When there were no semiconductors, a common technique for converting DC voltage into higher voltage for low-power projects was to transform it into AC voltage by means of vibrator circuits. Then, a step-up transformer was used to increase the output voltage level, followed by a rectifier circuit for performing the DC conversion. A combination of motor and generator was employed for applications where high power was required. The motor operates the generator, providing the required load voltage. These techniques were expensive and less efficient but used since no alternative method existed at that time.

The invention of power semiconductors and ICs led to different less expensive solutions, such as DC power supply, where high-frequency AC is input to a transformer. This device is less expensive and easy to handle. It varies the voltage that again converts into DC through rectifiers.

Operation of a DC-DC Converter

The switching technique is commonly used for the operation of converters. Switched-operated DC-DC converter transforms DC voltage from one level to another based on applications demands; the output voltage can thus be higher or lower than the input value. During conversion, the converter stores energy for some time and provides the output at different levels. The energy is stored in conductors’ transformers in form of a magnetic field and in capacitors in form of an electrical field. Switching voltage conversion is easier to process and more energy efficient than linear voltage conversion, where power is dissipated in heat. High-speed semiconductor devices with fast rise and fall times are needed to achieve efficiency. High-speed switching requires an accurate layout and makes circuit creation difficult. The high efficiency of the switched-mode converter reduces the need for heat sinks and increases the battery operating life. Using FETs increases the converter efficiency since these devices perform the switching process efficiently with fewer losses than BJTs.

Other improvements in the DC-DC converter include the replacement of the flyback diode with synchronous rectifier circuitry with power FET that has less resistance and decreases the losses during switching.

Most DC-DC converters are designed to direct the power flow in one direction, from the input to the output. However, all switching regulator circuits can be bidirectional and transfer power in any direction by replacing all diode-controlled active rectifier circuits. Bidirectional converters are good for applications like the regenerative braking of vehicles, where power is given to the wheel of vehicles during driving and received during braking.

Like in all high-frequency circuits, some measures must be taken to create DC-DC converters, like components selection, connections, and the switching noise that must be set at an acceptable level. Initially, the creation cost was higher than linear regulators, but then it decreased due to using the latest technique for circuit creation.

DC-DC converters are today based on ICs and require a few additional components. They also exist in a hybrid configuration used in several electronic structures.

Figure 1 shows the different types of DC-DC converters: buck, boost, buck-boost, and Ćuk circuits. The input of these circuits lies on the left side, while the output is provided to the load on the right. The power switches can be MOSFETs, IGBTs, or BJTs.

DC-DC converter characteristics

The main characteristics of the converter are described here.

Current rating

It is the highest value of current that the converter can provide to the load.

Temperature rating

It is the highest temperature value that the DC-DC converter must withstand during full-load operation. The operating temperature must be set according to the ratings, otherwise, the converter can be damaged and overheated.

Ripple voltage

This value defines the ripple voltage on the output of the converter.  Must be careful about these ratings during operations.

Voltage rating

DC-DC converter comes with some limits that define the voltage transformation ability of the converter.

Converter size and weight

Since switching converters can work at higher frequencies, they can be designed with a small footprint. During the design of the converter, a compromise between the size of the circuitry and its efficiency shall be found.

DC-DC Converters for Electric Vehicles

Many different types of vehicles moving around us generate air pollution, global warming, and depletion of the earth’s resources. To solve these issues, a solution is to replace conventional vehicles with EVs, Hybrid vehicles, and fuel-cell electric vehicles. Electric and hybrid vehicles use two types of energy storage devices. The first one is MES (Multi-Function Energy Storage), used to store high energy, while the second one is a rechargeable energy storage system (RESS) which provides high power capability ad reversibility. MES provides a high driving range and RESS has fine regenerative braking and acceleration system. Energy storage device changes output voltage based on load and the high voltage of the DC link creates problems for vehicle engineers during the configuration of energy storage components with traction drive. DC-DC converters can be used for interfacing components in electrical powertrains.

Automobile converters must be light, small, reliable, effective, and produce low electromagnetic interference.

