A voltage regulation circuit comprises a pass element, reference voltage, feedback circuit, and control circuit.
When a pass element operates at any point in the active region, the voltage regulation is an application of linear electronics. Such voltage regulators are called linear voltage regulators.
The variable conductivity of a pass element is utilized for voltage regulation in linear voltage regulation.
The pass element of linear voltage regulators operates in the linear region
There are two types of regulators used to supply constant voltage to a circuit: linear voltage regulators and switching voltage regulators. Linear voltage regulators have a pass element that operates as a linear electronic component. Factors such as heat dissipation, inefficiency, high operating temperatures, and large heat sinks limit the applications of linear electronics in voltage regulation schemes. In this article, we will learn why as we discuss linear voltage regulator characteristics and applications.
Power Supply Voltage Regulation
A wide range of applications—such as industrial utilities, consumer appliances, handheld devices, and communication systems—require power supplies. The power supply for these applications is a steady DC voltage, which is obtained by rectifying the AC power mains. The rectified AC voltage is pulsating, so filters must be used to smooth the supply.
Power supply solutions in today's designs are often required to maintain or regulate the output voltage constant, irrespective of the variations in the AC grid or load variations. Voltage regulators are integrated with the rectifier-filter combination to supply constant DC power. Generally, a voltage regulation circuit comprises a pass element, reference voltage, feedback circuit, and control circuit. The core function of the voltage regulation scheme is to sense the variation of the output DC voltage from the reference voltage. The difference between the output DC voltage and reference voltage forms an error. Depending on the error, the control signal is produced to drive the pass element such that the output DC voltage remains constant throughout.
Pass Elements in Voltage Regulators
The pass element in voltage regulators is a semiconductor device such as a BJT or MOSFET. The pass element can be operated in the active region or switching region. When the pass element operates at any point in the active region, the voltage regulation is an application of linear electronics. Such voltage regulators are called linear voltage regulators. The pass element is operated at the saturation region as a closed switch and in the cut-off region as an open switch to achieve voltage regulation. It might be interesting to note that a pass element operating as a switch forms a switching voltage regulator.
Let's take a look at the characteristics of linear voltage regulators and their appropriate applications.
Linear Voltage Regulators
Power management design can be a challenging task for circuit design engineers. There are several factors—such as efficiency, size, compactness, transient response, cost, and accuracy—to consider when choosing an appropriate power supply design. Depending on the application, the solution can be a linear voltage regulator or switching voltage regulator.
Linear voltage regulators employ a MOSFET or BJT as the pass element to maintain the constant output voltage. The variable conductivity of the pass element is utilized for voltage regulation in linear voltage regulation. The linear regulator acts as a voltage divider circuit, which is possible only due to the operation of the pass element in the active region, otherwise called the ohmic region or linear region.
There are two main types of linear voltage regulators:
Series linear voltage regulators: When a pass element is connected in series with the load, it forms a series linear voltage regulator. In a series voltage regulator, the voltage drop across the pass element is controlled to regulate the load voltage.
Shunt linear voltage regulators: A shunt linear voltage regulator is one in which the pass element is parallel to the load. The current through the pass element is controlled to maintain constant voltage in shunt linear voltage regulators.
Characteristics of Linear Voltage Regulators
Linear voltage regulators are available as highly integrated circuits such as ICs 7805, 7812, 7915, LM 317, LM309, and LM 2940. There are three basic designs in linear voltage regulators based on the minimum voltage drop required across the regulator for maintaining the output voltage constant. The classifications are:
- Standard regulator: The pass lament in a standard regulator is an NPN Darlington pair.
- Low dropout (LDO) regulator: There is a single transistor in LDO regulators.
- Quasi LDO regulator: A combination of PNP and NPN transistors forms the pass element in a quasi LDO regulator.
Linear regulators can output steady voltage with little to no noise. These regulators are most suitable for applications less than 20 W. Linear voltage regulator ICs simplify designs with minimum requirements for external components such as resistors and capacitors. However, several characteristics of these regulators limit their applications:
Offer very low efficiency in the order of 20-60%.
The linear mode operation of the pass element causes excessive power dissipation and heating.
Can only be used as step-down voltage regulators.
Line frequency transformers are employed for linear voltage regulation, and they increase the size and weight of the system.
In space-constrained applications, linear electronics-based voltage regulators should be replaced with switching regulators. The use of high-frequency transformers reduces the PCB space occupied by switching regulators more than linear regulators. In addition, switching voltage regulators are highly efficient, with step-up and step-down voltage regulation options.
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