What is a DC-DC converter ?

A DC-DC converter is an electrical system (device) which converts direct current (DC) sources from one voltage level to another. In other words, a DC-DC converter takes as input a DC input voltage and outputs a different DC voltage. The output DC voltage can be higher or lower than the DC input voltage. As the name implies, a DC-DC converter only works with direct current (DC) sources and not with alternative current (AC) sources.

A DC-DC converter is also called a DC-DC power converter or voltage regulator.

If we have two electrical systems, operating at different voltage levels, one high level (140 V) and another one low level (14 V), a DC-DC converter can convert the voltage between them, from high to low or from low to high. The conversion from one voltage level to another voltage lever is done with some power losses. Depending on the operating point of the DC-DC converter (voltage and current) and the type of converter, the efficiency can be between 75 % to 95 % or more.

The DC-DC converter in a Battery Electric Vehicle (BEV) is used to convert the high battery voltage (e.g. 400 V) to low direct current voltage (e.g. 12 V),  for conventional 12 V loads (lights, multimedia, power windows, etc.).

A DC-DC converter is a power converter that converts a source of direct current (DC) from one voltage level to another, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. The storage of the electric energy may be done in either magnetic field storage components (inductors, transformers) or electric field storage components (capacitors).

Efficiency of DC-DC converters

The electric power P [W] is the product between voltage U [V] and electrical current I [A].

\[P = U \cdot I \tag{1}\]

If, for example, the input voltage U_in = 120 V and the maximum current I_in = 5 A, this will give a input power of:

\[ P_{in} = 120 \cdot 5 = 600 \text{ W} \]

Since the electrical power is conserved (P_out = P_in) and we assume that the DC-DC converter has no losses (100 % efficiency), for an output voltage U_out = 14 V, we can calculate the output current as:

\[ I_{out} = \frac{P_{out}}{U_{out}} = \frac{600}{14} = 42.86  \text{ A} \]

In reality there will be some losses associated with the conversion and the maximum output current will be less than the one calculated for 100 % efficiency.

The efficiency of the DC-DC converter is calculated as:

\[\eta \text{ [%]} = \frac{P_{out}}{P_{in}} \cdot 100 \]

There are several types of DC-DC converters. The most common classification is from the ratio between the input and output voltage point of view:

• boost DC-DC converters
• buck DC-DC converters

In a boost DC-DC converter, the output voltage is higher than the input voltage. Due to power conservation (if we ignore the losses), the output current will be lower than the input current.

For this example, the efficiency of the boost DC-DC converter is:

\[\eta = \frac{780}{840} \cdot 100 = 92.86 \text{ [%]}\]

In a buck DC-DC converter, the output voltage is lower than the input voltage. Due to power conservation (if we ignore the losses), the output current will be higher than the input current.

For this example, the efficiency of the buck DC-DC converter is:

\[\eta = \frac{770}{840} \cdot 100 = 91.67 \text{ [%]}\]

Classification of DC-DC converters

There are multiple types of DC-DC converters. The simplest form of DC-DC converter is the linear converter, also called linear voltage regulator.

A linear voltage regulator can only work as a buck DC-DC converter, which means that will only lower a higher voltage level. Being a regulator, it also ensures that the output voltage is maintained at a specific value, even if the output load is variable.

A more efficient type of DC-DC converters is the switching DC-DC converter. There are several topologies of switching DC-DC converters, the most common being presented in the image below.

Prior to the switching DC-DC converters, linear converters were commonly used. The linear voltage regulator (DC-DC converter) comes in two main topologies: the shunt voltage regulator and the series voltage regulator. In this type of voltage regulators, transistors are operated in the active region as dependent current sources with relatively high voltage drops at high currents, dissipating a large amount of power. Due to high power dissipation, the efficiency of a linear voltage regulator is usually low. Linear regulators tend to be heavy and large, but have the advantage of low noise level and are suitable for audio applications.

Image: Simple shunt voltage regulator

Image: Simple series voltage regulator

The simple shunt voltage regulator, simply called shunt regulator, is a type of voltage regulator where the regulating component shunts the current to ground. The shunt regulator works by keeping a constant voltage across its terminals and it takes up the extra current to maintain the voltage across the electric load. One of the most common element of the shunt regulator contains the simple Zener diode circuit where the Zener diode has the role of shunt element.

The simple series voltage regulator, also called the series pass regulator, is the most common approach for providing the final voltage regulation in a linear regulated power supply. The series linear regulator is characterised by a high level of performance for the output voltage in terms of low ripple and noise.

The linear DC-DC converter only converts higher voltages to lower voltages. In terms of power dissipation let’s look at an example. If the input voltage is 42 V, output voltage 12 V and output current 5 A, the dissipated power P [W] is going to be calculated as:

\[P = I_{out} \cdot (V_{in} – V_{out}) = 150 W\]

All that dissipated power is going to be converted into heat. Without suitable cooling the linear DC-DC converter can overheat and destroy itself. For this reason, linear DC-DC converters are usually used for low power applications.

In switching DC-DC converters, transistors are operated as switches, which means that they dissipate much less power than transistors operated as dependent current sources. The voltage drop across the transistors is very low when they conduct high current and the transistors conduct a nearly zero current when the voltage drop across them is high. Therefore, the conduction losses are low and the efficiency of switching-mode converters is high, usually above 80% or 90%. However, switching losses reduce the efficiency at high frequencies, the higher the switching frequency, the higher the power losses.

Switch type DC-DC converters have better efficiency compared with linear converters because they are not continuously dissipating power.

