Converting from 24 V to 12 V? Understanding DC-DC Converters

table of contents

• ・Types of Power Supply ICs
• ・About DC-DC Converters
• ・Types of DC-DC Converters
• ・The Mechanism of Voltage Conversion by a DC-DC Converter
• ・Understanding Differences in the Systems and Methods of DC-DC Converters

In large trucks, 24 V is used, and someone wanting to use car navigation devices, audio equipment, and other products used in ordinary automobiles with 12 V power supplies, in such trucks is not able to, because of the voltage difference. A power supply enabling use of voltages suited to automotive products is needed. However, a DC-DC converter that converts 24 V to 12 V makes use of such automotive products and devices possible.

DC-DC converters are devices that convert between different voltages. In this article, the inner workings of DC-DC converters and the principles of operation when converting voltages are explained.

Types of Power Supply ICs

Power supplies are necessary for the operation of computers, game consoles, automobiles, and other electrical products. In ordinary households, 100 V AC (alternating current) power supplies can be used, but all electrical products do not operate using that 100 V AC power as-is. Many electrical products use various electrical circuits and integrated circuits, and appropriate electric power amounts are distributed to these circuits and used.

For example, the devices in a notebook computer, ranging from those that can be seen directly such as the display, mouse, and keyboard, to those operating inside such as the CPU and memory components, are all operating thanks to power supplied from an AC adapter or a battery. In actuality, in order to supply all the devices operating inside the computer with appropriate power, the 100 V AC supplied from a power outlet is allocated in power amounts appropriate to each of the electronic components.

An electronic component that adjusts the power supplied from a power source to an appropriate voltage is called a “power supply IC”. Power supply ICs include the following types of devices.

• ・Linear regulators and DC-DC converters, which supply stable voltages and power amounts
• ・Reset ICs, which monitor power supplied from a power source to ensure that it is appropriate
• ・Switching ICs, which switch the supply of power on and off

Many power ICs of these kinds are incorporated into the electronic circuits inside of electrical products, and are designed to supply appropriate voltages to the respective devices.

About DC-DC Converters

Following is an explanation of the actual workings of DC-DC converters, which supply stable voltages and power amounts.

DC-DC Converters are Devices that Convert Between Various Voltages

A DC-DC converter is a device that converts from DC (direct current) to DC, and more specifically, converts to another voltage. As was mentioned at the beginning, when the power supplied in a truck is at 24 V, this device is used to convert the voltage to 12 V.

In addition to lowering the voltage, a DC-DC converter can also convert the original voltage into various other voltages. A device that generates a voltage lower than the original voltage is called a “buck (step-down) converter”, and one that creates a voltage higher than the original voltage is a “boost converter”. There are various other types of converters as well, such as negative voltage converters which create negative voltages, and inverting converters.

Why are DC-DC Converters Needed?

Integrated circuits (ICs) and other electronic circuits that operate within electrical products have different voltage ranges and precisions within which they can operate. Hence if the appropriate power supply is not provided, malfunctions, breakdowns, and other problems may result.

Power sources for homes are 100 V AC (alternating current), while electrical products often use DC (direct current) power at various voltages, and so devices to perform voltage conversion are necessary in order to be able to accommodate many electrical products. In addition, in electrical products that can use batteries for operation apart from power supplied from an AC adapter, often the voltages of the power from the AC adapter and the power from the battery are different. Hence some means of providing stable electrical power from both of these power sources is required.

For example, the refrigerators, microwave ovens, vacuum cleaners, and other appliances that are used in homes get their power from electrical outlets, but the voltages these devices require are different. Hence refrigerators, microwave ovens, vacuum cleaners, and so on have internal AC-DC converters and DC-DC converters.

With this as background, we can say that AC-DC converters which convert electricity from AC to DC, and DC-DC converters which convert to the required voltage, are important devices for the operation of electrical products.

