A DC-DC converter refers to an electrical energy device that converts one voltage value into another voltage value in a DC circuit. New energy vehicle DC-DC converters are divided into three types: boost, buck and stabilizing. The buck type is installed in almost every new energy vehicle. Its main function is to convert the high voltage of the power battery into low voltage to charge the battery and supply power to the low-voltage electrical equipment of the entire vehicle.
Concept, principle and classification of DC-DC converter
A DC-to-DC converter, also known as a DC-DC converter, is a circuit or electromechanical device for power conversion that can convert a DC power supply into a DC (or near-DC) power supply of different voltages. Its power range can range from very small (small batteries) to very large (high-voltage power conversion). The output voltage of some DC-DC converters has the same reference point as the input voltage, while the output voltage of some DC-DC converters is isolated from the input voltage. According to the type and purpose of power conversion, AC and DC conversion circuits can be divided into five conversion forms: 1. DC to DC (DC-DC) converter, which converts power supplies of different voltages in the same phase. 2. Low dropout linear regulator (LDO), which is a converter that converts a DC voltage into a DC voltage slightly lower than the input, and is a linear (noise-free) DC converter. 3. Rectifier circuit, which is a converter that converts AC voltage into DC voltage. 4. Inverter, which is a converter that converts DC voltage into AC through periodic program control. 5. AC transformer is a converter that converts AC of a certain frequency into AC with different peak values. DC-DC refers to using semiconductor power devices as switches to convert unstable DC voltage into a stable value. We usually call it a switching converter or DC converter. Capacitor and inductor are two types of switching converters. The commonly used capacitive converter mainly refers to the charge pump, and the circuit uses the switch of the capacitive energy storage element to achieve the function of boosting and bucking; the inductive switching converter mainly includes boost converter, buck converter and buck-boost converter.
The DC-DC switching power converter includes a switching power stage and a control section. The power switch tube is usually divided into synchronous rectification and asynchronous rectification. The switch tube itself is a power tube that allows large current to pass through. The traditional converter is an asynchronous circuit, which uses a power MOS and a freewheeling diode to achieve voltage conversion. Usually, two power MOS are used as switch tubes and freewheeling tubes. Since the switch MOS has a small voltage drop and a small size, it can improve efficiency while saving chip area, thereby increasing integration and reducing costs. The energy storage devices in the circuit include inductors and capacitors. The filter circuit refers to the filter designed for input and output voltage regulation. The control circuit determines the on-time of the switch tube based on the output feedback voltage and the sampling current of the inductor, and achieves correct conversion by adjusting the output voltage.
Application of DC-DC in new energy vehicles
Taking the most commonly used buck converter form of DC-DC in new energy vehicles as an example, the control technology of buck converters is divided into three categories: voltage mode control, current mode control and ripple-based control mode. Since the switching frequency of voltage mode and current mode is fixed, their light load efficiency is low, and their transient response speed is limited by the circuit bandwidth. Relatively speaking, the ripple-based control mode does not require a complex compensation network to ensure the stability of the loop, and the ripple of the output voltage or inductor current is monitored in real time to achieve a fast transient response. In addition, the simple control structure can consume less quiescent current and can effectively improve the conversion efficiency of the converter. In the ripple-based control mode, the adaptive on-time control mode not only has the characteristics of fast transient response and simple design, but also can achieve high conversion efficiency under wide loads.
First: Working mode of DC-DC converter The brief topological structure of the buck converter is shown in the figure below. After the switch S1 is closed, the energy of the input source VIN will flow through the inductor L and store the energy. Since the freewheeling diode D1 has unidirectional conductivity, when the switch is turned off, the energy stored in the inductor will be transferred to the output capacitor and the load, and the inductor current will be maintained continuously. According to the on-off status of S1 and the existence of the inductor current IL, the basic operating modes of the converter in steady state are mainly divided into three types: CCM, DCM and critical conduction mode (CRM).
