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# Knowledge of common circuit design schemes for DC-DC power supplies

baijiahao | 19/05/2021

The DC/DC power supply is simply understood as a circuit that performs input and output voltage conversion. Common DC/DC power supplies are mainly divided into automotive, communications, industrial and consumer electronics, etc. The former use voltages generally 48V, 36V, 24V, etc., and the latter use voltages generally below 24V. The voltage used in different application fields will be different. For example, 12V, 5V, 3.3V are commonly used in PCs, 5V and 15V are commonly used in analog circuits, and 3.3V are commonly used in digital circuits. Current FPGAs and DSPs also use voltages below 2V. Such as 1.8V, 1.5V, 1.2V, etc.

The DC/DC power supply is also called the secondary power supply in the communication system. It is a DC input voltage provided by a primary power supply or a DC battery pack. After DC/DC conversion, one or more DC voltages can be obtained at the output. The DC/DC conversion circuit is mainly divided into three types: voltage stabilizer circuit, linear (analog) voltage stabilizer circuit and switching type voltage stabilizer circuit. Now let's talk about the common design schemes of these three types.

(1) Among many DC/DC conversion circuit schemes, the simplest is the voltage regulator circuit design scheme. The voltage regulator circuit of the voltage regulator tube has a simple structure, but the load capacity is poor, and the output power is small. Generally, it only provides a reference voltage for the chip, not as a power supply. The more commonly used is the shunt-type voltage stabilizing circuit, the circuit diagram is as shown in the figure below:

When choosing a voltage regulator tube, it can generally be estimated according to the following formula:

1. Uz=Vout

2. Izmax=(1.5-3)ILmax

3. Vin=(2-3)Vout

This circuit has a simple structure and can suppress the disturbance of the input voltage. However, due to the maximum operating current limit of the Zener tube and the output voltage cannot be adjusted arbitrarily, the circuit is adapted to the output voltage without adjustment, the load current is small, and the requirements are not high. It is often used as power supply for chips that do not require high supply voltage.

(2) Reference voltage source chip voltage stabilization circuit is another form of voltage stabilization circuit. Some chips have higher requirements for power supply voltage, such as the reference voltage of AD DA chip, etc. At this time, some commonly used voltage reference chips such as TL431, MC1403 , REF02, etc. TL431 is the most commonly used reference chip, a three-terminal adjustable shunt reference voltage source with good thermal stability. Its output voltage can be arbitrarily set between Vref (2.5V) and 36V with two resistors. Common circuit applications are shown in the following figure:

At this time Vo=(1+R1/R2)Vref, choosing different values ​​of R1 and R2 can get any voltage output from 2.5V to 36V, especially when R1=R2, Vo=5V. Several other reference voltage source chip circuits are similar.

(3) The series type voltage stabilizer circuit is a kind of DC voltage stabilized power supply. In fact, it is the more commonly used DC power supply method before the appearance of the three-terminal voltage stabilizer. Before the appearance of the three-terminal voltage stabilizer, the series voltage stabilizer is usually composed of The OP amplifier and the Zener diode constitute an error detection circuit, as shown in the following figure:

In the circuit, the reverse 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 voltage of the control transistor drops, so the output voltage decreases. Under normal circumstances, Vref=Vs=Vout*R2/(R1+R2). Adjust the ratio of R1 and R2 to set Set the required output voltage value. This is also the basic principle of a three-terminal regulator. In fact, the load size can be changed from a transistor to a Darlington, etc. The DC stabilized power supply composed of this series-type stabilized circuit is improperly handled and is prone to oscillation. Now engineers who don't have certain analog skills generally don't use this method, but directly use the integrated three-terminal voltage regulator circuit as a DC/DC conversion circuit.

(4) The commonly used design schemes of linear (analog) integrated voltage regulator circuits are mainly based on three-terminal integrated voltage regulators. There are two main types of three-terminal regulators: one output voltage is fixed, which is called a fixed output three-terminal regulator. The general products of the three-terminal regulator are 78 series (positive power supply) and 79 series (negative power supply). The output voltage is represented by the last two numbers in the specific model, including 5V, 6V, 8V, 9V, 12V, 15V, 18V , 24V, etc. The output current is distinguished by letters after 78 or 79, L means 0.1A, M means 0.5A, and no letter means 1.5A, such as 78L05 means 5V/ 0.1A.

Another type of output voltage is an adjustable linear voltage 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 and 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, the voltage between the output terminal and the adjustment terminal is 1.25V, the adjustment terminal The quiescent current is 50uA. The basic principle is the same, all adopt series type voltage stabilizing circuit. In the linear integrated voltage regulator, because the three-terminal voltage regulator has only three lead terminals, it has the advantages of fewer external components, convenient use, stable performance, and low price, so it is widely used.

