A DC-to-DC converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. It is a type of electric power converter. Power levels range from very low (small batteries) to very high (high-voltage power transmission).
History
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Before the development of power semiconductors, one way to convert the voltage of a DC supply to a higher voltage, for low-power applications, was to convert it to AC by using a vibrator, then by a step-up transformer, and finally a rectifier.[1][2] Where higher power was needed, a motor–generator unit was often used, in which an electric motor drove a generator that produced the desired voltage. (The motor and generator could be separate devices, or they could be combined into a single "dynamotor" unit with no external power shaft.) These relatively inefficient and expensive designs were used only when there was no alternative, as to power a car radio (which then used thermionic valves (tubes) that require much higher voltages than available from a 6 or 12 V car battery).[1] The introduction of power semiconductors and integrated circuits made it economically viable by use of techniques described below. For example, first is converting the DC power supply to high-frequency AC as an input of a transformer - it is small, light, and cheap due to the high frequency — that changes the voltage which gets rectified back to DC.[3] Although by 1976 transistor car radio receivers did not require high voltages, some amateur radio operators continued to use vibrator supplies and dynamotors for mobile transceivers requiring high voltages although transistorized power supplies were available.[4]
While it was possible to derive a lower voltage from a higher with a linear regulator or even a resistor, these methods dissipated the excess as heat; energy-efficient conversion became possible only with solid-state switch-mode circuits.
Uses
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DC-to-DC converters are used in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries primarily. Such electronic devices often contain several sub-circuits, each with its own voltage level requirement different from that supplied by the battery or an external supply (sometimes higher or lower than the supply voltage). Additionally, the battery voltage declines as its stored energy is drained. Switched DC to DC converters offer a method to increase voltage from a partially lowered battery voltage thereby saving space instead of using multiple batteries to accomplish the same thing.
Most DC-to-DC converter circuits also regulate the output voltage. Some exceptions include high-efficiency LED power sources, which are a kind of DC to DC converter that regulates the current through the LEDs, and simple charge pumps which double or triple the output voltage.
DC-to-DC converters which are designed to maximize the energy harvest for photovoltaic systems and for wind turbines are called power optimizers.
Transformers used for voltage conversion at mains frequencies of 50–60 Hz must be large and heavy for powers exceeding a few watts. This makes them expensive, and they are subject to energy losses in their windings and due to eddy currents in their cores. DC-to-DC techniques that use transformers or inductors work at much higher frequencies, requiring only much smaller, lighter, and cheaper wound components. Consequently these techniques are used even where a mains transformer could be used; for example, for domestic electronic appliances it is preferable to rectify mains voltage to DC, use switch-mode techniques to convert it to high-frequency AC at the desired voltage, then, usually, rectify to DC. The entire complex circuit is cheaper and more efficient than a simple mains transformer circuit of the same output. DC-to-DC converters are widely used for DC microgrid applications, in the context of different voltage levels.
Electronic conversion
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Comparison of non-isolated switching DC-to-DC converter topologies: buck, boost, buck-boost, and Ćuk. The input is on the left, the output with load (rectangle) is on the right. The switch is typically a MOSFET, IGBT, or BJT transistor.Switching converters or switched-mode DC-to-DC converters store the input energy temporarily and then release that energy to the output at a different voltage, which may be higher or lower. The storage may be in either magnetic field storage components (inductors, transformers) or electric field storage components (capacitors). This conversion method can increase or decrease voltage. Switching conversion is often more power-efficient (typical efficiency is 75% to 98%) than linear voltage regulation, which dissipates unwanted power as heat. Fast semiconductor device rise and fall times are required for efficiency; however, these fast transitions combine with layout parasitic effects to make circuit design challenging.[5] The higher efficiency of a switched-mode converter reduces the heatsinking needed, and increases battery endurance of portable equipment. Efficiency has improved since the late 1980s due to the use of power FETs, which are able to switch more efficiently with lower switching losses [de] at higher frequencies than power bipolar transistors, and use less complex drive circuitry. Another important improvement in DC-DC converters is replacing the flyback diode with synchronous rectification[6] using a power FET, whose "on resistance" is much lower, reducing switching losses. Before the wide availability of power semiconductors, low-power DC-to-DC synchronous converters consisted of an electro-mechanical vibrator followed by a voltage step-up transformer feeding a vacuum tube or semiconductor rectifier, or synchronous rectifier contacts on the vibrator.
