BASIC KNOWLEDGE - VOLTAGE REGULATOR Different types of voltage regulators and working principle
Voltage regulators are integrated circuits designed to regulate the voltage at their input to a constant, fixed voltage at their output, irrespective of changes in load current or input voltage.
Electronic designs/devices are usually made up of diverse electronic components that sometimes operate at different voltage levels. As such, to reliably meet the power requirements of a particular design or that of different components within the design, voltage regulators are usually employed in the power supply unit to regulate the voltage at the main source to what is needed across different sections of the device.
When designing the power supply unit for any device, there is always a ton of decisions to be made. One of those decisions, howbeit a difficult one, is the choice of voltage regulators, as they come in different "shapes and sizes" with different "bells and whistles" that make them a great choice when used in one circuit, but a disaster in other circuits.
As a result of this, selecting the right regulator for your project (and its constraints) requires a thorough understanding of the options, and today's article will focus on just that. We will evaluate the different types of voltage regulators, their operational principles, and when it makes sense to use one over the other.
Types of voltage regulators
Voltage regulators can be categorized based on different factors like their applications, voltages at which they operate, power conversion mechanisms, and many more.
In this article, we will focus on active voltage regulators and classify them into two broad categories, based on the mechanism they employ for regulation. The two categories include:
- 1. Linear voltage regulators
- 2. Switching voltage regulators
1. Linear voltage regulators
Linear voltage regulators use the principles of voltage dividers to transform the voltage at their input to the desired voltage at their output. They employ a feedback loop that automatically varies the resistance in the system to counter the effect of variations in the load impedance and input voltage, all to ensure the output voltage is kept constant.
Typical implementations of linear voltage regulators involve the use of FETs as one side of a voltage divider with a feedback loop connected to the gate of the transistor, driving it as required to ensure consistency in the output voltage.
While this use of transistors as resistors help simplify the design and implementation of linear regulators, it contributes largely to the inefficiency associated with the regulators. The reason for this is that the transistors convert the excess electrical energy (voltage difference between the input voltage and the output voltage) to heat, leading to power loss as a result of heating in the transistors.
In situations where the voltage at the input or the load current at the output is too high, the regulators could generate heat levels that may lead to its breakdown. To mitigate this, designers usually employ heat sinks whose size is determined by the amount of current(power) to be drawn through the regulator.
Another point worth talking about for linear regulators, is the need for the voltage at the input to be greater than the voltage at the output by a minimum value called the drop-out voltage. This voltage value (usually around 2 v) varies between regulators and is sometimes a major source of concern for designers working on low power applications due to power loss. As a way around this, use a type of linear voltage regulators called LDO (low-dropout) regulators as they are designed with the ability to function with as low as 100 mV difference between the input and output voltage.
Some popular examples of linear voltage regulators include the 78xx (e.g L7805(5v), L7809(9V)) series of voltage regulators.
Pros and Cons of the LM7805 linear voltage regulator
Some advantages of linear voltage regulators include:
- 1. Simple and easy to design and implement
- 2. Generates a lower amount of EMI and noise
- 3. Fast response time to changes in load current or input voltage conditions
- 4. Low ripple voltage at the output
Some disadvantages of linear voltage regulators include:
- 1. Low efficiency as a large amount of electrical energy is wasted as heat
- 2. Drop-out voltage requirement makes them a bad choice for low power applications
- 3. Occupy more space on printed circuit boards due to their need for heat sinks
- 4. Low efficiency as a large amount of electrical energy is wasted as heat
- 5. Drop-out voltage requirement makes them a bad choice for low power applications
- 6. Occupy more space on printed circuit boards due to their need for heat sinks
2. Switching voltage regulators
Although they feature a more complex design and require more companion components to function, switching voltage regulators are super-efficient regulators used in scenarios where power loss, as in linear regulators, cannot be condoned.
The voltage regulation mechanism in switching voltage regulators involves rapidly switching an element connected in series with an energy storage component (capacitor or inductor) to periodically interrupt the flow of current and transform the voltage from one value to the other. How this is done depends on the control signal from a feedback mechanism like the one employed in linear regulators.
Unlike in linear voltage regulators, the switching element is either in a fully conducting or switched-off state. It dissipates no power and allows the regulator to attain a high level of efficiency beyond that of the linear regulators.
A basic implementation of switching voltage regulator makes use of a "pass transistor" operated either at its cutoff or saturated state, as the switching element. When the pass transistor is in its cutoff state, no current flows through it, as such no power is dissipated, but when it's in the saturated state, a negligible voltage drop appears across it, accompanied by the dissipation of a small amount of power, with maximum current being forwarded to the load. As a result of the switching action and the energy saved during the cutoff state, the efficiency of a switched regulators is usually northwards of 70 %.
