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Theory and practice always complement each other to be perfect, of course, embedded programming and actual power system design must also be unified to make efficient and high-quality DC-DC DC conversion power supply. Sometimes engineers who engage in embedded systems tend to make MCUs, ARMs, DSPs, and FPGAs easy to use. Once the system is designed to supply power to the power system, it can also allow its well-designed programs to run, but for newcomers , Sometimes it may be inefficient, and often there is insufficient current or too much power supply to cause such problems. The eleven gold rules of this article make it easy to get the DCDC power conversion circuit design.
First, understand how DC/DC power supply matters?
The DC/DC power supply circuit is also called a DC/DC conversion circuit, and its main function is to perform input/output voltage conversion. Generally, we call the voltage conversion process with the input power voltage within 72V as DC/DC conversion. The common power supplies are mainly divided into automotive and communication series and general industrial and consumer series. The voltages used in the former are generally 48V, 36V, 24V, etc. The power supply voltage used in the latter is generally lower than 24V. Different application fields have different laws. For example, 12V, 5V, and 3.3V are commonly used in PCs, 5V and 15V for analog circuit power, and 3.3V for digital circuits. Currently, FPGAs and DSPs use voltages below 2V, such as 1.8V and 1.5V. V, 1.2V and so on. In the communication system, it is also called the secondary power supply. It is provided by a primary power supply or a DC battery pack to provide a DC input voltage and obtain one or several DC voltages at the output after DC/DC conversion.
Second, need to know the DC / DC conversion circuit classification
The DC/DC conversion circuit is mainly divided into the following three categories:
1 regulator regulator circuit. 2 linear (analog) regulator circuit. 3 switching regulator circuit
Third, the simplest regulator circuit design
The voltage stabilizing tube voltage stabilizing circuit has a simple circuit structure, but has a poor load carrying capacity and a small output power, and generally only provides a reference voltage for the chip, and does not use a power supply. More commonly used is a shunt regulator circuit. When choosing the voltage regulator tube, it can generally be estimated as follows: (1) Uz=Vout; (2) Izmax=(1.5-3)ILmax; (3) Vin=(2-3)Vout This circuit has a simple structure. The disturbance of the input voltage can be suppressed, but due to the maximum operating current limit of the regulator, and the output voltage can not be arbitrarily adjusted, this circuit is suitable for applications where the output voltage does not need to be adjusted, the load current is small, and the requirement is not high. For power supply voltage is not demanding chip power supply.
Article 4, reference voltage chip voltage regulator circuit
Another form of voltage regulator circuit, some chips have relatively high requirements for the power supply voltage, such as the reference voltage of the AD DA chip. At this time, some voltage reference chips such as TL431, MC1403, and REF02 are commonly used. The TL431 is the most commonly used reference source chip and has a good thermal stability of the three-terminal adjustable shunt reference voltage source. Its output voltage can be arbitrarily set to any value from Vref (2.5V) to 36V with two resistors.
Article 5 Circuit Recognition of Series Regulated Power Supply
Series regulator circuit is a kind of DC power supply, in fact, before the emergence of three-terminal regulator more common DC power supply method, before the emergence of three-terminal regulator, the series regulator usually has OP amplifier and The Zener diode constitutes an error detection circuit. In the general circuit, the inverting input terminal of the OP amplifier is connected to the detection signal of the output voltage, and the forward input terminal is connected to the reference voltage Vref, Vs=Vout*R2/(R1+R2). Because the amplified signal ΔVs is negative, the base-level voltage of the control transistor decreases, so the output voltage decreases. Under normal conditions, there must be Vref=Vs=Vout*R2/(R1+R2). The ratio of adjusting R1 and R2 can be set. Set the required output voltage value. In fact, the size of the load can replace the transistor with a Darlington tube, etc. This type of series regulator circuit does not properly handle the DC stabilized power supply, and is prone to oscillation. Engineers who do not have a certain degree of simulation skills now generally do not use this method, but instead use an integrated three-terminal regulator circuit directly for the use of DC/DC conversion circuits.
Article 6. Common Design Schemes for Linear (Analog) Integrated Regulator Circuits
Linear voltage regulator circuit design program mainly to three-terminal integrated voltage regulator. Three-terminal regulators, there are two main types:
An output voltage is fixed, called a fixed-output three-terminal regulator. The general-purpose products of the three-terminal regulator are the 78 series (positive supply) and the 79 series (negative supply), and the output voltage is the latter two in the specific model. The number represents 5V, 6V, 8V, 9V, 12V, 15V, 18V, 24V and other grades. The output current is differentiated by 78 (or 79) followed by a letter. L represents 0.1A, M represents 0.5A, and no letter represents 1.5A, such as 78L05 represents 5V 0.1A.
The other output voltage is an adjustable linear regulator circuit called an adjustable output three-terminal regulator. This type of chip represents the LM317 (positive output) and LM337 (negative output) series. The maximum input/output limit difference is 40V, the output voltage is 1.2V-35V (-1.2V--35V) continuously adjustable, the output current is 0.5-1.5A, and the voltage between the output terminal and the adjustment terminal is 1.25V. The quiescent current is 50uA.
The basic principle is the same and series voltage regulator circuits are used. In the linear integrated voltage regulator, the three-terminal regulator has only three lead-out terminals, has few external components, is convenient to use, has stable performance, and has low cost, and thus is widely used.