It should be noted that electrical products which are meant to generate heat such as heaters may operate on 100 V AC without modification. In such cases, after conversion by an AC-DC converter, if the voltage required is 100 V, a DC-DC converter may not be incorporated.

Types of DC-DC Converters

There are two types of DC-DC converters, differing in the methods they use to convert voltages. These are linear regulators and switching regulators. Their respective features are explained below.

Linear Regulators

Linear regulators have three terminals, which are an input terminal (V_IN), an output terminal (V_O), and ground (GND). They are also called three-terminal regulators.

Linear regulators are configured with a control element and a control circuit connected in series between the input and the output; the simplicity of the circuit is one feature. The control circuit monitors the output voltage, and when an input voltage is output, the control element adjusts a resistance value such that the voltage becomes a constant preset value. By this means, a voltage lower than the input voltage is created.

Because of its design, in which a resistance value is adjusted to change the voltage, a linear regulator can only change the voltage to a lower voltage. Moreover, some of the power which has been supplied is dissipated as heat.

Suppose that a linear regulator has an input voltage of 24 V, a load current of 1 A, and an input power is 24 W, and creates an output voltage of 12 V. In this case, the current flowing into the input and the output current are almost the same, so the difference in power between input and output, 12 W, is dissipated as heat inside the IC.

Because of this, linear regulators have the disadvantage of poor conversion efficiencies that are roughly 30% to 50% or so, and at very best are only about 70%.

Moreover, the greater the step-down ratio of the input voltage to the output voltage, the worse the efficiency, and in addition the more it is necessary to address the heat generated. Measures to deal with the heat include allowing heat to leave the housing and installing heat sinks to dissipate heat. Considering the conversion efficiencies and heat generation, we can say that linear regulators are suitable for power supplies with lower power levels.

Switching Regulators

In a switching regulator, a method of adjusting the voltage by using a switching IC to rapidly switch current on and off is employed. A linear regulator can only create a voltage lower than the input voltage (can only perform voltage step-down). In contrast, a switching regulator can create various voltages–can step voltages up or down, invert voltages, and the like.

In addition to an input terminal and an output terminal, a switching regulator uses a coil, a capacitor, a switching IC, and other components; an arbitrary voltage can be generated by changing the circuit. Hence there are diverse types of switching regulators: step-up and step-down models, inverting types, and so on.

In a switching regulator, a switching element (switching IC) is used instead of a control element; external devices used are an inductor (coil) and a capacitor. The switch is turned on and power is supplied, and after energy is accumulated in the inductor, the switch is turned off and the energy released by the inductor is accumulated in the capacitor. By repeating the switch on/off operation, the output voltage is adjusted.

In contrast with linear regulators and their heat generation, conversion using switching by a switching element achieves a high conversion efficiency of around 90%, and in addition to the impressive efficiency, there is the further feature of minimal heat generation.

However, because noise occurs due to the switching operation, a design that deals with noise is needed. Moreover, apart from the switching element, external components such as coils and capacitors are also necessary, and so there is the disadvantage that, when noise countermeasures are included, the design tends to become complex. However, among recent switching ICs there are products that incorporate necessary components such as coils and capacitors, simplifying circuit design.

Because switching regulators that use circuits with little heat generation can be created, they are well-suited to the power supplies of digital circuits which require low voltages and large currents.

Examples of Applications of DC-DC Converters

Examples of applications of DC-DC converters are introduced. As an actual power supply intended for a major appliance, a DC-DC converter that uses a 24 V or 15 V power supply to generate 5 V and 3.3 V is employed to operate devices such as sensors and monitors. In the following diagram, buck DC-DC converters (switching regulators) are used.

Because a linear regulator can output a voltage with minimal noise, these devices are well-suited to power supplies for components that are sensitive to noise such as sensors. However, if the input/output voltage difference is large, heat dissipation problems occur, and measures to deal with heat become necessary. In other words, one can say that optimal applications are those in which the load is susceptible to noise and power consumption is low.

