Types of power supply:
Power supply is used as an energy source in various circuits. It works by converting the AC mains into DC voltage. This becomes a fixed or variable voltage as applied by you into the circuit.
There are 2 main types of power supplies used commonly:
- The Linear power supply is most commonly used.
Linear power supply is used in simple circuits that are not complicated. It is large and has low efficiency of only about 50% or more. With its working, a lot of energy is lost in the form of high heat.
- Switching power supply:
it has been used in many circuits. This type of power supply has high efficiency which is about 85% or more. Imagine we apply 100% electric energy to a circuit. It can be transformed into 85% of energy while only 15% is lost in the form of heat.
But one thing noticeable about it is that the switching supply circuit is quite complex. I have tried to avoid explaining it previously because I wasn’t sure if I could explain it easily.
Are you ready to get started?
Lets start by looking at the block diagram of the switching power supply. Its structure looks quite complicated. Once we separate the circuit into its parts, it becomes easier to understand.
The highlight of this circuit is that it works with high frequency. Therefore a smaller transformer is used. With high frequencies, there is a switching system. Input and output circuits include the rectifier and filter circuit. There is an error voltage detector to control the stable voltage.
You might not understand all this initially. But after reading the next section, my dear friends you will understand more.
Switching power supply has 4 types of rectifier circuits.
Meet Rectifier AC to DC:
The switching power supply has the rectifier circuit attached to both input and output. This type is called a bridge rectifier circuit.
The parts that convert AC to DC are called Rectifier. In a linear circuit, this type of arrangement is important. In the switching supply circuit, the rectifier circuit is very important.
Another important device is the diode, which is a semiconductor device that allows current to flow only through it in one direction. Then, the DC voltage flows through the filer to smooth the current.
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Switching power supply consists of 4 types of rectifier circuits:
1# AC main to DC pulse Bridge rectifier
Normally, we will find the rectifier circuit first. The input side of the switching power supply is shown in the circuit diagram below.
2# Half-wave Rectifier from RF AC signal
In a switching power supply, with high-frequency RF the DC signal input will be switched. Then, the step-down transformer transforms it into low AC voltage. Next, it flows to a half-wave rectifier to a DC pulse.
3# Full-wave Rectifier using center tap transformer
This is developed by a half-wave rectifier. We often see a rectifier like this. We might have noticed that it uses the center tap of the secondary transformer. This provides a reference to the ground.
4# Full-wave Bridge Rectifier from a step-down transformer
This circuit does not use a center tap transformer; hence we need to use 2 more diodes.
Selection of diodes for the rectifier circuit
There are 2 important factors which must be kept in mind.
The peak Inverse voltage- PIV
It is the maximum voltage that the diode can tolerate. Whether it is receiving a reverse bias or while the Diode is OFF.
The PIV value of the diode that is used here must be able to withstand at least 2 times of the operating voltage. When you are calculating, the security should be increased by 50% as well.
At AC input voltage of 220Vrms, the peak voltage is 1.414 x Vrms = 311Vpk.
We must choose a diode with value:
Piv = (311Vpkx2) + (311Vpkx0.5)
The current that the diode allows to flow through it when receiving a forward without damage is called the forward current. And more importantly, here a safety value with 50% must be added.
For example, if the input rectifier has a current of 1A, we should choose a diode with forwarding current:
IF = 1+ (1×0.5) = 1.5A
Why is filter important
The voltage from the rectifier is DC but it cannot be used. We need to smooth it by using a filter capacitor. Both linear and switching power supply must be used here.
A capacitor is a device used to store energy. It charges the energy present in it until it reaches the maximum value of the pulse voltage. Then it releases when loaded.
The working voltage Rate
We need to use the working voltage rate of the capacitor more over the voltage when the operating current is approximately 50%
A transformer is a device that converts a high voltage on a primary to a low voltage on the secondary as shown in the image below.
What is RF switching regulator
The heart of the switching power supply is the RF Regulator which is also known as the switching regulator.
Pulse Width Modulation Switching Regulator
Many different switching circuits are used these days but the most commonly used is PWM-Pulse Width Modulation.
This is a Basic block diagram of the Pulse Width Modulation (PWM) switching regulator. This device is responsible for maintaining the voltage level with a closed-loop form.
This circuit will detect the voltage error to get a constant output voltage. This error signal detected is used to control the pulse width of the switching circuit. It causes a change in the pulse width of the oscillator circuit within the regulator.
The width of the pulses that are altered from the oscillator is sent to drive the transistor acting as a switch. Here the changing pulse width causes the average voltage of the output to change accordingly.
The high-frequency transformers cause the lowering of voltage into the AC signal, and then it becomes rectified and filtered again.
The output will be randomized again for the final output of the DC voltage. This output will adjust the error signal that will follow until maintaining the constant voltage is needed.
This process shows that the circuit will operate in a closed loop. The output voltage is continuously controlled until it starts to work normally.
Now, we understand the basic working principle of the switching regulator. but whats next? I guess it’s probably time for us to apply it.
Hybrid Switching Regulator Working principle
It must be kept in mind that it is not always necessary to use a High-Frequency Transformer to design a Switching Power Supply.
The transformer is normally used to change the voltage of the pulse from a high voltage to a low voltage. If a DC input voltage is present somewhere close to the actual operating voltage there is no need to use a high-frequency transformer. In this situation, we can use 50Hz Step-down voltage transformer to reduce the voltage to a lower value before it can be fed to the input of the rectifier circuit.
Shown in the diagram is a circuit that has a Hybrid Switching Regulator. The input of the circuit has similar characteristics to the linear power supply but has improved performance.
5V 500mA Hybrid Switching Regulator
Look at the actual usage examples. A 5V 500mA Hybrid Switching Regulator is used. In this circuit, it uses LM341 of NS. Generally, it is the 3 Terminal Positive Voltage Regulator.
I don’t normally enjoy reading the text. But to learn its operation with a circuit and a few block diagrams in between reading text becomes easier. Do you have the same point of view? Let’s take a look at the circuit to help with our understanding.
This circuit serves as the oscillator. The oscillator frequency in the circuit is determined in the ratio of resistance R2 and R3. The output voltage is fed back by the inductor L1. The transistor Q1 serves as the real switching device in the circuit
Learn Flyback Switching Regulator Works
If your load requires a power under 100 watts and you need a switching regulator that uses a few components, you must look at the block circuit diagram below.
It is called a flyback switching power supply circuit.
A high-frequency transformer is very important in this circuit because it has 3 main functions as follows:
- It reduces the voltage.
- It separates the input and output circuits.
- It limits the AC line current too.
In this device, the primary and secondary coils are wrapped in opposite directions.
A transistor runs when there is a pulse control signal to bias. This will drive the current through a high-frequency transformer but the output rectifier doesn’t conduct a current.
The primary voltage will reverse when the transistor is off. This will result in a flyback current that flows through and goes to the rectifier output and filter output. We can control the pulse width via the transformer to keep a constant output voltage.
The flyback switching power supply has a limited power range of 100 watts because of the current of the transformer which is the limit of per peak current value of switching transistor.
For applications that require over 100 watts we will use other switching regulator circuits. This is explained in the next circuit.
Hand-picked related circuits you may want to read:
Forwards switching regulator circuit of 80 to 200 watts
Look at a forward Switching Regulator circuit in the block diagram below. It has a high power of 80W to 200W. Here we can improve the ripple to become lower because we have used a bridge rectifier circuit. This circuit has the ripple lower than the half-wave rectifier of the flyback switching regulator.
We can also reduce the ripple even more by connecting a choke inductor in series with a capacitor filter. When a transistor runs (ON), the output of the circuit starts to conduct the current and develops voltage across itself. When the transistor stops (OFF), the current will stop to flow in the output rectifier. The voltage across the choke will have reverse polarity and will supply it to a load. This is why the ripple becomes lower.
There is a small difference in the pulse control circuit of a forward switching regulator.
Practically it is necessary to change the Pulse-timing of the output to suit the different output sizes to achieve the best results.
Here are a related post you might want to read:
Push-pull switching power supply
If you need a power of more than 200 watts the following circuit is designed to provide a power up to 600 watts.
Look at the Block diagram below. It shows a 2 Pulse Width Modulation Switching Regulator that is working to drive the switching transistor on each side.
This type of circuit connection allows the driving of more current.
The ripple appearing in the push-pull switching circuit can be reduced in size by providing a circuit for each pulse with a wide modulation to be balanced.
Typically, the push-pull switching circuits show the least ripple as compared to other switching supply circuits.
Rectifiers and modulation pulse filtering circuits behave the same. They display the error voltage of the output at the same point.
The switching power supply has a minor disadvantage. It produces an RF noise signal which is propagated. This noise interferes with other circuits if it is not well shielded.
It has regulation and ripple values similar to linear circuits.
In summary, switching power supply is suitable for applications that require a small size with high efficiency with minimum heat.