Product Announcement from American Microsemiconductor, Inc.
Is Zener tolerance enough? THE TEMPERATURE COMPENSATION IS A VERY IMPORTANT CONSIDERATION IN ZENER DIODE SELECTION
The zener diode temperature coefficient is a most important zener diode consideration. Usually specified in parts per 10,000 per degree C, it can be more important than a initial zener diode tolerance.
Temperature-compensated zener reference diodes mostly utilize a single chip oxide passivated junction for long-term voltage stability. A rugged, glass-enclosed, hermetically sealed structure.
To begin with the true zener breakdown ends at about 5.6 Volts. "So called" higher voltage zeners depend upon the intrinsic avalanche breakdown characteristics of a diode rather than true zener breakdown. So a 10V zener diode or a 50V zener diode depend upon an avalanche breakdown , nonetheless they are still referred as zener diodes.
Now to continue with temperature coefficients: zener voltages below 5.6 volt have a negative temperature coefficient while voltages above 5.6 volts have a positive TC. The exact TC depends upon the doping level of the diode which also determines,the breakdown voltage.
To obtain a lower TC, a composite of two or more zener diodes may be used in series to reduce the overall TC effect. One having a positive TC and the other a negative. Also since the Vf of a standard diode has negative temperature, it is frequently used in series with a positive TC zener in order to reduce the resultant TC.
Finally to show the effect of say a 100 degree rise of a zener diode that has a TC of .08%/degreec C: the breakdown voltage will increase by 8%, far outweighing an 5 % initial tolerance.
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