What is a switching power supply?
A power supply is an electrical device that converts the electric current from a power source to the voltage required to power a load, such as a motor or electronic equipment.
What is a switching power supply?
A switching power supply design is a newer method that was made to fix many of the problems with linear power supply design, such as the size of the transformer and how the voltage is controlled. In switching power supply designs, the input voltage is no longer lowered; instead, it is rectified and filtered at the input. The voltage is then sent through a chopper, which turns it into a pulse train with a high frequency. Before the voltage gets to the output, it is once again filtered and rectified.
How Does a Switching Power Supply Work?
Linear AC/DC power supplies have been changing AC power from the utility grid into DC voltage for many years. DC voltage is used to power home appliances and lights. Because high-power applications need smaller power supplies, linear power supplies are now only used in certain industrial and medical applications where their low noise is still needed. Switching power supplies, on the other hand, have taken over because they are smaller, more efficient, and can handle more power. Figure 1 shows how a switching power supply changes from an alternating current (AC) to a direct current (DC).
Figure 1: Isolated Switched-Mode AC/DC Power Supply
Input Rectification
The process of converting AC voltage to DC voltage is called "rectification." In switched-mode AC/DC power supplies, the first step is to correct the signal coming in. People usually think that DC voltage is a steady, straight line of voltage, like what comes out of a battery. But what makes direct current (DC) what it is, is that electric charge flows in only one direction. This means that the voltage flows in the same direction, but it is not always the same.
AC's most common waveform is a sine wave, which is positive during the first half-cycle and negative during the rest of the cycle. If the negative half-cycle is turned around or taken away, the current stops switching back and forth and becomes a direct current. The process for doing this is called "rectification."
Using a diode to get rid of the negative half of the sine wave in a passive half-bridge rectifier is one way to get rectification (see Figure 2). The diode lets current flow through it when the wave is going in the right direction, but it stops the current when it is going in the opposite direction.
Figure 2: Half-Bridge Rectifier
After rectification, the resulting sine wave will have low mean power and won't be able to power devices well. A much better method would be to change the polarity of the negative half-wave and make it positive. This method is called full-wave rectification, and it only needs four diodes in a bridge configuration (see Figure 3). No matter which way the voltage goes in, this setup keeps the direction of the current flow stable.
Figure 3: Full-Bridge Rectifier
The mean output voltage of a fully rectified wave is higher than that of a half-bridge rectifier, but it is still a long way from the constant DC waveform that electronic devices need to work. Even though this is a DC wave, it would not be a good way to power a device because the shape of the voltage wave changes very quickly and often. This change in DC voltage over time is called a "ripple." For a power supply to work well, the ripple must be reduced or eliminated.
A large capacitor at the rectifier's output, called a reservoir capacitor or smoothing filter, is the easiest and most common way to reduce ripple (see Figure 4).
The capacitor stores voltage during the wave's peak, then sends current to the load until its voltage is lower than the now-rising rectified voltage wave. The shape of the resulting wave is much closer to what was wanted, and it can be thought of as a DC voltage with no AC part. DC devices can now be powered by this final voltage waveform.
Figure 4: Full-Bridge Rectifier with Smoothing Filter
Passive rectification uses semiconductor diodes as switches that are not controlled. It is the easiest way to change an AC wave into a DC wave, but it is not the most efficient.
Diodes are pretty good switches because they can turn on and off quickly with little loss of power. The only problem with semiconductor diodes is that they have a forward bias voltage drop of 0.5V to 1V, which makes them less efficient.
In active rectification, MOSFETs or BJT transistors are used instead of diodes to control the flow of electricity (see Figure 5). There are two good things about this: First, transistor-based rectifiers don't have the fixed voltage drop of 0.5V to 1V that semiconductor diodes do. This is because their resistances can be made as small as needed, so the voltage drop is small. Second, transistors are switches that can be turned on and off. This means that the frequency of switching can be controlled and therefore optimized.
On the other hand, active rectifiers need more complicated control circuits to do their job. This means that they need more parts, which makes them more expensive.
Figure 5: Full-Bridge Active Rectifier
Power Factor Correction (PFC)
The second step in making a switching power supply is to correct the power factor (PFC).
PFC circuits don't do much to change AC power into DC power, but they are an important part of most commercial power supplies.
Figure 6: Voltage and Current Waveforms at the Rectifier Output
If you look at the current waveform of the rectifier's reservoir capacitor (see Figure 6), you'll see that the charging current flows through the capacitor for a very short time, from the point where the voltage at the input of the capacitor is higher than the capacitor's charge to the peak of the rectified signal. This causes a series of short current spikes in the capacitor. Because these current spikes add a lot of harmonics to the power grid, they cause a big problem not just for the power supply but for the whole grid. Harmonics can cause distortion, which can affect other power supplies and devices that are connected to the grid.
The goal of the power factor correction circuit in a switching power supply is to filter out as many of these harmonics as possible. There are two ways to do this: active power factor correction and passive power factor correction.
Isolation: Isolated vs. Non-Isolated Switching Power Supplies
Whether a PFC circuit is present or not, the last step in power conversion is to step down the rectified DC voltage to the right size for the application.
Since the AC waveform at the input has already been rectified, the DC voltage at the output will be high. If there is no PFC, the DC voltage at the output of the rectifier will be about 320V. If the PFC circuit is working, the output of the boost converter will be a constant DC voltage of 400V or more.
