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Albert Einstein He developed the general theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics).


The first integrated circuit invented by the engeneer Jack S. Kilby1 (1923-2005) contained only a few transistors. Texas Instruments.
We have 3 types of non-insulated DC to DC convertes: Buck, Boost and Buck-Boost configurations.
buck converter circuit
Buck converter
boost converter circuit
Boost converter
Buck-Boost converter circuit
Buck-Boost converter




DC to DC buck converter

A buck converter (step-down converter) is a DC-to-DC power converter which steps down voltage (while stepping up current) from its input (supply) to its output (load). It is a class of switched-mode power supply (SMPS) typically containing at least two semiconductors (a diode and a transistor, although modern buck converters frequently replace the diode with a second transistor used for synchronous rectification) and at least one energy storage element, a capacitor, inductor, or the two in combination. To reduce voltage ripple, filters made of capacitors (sometimes in combination with inductors) are normally added to such a converter's output (load-side filter) and input (supply-side filter).

In this tutorial we will learn how to build and how a DC to DC buck converter works. The circuit is very basic using just one diode, an inductor and a capacitor. The switch will be a MOSFET transistor and to create the PWM signal we will use a 555 timer in the PWM configuration, boost adjustable controller or one Arduino NANO. But first let's study a little bit of theory. We have the Buck converter circuit in the next figure where we can see the switch, inductor and capacitor and of course we add a load to the output.



Buck converter theory

Ok, so we have the next circuit. In order to study how it works, we will divide it in two stages. The ON and OFF stages. In the ON part, the switch is closed as we can see in the next figure where the diode is open becasue the cathode voltage is higher than the anode. When the switch is first closed (on-state), the current will begin to increase, and the inductor will produce an opposing voltage across its terminals in response to the changing current. This voltage drop counteracts the voltage of the source and therefore reduces the net voltage across the load. Over time, the rate of change of current decreases, and the voltage across the inductor also then decreases, increasing the voltage at the load. During this time, the inductor stores energy in the form of a magnetic field. If the switch is opened while the current is still changing, then there will always be a voltage drop across the inductor, so the net voltage at the load will always be less than the input voltage source. When the switch is ON the inductor will charge up and the voltage on the inductor will be the difference between the output and the input. But we also know that the inductor voltage is the inductance L multiplied by the inductor current derivate. As we can see in the next figure we obtain the ON current through the inductor.

When the switch is opened again (off-state), the voltage source will be removed from the circuit, and the current will decrease. The decreasing current will produce a voltage drop across the inductor (opposite to the drop at on-state), and now the inductor becomes a Current Source. The stored energy in the inductor's magnetic field supports the current flow through the load. This current, flowing while the input voltage source is disconnected, when concatenated with the current flowing during on-state, totals to current greater than the average input current (being zero during off-state). The "increase" in average current makes up for the reduction in voltage, and ideally preserves the power provided to the load. During the off-state, the inductor is discharging its stored energy into the rest of the circuit. If the switch is closed again before the inductor fully discharges (on-state), the voltage at the load will always be greater than zero.

In this case the voltage across the inductor is the output voltage. So once again using the next figure formulas we obtain the current of the OFF part.

Ok, now if we want to obtain the output depending on the input and the duty cycle of the PWM all we have to do is to make the sum of the On and Off current equal to 0. That means that the On current is equal to the Off current. So the will give us:

So we've obtain that the output is the input multiplied by the duty cycle. The duty cycle of the PWM can have values between 0 and 1. So te only posible output will be equal or lower than the input. That's why this configuration is called step down converter.






Buck converter circuit 555 timer

This 555 configuration will create a PWM signatl and apply that signal to the MOSFET gate. The circuit works ok but it has a big problem. The output will change if we change the output load because the circuit has no feedback. Ok so we will use the next schematic for our buck converter. To create the PWM signal we will use the 555 timer with the PWM configuration. With the P1 potentiometer we can change the duty cycle of the PWM signal, and at the same time the output value. For the MOSFET you could use the IRF4905 P channel mosfet. You could always try different inductance values for the inductor and see the results.


The input could be in range of 5 to 15 volts. Don't apply higher voltage or you could burn the 555 timer. Connect the PWM (pin 3 of the 555 timer) to the MOSFET (switch) gate. Add an output load and test the circuit. You could obtain output valuew between 1V and 15V.






Buck converter LM2576T-ADJ circuit

With this component we have feedback and the output will stay the same using different loads. Just make the connections, add the input capacitor to have a steady input and you're done.


The input could be in range of 5 to 55 volts. Don't apply higher voltage or you could burn LM2576T-ADJ component. In this case we need no external switch since the LM2576T-ADJ already has it inside it. With the feedback pin connected to the output voltage divider, the LM2576T-ADJ will change the width of the pulse depending of the output in order to keep it constant. In this case use a Schottky Barrier Rectifier diode because it has a low forward voltage. This diode will live the current flow when the switch is open.






Buck converter Arduino NANO

Sincerely, this circuit has no sense but to learn. The Arduino NANO already has a 5V linear voltage regulator that will lower the efficiency of the circuit. So the main goal is to learn how the circuit, the feedback and the PWM signal work in order to achive the desired output.


As you can see we have a potentiometer connected to the analog input A0. With this potentiometer we will choose the output value between 1 and 15 volts. At the output of the circuit we have a voltage divider that will lower the voltage from 15V to under 5 volts because that's the maximum input voltage of the Arduino ADCs. In the code we compare this two voltages and increase or decrease the PWM width in order to keep the output constant. Just copy and upload the next code to the Arduino for this example.




Arduino code for buck converter


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int potentiometer = A0;
int feedback = A1;
int PWM = 3;
int pwm = 0;

void setup() {
  pinMode(potentiometer, INPUT);
  pinMode(feedback, INPUT);
  pinMode(PWM, OUTPUT);  
}

void loop() {  
  float voltage = analogRead(potentiometer);
  float output  = analogRead(feedback);

  if (voltage > output)
   {
    pwm = pwm+1;
    pwm = constrain(pwm, 0, 255);
   }

  if (voltage < output)
   {
    pwm = pwm-1;
    pwm = constrain(pwm, 0, 255);
   }

   analogWrite(PWM,pwm);
}
 




Boost converter page: