What is switching power supply vs linear, how does it work?

When we need a high-efficiency power supply in a small size. Many people pick a switching power supply. In the past, I liked a Linear Power Supply. But sometimes I should try other ways.

In this post, we will learn what is switching power supply vs linear, how does it work?

You maybe like it the same as me. After reading this article.

What is switching power supply how does it work

What are the types of power supply

The power supply is the source of energy for various circuits. It will convert the AC mains into DC voltage. This is a fixed or variable voltage as applied as your works.

There are 2 main types of power supplies:

  • The Linear power supply is commonly used.
    It is simple circuits are not complicated. But they are large and low efficiency only about 50% or more. While they work are a loss in the form of high heat.
  • Switching power supply Currently,
    Many works, choose this type of power supply. Because small High efficiency is about 85% or more. Imagine we enter 100% electric energy. It can be transformed into 85% of energy. And, 15% is lost energy in the form of heat.

But the switching supply circuit is quite complex. Which previously I tried to avoid it because I wasn’t sure if I could explain it easily.

Ready to get started?

We should start by looking at the block diagram of the switching power supply. Although the structure looks complicated. But if the circuit can be separated into parts It can be easier to understand.

Block diagram of a switching power supply

The highlight of this circuit is working with high frequency. Therefore has a smaller transformer There is a switching system with high frequencies.

And input and output circuit include the rectifier and filter circuit. and error voltage detector to control the stable voltage.

Of course, now you may not understand. But when reading in the next section, friends will understand more.

What’s more?

In switching power supply has 4 types of rectifier circuits

Meet Rectifier AC to DC easy but so helpful

The switching power supply will have the rectifier circuit both input and output. Most of this is a bridge rectifier circuit.

The parts convert AC to DC is Rectifier. In a linear circuit, this circuit is important. In the switching supply circuit, the rectifier circuit is also important.

The important device is the diode, which is a semiconductor device that allows current to flow only through one direction. Then, the DC voltage will flow through the filer to smooth the current.

Recommended: How Rectifier circuit works

In switching power supply has 4 types of rectifier circuits:

1# AC main to DC pulse Bridge rectifier

Normally, we find the rectifier circuit first. The input side of the switching power supply as the circuit diagram below.

AC input to DC pulse voltage using 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, DC signal input will be switched with high-frequency RF. Then, the step-down transformer transforms it into AC low voltage. Next, it flows to a half-wave rectifier to DC pulse, too.

3# Full-wave Rectifier using center tap transformer

Full-wave-Rectifier using center tap transformer

This developed from a half-wave rectifier. We will often see a rectifier like this. And, notice that it uses the center tap of the secondary transformer. It is 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 need a center tap transformer, but we need to use 2 more diodes.

Selection of diodes for the rectifier circuit

There are 2 important factors which are

The peak Inverse voltage- PIV

It is the maximum voltage that the diode can tolerate. While it receives a reverse bias. Or while Diode is OFF.

The PIV value of the diode used should withstand at least 2 times the operating voltage. And when 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 Should choose a diode with a value:

Piv = (311Vpkx2) + (311Vpkx0.5)
= 777.5Vpiv

Forward Current-IF

It is the current that the diode allows to flow through it when receiving a forward without damage. And more importantly, do not forget to add a safety value with 50%.

For example, an input rectifier with a current of 1A. We should choose a diode with forwarding current:
IF = 1+ (1×0.5) = 1.5A

How is filter important

The voltage from the rectifier is DC. But we cannot use it. We need to smooth it by the filter capacitor. Both linear and switching power supply need to use it.

The capacitor is a device used to store energy. It charges the energy within it until it reaches the maximum value of the pulse voltage. And will release when loaded.

Effect of DC pulse filtering of load
The effect of pulsed DC signal filtering and the response load current

The image shows the filter effect of the capacitor in both charging and discharging rhythm. When connected to the load. The ripple voltage across the capacitor is called the Ripple.

  • There is High ripple. If the high load current
  • In contrast, low ripple. If it is a low current of the load.

And if we take a look at the block diagram working. In the filter circuit for AC voltage 50-60Hz. We will use the capacitor quite large.

Usually in the range of 1,000uF to 2,000uF. It depends on the load current.

Read next: How to design the unregulated power supply

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

The working voltage Rate
Importantly, we need to use the working voltage rating of the capacitor more over the voltage at the operating current is approximately 50%

High-frequency transformer

A transformer is a device that is used to convert a high voltage on a primary to a low voltage on the secondary as the image below.

RF transformers coupling between input output
RF High-frequency transformers connect coupling between input and output

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

Typically 50Hz transformers that are commonly used will not be able to be used at high frequencies.

Although the size and shape of the switching transformers are different from the 50Hz transformers. But the operation still uses the same basic principles of magnetic field coupling.

