Boost Converter: Basics, Working, Design & Application

Introduction

The boost converter is a DC-to-DC converter designed to perform the step-up conversion of applied DC input. In the Boost converter, the supplied fixed DC input is boosted (or increased) to adjustable DC output voltage i.e. output voltage of the boost converter is always greater than the input voltage. So, a Boost converter is also called a step-up converter or step-up chopper. It is given the name “boost” because the obtained output voltage is higher than the supplied input voltage. It performs the reverse operation of the buck converter which converts higher DC input into lower DC output.

The boost converter is used to step up an input voltage to some higher level as per the requirement of the load. This step-up conversion in the boost converter is achieved by storing energy in the inductor and releasing it to the load at a higher voltage. Boost converters are widely used in battery-powered devices where perhaps a pair of batteries deliver 3V but need to supply a 5V circuit.

As we know, the product of voltage and current results in power, the increase in voltage at the output of the boost converter means a decrease in the output current through the circuit.

There are at least two semiconductors (such as a diode and transistor) and at least one energy storage element (such as an inductor or capacitor or both). Other semiconductor devices like power MOSFET, power BJT, IGBT, etc. are used as a switch in boost converter circuits. Thyristors are not used generally for DC-to-DC converters because another external communication circuit is required when using thyristors.

Boost Converter Operating Principle

The operation of the boost converter is based on the principle of storing energy in an inductor. The voltage drop across an inductor is proportional to the change in the electric current flowing through the device. The circuit arrangement operates in such a way that it helps in maintaining a regulated and increased dc output at the load.

The circuit diagram for a typical boost converter is shown in the figure below.

In this circuit, the solid-state device such as power MOSFET which operates as a switch is connected across the source. A diode is used as a second switch. The diode is connected to the capacitor and the load. The capacitor and load are connected in parallel as shown in the above circuit diagram. The inductor is connected in series with the supply voltage source which leads to a constant input current. So the boost converter acts as a constant current input source and loads act as a constant voltage source.

The controlled switch S is turned on and off by using PWM (Pulse Width Modulation). PWM can be time-based or frequency based. Time-based Modulation is mostly used for Boost Converter because it is simple to construct and use. The frequency remains constant in this type of PWM modulation. Whereas Frequency-based modulation has a wide range of frequencies to achieve the desired control of the switch and has a complicated design for the low-pass LC filter.

There are two modes of operation of the Boost converter. They are:

Mode I: Switch S is ON and Diode D is OFF
Mode II: Switch S is OFF and Diode D is ON

Mode I: Switch S is ON and Diode D is OFF

In this mode of operation, switch S is in closed condition i.e. ON state, and diode D is in open condition i.e. OFF state. Thus switch S allows the flow of current through it. All the current will flow through the closed path including inductor L, switch S, and back to the dc input source. The circuit diagram for this mode is shown in the figure below.

Here, the polarity of the inductor will be according to the direction of the flow of current. In this mode of operation, the diode D is in reverse biased condition so that diode does not allow the flow of current through it to the circuit. In this condition, the voltage across the switch S will appear across the load resistance and hence output voltage.

Let us say the switch S is ON for a time Ton and it is OFF for a time Toff.

Then the total time period T is a combination of Ton and Toff time.

Then the switching frequency is given by:

And the duty cycle is given by:

Applying KVL in the above circuit, we get:

We have:

Also,

Since switch S is on closed condition for a time Ton = DT, so 𝚫t = DT.
Then,

This equation gives the change in current through the circuit when the switch S is closed. i.e. in Mode I.

Mode II: Switch S is OFF and Diode D is ON

In this mode of operation, switch S is in open condition i.e. OFF state and diode D is in closed condition i.e. ON state. Thus switch diode D allows the flow of current through it, whereas switching S blocks the current flow through it. The circuit diagram for this mode is shown in the figure below.

As we know, the inductor in the circuit store energy in the form of the magnetic field, the inductor acting as the source when the switch S is open. Hence diode D becomes closed. In this mode of operation, the inductor releases the energy stored in the previous mode when switch S was closed. During releasing of energy stored in the inductor, the polarity of the inductor gets reversed which causes the diode D to come in forward biased condition. So it allows the flow of current in the circuit through diode D. The way of current flow is shown in the above figure.

The released energy is ultimately dissipated in the load resistance which helps to maintain the flow of current in the same direction through the load and also steps up the output voltage. The current through the inductor is of decreasing nature and will die out after the point in time.

Now, applying KVL in the above circuit, where we kept the original convention to analyze the circuit, we get:

This equation gives the change in current through the circuit when the switch S is open. i.e. in Mode II.

Since the net change in current through the inductor in one complete cycle is zero i.e. the summation of the rate of change of current in Mode I and Mode II becomes zero

On simplifying, we get:

We know that the value of duty cycle D varies between 0 and 1. For this range of D, the output voltage is greater than the input voltage. Hence in this way boost converter steps up the input voltage. However, for D=1, the ratio of output voltage and input voltage at a steady state goes to infinity, which is not physically possible. Practically, the value of D is kept at greater than 0.7 leading to instability since the boost converter is a non-linear circuit.

The waveform of the Boost converter is shown in the figure below: