degradation of converter

performance occurs.

THERMAL CHARACTERISTICS

Junction and Case Temperatures: It is important for the

user to know how hot the hottest semiconductor junctions

within the converter get and to understand the relationship

between junction, case, and ambient temperatures. The hottest

semiconductors in the 100 W product line of Analog Devices’

high density power supplies are the switching MOSFETs and

the output rectifiers. There is an area inside the main power

transformers that is hotter than these semiconductors, but it is

within NAVMAT guidelines and well below the Curie tempera-

ture of the ferrite. (The Curie temperature is the point at which

the ferrite begins to lose its magnetic properties.)

Since NAVMAT guidelines require that the maximum junction

temperature be 110°C, the power supply manufacturer must

specify the temperature rise above the case for the hottest semi-

conductors so the user can determine what case temperature is

required to meet NAVMAT guidelines. The thermal charac-

teristics section of the specification table states the hottest junc-

tion temperature for maximum output power at a specified case

temperature. The unit can operate to higher case temperatures

than 90°C, but 90°C is the maximum temperature that permits

NAVMAT guidelines to be met.

Case and Ambient Temperatures: It is the user’s responsi-

bility to properly heat sink the power supply in order to maintain

the appropriate case temperature and, in turn, the maximum

junction temperature. Maintaining the appropriate case tem-

perature is a function of the ambient temperature and the

mechanical heat removal system. The static relationship of

these variables is established by the following formula:

where

TC = TA + (PD × RθCA)

TC = case temperature measured at the center of the package

bottom,

TA = ambient temperature of the air available for cooling,

PD = the power, in watts, dissipated in the power supply,

RθCA = the thermal resistance from the center of the package

to free air, or case to ambient.

The power dissipated in the power supply, PD, can be calculated

from the efficiency, h, given in the data sheets and the actual

output power, PO, in the user’s application by the following

formula:

PD

=



PO 

1

η

– 1

For example, at 80 W of output power and 80% efficiency, the

power dissipated in the power supply is 20 W. If under these

conditions, the user wants to maintain NAVMAT deratings

(i.e., a case temperature of approximately 90°C) with an ambi-

ent temperature of 75°C, the required thermal resistance, case

to ambient, can be calculated as

90 = 75 + (20 × RθCA) or RθCA = 0.75°C/W

This thermal resistance, case to ambient, will determine what

kind of heat sink and whether convection cooling or forced air

cooling is required to meet the constraints of the system.

SYSTEM INSTABILITY CONSIDERATIONS

In a distributed power supply architecture, a power source

provides power to many “point-of-load” (POL) converters. At

low frequencies, the POL converters appear incrementally as

negative resistance loads. This negative resistance could cause

system instability problems.

REV. A

–11–

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