The switching and PWM based control gives quite a lot of flexibility that allows switching voltage regulators to work in different modes and exist in various types, including:
- Buck switching regulators
- Boost switching regulators
- Buck/Boost switching regulators
1. Buck switching voltage regulators
Buck switching regulators, also known as step-down regulators, convert a high voltage at their input terminals to a lower voltage at their output terminal. This operation is like that of linear regulators except for the fact that the buck regulators operate at a higher degree of efficiency. An image illustrating the arrangement of components in buck regulators is provided below.
2. Boost switching voltage regulators
Boost switching regulators, also known as step-up regulators, can convert low voltages at their input to a higher voltage at their output. Their configuration is one of the major differences between linear regulators and switching regulators as no regulation is experienced if the voltage at the input of linear voltage regulators is greater than the voltage required at their output. A circuit illustrating boost switching voltage regulators is provided below.
3. Buck/boost switching voltage regulator
A buck/boost regulator combines the characteristics of the two regulators above. It can provide a fixed output voltage irrespective of the difference (+ or -) between the input and output voltage. They are very useful in battery applications where the voltage at the input, which may be higher than the output voltage at the beginning, decreases to a level below the output voltage over time. A circuit illustrating buck/boost switching regulator is provided below:
Pros and Cons
As efficient and perfect as switching voltage regulators appear, they have shortcomings, some of which include:
- 1. Complicated design
- 2. Requires more additional components
- 3. Expensive
- 4. High EMI and noise generation rates that could affect product certification if not properly managed
- 5. High output voltage ripple
- 6. Slower transient recovery time compared to linear regulators
Depending on your application, the benefit of switching regulators may outweigh their disadvantages. Some of the advantages include:
- 1. Small size
- 2. High efficiency
- 3. They can provide an output voltage that is greater than or less than the input voltage
- 4. Fits low power applications
- 5. Small size
- 6. High efficiency
- 7. They can provide an output voltage that is greater than or less than the input voltage
- 8. Fits low power applications
Selecting the right voltage regulator for your project
Selecting the right voltage regulator for your project is usually not a problem of selecting between linear or switching voltage regulators. Choosing between the two can be done by simply considering their pros and cons and deciding which one best suit you. However, other specific properties of the regulator (either switching or linear) need to be verified to ensure it's a perfect fit for your project. Five of these basic properties are described below:
1. Output voltage (or voltage range)
This is probably the first thing to look out for in a regulator. Ensure the output voltage (or voltage range) of the regulator matches the required value for your application. For some regulators, external components may be required to hold the output voltage steady at your desired voltage level. All of this should be confirmed before rubber-stamping a regulator for your project.
2. Output current
Voltage regulators are designed with specific current ratings in mind. Connecting them to a load with current requirements that are greater than their current rating could lead to the regulator being damaged or the load malfunctioning. This is even more important in the case of linear voltage regulators since the current has a direct effect on the power loss.
Always ensure your regulator of choice can withstand the prospective load current.
3. Input voltage range
This refers to the tolerable range of input voltages supported by the regulator. It is usually specified within the datasheet, and as a designer, it's important to ensure that the possible input voltage for your application falls within the range. One mistake most junior designers make around this is to focus on the maximum input voltage only, forgetting that an input voltage below the specified minimum voltage could lead to regulation errors, especially in the case of linear regulators. Knowing these values will help you estimate the conditions under which the regulator will fail either due to excessive heat being generated in the case of linear regulators or malfunctions in the case of Switching regulators.
4. Operating temperature range
Defined in most datasheets as the ambient temperature (Ta) or the junction temperature, the operating temperature range is the range of temperature within which the regulator functions properly. In more specific terms, the junction temperature usually refers to the highest operating temperature of the transistor. In contrast, the ambient temperature refers to the temperature of the environment around the device. Both values are important, especially for linear regulators, as they contribute to the process of selecting the perfect heat sink for the regulator.
5. Drop-out voltage
This is important when selecting linear voltage regulators. As explained earlier, the drop-out voltage refers to the amount by which the input voltage must be greater than the output voltage for regulation to occur. While this may not be an important consideration for most applications, for applications where efficiency and low power considerations are important, it makes sense to use voltage regulators with low drop-out voltage.
Other factors like efficiency, package size, transient response, and potential EMI/noise generation are also to be considered.
In conclusion, an easy way to decide what regulator to use, is first to decide if a linear or switching voltage regulator will be the best choice based on their pros and cons. After this decision level, further investigations can then be made to properties of the regulator, as it may affect your design. As unnecessary as this due diligence sometimes feels, it could be critical to the success of your project.