Most DC-to-DC converters are designed to move power in only one direction, from dedicated input to output. However, all switching regulator topologies can be made bidirectional and able to move power in either direction by replacing all diodes with independently controlled active rectification. A bidirectional converter is useful, for example, in applications requiring regenerative braking of vehicles, where power is supplied to the wheels while driving, but supplied by the wheels when braking.
Although they require few components, switching converters are electronically complex. Like all high-frequency circuits, their components must be carefully specified and physically arranged to achieve stable operation and to keep switching noise (EMI / RFI) at acceptable levels.[7] Their cost is higher than linear regulators in voltage-dropping applications, but their cost has been decreasing with advances in chip design.
DC-to-DC converters are available as integrated circuits (ICs) requiring few additional components. Converters are also available as complete hybrid circuit modules, ready for use within an electronic assembly.
Linear regulators which are used to output a stable DC independent of input voltage and output load from a higher but less stable input by dissipating excess volt-amperes as heat, could be described literally as DC-to-DC converters, but this is not usual usage. (The same could be said of a simple voltage dropper resistor, whether or not stabilised by a following voltage regulator or Zener diode.)
There are also simple capacitive voltage doubler and Dickson multiplier circuits using diodes and capacitors to multiply a DC voltage by an integer value, typically delivering only a small current.
Magnetic
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In these DC-to-DC converters, energy is periodically stored within and released from a magnetic field in an inductor or a transformer, typically within a frequency range of 300 kHz to 10 MHz. By adjusting the duty cycle of the charging voltage (that is, the ratio of the on/off times), the amount of power transferred to a load can be more easily controlled, though this control can also be applied to the input current, the output current, or to maintain constant power. Transformer-based converters may provide isolation between input and output. In general, the term DC-to-DC converter refers to one of these switching converters. These circuits are the heart of a switched-mode power supply. Many topologies exist. This table shows the most common ones.
In addition, each topology may be:
Magnetic DC-to-DC converters may be operated in two modes, according to the current in its main magnetic component (inductor or transformer):
A converter may be designed to operate in continuous mode at high power, and in discontinuous mode at low power.
The half bridge and flyback topologies are similar in that energy stored in the magnetic core needs to be dissipated so that the core does not saturate. Power transmission in a flyback circuit is limited by the amount of energy that can be stored in the core, while forward circuits are usually limited by the I/V characteristics of the switches.
Although MOSFET switches can tolerate simultaneous full current and voltage (although thermal stress and electromigration can shorten the MTBF), bipolar switches generally can't so require the use of a snubber (or two).
High-current systems often use multiphase converters, also called interleaved converters.[9][10][11] Multiphase regulators can have better ripple and better response times than single-phase regulators.[12]
Many laptop and desktop motherboards include interleaved buck regulators, sometimes as a voltage regulator module.[13]
Bidirectional DC-to-DC converters
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Specific to these converters is that the energy flows in both directions of the converter. These converters are commonly used in various applications and they are connected between two levels of DC voltage, where energy is transferred from one level to another.[14]
Multiple isolated bidirectional DC-to-DC converters are also commonly used in cases where galvanic isolation is needed.[15]
Capacitive
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Switched capacitor converters rely on alternately connecting capacitors to the input and output in differing topologies. For example, a switched-capacitor reducing converter might charge two capacitors in series and then discharge them in parallel. This would produce the same output power (less that lost to efficiency of under 100%) at, ideally, half the input voltage and twice the current. Because they operate on discrete quantities of charge, these are also sometimes referred to as charge pump converters. They are typically used in applications requiring relatively small currents, as at higher currents the increased efficiency and smaller size of switch-mode converters makes them a better choice.[16] They are also used at extremely high voltages, as magnetics would break down at such voltages.
Electromechanical conversion
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A motor generator with separate motor and generator.A motor–generator set, mainly of historical interest, consists of an electric motor and generator coupled together. A dynamotor combines both functions into a single unit with coils for both the motor and the generator functions wound around a single rotor; both coils share the same outer field coils or magnets.[4] Typically the motor coils are driven from a commutator on one end of the shaft, when the generator coils output to another commutator on the other end of the shaft. The entire rotor and shaft assembly is smaller in size than a pair of machines, and may not have any exposed drive shafts.