Switching regulators generate less heat than do linear regulators even when the input/output voltage difference is large, and have the further advantage of high conversion efficiency. In addition to voltage step-down, they are also capable of other kinds of conversion such as boost and inversion.

On the other hand, the higher noise levels are a drawback. Measures to address noise must be considered.

Differences Between Linear Regulators and Switching Regulators

Up till now, we have explained the two types of DC-DC converters, that is, linear regulators and switching regulators. The differences between the two are summarized below.

Switching regulators are capable of various types of conversions, and efficiencies are high; consequently in some cases the term “DC-DC converter” is used to refer to switching regulators. However, due to the need for noise measures, the higher cost, and other disadvantages of switching regulators, there are also cases in which linear regulators are used.

Because linear regulators and switching regulators each have their advantages and disadvantages, they should be selected according to the purpose.

The Mechanism of Voltage Conversion by a DC-DC Converter

From this point, we explain DC-DC converters, and in particular the mechanism of voltage conversion by a switching regulator. Switching regulators are capable of various kinds of conversion–boost, buck, inverting, and so on–but the conversion mechanism is different in each case.

Buck (Step-Down) Converter

Voltage step-down (buck) can also be performed by a linear regulator, but here step-down is explained using a basic circuit for voltage step-down by a switching regulator.

In this circuit configuration, time division of the input voltage V_IN is performed by the switches S₁ and S₂, and step-down is achieved by converting the voltage to the required voltage using the inductor (coil) L₁ and capacitor C₁.

When S₁ is on and S₂ is off, a current flows from the input V_IN, and when S₁ is off and S₂ is on, a zero-volt state is entered. The pulse waveform that is output by switching S₁ and S₂ rapidly is smoothed by the inductor and the capacitor.

The output voltage is determined by the ratio of the time (T_ON) during which S₁ is on and S₂ is off to the time (T_OFF) during which S₁ is off and S₂ is on. For example, if V_IN is 5 V and T_ON and T_OFF are the same (ratio 1:1, 50%), then 2.5 V is generated. To increase the output voltage, the time ratio is increased.

Expressed as an equation, we obtain the following.

\(V_{OUT} = V_{IN} \times \displaystyle \frac{T_{ON}}{T_{ON} + T_{OFF}}\)

Boost (Step-Up) Converter

“Boost” means outputting a voltage higher than the input voltage; it can be performed only by switching regulators. The same mechanism is used for step-up as for step-down, but the circuit configuration is different: the inductor is placed on the input voltage side, and one switch is placed after the inductor. When the switch is turned on, power is accumulated in the inductor, and when it is turned off, the high voltage due to the inductor properties (the action causing current to flow due to a voltage higher than the voltage originally input) is used to cause a higher voltage to appear in the converter.

From this setup, by repeating switch on/off operations, a higher voltage than the input voltage can be output.

Let’s consider the amount of current output. Suppose that the input voltage is V_IN, the time during which S₁ is turned on is T_ON, and the inductor value is L. The longer T_ON is, the greater the increase in the current that is accumulated in L.

\(I_{ON} = \displaystyle \frac{1}{L} \times V_{IN} \times T_{ON}\)

When the switch is off, the energy accumulated in the inductor is released, and so the inductor current declines. The reduced current amount is as follows.

\( I_{OFF} = \displaystyle \frac{1}{L} \times (V_{OUT} – V_{IN}) \times T_{ON} \)

The voltage rises accordingly as the current increase while the switch is on and the current decrease while the switch is off are the same. Calculating V_OUT when I_ON and I_OFF are the same gives the following.

\( V_{OUT} = V_{IN} \times \displaystyle \frac{T_{ON} + T_{OFF}}{T_{OFF}} \)

The increase in voltage means that a voltage higher than the input voltage is obtained, but when the output voltage rises, the output current falls. The power is calculated as voltage (V) × current (I). Because the input power and the output power are the same, if the voltage increases, the current decreases.

