Why are DC-DC converters so useful?
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Why are DC-DC converters so useful?

Posted Date: 2024-01-14

1. What is a DC-DC converter?

A DC-DC converter is a power electronic circuit that efficiently converts direct current from one voltage to another.

DC-DC converters play an integral role in modern electronics. This is because they offer several advantages over linear regulators. Linear regulators in particular dissipate a lot of heat and are very inefficient compared to switching regulators in DC-DC converters.

DC-DC boost circuit

Before introducing the working principle of DC-DC converter, let's look at an example. Why is DC-DC converter so useful? Assume building a circuit with the following requirements:

DC-DC boost circuit

2Ω load resistance

12V DC power supply

5V load voltage

We need to reduce the voltage of the 12V battery to provide 5V to the load. We can put a 2.8Ω resistor in series with the load to provide the required voltage.

First calculate the efficiency of the circuit as follows:

DC-DC boost circuit

From these calculations, we can see that the load consumes only 12.5W of the input power and the remainder (30 – 12.5 = 17.5 W) is converted into heat.

From this point of view, it is actually a bit wasteful. If you touch the series resistor, it will be a bit hot. A mechanism needs to be combined to cool the circuit. In order to get a better solution, you can look at the following circuit:

DC-DC boost circuit

When the switch is off, the input voltage is 0V; when the control is in the ON position, the input voltage is 12V. The figures below show the equivalent circuits for the switch positions ON and OFF respectively.

DC-DC boost circuit equivalent circuit

If we control the switch as shown in figure (a) below, we get the voltage diagram as shown in figure (b) below. T is the switching period, in milliseconds or microseconds.

DC-DC boost circuit

In this case, the average output voltage for this switching behavior is 5V because:

DC-DC boost circuit

The average output voltage of this circuit is 5V, but we can improve the output waveform by using an RC filter circuit to remove the harmonics.

If we assume that the switch is ideal (an ideal switch is a switch that consumes or dissipates no power), we can calculate that the efficiency of this circuit is 100%. When the switch is in the ON position, the current flowing through the circuit is 6A.

Since we have an ideal switch, the power dissipated is P_diss = RI 2 = 0 * 9 2 = 0W. When the switch is in the off position, no current flows through the switch, so the power dissipated is also 0 in this case.

However, in practical applications, it is more difficult to find an ideal switch, which means that there will actually be some power consumption. Although there is power consumption, the conversion efficiency is still very high.

2. DC-DC boost circuit

The DC-DC boost circuit mainly increases the voltage of the power supply. For example, the boost converter can take a 5V power supply and boost it to 25V. Typically, you'll find DC-DC boost converters in battery chargers or solar panels. They can also be used to power components with different operating voltages from the same battery.

This configuration increases the DC voltage to a level determined by the selection of components in the circuit. This is a general schematic diagram of a boost converter.

1. Boost switch is on

Boost switch ON status

2. Boost switch closed state

Boost switch closed state

The basic configuration includes DC power supply (Vin), inductor (L), diode (D), switching device (SW), smoothing capacitor (C) and load resistor (Load). Vout is the output voltage.

Switches are typically power electronic devices such as MOSFETs or BJT transistors controlled by a PWM signal. This PWM signal works by switching transistors very quickly, typically thousands of times per second.

3. Working principle of DC-DC boost circuit

Assume that the current voltage is 5V, and 5V needs to be converted to a higher voltage value. This can be achieved with a DC-DC boost circuit. Here we assume that we are a plumber.

1. Turbo acceleration

First we need to speed up the turbo. To do this, the throttle is opened and water is rapidly discharged, transferring part of the energy to the turbine, as a result of which the turbine begins to rotate.

Working principle of DC-DC boost circuit

2. Fill the pressure storage tank

The throttle is closed, and part of the water pushed by the rotating half of the turbine flywheel opens the valve and fills the water storage tank. The other part of the water flows to the consumer under the high pressure provided by the water storage tank, while the valve prevents the water from flowing back.

Working principle of DC-DC boost circuit

3. Generate electricity from pressure storage tanks and accelerate turbines

The speed of the turbine begins to decrease. The water can no longer push the valve, and the water storage tank still has enough energy accumulation. Then the throttle is opened again and the water starts spinning the turbine rapidly. Since the consumer receives energy from the tank, the flow of energy to the consumer does not stop and the cycle repeats.

Now that the working principle is clear, we will switch from plumbing equipment to electronic equipment.

Working principle of DC-DC boost circuit

We replaced the turbine with an induction throttle. Transistors are used in place of throttle valves that control the flow of water. The diode acts as a valve, replacing the pressure storage tank with electricity.

Below you can have a good understanding of the working principle of the DC-DC boost circuit.

1. Accumulated charge in the inductor

The switch is closed and the inductor accumulates energy by receiving current from the source.

Working principle of DC-DC boost circuit

2. Transfer energy to the capacitor

The switch opens and the coil retains the energy accumulated in the magnetic field. The current tries to stay at the same level, but the extra energy from the inductor increases the voltage, opening a path through the diode. A portion of the energy flows to the consumer, while the remaining energy accumulates in the capacitor.

Working principle of DC-DC boost circuit

3. Accumulate energy in the inductor and transfer charge to the consuming circuit

The switch is then locked and the coil starts accumulating energy again, while the dissipated one receives energy from the capacitor.

Working principle of DC-DC boost circuit

4. How to build a DC-DC boost circuit

In the following, build a 1.5V to 5V DC-DC boost converter,

1. Components required to build a DC-DC boost circuit:

1.5V DC power supply

One 180uH inductor

1 1N3491 diode

1 33uF capacitor

A 150 Ω resistor

A MOSFET or JFET switching transistor

PWM source, such as Arduino Uno or 555 timer, can generate 50KHz, 5V, 75% duty cycle

2. Working principle diagram of DC-DC boost circuit

DC-DC boost circuit

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Review Editor: Huang Fei


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