Many people pick a switching power supply when they need a high efficiency power supply that is available in a small size. I have given different types of power supplies a chance but I have liked a Linear Power Supply in the past and have used it a lot. Sometimes its better to give other types a try.

In this post, we will find a comparison between a switching power supply vs linear power supply. You will also find out how they work. You will also start to like them just as I do after reading this article.

What is switching power supply how does it work

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.

Block diagram of a switching power supply

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.

AC input is converted to DC pulse voltage using a Bridge rectifier.

Input AC voltage 220V RMS or 311 Vpk is rectified to DC pulse voltage of 160Vpk. Then, it comes to RF Switch circuit diagram.

2# Half-wave Rectifier from RF AC signal

Half-wave-Rectifier from AC high RF 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

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

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)
= 777.5Vpiv

Forward Current-IF

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.

Effect of DC pulse filtering of load

The effect of pulsed DC signal filtering and the response load current is shown in the diagram.

There is a filter effect of the capacitor in both charging and discharging rhythm. When connected to the load the voltage across the capacitor is called the Ripple voltage.

  • If the current has high load, the ripple will be high.
  • In contrast there is a low ripple, if the load of the current is low.

If we take a look at the block diagram, it is shown that the filter circuit for AC voltage is 50-60Hz. This means that we will use a large capacitor. It must usually be in the range of 1,000uF to 2,000uF. It depends on the load current.

Increasing its value (In parallel) reduces the discharging time between pulses. This Results in less ripple voltage values ​​as well.

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%

High-frequency transformer

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.

RF transformers coupling between input output

RF is the high-frequency transformer connected in coupling between input and output.

It provides a form of connection of the transformer to the input and output. We use it as switching power supply for switching at high frequencies of 20KHz or more.

Normally the 50Hz transformers that are commonly used, cannot be used at high frequencies. The size and shape of the switching transformers is different from the 50Hz transformers but the operation still uses the same basic principles of magnetic field coupling.

This shows a high voltage connected to the primary coil and it will store energy and create magnetic fields alternating between the On and Off phase.

The transformer core acts as a magnetic field induced to secondary in the form of a coupling transformer.

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.

Basic block diagram of Pulse Width Modulation circuit

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.

Hybrid Switching Regulator Working principle

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.

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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.

Forwards switching regulator circuit

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.

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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.

Push-pull switching power supply block diagram

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.

Conclusion

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.

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