ADDC02812DA/ADDC02815DA

REV. A

–13–

NAVMAT DERATING

NAVMAT is a Navy power supply reliability manual that is

frequently cited by specifiers of power supplies. A key section of

NAVMAT P4855-1A discusses guidelines for derating designs

and their components. The two key derating criteria are voltage

derating and power derating. Voltage derating is done to reduce

the possibility of electrical breakdown, whereas power derating

is done to maintain the component material below a specified

maximum temperature. While power deratings are typically stated

in terms of current limits (e.g., derate to x% of maximum rating),

NAVMAT also specifies a maximum junction temperature of the

semiconductor devices in a power supply. The NAVMAT

component deratings applicable to the ADDC02812DA and

ADDC02815DA are as follows:

Resistors

80% voltage derating

50% power derating

Capacitors

50% voltage and ripple voltage derating

70% ripple current derating

Transformers and Inductors

60% continuous voltage and current derating

90% surge voltage and current derating

20

°C less than rated core temperature

30

°C below insulation rating for hot spot temperature

25% insulation breakdown voltage derating

40

°C maximum temperature rise

Transistors

50% power derating

60% forward current (continuous) derating

75% voltage and transient peak voltage derating

110

°C maximum junction temperature

Diodes (Switching, General Purpose, Rectifiers)

70% current (surge and continuous) derating

65% peak inverse voltage derating

110

°C maximum junction temperature

Diodes (Zeners)

70% surge current derating

60% continuous current derating

50% power derating

110

°C maximum junction temperature

Microcircuits (Linears)

70% continuous current derating

75% signal voltage derating

110

°C maximum junction temperature

The ADDC02812DA and ADDC02815DA, with one excep-

tion, can meet all the derating criteria listed above. However,

there are a few areas of the NAVMAT deratings where meeting

the guidelines unduly sacrifices performance of the circuit.

Therefore, the standard unit makes the following exceptions.

Common-Mode EMI Filter Capacitors: The standard

supply uses 500 V capacitors to filter common-mode EMI.

NAVMAT guidelines would require 1000 V capacitors to meet

the 50% voltage derating (500 V dc input to output isolation),

resulting in less common-mode capacitance for the same space.

In typical electrical power supply systems, where the load

ground is eventually connected to the source ground, common-

mode voltages never get near the 500 V dc rating of the stan-

dard supply. Therefore, a lower voltage rating capacitor (500 V)

was chosen to fit more capacitance in the same space in order to

better meet the conducted emissions requirement of MIL-STD-

461D (CE102). For those applications which require 250 V or

less of isolation from input to output, the present designs would

meet NAVMAT guidelines.

Switching Transistors: 100 V MOSFETs are used in the

standard unit to switch the primary side of the transformers.

Their nominal off-state voltage meets the NAVMAT derating

guidelines. When the MOSFETs are turned off, however,

momentary spikes occur that reach 100 V. The present genera-

tion of MOSFETs are rated for repetitive avalanche, a condition

that was not considered by the NAVMAT deratings. In the

worst case condition, the energy dissipated during avalanche is

1% of the device’s rated repetitive avalanche energy. To meet

the NAVMAT derating, 200 V MOSFETs could be used. The

100 V MOSFETs are used instead for their lower on-state resis-

tance, resulting in higher efficiency for the power supply.

Output Rectifiers (ADDC02815DA only): Schottky diodes

are used as output rectifiers for the

± 15 V dc converter. The

reverse voltage stress on these diodes under normal operating

conditions is 75% of their maximum rating, compared to a

NAVMAT derating guideline of 65%.