That means that the On current is equal to the Off current. 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. So once again using the next figure formulas we obtain the current of the OFF part.įigure 3: OFF state of the buck converter In this case the voltage across the inductor is the output voltage. If the switch is closed again before the inductor fully discharges, the voltage at the load will always be greater than zero. During the off-state, the inductor is discharging its stored energy into the rest of the circuit. The "increase" in average current makes up for the reduction in voltage, and ideally preserves the power provided to 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 stored energy in the inductor's magnetic field supports the current flow through the load. 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. When the switch is opened again (off-state), the voltage source will be removed from the circuit, and the current will decrease. As we can see in the next figure, we obtain the ON current through the inductor. But we also know that the inductor voltage is the inductance L multiplied by the inductor current derivate. 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. 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. During this time, the inductor stores energy in the form of a magnetic field. Over time, the rate of change of current decreases, and the voltage across the inductor also then decreases, increasing the voltage at the load. This voltage drop counteracts the voltage of the source and therefore reduces the net voltage across the load. 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 is because the circuit acts rather like a mechanical flywheel that, given regularly spaced pulses of energy keeps spinning smoothly (outputting energy) at a steady rate. The circuit operation depends on what is sometimes also called a Flywheel circuit. To maintain a continuous output, the circuit uses the energy stored in the inductor L, during the on periods of the switching transistor, to continue supplying the load during the off periods. The switching transistor between the input and output of the buck converter continually switches on and off at high frequency. In the ON state, the switch is closed as we can see in the next figure, where the diode is open because the cathode voltage is higher than the anode.įigure 1: Basic circuit for a DC-DC buck converter In order to study how it works, we will divide it in two stages. Theory of DC-DC Buck Converter Operation ¶Ī basic circuit of a buck converter is provided. Finally, the schematic and the circuit of the DC-DC converter is provided. Second, the list of components used in this experiment is provided. 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).įirst, the theory of DC-DC buck converter is provided. 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. The pulse width modulation (PWM) switching signal is generated using Arduino Uno and drives the gate of P-Channel Metal oxide silicon field effect transistor (MOSFET) through a bipolar junction transistor (BJT).Ī buck converter or a 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). In this experiment, we want to build a cheap DC-DC buck converter using the common electronic components available online. You can watch the video of this experiment by following this link: DC-DC Buck Converter with Arduino Uno ¶ Video of the Experiment ¶
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