Rule One
Understand what is the DC / DC power supply and DC / DC conversion circuit classification
The DC/DC power supply circuit is also called a DC/DC conversion circuit, and its main function is to perform input and 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.
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
Rule two
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.
When selecting the regulator, it can be estimated as follows: (1) Uz=Vout; (2) Izmax=(1.5-3)ILmax (3)Vin=(2-3)Vout Suppression of the input voltage disturbance, but due to the maximum operating current regulator voltage limit, while the output voltage can not be arbitrarily adjusted, so the circuit is adapted to the output voltage does not need to adjust, load current is small, less demanding occasions, the circuit is often used as a pair Power supply voltage is not required for chip power supply.
Rule Three
Voltage Reference Chip Regulator
Another form of voltage regulator circuit, some chips have relatively high requirements on 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. The most common circuit application is as follows, where Vo=(1+R1/R2)Vref. Selecting different values ​​for R1 and R2 results in an arbitrary voltage output ranging from 2.5V to 36V. In particular, when R1=R2, Vo=5V.
Several other reference voltage source chip circuits are similar.
Rule Four
Series regulated power supply circuit recognition
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, as shown in the following diagram. In this 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). Since the amplification 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) to adjust the ratio of R1 and R2. The required output voltage can be set.
The figure shows that this is the basic principle of the three-terminal regulator. In fact, the load can be replaced with a Darlington transistor, etc. This type of series regulator circuit is not properly handled by the DC stabilized power supply. Easy to produce oscillations. Engineers who do not have a certain degree of simulation skills now generally do not use this method. Instead, they use an integrated three-terminal voltage regulator circuit directly for the use of DC/DC converter circuits.
Rule Five
Linear (analog) integrated voltage regulator circuit commonly used design
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 product of the three-terminal regulator is the 78 series (positive power supply) and the 79 series (negative power supply), and the output voltage is from the rear two of 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 the advantages of few external components, convenient use, stable performance, low cost, and is therefore widely used.
Rule 6
DC/DC Switching Regulator Circuit Design
The above-mentioned several kinds of DCDC conversion circuit belong to the series feedback type voltage stabilizing circuit, in this working mode, the regulating tube operates in the linear amplification state in the integrated voltage regulator, so when the load current is large, the loss is relatively large, ie, the conversion 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 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 buck, boost, buck-boost, flyback, forward, bridge, and so on.
Non-isolated DCDC switching circuit design. Isolated DCDC switching circuit design.
Rule Seven
Non-isolated DCDC switching integrated circuit chip circuit design
DCDC switch conversion integrated circuit chip, the use of this type of chip is very similar to the LM317 in the sixth article, here to illustrate with the L4960, generally using a 50Hz power transformer for AC-AC conversion, the ~220V down to the switching power supply integration Conversion chip input voltage range, for example, 1.2 ~ 34V, by the L4960 DC-DC conversion, then adjust the output voltage to 5V under the range, up to 40V, the maximum output current up to 2.5A (can also be connected to a large power switch Expanded flow), and built-in protection features such as over-current protection, over-temperature protection, etc. 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 up to 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.
Rule 8
Isolated DC/DC 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. It 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 solution can make the power very large. Compared with the previous several design schemes, the circuit structure is complex, the component parameters are difficult to determine, 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.
Rule 9
DCDC 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. Latch-up 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.
Rule ten
DCDC power conversion scheme selection considerations
This article is also a summary of this article, it is very important. This article mainly introduces 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 linear voltage regulator circuit circuit structure is simple, but due to low conversion efficiency, it can only be used in small power regulated power supply;
3 switch-type voltage regulator circuit has high conversion efficiency and can be used in high-power applications, but its limitations are relatively complex in circuit structure (especially in high-power circuits), which is not conducive to miniaturization.
From the above ten rules, we can see that in the design process, we can choose the right design based on actual needs.
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