(5) The several DC/DC conversion circuits mentioned above are all series feedback voltage stabilized circuits. In this mode of operation, the regulator tube in the integrated voltage stabilizer works in a linear amplification state. Therefore, when the load current is large, the loss Relatively large, that is, the conversion efficiency is not high. Therefore, the power of the power supply circuit using the integrated voltage regulator will not be very large, generally only 2-3W, this design scheme is only suitable for low-power power supply circuits.

The DC/DC conversion switch-type voltage stabilizing circuit design scheme adopts a DCDC conversion circuit designed by a switching power supply chip, which has high conversion efficiency and is suitable for larger power power supply circuits. Has been widely used at present, divided into two types of non-isolated switching power supply and isolated switching power supply. The basic topologies of switching power supplies include buck, boost, buck-boost, flyback, forward, bridge changes, and so on.

(6) Non-isolated DC/DC switching integrated circuit chip circuit design scheme, the use of this type of chip is very similar to LM317. Here L4960 is used as an example. Generally, a 50Hz power transformer is used for AC-AC conversion, and 220V is reduced to the input voltage range of the switching power supply integrated conversion chip, such as 1.2~34V. Then the L4960 performs DC-DC conversion. At this time, the output voltage can be adjusted down to 5V and up to 40V, and the maximum output current can reach 2.5A (can also be connected to a high-power switch tube for current expansion), and the internal equipment is perfect Protection functions, such as overcurrent protection, overheating protection, etc.

Although the use method of L4960 is similar to LM317, the efficiency of L4960 of switching power supply is different from that of LM317 of linear power supply. L4960 can output up to 100W of power (Pmax=40V*2.5A=100W), but it only consumes 7W at most, so the radiator is small and easy to manufacture. L296 is similar to L4960, its basic parameters are the same as L4960, but the maximum output current can be as high as 4A, and it has more protection functions, and the package form is different. There are many such chips, such as LM2576, TPS54350, LTC3770 and so on. Generally when using these chips, the manufacturers will have detailed instructions and typical circuits for reference design.

(7) Circuit design scheme of isolated DC/DC switching power supply module. Commonly used single-ended flyback DC/DC conversion circuits, such isolated control chip models are also many, the typical representative of the control chip is the commonly used UC3842 series. This is a high-performance fixed-frequency current controller, which is mainly used to isolate AC/DC switching power supply modules and DC/DC switching power supply modules. The main application principle is: the circuit is composed of 4 parts: main circuit, control circuit, starting circuit and feedback circuit. The main circuit uses a single-ended flyback topology, which is formed by adding an isolation transformer after the evolution of the buck-boost chopper circuit. The 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 adopts peak current type double-loop control, that is, adding peak current feedback control in the voltage closed-loop control system. This kind of scheme chooses the right transformer and MOS tube to make the power very large. Compared with the previous design schemes, the circuit structure is complicated, the component parameters are difficult to determine, and the development cost is relatively high. Therefore, when this solution is needed, the cheaper DC/DC isolation switching power supply module on the market can be preferred.

(8) DC/DC switch integrated module power supply scheme. Many microprocessors and digital signal processors (DSP) require core power and an input/output (I/O) power supply. These power supplies 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 performance specifications specified by the manufacturer. Without proper power sequencing, lock-up or excessive current consumption may occur, leading to microprocessor I/O ports or memories, programmable logic devices (PLD), field programmable gate arrays (FPGA), or data converters The I/O port of the supporting device is damaged. To ensure that the I/O load is not driven before the core voltage is properly biased, core power and I/O power tracking are required.

Now there are specialized module power supply companies that can tailor some special module power supplies, mainly for some conventional electrical performance indicators, small size, high power density, high conversion efficiency, low heat generation, long MTBF, and good reliability. , DC/DC module power supply with lower cost and higher performance. These modular power supplies can be plug-and-play, and most or all of the components required by the solution can replace up to 40 different components. This simplifies the integration and speeds up the design, while reducing the board space of the power management part.

The DC/DC power supply is a power supply that effectively outputs a fixed voltage after converting the input voltage. Most of the components selected by the standard circuit are components that can exhibit general characteristics under standard use conditions. Therefore, it is not that they are the best under various use conditions. The best components are selected. Therefore, in each design, design changes must be made from standard circuits according to their respective requirements specifications (such as efficiency, cost, mounting space, etc.). But to be able to design a circuit that meets the required specifications, sufficient knowledge and experience accumulation is required.

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