SMPS repair series #10: Temperature compensated Zener diode in linear regulators
975We will cover the topic of the positive thermal coefficient of Zener diodes and the negative thermal coefficient of standard forward biased Silicon diodes. Namely, when Zener diodes with Zener voltages above 6V are being heated up with one Centigrade, their Zener voltage will increase with about 3mV. In contrast to this behavior, when we heat up a Silicon diode with one Centigrade, its forward voltage will go down with about 2mV.
Therefore, it was a traditional design trick to put a Zener diode in series with a Silicon diode, and have the two diodes located close to each other on the printed circuit board. Namely, this way the positive temperature coefficient of the Zener will be partially compensated by the negative temperature coefficient of the Silicon diode. This way the drift of the output voltage will be considerably lower, and the power supply will provide a more stable output voltage for a wider temperature range. With modern components like the TL431 precision voltage reference nowadays one can build even more stable power supplies without paying much attention to temperature compensation. However, temperature compensation for old designs was often based on the solution of a Zener put in series with a Silicon diode. Some older equipment might have even used oven stabilized Zeners.
There is an other ramification of adding the series Silicon diode to the circuit. Namely, when we use a power transistor in an emitter follower configuration, the output voltage will be now almost the same as the Zener voltage. This is because the reference voltage will increase with about 0.75V (drop on the Silicon diode). This will be lost due to forward drop on the Base-Emitter junction of the bipolar transistor operating in the emitter follower configuration.
One has to point out, that Zener diodes which have Zener voltages below 6V will most often have a negative thermal coefficient, since in these diodes the "Avalanche Effect" is very small. Therefore, combining Zener diodes with lower voltages below 6V with a series Silicon diode will not help with the temperature compensation, in fact it might make the situation worse. Thus, one has to check the data-sheet of the given Zener before trying to apply thermal compensation.