Both of these situations are very dangerous and don't work for most applications, which usually need much lower voltages. Table 1 shows several converter and application factors that should be thought about when choosing the right isolation topology.
| Isolated AC/DC Power Supplies | Non-Isolated AC/DC Power Supplies | |
|---|---|---|
| Topology | Flyback converter | Buck converter |
| Safety | Galvanic isolation offers increased user safety | Potential current leaks could cause significant harm to users or loads |
| Size and Efficiency | Transformers add size and weight | Only one inductor needed, much smaller circuit |
| Efficiency | Transformer iron and copper losses affect efficiency | A single inductor is much more efficient than an entire transformer |
| Complexity | Control circuitry is needed for both | |
When deciding which step-down method to use, safety is the most important thing to think about.
At the input, the power supply is connected to the AC mains. This means that if there was a leak of current to the output, an electric shock of this size could kill or seriously hurt someone and damage any device connected to the output.
By magnetically isolating the input and output circuits of an AC/DC power supply that is plugged into the wall, safety can be achieved. Flyback converters and resonant LLC converters are the most common types of isolated AC/DC power supplies. This is because they have galvanic or magnetic isolation (see Figure 7).
Figure 7: Flyback Converter (Left) and LLC Resonant Converter (Right)
Since a transformer is being used, the signal can't just be a flat DC voltage. Inductive coupling, on the other hand, needs a change in voltage and, by extension, a change in current in order to move energy from one side of the transformer to the other. So, both flyback and LLC converters "chop" the DC voltage coming in and turn it into a square wave, which a transformer can then use to step down. Then, the output wave has to be changed back to a sine wave before it can be sent to the output.
Most of the time, low-power applications use flyback converters. A flyback converter is an isolated buck-boost converter, which means that the output voltage can be higher or lower than the input voltage, depending on the turn’s ratio between the primary and secondary windings of the transformer. The way a flyback converter works is a lot like how a boost converter works.
When the switch is shut, the input charges the primary coil, making a magnetic field. When the switch is open, the charge in the primary inductor moves to the secondary winding. This sends a current into the circuit, which powers the load.
Flyback converters are easy to make and require fewer parts than other converters. However, they are not very efficient because they force the transistor to turn on and off at random, which wastes a lot of energy (see Figure 8). This is bad for the life of the transistor, especially in high-power applications, and wastes a lot of power, which is why flyback converters are better for low-power applications, usually up to 100W.
Most of the time, resonant LLC converters are used in high-power applications. A transformer also keeps these circuits from each other magnetically. LLC converters are based on the idea of resonance, which is when a certain frequency gets stronger when it matches the natural frequency of a filter. In this case, the resonant frequency of an LLC converter is set by an inductor and a capacitor connected in series (called a "LC filter"), plus the effect of the transformer's primary inductor (L), which is why the device is called a "LLC converter."
LLC resonant converters are used in high-power applications because they can switch with no current. This is also called "soft switching" (see Figure 8). This method of switching turns the switch on and off when the current in the circuit gets close to zero. This keeps the switching losses of the transistor to a minimum, which reduces EMI and increases efficiency. Unfortunately, this improved performance comes at a cost: it is hard to design an LLC resonant converter that can do soft switching for a wide range of loads.
Figure 8: Hard Switching (Left) vs. Soft Switching (Right) Losses
In the first part of this article, we talked about how the size and weight of the input transformer is one of the problems with AC/DC power supplies. This is because the low operating frequency (50Hz) requires large inductors and magnetic cores to keep the transformer from becoming saturated.
In switching power supplies, the voltage oscillation frequency is much higher (above 20kHz at the very least). This means that the step-down transformer can be smaller because linear transformers lose less magnetic energy when high-frequency signals are sent through them. By making input transformers smaller, the system can be shrunk down to the point where a whole power supply can fit into a case the same size as the ones we use to charge our phones.
Some DC devices don't need the isolation that a transformer provides. This is common in devices that don't need to be touched directly by the user, like lights, sensors, IoT, and more. The device's settings are usually changed from a separate device, like a phone, tablet, or computer.
This is great in terms of weight, size, and how well it works. A high-voltage buck converter, also called a step-down converter, is used in these converters to lower the output voltage. This circuit could be thought of as the opposite of what was said about the boost converter. In this case, when the transistor switch is closed, the current flowing through the inductor creates a voltage across the inductor. This voltage works against the voltage from the power source, which lowers the voltage at the output. When the switch opens, the inductor releases a current that flows through the load, maintaining the voltage value at the load while the circuit is cut off from the power source.
A high-voltage buck converter is used in AC/DC switching power supplies because the MOSFET transistor that acts as a switch must be able to handle big changes in voltage (see Figure 9). When the switch is closed, the voltage across the MOSFET is close to 0V. When the switch is open, the voltage across the MOSFET goes up to 400V for single-phase applications or 800V for three-phase converters. Normal transistors could be broken by these big, sudden changes in voltage, so high-voltage MOSFETs are used instead.
Figure 9: Non-Isolated AC/DC Switching Power Supply with Active PFC
Buck converters are much easier to put together than transformers because they only need one inductor. They are also much better at lowering voltage, with an average efficiency of 95% or more. This level of efficiency is possible because transistors and diodes almost never lose power when switching. The only thing that loses power when switching is the inductor.
Conclusion
AC/DC switching power supplies are the best way to change AC power to DC power at the moment. In three steps, the power is changed:
When making a switching power supply, there are many things to think about, including safety, performance, size, weight, etc. Switching power supplies also have more complicated control circuits than linear power supplies. This is why many designers find it helpful to add integrated modules to their power supplies.