Article 7, DCDC Switching Regulator Circuit Design
The above-mentioned several kinds of DCDC conversion circuit belong to the series feedback type voltage stabilizing circuit, in this kind of work mode, the regulating tube works in the linear amplifying state in the integrated voltage regulator, so when the load current is large, the loss is relatively large, namely changeover low efficiency. Therefore, the power supply circuit using the integrated voltage regulator is not very large, generally only 2-3W, this design is only suitable for low-power power circuits.
The DCDC conversion circuit designed with a switching power supply chip has a high conversion efficiency and is suitable for a relatively large power supply circuit. At present, it has been widely used, and it is divided into non-isolated switching power supply and isolated switching power supply circuit.
DCDC transfer switch type voltage stabilizing circuit design scheme, adopts switching power supply chip design DCDC conversion circuit conversion efficiency is high, suitable for larger power supply circuit. At present, it has been widely used, and it is divided into non-isolated switching power supply and isolated switching power supply circuit. Of course, the basic topology of switching power supplies includes step-down, step-up, step-up and step-down, flyback, forward, bridge, and so on.
Article 8, Non-isolated DCDC Switching Integrated Circuit Chip Circuit Design
DC-DC switching integrated circuit chip, the use of this type of chip is very similar to the LM317 in Article 6, here to illustrate with the L4960, the general is to use a 50Hz power transformer for AC-AC conversion, the ~220V to the switch The input voltage range of the power integrated conversion chip is, for example, 1.2 to 34 V. The DC-DC conversion is performed by the L4960. At this time, the output voltage can be adjusted to 5 V under the variation range, and it can be adjusted up to 40 V. The maximum output current can reach 2.5 A (high power can also be connected. The switch tube carries out the flow expansion, and has built-in protection functions such as over-current protection and over-heat protection. Although the L4960 is used in much the same way as the LM317, the efficiency of the L4960 switching power supply is not the same as that of the linear power supply LM317. The L4960 can output a maximum of 100W (Pmax=40V*2.5A=100W), but it is the most Only consumes 7W, so the heat sink is small and easy to make. L296 is similar to the L4960. Its basic parameters are the same as those of the L4960, except that the maximum output current can be as high as 4A, and it has more protection functions and different package formats. This kind of chip is more, for example, LM2576 series, TPS54350, LTC3770 and so on. When using these chips, manufacturers will use detailed instructions and typical circuits for reference.
Article 9, Isolated DCDC Switching Power Supply Module Circuit Design
The commonly used isolation DC/DC conversion is mainly divided into three major categories: 1. Flyback conversion. 2. Forward transformation. 3. Bridge transformation
Commonly used single-ended flyback DC/DC conversion circuits, there are also many types of isolated control chips. The typical representative of the control chip is the commonly used UC3842 series. This is a high-performance fixed-frequency current controller that is mainly used to isolate AC/DC, DC/DC conversion circuits. Its main application principle is: The circuit consists of the main circuit, control circuit, start-up circuit and feedback circuit. The main circuit uses a single-ended flyback topology, which is composed of an isolation transformer after the evolution of the buck-boost chopper circuit. This circuit has the advantages of simple structure, high efficiency, and wide input voltage range. The control circuit is the core of the entire switching power supply, and the quality of the control directly determines the overall performance of the power supply. This circuit uses peak current type double loop control, which adds peak current feedback control to the voltage closed loop control system. The selection of suitable transformers and MOS transistors for this type of scheme can make the power very large. Compared with the previous design schemes, the circuit structure is complex, the parameter determination of the components is difficult, and the development cost is high. Therefore, when this scheme is needed, it can be preferred. The cheaper DC/DC isolation module on the market.
Article 10, DC-DC Switch Integrated Power Module Solution
Many microprocessors and digital signal processors (DSPs) require a core power supply and an input/output (I/O) power supply, which must be sequenced at startup. Designers must consider the relative voltage and timing of the core and I/O voltage sources during power-up and power-down operations to meet the manufacturer's specified performance specifications. Latchup or excessive current consumption may occur if there is no proper power sequencing, which may result in microprocessor I/O ports or memories, programmable logic devices (PLDs), field programmable gate arrays (FPGAs), or data conversions The I/O port of the supporting device such as the device is damaged. Kernel power and I/O power tracking are required to ensure that the I/O load is not driven before the core voltage is properly biased. Now there are specialized power supply module companies tailor-made for some special switching power supply modules, mainly those that eliminate the conventional electrical performance indicators, its small size, high power density, high conversion efficiency, less heat, average failure-free long working hours , Reliability, and lower cost, higher performance DC/DC power modules. These modules combine most or all of the components needed to implement a plug-and-play solution and can replace up to 40 different components. This simplifies integration and speeds up the design while reducing the footprint of the power management section.
The most traditional and most common non-isolated DC/DC power module is still a single in-line (SiP) package. These open-frame solutions do make progress in reducing design complexity. However, the simplest is to use standard packaged components on a printed circuit board.
Article 11. Precautions for the selection of DCDC power conversion scheme
The last one is the most important one. In the article, we mainly introduce several commonly used design methods for the three circuit modes of Zener voltage regulator, linear (analog) regulator, and DCDC switch regulator. 1 It should be noted that the Zener voltage regulator circuit can not be used as a power supply, and can only be used for power supply of chips without power requirements; 2 The linear voltage regulator circuit has a simple circuit structure, but can only be used for low power stability due to its low conversion efficiency. The power supply; 3 switch-type voltage regulator circuit conversion efficiency, can be applied in high power applications, but its limitations in the circuit structure is relatively complex (especially high-power circuits), is not conducive to miniaturization. Therefore, in the design process, suitable design options can be selected according to actual needs.