In one of our previous articles we discussed the advantages and applications of integrated voltage converters. Today we would like to continue this topic and introduce you to another important issue related to power electronics.
Voltage converters and linear regulators – as you may already know, they perform similar tasks, but in completely different ways. In today's text, we will look in detail at the differences between them. We will also explain with examples when it is best to choose each solution.
How does a voltage converter work?
A voltage converter is an electronic circuit whose function is to convert voltage using pulse width modulation (PWM). It consists primarily of a transistor that acts as a switch, an inertial element – usually an inductor, a diode, and a control circuit.
Figure 1: Simplified schematic of a step–down converter
The pass transistor works like a controlled switch – it alternates between conduction (saturation) and cutoff mode. Thanks to this, it dissipates almost no power, allowing for high efficiency. The average output voltage is controlled by the duty cycle of the PWM signal at the transistor gate.
In the next stage, a series of pulses is converted back to direct current. The inductor acts as a "flywheel" – when the switching transistor is conducting, it stores energy in a magnetic field. When the transistor is not conducting, it becomes a temporary energy source. The capacitor, on the other hand, compensates for voltage fluctuations by storing electrical charge. Together they form a LC filter – a low-pass filter that "reconstructs" the DC current from a series of pulses.
The most common switching regulator topologies include a buck (step-down) converter and a boost (step-up) converter. There are also configurations combining both of these functions – the so-called buck–boost, as well as regulators inverting the polarity of the voltage.
What is the difference between a switching converter and a linear regulator?
The essential building blocks of the regulator are a pass transistor and a reference voltage source – typically using a Zener diode. The circuit is controlled by a negative feedback loop to maintain a constant output voltage.
Figure 2: Simplified schematic of a linear regulator
The transistor regulates the current passing through it depending on the difference between the input voltage and the output voltage. It can be said that the circuit works like a variable voltage divider that constantly adjusts its resistance, depending on the input voltage and the output load. This allows to maintain constant output voltage while unnecessary energy is lost in the form of heat. However, this results in relatively low efficiency – the biggest drawback of linear regulators. More precisely, the efficiency of voltage conversion is given by the ratio of the output voltage to the input voltage of the regulator. The greater the difference between these voltages is, the lower the conversion efficiency will be. Furthermore, a regulator of this type is only capable of stepping down the voltage – other configurations possible with switching regulators are not achievable here.
Today, most linear regulators come in the form of integrated circuits. This gives them great flexibility and makes them easy to use. Some of them also use an external resistor voltage divider, which allows for easy adjustment of the output voltage.
When should a voltage converter be used?
In the following, we would like to introduce you to some typical applications where the advantages of boost converters prove to be particularly relevant.
High power applications
Higher voltage conversion efficiency is the main reason why switching converters are often chosen over other solutions. This feature is especially important in case of high power applications. Smaller energy losses means that less heat needs to be dissipated. Moreover, it also reduces power requirements of the entire system as well. This is the main reason why switching converters are commonly used in power supplies, inverter welders or electric vehicle chargers.
Large voltage differences
As we mentioned earlier, the efficiency of a linear regulator is determined by the ratio of the output voltage to the input voltage. This means that the greater the difference in these voltages, the lower the conversion efficiency becomes. In other words, the more we lower the voltage, the more energy is lost as heat.
To better explain this to you, let's consider the following example: in a device powered by 24V supply, we want to generate a 3.3V rail for low voltage logic. If we use a linear stabilizer for this purpose, the conversion efficiency will be:
This means that the vast majority of the energy delivered will be dissipated as heat. Such high losses will result in a dramatic increase in the energy demand of the entire system. In addition, large amounts of extra heat can make cooling problematic.
Meanwhile, our PSR1-783R3C integrated converter can perform this task with up to 90% efficiency. In addition, it features a wide range of input voltages, and the compact, integrated SIP enclosure makes it extremely easy to implement. We encourage you to take a look at our offer of AC and DC converters of renowned German brand PEAK Electronics.
Battery–powered devices
With battery–powered devices, energy efficiency also becomes very important. When designing such a device, we usually aim to achieve the longest possible battery life. The higher voltage regulation efficiency of boost converters is a huge advantage. Some of them are able to operate at efficiency up to 96%, which is practically unachievable with linear regulators.
Another issue is the voltage drop, which is characteristic for all regulators. Typically it is in the range of about 1V to 2V – this means that the input voltage must be higher than the target output voltage by at least this value. In battery–powered devices, this is a significant problem because the cell voltage drops as the load increases, and also as the cell discharges. For example, for a 1.5V LR6/AA alkaline battery the voltage can drop as low as 0.8V. Therefore, in order to achieve the target output voltage of the regulator, it is necessary to connect many cells in series.
