A DC to DC converter (DC-DC) is a power electronics circuit that efficiently converts a direct current from one voltage level. Without a doubt, DC-DC converters play an integral role in modern electronics. This is because they offer several advantages compared with linear variable structure systems such as voltage regulators.
Linear regulators, in particular, dissipate a lot of heat, and they have a very low efficiency compared with switching regulators.
In this tutorial, we will talk about DC-DC converters. There are many types of power converters, but we will limit the scope of this tutorial to boost and buck converters.
Why we need DC-DC Converters
Before we dive deep into the operating principles of DC-DC converters, let’s talk briefly about why we need them in the first place. Suppose we have the following requirements:
- 2 ohm load
- 12V DC supply
- 5V load input voltage
We need to step down the battery voltage to supply the load with 5V. So, according to these specifications, we can place a 2.8-ohm resistor in series with the load to provide the required voltage.
Calculating the Efficiency
We can calculate the circuit efficiency easily as follows:
From these calculations, we can see that the load consumes only 12.5W of the input power. The remaining part (30 – 12.5 = 17.5 W) is converted into heat.
Now, this is too wasteful. If you touch the series resistor, it will be hot, and you may need to incorporate mechanisms to cool down your circuitry. As an attempt to obtain a more efficient solution, you may use a circuit shown in the diagram below:
When the switch is OFF, the input voltage is 0 V, and when the control is at the ON position, the input voltage is 12V. The diagram below shows the equivalent circuits for switch positions ON and OFF, respectively.
If we control the switch as shown in diagram (a) below, we obtain a voltage graph as shown in diagram (b) below. T is the switching period, and its units are in milliseconds or microseconds.
In this case, the average output voltage of this switching behavior is 5V since:
The average output voltage of this circuit is 5V, but we can improve the output waveform by using RC filter circuits to get rid of harmonics.
If we assume the switch to be ideal (an ideal switch is a switch that does not consume or dissipate power from the source), we can calculate the efficiency of this circuit to be 100%. When the switch is at position ON, the current flowing through the circuit is 6A. Since we have an ideal switch, the dissipated power is P_diss = RI2 = 0 * 92 = 0W. When the switch is at position OFF, no current flows through the switch, so in this case, the dissipated power is 0, too.
In a real-world scenario, however, it may be challenging to find an ideal switch. There is some dissipated power in actual control, but the conversion efficiencies are high despite these dissipations.
Suppose we want to increase the voltage of our DC power supply efficiently. We use a power electronics configuration called a DC-DC boost converter. This configuration steps up the DC voltage to a level determined by choice of components in your circuit. Here is a general schematic of the boost converter.
The basic configuration consists of a DC power source, an inductor, a diode, a switching device, a smoothing capacitor, and the load. This configuration increases the output voltage and reduces the number of cells. Typically, you find DC-DC boost converters in applications such as battery chargers or solar panels. One of the primary uses of a DC-DC boost converter is to supply components with different operating voltages from the same battery.
For example, a breadboard project or a PCB may have other features that use various voltages. It is expensive and impractical to have a different power supply for each voltage rail. Hence we use a DC-DC converter to supply power to multiple components at their respective voltage levels.
How Boost Converters Work
If we look at a typical boost converter configuration, we find essential electronic components such as a switch, an inductor, a capacitor, and a diode. We also have the load, represented here by a 2-ohm resistor. A switch is typically a power electronics device such as a MOSFET or BJT transistor controlled by a PWM signal in a real converter. This PWM signal works by switching the transistor very fast, usually thousands of times per second.
To understand how a DC-DC converter works, let’s first understand how an inductor works.
Recall that an inductor is a passive electronic component that can store electrical energy in the form of magnetic energy. We can use this property to control the output voltage of our circuit. Here is how it works:
- If we OPEN the switch, as shown in the “Boost switch OFF State” diagram above, a smaller current flows from the battery, through the inductor, through the diode and charging the capacitor.
- When we CLOSE the switch, as shown in the “Boost switch ON State” diagram above, a larger current will flow from the battery, through the inductor and through the switch because we now have a path of least resistance. Now, the inductor behaves interestingly. Since we now have a larger value of current flowing through the circuit, the magnetic field of the inductor will expand. This means that the inductor is storing energy, and during this process, the potential across the inductor will be positive on the left and negative on the right.
- When we OPEN the switch again, the current flowing through the circuit will no longer be a large current due to the high impedance. When the current flowing through the circuit decreases, the magnetic field across the inductor will collapse. In the process, the electrical energy that was stored is now being released. This causes the polarity of the inductor to change. We now have a negative polarity on the left side and positive on the right side of the inductor. If you look closely, the inductor is now in series with the battery.
The current through an inductor cannot change instantaneously. Therefore, the inductor will try to support that change by generating a large voltage. This means that we now have the voltage generated by the inductor and the voltage from the battery at the capacitor. Suppose we keep turning the switch ON and OFF. We will have an output voltage that is higher than the battery voltage.
Sample Schematic of a Boost Converter
Here, I used NI Multisim software to simulate 1.5V to 5V DC-DC Boost converter. Here are the components that you need if you want to reproduce this simulation:
- DC_POWER – 1.5V
- Inductor – 180uH
- DIODE – 1N3491
- Capacitor – 33uF
- Load resistor – 150-ohm
- CLOCK_VOLTAGE (50KHz, 5V, 75% Duty Cycle)
Suppose you want to reduce the voltage of your battery without using linear regulators. You can make use of a DC-DC Buck converter. This is a power electronics circuit that steps down the DC voltage to a level determined by the choice of components in your circuit.
Here is a general schematic of the buck converter.
If you look closely, these are the same components that we find in a boost converter, but their arrangement is different. Unlike linear regulators that reduce the voltage by dissipating power as heat, the buck converter reduces the voltage by increasing the current.
How Buck Converters Work
To obtain an output voltage lower than the battery, we connect the switch before the inductor. When we turn ON and OFF the controller with the circuit in this configuration, the average output voltage will be lower than the battery’s voltage.
Here is what happens:
- If the switch (SW) is closed, current will flow through the switch to the circuit. As the current increases, the magnetic field of the inductor will expand. While that happens, the inductor is storing energy in its magnetic field. As before, the polarity of the inductor will be positive on the left and negative on the right. This opposing voltage counteracts the voltage of the source and, therefore, reduces the net voltage across the load.
- When we open the switch, the magnetic field of the inductor will collapse, and the current will flow from the inductor through the diode. This current will add to the current that flows during step one, the OFF state. That is why the average current increases on this type of converter. This also makes up for the reduced voltage, therefore, preserves the power supplied to the load.
Sample Schematic of a Buck Converter
Here, I have used NI Multisim software to simulate a 12V to 5V DC-DC Buck converter. Here are the components that you need if you want to reproduce this simulation:
- DC_POWER – 12V
- DIODE – 1N3491
- Inductor – 100uH
- Capacitor – 50uF
- Load resistor – 4.7-ohm
- DC_POWER – 470mV (This voltage controls the duty-cycle of the PWM. 1V gives a duty cycle of 100%).
In this tutorial, we have talked about the two main types of DC-DC converters. You now know how boost and buck converters work in simple terms. Most importantly, I have shown you two circuits that simulate efficient power conversion.
There are a lot of ICs that you can use for power conversion. Linear Technology is an industry leader in this regard.