That is the high voltage connected to the primary coil. And it will store energy and create magnetic fields alternating between the On and Off phase.

Which the transformer core acting 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. Also known as the switching regulator.

Pulse Width Modulation Switching Regulator

Although there are many different switching circuits. But 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. It maintains the voltage level with a closed-loop form.

To get a constant output voltage. This circuit will detect the voltage error. This error signal is used to control the pulse width of the switching circuit. It is the change in the pulse width of the oscillator circuit within the regulator.

The width of the pulses changed from the oscillator is sent to drive the transistor acting as a switch. In which the changing pulse width causes the average voltage of the output to change accordingly.

The high-frequency transformers lower the voltage into the AC signal, then it is rectified and filtered again.

For the final output of the DC voltage. The output will be randomized again. And will adjust the error signal followed. Until receiving the constant voltage as needed.

Which means the circuit will operate in a closed loop. The output voltage is continuously controlled Until working normally.

Now, we can know the basic working principle of the switching regulator. How does it work? So what next? It’s probably time for us to apply it.

Read Also: 0-45V 8A DC switching power supply circuit

Hybrid Switching Regulator Working principle

It is not always necessary to use a High-Frequency Transformer to design a Switching Power Supply.

Normally, the transformer is used to change the voltage of the pulse from a high voltage to a lower voltage.

If a DC input voltage is close to the actual operating voltage. The high-frequency transformer is not necessary.

We can use the 50Hz Step-down voltage transformer to reduce the voltage to a lower value. Before feeding it to the input of the rectifier circuit.

Hybrid Switching Regulator Working principle

Look in the circuit is Hybrid Switching Regulator that the input of the circuit has similar characteristics to the linear power supply. But it improves performance.

5V 500mA Hybrid Switching Regulator

Look at actual usage examples, 5V 500mA Hybrid Switching Regulator. In the circuit, it uses LM341 of NS. Generally, it is the 3 Terminal Positive Voltage Regulator.

I don’t like reading the text. But I like to learn its operation with circuit and block diagrams. Are you the same as me? Let’s look in the circuit. We will more understand.

But this serves 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 devices in the circuit.

Check out these related articles, too:

Learn Flyback Switching Regulator Works

If you need a switching regulator that uses a few components. And your load requires a power under 100 watts.

Look at the block circuit diagram below.

It is a flyback switching power supply circuit.

The High-frequency transformer is very important in this circuit. Because it has 3 main functions which are:

  • Reduce the voltage down.
  • Separate the input and output circuits.
  • Limit the AC line current too.

In which the primary and secondary coils are wrapped in opposite directions.

When there is a pulse control signal to bias a transistor runs. The current will drive through a high-frequency transformer. But the output rectifier doesn’t conduct a current.

In contrast, when the transistor is off. The primary voltage reverses. And, this result causes a flyback current flows through 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 limited a power rated of 100 watts. Because of the current of the transformer. And the limit per peak current value of switching transistor.

For applications over 100 watts. We will use others switching regulator circuits. This will be 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 forwards Switching Regulator in the block diagram below. It is a high power of 80W to 200W. We can improve a ripple to lower. Because we use a bridge rectifier circuit. Which it has the ripple lower than the half-wave rectifier of the flyback switching regulator.

Forwards switching regulator circuit

In addition, we can 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 will conduct the current and has the voltage across itself.

And when the transistor stop (OFF). The current will stop flows in the output rectifier. The voltage across the choke will reverse polarity. And supplies to a load. This is why it lower ripple.

There is a slight difference in the pulse control circuit of forwards switching regulator.

In practice, it is necessary to change the Pulse-timing of the output to suit the different output sizes. For the best results.

Here are a few related posts you might want to read:

Push-pull switching power supply

If you need power more than 200 watts. This circuit is designed to be able to provide power up to 600 watts.

Look at the Block diagram. It consists of 2 Pulse Width Modulation Switching Regulator working together to drive the switching transistor on each side.

Push-pull switching power supply block diagram

This type of circuit connection will allow driving more current.

The ripple in the push-pull switching circuit can be reduced in size. By providing a circuit for each pulse wide modulation to be balanced.

Typically, push-pull switching circuits have the least ripple. When compared to other switching supply circuits.

Both rectifiers and pulsed modulation pulse filtering circuits are the same. With the point get the error voltage of the output is the same point.

Conclusion

The switching power supply has a disadvantage is RF noise signal. Which it can propagate and interfere with other circuits. If not well shielded.

Regulation and Ripple values ​​are similar to linear circuits.

In summary, switching power supply is suitable for applications requiring small size and high efficiency, low heat.

Also, here are a couple of related posts you should read, too:

How to learn electronics with simple ways

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