Howto repair switch mode power supplies #6: Basics of Linear Regulators and voltage references

In the sixt video of the Switch Mode Power Supply (SMPS) repair series we look at the basic ideas and priciples of operation of linear serial regulators. Serial regulators are used in every power circuit in order to generate stable supply voltages. Even in SMPS we will rely on linear regulator circuits to get stable voltages in order to feed integrated circuits like the APFC controller, the Pulse Width Modulation (PWM) controller chip, or the supervisor chip. Furthermore, these integrated circuits will contain highly precise reference-voltage generators, which are also based on linear regulators.

The video is built up such, that we start from a simple circuit with an incandescent lamp, and we introduce step-by-step new elements into the circuit to get a high power linear regulator. First the functionality of the series drop resistor is discussed. Next, we look into the case when the input voltage is fluctuating over time. In such cases for low power applications we can use a Zener diode in series with a fixed resistor. The functionality and the main property of the Zener diode is briefly introduced (without going into detail about avalanche current, etc.).

Next, we focus on medium and high power applications, where a bipolar power transistor is being used as a variable resistor, where the equivalent resistance between the Collector and the Emitter can be adjusted through a driver signal fed into the Base electrode.

In real-life circuits, most often a voltage reference circuit formed by a Zener diode and a fixed resistor is used to provide a stable voltage. This reference voltage is than fed into the Base of a power transistor, which in turn is able to provide stable output voltage even for medium and high power applications. Thus, practical circuits are combining Zener reference diodes with power transistors. For example, serial regulators available in three pin power packages, like the LM317, or the classical 7805, they use a similar inner circuit, where the three pins correspond to the power input, power output, and to the ground for the reference diode.

One major drawback of linear regulators is, that they dissipate a large portion of the input power in form of heat. This makes linear regulators rather inefficient. Due to the low efficiency, one also need to design linear series regulators with large heatsinks and even with active fans to extract the waste heat. To reduce the amount of power wasted, and to make more efficient power supplies, SMPS use instead a technique called Pulse Width Modulation.