One possible solution is to use LDO (Low Dropout) regulators, with much lower voltage drop – even below 200mV. However, in many cases this does not completely eliminate the problem. Let's consider the following example – we design a 3.3V circuit powered by a Li-Ion NCM14500-850mAh battery with a nominal voltage of 3.6V. In order to generate the desired voltage, we use an LDO stabilizer with 120mV dropout voltage. Its input and output voltage as the battery discharges will then change overtime as follows:
As you can see from the graph above, the LDO is unable to maintain the desired voltage throughout the battery life – after some time it starts to drop below 3.3V. Assuming that our circuit is unable to function properly at lower voltages, we lose about 25% of battery runtime.
So how should this problem be solved? The answer is to use a converter – to be more precise, in buck–boost configuration. It will allow you to maintain a constant output voltage regardless of the battery level, thus fully utilizing its capacity.
Is a voltage converter always a better solution than a linear regulator?
Although switching regulators seem to be a better solution overall, linear regulators are still commonly used in many places. Curious as to why? Below we'll consider cases where an ordinary stabilizer may be a much better choice than a switching converter.
Devices sensitive to interference
As we explained earlier, the output voltage of a switching converter rises and falls with its each cycle. The bypass capacitor at the output smooths out these variations, but in practice it is not able to remove them completely. Therefore, the output voltage will always contain some amount of high-frequency interference – it is referred to as ripple or noise. In most cases, the noise is not large enough to negatively affect the load, but some systems may be particularly susceptible to it. These include, among others, audio equipment, as well as precise measurement instruments. Having to deal with such a system, one must take care to design a well–stabilized power supply. In order to reduce the level of high–frequency noise of a switching converter, feedthrough capacitors, ferrite beads, as well as snubbers can be used. However, all of this adds to system complexity and cost – so this is another case where linear regulators may be a much better solution.
Interference emitted in the form of electromagnetic waves can also be a problem. Switching regulators typically operate at high frequencies, up to tens of MHz. This makes them a significant source of radiated electromagnetic interference (EMI). Excessive emissions can interfere with nearby systems and cause problems during EMC compliance testing. Choosing a linear regulator instead allows to avoid such difficulties.
Ease of implementation and low system complexity
One of the indisputable advantages of linear regulators is their ease of use. They are commonly available as integrated circuits, and the entire circuit usually consists of no more than three components. Implementation should pose no problems for even less experienced electronics engineers. The only challenging aspect may be efficient heat dissipation, especially at higher power levels.
Read also: Cooling of electronic components
In the case of voltage converters, implementation can be much more difficult. The engineer must take care to select components that meet strict parameters. Using inappropriate parts may cause the circuit not to work properly. Equally important is the proper PCB layout – placing the components close together and connecting them in a proper way. The engineering experience and knowledge required to complete this task is much greater.
Greater circuit complexity also means higher costs. The PCB space utilization may also be an important aspect – in case of compact mobile devices even every square millimeter of its surface counts. A larger number of components clearly does not make space management easier.
Very low power consumption
Each voltage converter constantly draws a small amount of current – it is referred to as its quiescent current. The circuit needs it to perform its functions – monitor the output voltage, generate internal reference voltages, switch transistors, etc. What is important, this current is consumed regardless of the output load – even with no load, the converter draws a small amount of energy. This may result, for example, in draining the battery if the device is left inactive for a long time. Linear regulators, on the other hand, due to a different structure, are able to achieve a much lower quiescent current. This is particularly important in systems that are in standby mode for a long period of time – then it is the linear regulators that allow for lower power consumption.
It is also worth noting that the switching converter reaches its highest efficiency only at optimum load – otherwise its efficiency decreases. In extreme cases, the efficiency of the converter may become lower than that of a linear regulator. For this reason, it is always a good idea to consult the data sheets – the efficiency of the converter can be easily determined from the graphs of efficiency vs. the load current.
Summary
Coming back to the question asked in the title – which solution should I choose? Which option will work better in my application or will allow for a higher efficiency? Unfortunately there is no clear answer here. Each case must be considered separately through appropriate calculations or estimates.
In electronics engineering, these types of nuances arise a lot, and making correct decisions requires a lot of knowledge and experience. This is one of the reasons why more and more companies decide to outsource this task to external design firms. If you are also considering this option, you do not have to look further – our awesome engineering team at InterElcom will gladly take your order! We offer complete solutions – we take on all the burden of carrying out the project, from the initial considerations, all the way to production. Moreover, our offer of electronic components is one of the most attractive on the European market. We encourage you to take a look at our offer, as well as to get a free valuation of your project.
