1.1 IDC. The chassis surface area required: A=Pmax32 in for a 40°C. rise.
SERIES PASS TRANSISTORS
The power transistor required must be rated for a voltage of 2 x V in nominal and a current of at least 1.5 x the required current with adequate worst case DC current gain to require less than 25ma of drive current. The power requirements will be met if the transistor meets these requirements:
Tj-c – 0jc x Pmax
Tj-c ≤Tjmax – [60 + Tamb max + (.5 x Pmax)]
60 = 40°C. chassis rise + 20°C. safety margin (.5 x Pmax) = Ths-c
If this requirement is not met, a higher powered transistor with lower θj-c or parallel transistors must be employed.
The transistor parameters must also be checked to assure that all combinations of voltage and current through the overload and short circuit points fall within the safe operating area of the transistor.
In high temperature environments it is generally undesirable to use plastic power transistors at high power levels. If the transistor is mounted to the chassis and the pins are into a PC Board, this places mechanical stress on the lead junction and thermal cycling may eventually fracture the seal causing failure. On low power levels where there are no mechanical stresses on the transistor there is no objection to plastic devices.
RECTIFIER FAILURE
One of the most neglected areas of power supply design is the rectifier filter requirements. Even many O.E.M. manufacturers will attempt to use parallel 3 amp rectifiers in a 6 amp supply. During the initial turn on of a power supply, the discharged filter capacitor appears as a short circuit and the peak rectifier current is limited only by the source impedance of the transformer. This first half cycle surge will usually be about 10 x the rated DC current and on heavy supplies, can approach 20 x the DC rating. The surge current rating of a rectifier decreases with temperature in accordance with its power derating curve. 3 amp diodes with surge current ratings as high as 200 amps at 25°C. (derated to 50 amps at 125°C. where the rectifier is usually working) are available; however, the lower cost 3 amp diodes have surge ratings as low as 50 amps at 25°C. The most common failure on these supplies is rectifier failure when the supply is repeatedly switched on and off while hot. For this reason, Adtech uses large 30 amp dual rectifiers with a surge rating of 250 amps at 100°C. even on their 6 amp
–83–
