DC to DC Buck converter working principle

This is a DC to DC Buck converter working principle. We often use it as a step down DC to DC converter circuit. Because it is a switching mode power supply.

It works by switching on and off. So it can get a much higher power efficiency than a linear power supply. Which it steps down voltage from a high input to a low voltage output. While it will step up output current to load. So it lets save power source.

You may think that the switching supply is hard for a beginner or an electronics hobbyists.

But wait, try to read this. It may change your mind. We can try to play or real use. We do not need to use expensive parts. You already have the equipment exactly.

A power supply on the switch mode power supply. It is very popular. Because it is power saving circuit.

The main principle of Buck DC to DC converters

The DC to DC converters will circuit will change the voltage source. To higher or lower or something else. By purpose of the designer or our circuit ideas.

When we see a basic buck converter circuit as Figure 1. It is easy to understand with:

  • Uin is an input voltage as popular of a European country. And “Vin” of the United States.
  • Uout is an output voltage of European countries. And “Vout” on the United States.

In this circuit, it consists of 3 main components only.

  • S is a switch, in the real circuit, we use a transistor.
  • D is a diode.
  • L is a coil or indictor.
  • C is a Capacitor.

Figure 1 Buck converter working principle

Figure 1 is basic buck converter circuit. It powers a certain output voltage. Other converter systems may call that a step down.

First, the Uin charges into capacitor until full. The its voltage is same as the power supply input.

Next, switch closed into the circuit. Therefore the positive voltage to drop across the coil L.

The current flow through coil to increase up in linear rate. While there is energy stored in the coil.

Then, the S open up. So, the current of L flow to the output capacitor.

And it flows through Diode (D). It makes the voltage drop across the coil L in backward (negative).

And, the current through the coil reduces in linear. The energy stored at the output capacitor.

When the switch (S) connected to the circuit again. The system started working the new one. To be able to supply the load continuously.

In Practice, It is difficult to control the switch(S). Which It will make the energy to the output. How to do it?

3 types of DC converter current-mode
Figure 2 Three types of current mode

In normal principle, the electricity flows in 3 modes. As shown in Figure 2.

  • Continuous mode (CM): When switch is closed. Then, the current is continuously flowing in coil. See Figure 2A
  • Discontinuous mode (DM): But if we control times to switch on-off .
  • CM/DM boundary: This mode, the switch turn-on immediately, if current of coil drop to zero.

There are too many principles to control the switches ON-OFF. But we do not need use all principles in one working. Because our some project use a few principles only. We do want to know what we learn just one only.


Let’s see a true circuit. To tell the story. It is the best solution. We would like to actually get into the next circuits.

Keep reading: ‘Switch mode LED driver circuit’ »

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