I DC-DC converter history|
|II DC-DC converter classification|
|III DC-DC converter application|
If you needed to convert a direct current of low power into higher voltage direct current before the power semiconductor component and power electronic technology came into being, you should first convert the low-power direct current into alternating current using an oscillating circuit, then use a step-up transformer to boost its voltage, and finally convert it into direct current using a rectifier. If a higher power DC voltage was needed to be converted, a motor should be used to drive the generator (sometimes the motor and the dynamo were integrated into a dynamotor module, of which one winding drove the motor and the other winding generated the output voltage.) These were relatively inefficient methods, and their costs were more expensive. But there were no better methods at the time, such as driving the early car audio (the working voltage of the thermionic tubes or vacuum tubes used in them was much higher than the 6V or 12V in the car.) With the advent of power semiconductors and integrated circuits, the cost of using some new circuits begun to drop and became relatively cheap, so it was affordable for general applications. These new circuits included converting direct current into high-frequency alternating current, then using a smaller, lower and cheaper transformer to convert the alternating voltage, and finally using a rectifier to convert it to direct current. In 1976, car radios began to use transistors that did not require high voltage. Power supply units that used transistors are available, but some amateur radio users still use oscillating circuits and dynamotor power supplies as a power source for radio transmitters that require high voltage.
It was possible to derive a lower voltage from a higher direct current voltage with a linear regulator or even a resistor. These methods dissipated the excess energy as heat, but it was inefficient. It was until the advent of solid-state switch-mode circuits that energy-efficient conversion became possible.
Electronic DC-DC converter
The actual application of electronic DC-DC converters will use switching technology. The DC-DC switching power supply can temporarily store energy, and then release it through the output voltage, which can convert the DC voltage into a higher or lower voltage direct current. Energy can be stored in electric fields (capacitors) or magnetic fields (inductors or transformers). This conversion method can boost or step down voltage, and the switching efficiency can reach 75% to 98%, which is better than the efficiency of a linear voltage regulator (which consumes unnecessary energy as heat). Considering the efficiency, the speed of turning on or off of the semiconductor components is quite fast, but because of the fast transients and the stray components in the circuit layout, the circuit design is more challenging. The high efficiency of the switching power supply reduces the size or volume of the heat sink and increases the operating time of the portable device when it is powered by a battery. In the late 1980s, due to the emergence of power-level FETs, it can have lower switching losses than power-level bipolar transistors at higher frequencies, so the efficiency can be further improved, and the drive circuit of the FET is easier. Another important breakthrough of the switching power supply is to use the synchronous rectification technology of the power-level FET to replace the flywheel diode, which has a lower conduction circuit and can also reduce the switching loss. Before power semiconductors were widely used, low-power DC-DC synchronous rectifiers included an electromechanical oscillator. The oscillated electricity was passed through a step-down transformer and output to a vacuum tube, a semiconductor rectifier, or a synchronous rectifier connected to the oscillator.
Most DC-DC converters are designed for unidirectional conversion, and power can only flow from the input side to the output side. However, the topology of all switching voltage converters can be changed to bidirectional conversion, allowing power to flow from the output side back to the input side by changing all diodes to independently controlled active rectification. Two-way converters can be used in applications such as vehicles that require regenerative braking. When the vehicle is running, the converter supplies power to the wheels, but when braking, the wheels will in turn supply power to the converter.
Switching converters are actually more complicated from an electronics point of view, but because many circuits are encapsulated in integrated circuits, they require fewer parts. In the circuit design, in order to reduce the switching noise (EMI / RFI) to the allowable range and to make the high-frequency circuit operate stably, careful design of the circuit and the layout of the actual circuit and components are required. In a step-down application, the cost of the switching converter is higher than that of the linear converter. However, with the advancement of chip design, the cost of the switching converter is gradually decreasing.
The DC-DC converter can be composed of an integrated circuit (IC) and several parts, and some converters themselves are complete integrated circuit modules, which only need to be assembled on a circuit board for use.
The linear voltage regulator can convert a stable DC voltage from a high voltage but possibly unstable DC voltage source and the power corresponding to the input and output voltage difference is converted into thermal energy to dissipate according to Joule's law. In terms of definition, they can be regarded as DC-DC converters and are rarely called linear voltage regulators in practice. The resistor divider circuit can also generate an output voltage that is different from the input voltage. A regulator or Zener diode may be added to adjust the output voltage, but it is rarely called a DC-DC converter.
Electromechanical DC-DC converter
The motor-generator set is a commonly used system in the past, which is composed of a set of coupled electric motors and generators.
Dynamotor goes a step further, putting the motor and generator in the same unit. The motor and generator windings will be wound on the same rotor, and the motor and generator coils share the same outer field winding or magnet. Generally speaking, the motor coil will be driven by the inverter on the motor shaft, and the generator coil will be output by the inverter on the other side of the shaft. The size of Dynamotor will be smaller than a set of motors and generators, and there will be no exposed rotating shaft.
The electromechanical converter can convert the voltage between AC of any voltage/frequency or DC of any voltage, and it can also perform AC-AC conversion or DC-DC conversion. Large motor-generator sets will be used to convert industrial-grade electricity, and small motor-generator sets can convert battery power (DC 6V, 12V, or 24V) into a higher DC voltage, which can drive vacuum tube equipment.
If some low-power automotive applications require a higher voltage than the voltage that the car battery can generate, a power supply with a mechanical vibrator will be used. There are contacts on the mechanical vibrator, which exchange the polarity of the battery connected to the power supply at a speed of several times a second. Equivalently, it converts direct current to alternating current of a square wave, which can then be sent to the transformer to generate the required voltage, but there will be noise from mechanical vibrators.
Electrochemical DC-DC converter
The DC-DC conversion of several kilowatts to one million watts can be carried out with flow batteries, such as all-vanadium redox flow batteries.
DC-DC converters are commonly used in mobile devices which are mainly powered by batteries such as mobile phones and notebook computers. There are often many sub-circuits in this type of electronic device, and the required supply voltage is also different from that provided by a battery or an external power supply. And when the battery's power decreases, its voltage also drops. The switching-type DC-DC converter can be used with a battery whose voltage has dropped so that the voltage of the supply circuit can be maintained within a certain range, so there is no need to use multiple batteries to achieve this purpose. Most DC-DC converters also stabilize the output voltage, but there are some exceptions.
DC-DC converters can also be used in conjunction with photovoltaic arrays or wind engines in order to collect the most energy. This type of equipment is called a power optimizer.
If the power of the transformers using mains supply with 50-60Hz exceeds a few watts, their volume will be very large and heavy, and the copper loss of the winding and the eddy current of the iron core will cause energy loss. DC-DC converters will design circuits so that transformers or inductors can work at higher frequencies, so the components are smaller, lighter, and cheaper. Even this kind of component will be used in some occasions where traditional mains supply frequency transformers were originally used. For example, household electrical equipment often first rectifies the mains supply into direct current, uses the technology of a switching power supply to convert it into a high-frequency alternating current of the required voltage, and finally rectifies it to a direct current of the corresponding voltage. The whole circuit is more complicated than traditional systems with transformers and rectifiers, but it is cheaper and more efficient.