Hence it must be noted that the larger the step-up ratio (the ratio of the boosted output voltage to the input voltage), the lower is the maximum output current.

Inverting Converter

An inverting converter can invert the polarity of a power supply voltage.

When S₁ is turned on, current flows from the input V_IN through the inductor to GND, current flows in the coil, and energy is accumulated.

After switching S₁ off and S₂ on, current tries to continue flowing in the inductor. Due to energy released from the inductor, the output capacitor is discharged and the output voltage falls. By alternating between these operations, Step 1 and Step 2, the output voltage V_OUT falls below 0 V.

If the interval while S₁ is on is T_ON, the inductor value is L, and the input voltage is V_IN, then the current increase amount is

\( I_{ON} = \displaystyle \frac{1}{L} \times V_{IN} \times T_{ON} \)

If the interval while S₂ is on is T_OFF, and the voltage applied across the inductor ends is |V_OUT|, then the current decrease amount is

\( I_{OFF} = \displaystyle \frac{1}{L} \times |V_{OUT}| \times T_{ON} \)

When the amounts of current increase and decrease in the inductor have become equal, the decline in output voltage stops, and so

\( |V_{OUT}| = \displaystyle \frac{T_{ON}}{T_{OFF}} \times V_{IN} \)

In this way, in addition to operating the switches in a switching regulator, by using the inductor and capacitor to perform smoothing, a constant adjusted voltage can be output.

DC-DC Converter Control

Among DC-DC converters, in addition to differences in functions such as voltage boost and buck (step-down) and inverting, there are also differences in operation modes that control output, feedback control methods to stabilize output, and so on.

There are two operation modes for controlling output voltage: PWM (Pulse Width Modulation) and PFM (Pulse Frequency Modulation).

In the buck, boost, and inverting operations explained above, a method is used in which a constant voltage is generated by controlling the ratios of switch on and off times. This is a control method called PWM (Pulse Width Modulation). The switching period is constant, and voltage is regulated by adjusting the ratio of the on and off times within a period.

There is also a control method in which the on and off times within a period are constant, and the switching period is adjusted to regulate the voltage. This method is called PFM (Pulse Frequency Modulation).

There are three feedback control methods to stabilize the output: current mode, voltage mode, and hysteresis control.

Voltage mode is the most basic method: only the output voltage is used for feedback. After the output voltage and a reference voltage are compared by an error amplifier, the difference is compared to a triangular wave, and the PWM signal pulse width is determined. This control method is relatively simple, with comparatively good noise tolerance.

Current mode differs from voltage mode, which control the output voltage, in being a method of controlling the current required for a preset output voltage. Instead of the triangular wave of voltage mode, the circuit’s inductor current is fed back. This method has the advantage of a greatly simplified design for the phase compensation circuit, but susceptibility to noise is a drawback.

The hysteresis control method uses a comparator rather than an error amplifier. The comparator is used to compare the output voltage with a reference voltage in order to control the timing of switch turn-on/off. Compared with an error amplifier, there is the advantage of much faster response to changes in the load. However, the fact that the switching frequency changes, and the need for an output capacitor with a high ESR (equivalent series resistance), are disadvantages.

Understanding Differences in the Systems and Methods of DC-DC Converters

In this article we have explained the systems and operating principles of DC-DC converters that adjust voltages. DC-DC converters are grouped into linear regulators and switching regulators according to differences in the methods used, and each type has its advantages and drawbacks. It is important to understand the features of each and to understand where they may be used appropriately.

Linear regulators use control elements to perform voltage step-down, but require measures to deal with heat. In contrast, switching regulators use switching elements, inductors, and capacitors to perform various conversions–voltage buck (step-down) and boost, inverting, and so on. However, because switching operations cause noise, it should be remembered that measures to deal with noise are necessary.

There are many, many types of DC-DC converters. Hence persons wishing to delve further into their operation should perform searches for related information.