Types of DC-DC converters for electric vehicle

In Figure 2, we can see the different types of converters used in electric vehicles. It shows how at least one DC-DC converter interfaces the DC link with the fuel cell battery or supercapacitors module. Electric vehicles have different energy sources, like fuel cells and battery supercapacitors. In EVs, one or more energy storage devices are used.  That reduces the overall cost and volume and provides good operation. Commonly used energy storages are batteries and supercapacitors. These devices are configured with fuel cell stacks in different configurations. The normal configuration is a direct connection of two modules in parallel combination: fuel cell or battery, fuel cell or supercapacitors, or battery or supercapacitors. In this method, power is used not in control but can be measured through component impedance value. Impedance depends on different parameters such as efficiency and device health condition. The voltage parameters also match the two components’ specifications. This is the case of a fuel cell/battery combination, where the fuel cell must have the same power during the complete time due to the fixed voltage of the battery. In the case of a battery or supercapacitors combination, only very small energy exchange features of capacitors can be used. This is all about the constant value of battery voltage. The DC-DC converter output voltage can be selected and the power of every device can be regulated.

DC-DC converter is a power conversion circuit that transforms energy at different levels and stores energy in magnetic and electrical fields. Here, different types of DC-DC converters applied to EVs will be discussed.

Non-isolated converters

The non-isolated converters are used for applications where voltage is increased or decreased in a ratio of 4:1. With that, the output and input have no dielectric isolation. Non-isolated converters also have sub-types: charge pump, buck, cuk, buck-boost, buck, and boost. Buck is used for voltage decrease and boost for voltage increase, or step-up. Buck-boost and Cuk converters are used for voltage increase and decrease. The charge pump converter is used for voltage step-up and voltage inversion and is part of some projects.

Magnetic Converters

DC-DC converters store energy periodically and release it as a magnetic field in inductors or transformers with a frequency value of three hundred kilohertz to ten-kilo megahertz. Through stability, the duty cycle of charging the amount of power required to transmit the load can be controlled.

With that control can be used across the input and output current and for maintaining the constant power through circuitry. A converter having a transformer can provide the operation between input and output.

Isolated converters

In these converters, the high-frequency transformer is used. For applications where output is completely separated from input, this converter is used. There are different types of isolated converters such as half-bridge, full-bridge, fly black forward, and push-pull DC-DC converters. These converters can be used as a bi-directional converter.

Properties of EV DC-DC converter

For interfacing, the fuel cell DC-DC converters boost the fuel cell voltage and regulate the DC link voltage. Though reversible dc to dc converter is required for interfacing the super capacitors. There are different types of DC-DC converters with certain features designed and applied. But in the case of vehicles, there are some consideration are follows that are listed here

• The efficiency of the converter used for the vehicle must be high
• The converter must have a small size and less weight
• There is less electromagnetic interference
• There is less ripple drawn from the fuel cell
• The function must provide the voltage rise or step-up function
• The regulation of the converter flow of power provides the different voltage changes in the converter input

Every converter type has its benefits and disadvantages. Such as DC-DC boost converters not following electrical isolation.  High changes in the magnitude of input and output cause stress on the switch and this circuitry effects by high current and voltage ripples and also faces high weight issues.

The simple multichannel converter helps to decrease input and output current and voltage ripples that increase the weight of the circuitry and enhance efficiency.  This circuit is unsuitable for a high voltage step-up ratio as the duty cycle is restricted through the impedance of the circuitry causing the highest step-up ratio of about four. So, two series configured step-up converters are needed to get a certain voltage gain for the project. The full bridge DC-DC converter is used in applications requiring electrical isolation. The full bridge DC-DC converter is good for high-power transmission since voltage and current values are low. It faces the small voltage and current ripples at the input and output.

Advantages of DC-DC converter

• The use of converters simplifies the circuitry of the power supply
• The use of converters deceased the space of battery in circuits and projects
• It provides organized output in both negative and positive polarities
• DC controls circuits use this device and voltage regulators use this module
• It comes in hybrid circuits that have all components of circuits in a single structure or casing
• It is helpful for the extension of voltage when needed
• The main benefit of this device is that it provides isolation between the input and output sides of circuits.

Disadvantages of DC-DC converter

• These converters cause noise during the operation
• It has a high ripple current and high loess and a high value of input and output capacitance
• The circuitry of converters is expensive
• These circuits provide unsteady voltage and current supply.

Conclusion

DC-DC converter is an electronic device that is helpful for DC voltage conversion to other levels, higher or lower, according to circuits and project requirements. It is a basic part of power supplies that changes the voltage value to operate different applications that need either low or high DC voltage. It is the main part of electric vehicles, which consist of batteries that provide a high DC voltage. Since EV components operate on different voltage levels (high or low), to maintain the voltage value for these components DC-DC converters are used. DC-DC converters have become a crucial part of electrical vehicles and other electronic projects that operate at different DC levels.