Motor–generators can convert between any combination of DC and AC voltage and phase standards. Large motor–generator sets were widely used to convert industrial amounts of power while smaller units were used to convert battery power (6, 12 or 24 V DC) to a high DC voltage, which was required to operate vacuum tube (thermionic valve) equipment.
For lower-power requirements at voltages higher than supplied by a vehicle battery, vibrator or "buzzer" power supplies were used. The vibrator oscillated mechanically, with contacts that switched the polarity of the battery many times per second, effectively converting DC to square wave AC, which could then be fed to a transformer of the required output voltage(s).[1] It made a characteristic buzzing noise.
Electrochemical conversion
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A further means of DC to DC conversion in the kilowatts to megawatts range is presented by using redox flow batteries such as the vanadium redox battery.
Chaotic behavior
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DC-to-DC converters are subject to different types of chaotic dynamics such as bifurcation,[17] crisis, and intermittency.[18][19]
Terminology
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References
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DC-to-DC converters are devices that temporarily store electrical energy for the purpose of converting direct current (DC) from one voltage level to another. In automotive applications, they are an essential intermediary between systems of different voltage levels throughout the vehicle.
Control circuitry played the role of DC-to-DC converter in the traditional 12V electrical architecture that dominated the automotive industry starting in the 1950s. Over the decades, new features and innovations increased the complexity of vehicle electrical/electronic architectures, including the introduction of cruise control in the 1950s, emission control features in the ’70s and electrical centers in the ’90s. DC-to-DC converters enabled this growth by stepping down power from the 12V battery to lower-voltage electrical systems such as the instrumentation panel, entertainment system, LED lighting and sensors (which can require as little as 3.3V). These low-voltage DC-to-DC converters are still an essential part of the control circuitry in all vehicles today — both in internal combustion engine vehicles and battery electric vehicles (BEVs).
BEVs introduce a much greater level of electrical power, requiring a more robust DC-to-DC converter. Systems higher than 60V are considered high voltage; typical BEV batteries range from 400V to 800V. The voltage has to step down to, say, 48V to power an air conditioning unit, and down to 12V and lower to power numerous electronics throughout the vehicle. The voltage may also have to be stepped up — for example, if a battery running at 400V is connected to a charging station running at 800V.
The expansion of software-enabled features, including active safety, connectivity and infotainment, has only added to the complexity of the low-voltage architecture. BEVs must deliver enough power to drive the wheels of the car while also being able to step down current to run all of the low-voltage devices that make up the software-defined vehicle. And they need to be reliable enough to meet the functional safety demands of autonomous driving features and advanced driver-assistance systems.
DC-to-DC converters for high-voltage applications
High-voltage DC-to-DC converters are much larger and heavier than their low-voltage counterparts due to the extra shielding required to protect nearby components from the electromagnetic interference generated from increased current. Because EV designers are looking to reduce size and weight wherever possible to extend the vehicles’ range, they are turning to DC-to-DC converters with a higher power density, as measured in kilowatts of power per unit of volume.
To step 400V or 800V down to 12V requires a DC-to-DC converter with power ranging from 700W to 4kW — or even up to 12kW for a commercial vehicle.
The challenge is to optimize for space while maintaining the highest levels of safety and efficiency possible. While some automakers have retained a 12V battery in addition to the main 400V or 800V battery, emerging designs are achieving greater efficiencies by combining the larger battery with a more sophisticated DC-to-DC converter, thereby eliminating the weight, cost and maintenance of a separate 12V battery.
The software that runs the DC-to-DC converter is key to ensuring that conversion remains efficient, and knowledge of the entire vehicle architecture informs the software and hardware design. While high-voltage vehicle components are relatively new territory for the industry, Aptiv is building on decades of knowledge about what OEMs need to lead the way in vehicle electrification solutions. The feature-rich software-defined vehicles of tomorrow must be space- and energy-efficient enough to enable the functions that OEMs and consumers expect, like over-the-air updates, cybersecurity, autonomous driving, advanced safety and state-of-the-art user experiences.
Aptiv’s expertise with both the brain and the nervous system of the vehicle enables us to optimize electrical/electronic systems and packaging space while still fulfilling the performance, functional safety, and compute and power needs.