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DC-DC Converters

DC-DC converters are power electronics circuits that convert one dc voltage to another dc voltage level. There is usually a regulated voltage at the output of these circuits. In this tutorial, we will introduce dc-dc converters and give information about their types.

Differences Between Linear Regulator and DC-DC Converters

First of all, DC-DC converters are different from linear regulators. A linear regulator takes the input voltage and reduces it to the specified voltage level. But it wastes energy by converting excess voltage into heat. Therefore, the efficiency is low in linear regulators. DC-DC converters are not wasted energy while reducing voltage and therefore, efficiency is high. In addition, linear regulators can only reduce the voltage, but DC-DC converters can also increase the voltage.

Simple Switch Converter

Basic switch converter circuit.
Basic Switch Converter Circuit

Let’s examine the circuit above to show how we can simply decrease a DC voltage. A Vs is a DC source in the circuit that is connected to the output load by a switching element (transistor). With the switching element here, we can switch the connection between the DC source and the load on and off. When the switch is open, the connection between the DC source and the load is broken and at the output, we obtain a voltage of 0V (for the above circuit). When the switch is triggered and turned off, the output voltage is equal to the Vs voltage. This process is shown in the graph below.

Basic DC-DC converters graph.
Basic Switch Converter Graph

If we switch the switch on and off continuously for a certain period and if we can do this very quickly, we get a voltage at the output between the Vs and 0V in high frequency. If we use a filter to regulate this voltage, we will obtain a regulated dc voltage between 0V and Vs at our output. That is the principle of a simple Buck Converter.

When we take the integral of the graph above, the output voltage Vo is equal to;

Average output voltage equation.
Average Output Voltage Equation

D is the ratio of the switch-on time to the period. For example, if period 1s and switch on time is 0.4s, then, D = 0.4 / 1 = 0.4 and output voltage Vo = 0.4Vs. If the input voltage is 10V, the output voltage will ideally be 4V.

DC-DC Converter Types

The DC-DC converters are divided into 3 part these are;

  1. Buck Converter (Step Down)
  2. Boost Converter (Step Up)
  3. Buck-Boost Converter (Step Down – Step Up)

1. Buck Converter

Buck converter is used to reduce the input voltage. The most important elements in this converter are the switching element (usually MOSFET), the diode, the coil, and the capacitor. It is used to regulate the output voltage by using a low pass LC filter with a coil and capacitor while voltage control with MOSFET. For example, we can use a buck converter to use a 9V battery in a 5V circuit. Thus, the battery will give power to the circuit for a long time as they are very efficient compared to linear regulators.

2. Boost Converter

The boost converter is used to increase the input voltage. The elements used in this converter are the same as those used in the buck converter. Ideally, the input voltage can be increased to infinite voltage, while in fact the input voltage can be increased up to 5-10 times. If you want to increase the voltage further, a boost converter can be added in series. Let’s say we have a 3V battery and we want to run a circuit that works with 5V again. In this case, a boost converter can be used.

3. Buck-Boost Converter

Buck-Boost converter is used for both decreasing and increasing the input voltage. The elements used in this converter are the same as those used in the buck and boost converter. Consider the example of the Buck converter. When our 9V battery reaches to 5V, our circuit will no longer work. But using a buck-boost converter instead of buck, we can make the circuit work by raising the voltage even when the voltage drops below 5V. So, we will ideally use all the power of the battery.

In this tutorial, we have introduced the DC-DC converters. In other tutorials, we will explain these converters in detail and make examples. I hope it’s been useful, stay healthy.


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