SMPS repair series #9: Zener diode and power transistor without feedback loop
1,132Today we will look into the behavior or a basic linear regulator circuit based first of a Zener diode, after which we extend the circuit with a power transistor. The goal is to understand how do these devices behave in a simple configuration, and how are they used in practice. For this, we will build simple circuits on a prototyping board. Also, we will look at the regulated output voltage as a function of the input voltage and comment on the relative stability of the regulator circuits.
Although Zener diodes offer a relatively stable output voltage, the Zener voltage will slowly drift because of two major effects: the Zener diode will heat up, changing the characteristics of the device, and also the avalanche current will gradually increase as we increase the input voltage. In the 1980s thermally compensated Zeners were still used in liner power supplies. However, due to this drift of the output voltage, Zener diodes are rarely used in modern circuits. In fact, in switch mode power supplies instead of Zeners nowadays we use precision voltage references, which are integrated devices containing a bandgap reference and other auxiliary components.
In addition to the voltage drift, an other major drawback of a simple linear regulator based on a series resistor and a Zener diode is, that the maximum current what we can draw from such a circuit is limited to 50--100mA. Therefore, such a circuit is not usable for practical applications whenever we are trying to power consumers which need higher currents.
As a workaround to this limitation, we will use a power transistor in an Emitter-follower configuration: the Collector is connected to the input voltage, the Base to the stabilized Zener voltage, and the output voltage on the Emitter will follow the voltage fed into the Base. We will discuss how the voltage output differs compared to the Zener diode due to the forward voltage drop between the Base and the Emitter of the NPN power transistor. Because of this, the output voltage will be shifted to about 0.6--0.8 V lower compared to the output of a Zener diode.
After this, on the resulting diagram we will explain that the voltage regulation of the Zener diode is considerably better than the combination of a Zener diode with a power transistor. This is because the circuit is not using any feedback from the output, and it does not contain an error amplifier. These two components will be presented in the follow-up video.