 

Buck DC-DC converter, also called step-down DC-DC converter, is a DC-DC power converter which lowers the output voltage, while it’s increasing the output current. It consists of at least four components:

• a power transistor used as a switching element (S)
• a rectifying diode (D)
• an inductor (L) as energy storage element
• a filter capacitor (C)

The relationships between input and output voltage, current and power are as follows:

• U_out < U_in
• I_out > I_in
• P_out = P_in – P_loss

In electric vehicle applications, buck DC-DC converters are used to lower the high voltage of the main battery (e.g. 400 V) to lower values (12-14 V) required by the auxiliary systems of the vehicle (multimedia, navigation, radio, lightning, sensors, etc.).

Boost DC-DC converter, also called step-up DC-DC converter, is a DC-DC power converter which increases the output voltage, while it’s decreasing the output current. It contains the same components as a buck DC-DC converter but arranged in a different topology.

The relationships between input and output voltage, current and power are as follows:

• U_out > U_in
• I_out < I_in
• P_out = P_in – P_loss

In some Hybrid Electric Vehicle (HEV) applications, boost DC-DC converters are used to step up the voltage from the battery from 202 V to 500 V. The voltage of the battery in a hybrid electric vehicle (HEV) application is limited by the number of battery cells in series. Due to limited space, the batteries are limited in number of cells in series therefore to output voltage is limited as well. Using boost DC-DC converters, the battery voltage can be increased to the higher voltage required by the electric machine.

In Buck DC-DC converters the output voltage is always less than the input voltage. On the other hand, in DC-DC Boost converters, the output voltage is always greater than the input voltage. A Buck-Boost DC-DC converter combines the two and can have its output voltage both higher and lower compared to the input voltage, depending on the duty ratio applied to the switch.

The inverting topology buck-boost DC-DC converter outputs a voltage with opposite polarity compared to the input voltage. The output voltage is adjusted function of the duty cycle of the switching element (transistor).

The Ćuk DC-DC converter is another type of buck-boost converter which outputs a zero-ripple current. Ćuk converter can be seen as a combination of boost converter and buck converter, having one switching device and a mutual capacitor, to couple the energy. Similar to the buck-boost converter with inverting topology, the output voltage of non-isolated Ćuk converter is typically inverted, with lower or higher values with respect to the input voltage. Usually in DC-DC converters, the inductor is used as a main energy-storage component, while in Ćuk converter the main energy-storage component is the capacitor [8].

The Single-Ended Primary-Inductor Converter (SEPIC) DC-DC converter allows the electrical potential (voltage) at its output (U_out) to be greater or less than the input voltage (U_in). The output of the SEPIC DC-DC converter is controlled by the duty cycle of the control switch (S).

A SEPIC consists of a boost converter followed by an inverted buck-boost converter, therefore it is similar to a traditional buck-boost converter, but has advantages of having non-inverted output (the output has the same voltage polarity as the input), using a series capacitor to couple energy from the input to the output (and thus can respond more gracefully to a short-circuit output), and being capable of true shutdown: when the switch S is turned off enough, the output (U_out) drops to 0 V, following a fairly hefty transient dump of charge [9].

Similar to the SEPIC DC/DC converter topology, the Zeta DC-DC converter topology provides a positive output voltage from an input voltage that varies above and below the output voltage. The Zeta converter also needs two inductors and a series capacitor, sometimes called a flying capacitor. Unlike the SEPIC converter, which is configured with a standard boost converter, the Zeta converter is configured from a buck controller that drives a high-side PMOS FET. The Zeta converter is another option for regulating an unregulated input-power supply [10].

In a DC-DC converter the switching devices (S) have to open and close an electrical circuit. Hence, they have two roles: as an electrical conductor to close the circuit, as well as an electrical insulator to break/open the circuit. This dual function defines what a semiconductor is: a device which is able to conduct current in an efficient way, as well as to block it.

Semiconductors are rated in terms of the maximum voltage they can handle and still behave as an insulator, and the maximum current that can circulate through them without damaging the device. Maximum allowed current does not only depend on the module rating but also on the thermal properties of the semiconductor. Thus, according to the power module packaging, as well as the used heat sink, maximum allowed current can vary for the same device.

For automotive applications, a DC-DC converter must meet several design requirements, like:

• light weight
• high efficiency
• small volume
• reject electromagnetic interference
• low output current ripple

In the following articles we are going to discuss about the modes of operation of DC-DC converters, derive the their mathematical models and perform simulations using Scilab/Xcos.

References:

[1] Ali Emadi, Advanced Electric Drive Vehicles, CRC Press Taylor & Francis Group, 2015.
[2] Seref Soylu, Electric Vehicles Modelling and Simulations, IntechOpen, 2011.
[3] Branko L. Dokić, Branko Blanuša, Power Electronics Converters and Regulators, 3rd Edition, Springer, 2015.
[4] Marian K. Kazimierczuk, Pulse-Width Modulated DC-DC Power Converters, 2nd Edition, Wiley, 2016.
[5] Narayanaswamy P. R. Iyer, Power Electronic Converters – Interactive Modelling Using Simulink, CRC Press, 2018.
[6] Seddik Bacha, et al, Power Electronic Converters Modeling and Control with Case Studies, Springer, 2014.
[7] Erik Schaltz, Electrical Vehicle Design and Modeling, IntechOpen, 2011.
[8] https://en.wikipedia.org/wiki/%C4%86uk_converter
[9] https://en.wikipedia.org/wiki/Single-ended_primary-inductor_converter
[10] Jeff Falin, Designing DC/DC converters based on ZETA topology, Texas Instruments, 2010.