The function of a DC power source is to maintain a constant output voltage and supply power to a load regardless of changes in the input voltage or the current demands of the load. The most common requirement is to convert a higher input voltage into a lower output voltage, and there are two fundamental methods of accomplishing this goal. This article will review these methods and compare their relative advantages and disadvantages.
How Does a Linear Voltage Regulator Work?
A linear regulator is the simplest way to produce a regulated voltage. The input is always connected to the output through the pass element, which acts as a variable resistor. The error amplifier compares the divided-down output voltage to a reference voltage and uses the difference to control the pass element, increasing or decreasing its resistance as needed. The desired value of VOUT determines the values of R1 and R2.
The pass element can be a transistor or MOSFET; it operates in the linear region and regulates the current flow from source to load. The simplest version of the linear regulator has only three pins - input, output and ground, although optional features may add additional pins, as discussed below.
The linear regulator requires a minimum dropout voltage, VDROPOUT, between VIN and VOUT in order to operate. If VIN - VOUT drops below VDROPOUT, the part can no longer maintain a regulated output voltage. The first linear regulators required a minimum dropout voltage of one volt or more, but today most devices are low-dropout (LDO) designs that can operate with VDROPOUT of a few hundred millivolts. VDROPOUT depends on load current since it equals ILOAD X RON, where RON is the resistance of the pass device.
Linear Regulator Efficiency
The linear regulator has many advantages. Since all components are operating in a linear region, there are no fast transients to generate noise, and the LDO has an excellent response to load variations. For low-current applications, the simple design results in a very small package.
The prime disadvantage of a linear regulator is its low efficiency. For an input current IIN, the input power PIN = VIN x IIN; the output power POUT = VOUT x ILOAD.
Ignoring the current needed to operate the regulator, ILOAD = IIN, so the efficiency η is given by
η = POUT/PIN = VOUT/VIN.
The difference between PIN and POUT is a power loss, dissipated as heat across the pass transistor; the loss is a function of the voltage difference between input and output, or PLOSS = I x (VIN - VOUT).
If VIN >> VOUT, the linear regulator can generate considerable heat even at low output currents. Disposing of the heat can cause problems in the design, requiring a larger printed-circuit-board area or the addition of a heat sink.
LDO Design Basics
Manufacturers offer several variations to the basic three-terminal LDO design. Fixed-output devices include the resistor divider internally, but a variable-output regulator allows the designer to select the values. Many devices allow for operation without an output capacitor, or include features such as enable pins, or soft-start operation; other devices offer an option to add an external capacitor to filter the output of the voltage reference, the primary internal noise source, for extremely noise-sensitive applications.
What is a Switching Regulator?
Figure 1: The simple buck regulator has two operating modes: Q1 on and Q1 off. (Image Source: Analog Devices)
A switching regulator differs from a linear regulator because it delivers power to the load in bursts. In contrast to the continuously-varying operation of the linear pass transistor, the power device in the switching regulator switches off and on at a high rate; depending on the design this can range from hundreds of kilohertz up to megahertz.
Figure 2 illustrates the operation of the step-down, or buck, switching converter with input voltage VIN and output voltage VO. For simplicity, we’ll assume all components are ideal. When switching transistor Q1 is turned on, the switching-node voltage VSW = VIN and inductor L current is being charged up by (VIN – VO), as shown in figure X(a). When Q1 is turned off, inductor current flows through diode D1, as shown in Figure X(b). The switching node voltage VSW = 0V and the inductor current flows to the load. More details are available in this application note.
Switching Voltage Regulator Design
The switching topology has several advantages. By switching between fully on and fully off, the power device is operating in two states with minimum power dissipation, so a switching regulator can have an overall efficiency well over 90%. The result is a smaller design than an equivalent linear solution, with lower heat dissipation.
On the other hand, the design of a switching regulator is far more complex than that of a linear device. A controller must measure the output voltage and continuously adjust the duty cycle D to maintain the desired value of VO in response to changes in both VIN and the load.
The calculation of the losses is also complicated. The buck converter in Figure 2 has both DC conduction losses and AC switching losses. The DC conduction losses arise from voltage drops across the diode, the transistor and the inductor when they are conducting current. Since these devices are only conducting part of the time, the conduction losses are a function of the duty cycle D, defined as the ratio between the on-time and the switching period. The AC switching losses include MOSFET switching losses, the inductor core loss, the loss due to the switching transistor gate drive, and a loss from the MOSFET’s body diodes.
Switching Regulator Applications
Designer often add enhancements to the basic design in order to reduce losses. The synchronous buck converter, for example, replaces the diode with a low-loss MOSFET, but adds complexity in the form of another driver.
The fast power switching makes a switching design much noisier than an LDO, so the designer must add appropriate filtering and carefully route sensitive traces away from the switching circuitry.
Manufacturers offer designers a wide range of switching options to simplify the design task. These include all-in-one devices that integrate all components into a single package, switching converters with integrated power switches, and switching controllers for use with external devices.
In addition to the simple buck converter, there are many switching topologies for different applications, with varying feature sets. Not all the designs are step-down: the boost converter can step up the input voltage to a higher output voltage; and a buck-boost converter, as the name suggests, can generate an output DC voltage both above and below the input voltage.
Linear vs. Switching Voltage Regulators: Which Approach is best?
The linear regulator is conceptually simple, with low cost, low noise, plus excellent dynamic response and load regulation. The efficiency, though, depends on the ratio between VIN and VOUT. If the two are close, the efficiency can exceed 90%; if not, the efficiency drops, and the device generates a large amount of heat that must be dissipated.
The switching regulator exhibits excellent efficiency over the full range of input voltage and output load. But it’s a much more complex design with many more components and requires careful attention to detail to maintain stability and minimize losses.
Switching topologies are overwhelmingly preferred for high-power applications due to their higher efficiency. They’re also the only option if the application requires more than a simple step-down conversion. Their noise makes them unsuitable for powering sensitive analog components such as sensors or high-resolution data acquisition systems.
Many applications make use of both approaches: using a switching design for the primary power conversion and adding linear regulators to supply low-noise power where needed.
High-Efficiency Switching Regulators: Alternatives to Changing Boards
Many linear regulators in legacy designs remain on the board because it’s not worth incurring the cost of a design and a PCB layout change, even though a switching regulator would reduce power consumption, eliminate a costly heat sink, and increase reliability by lowering the temperature.
New switching regulators now make it possible to replace an inefficient linear regulator with a switching device without changing the board. RECOM Power’s R-78E-1.0, for example, is a 1-A switching device with 5 V output that’s a drop-in replacement for the venerable 7805, a design that first appeared in 1972. Available from Arrow, RECOM offers several versions of the R-78 with output voltages of 3.3, 5, 9, 12, and 15 V and output currents of 500 mA or 1 A depending on model.
View a demonstration video here.
Conclusion
The key function of a regulator is to generate a DC output voltage that remains stable despite variations in input voltage or output load. Linear and switching regulators are two fundamentally different methods of accomplishing this goal. This article has reviewed the basics of each, their advantages and disadvantages, and compared their performance in several key areas.