10 rules for DC/DC conversion circuit design
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# 10 rules for DC/DC conversion circuit design

Posted Date: 2024-01-26

DC/DC power supply circuit is also called DC/DC conversion circuit, and its main function is to convert input and output voltages. Generally, we call the voltage conversion process when the input power supply voltage is within 72V DC/DC conversion. Common power supplies are mainly divided into automotive and communication series and general industrial and consumer series. The voltage used by the former is generally 48V, 36V, 24V, etc., while the power supply voltage used by the latter is generally below 24V. Different application fields have different rules. For example, 12V, 5V, and 3.3V are commonly used in PCs, 5V and 15V are commonly used in analog circuit power supplies, and 3.3V is commonly used in digital circuits. Current FPGAs and DSPs also use voltages below 2V, such as 1.8V and 1.5V. V, 1.2V, etc. Also called secondary power supply in communication systems, it is a DC input voltage provided by a power supply or a DC battery pack. After DC/DC conversion, one or several DC voltages are obtained at the output end.

DC/DC conversion circuits are mainly divided into the following three categories:

①Volume regulator tube voltage stabilizing circuit.

②Linear (analog) voltage stabilizing circuit.

③Switching voltage regulator circuit rule 2: Simple regulator tube circuit design scheme The regulator tube regulator circuit has a simple circuit structure, but has poor load capacity and low output power. It generally only provides a reference voltage for the chip and is not used as a power supply.

When selecting 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 structure is simple and can Suppresses the disturbance of the input voltage, but due to the operating current limit of the voltage regulator tube, and the output voltage cannot be adjusted arbitrarily, this circuit is suitable for occasions where the output voltage does not need to be adjusted, the load current is small, and the requirements are not high. This circuit is often used for power supply Power supply for chips with low voltage requirements.

Rule 3: Reference voltage source chip voltage stabilizing circuit is another form of voltage stabilizing circuit. Some chips have relatively high requirements for power supply voltage, such as the reference voltage of AD DA chips. At this time, some commonly used voltage reference chips such as TL431, MC1403, REF02 wait. TL431 is a commonly used reference source chip, a three-terminal adjustable shunt reference voltage source with good thermal stability. Its output voltage can be arbitrarily set to any value from Vref (2.5V) to 36V using two resistors. Commonly used circuit applications are as shown below, at this time Vo=(1+R1/R2)Vref. Selecting different values ​​of R1 and R2 can obtain any 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 4: Understanding the Circuit of Series Stabilized Power Supply The series stabilized circuit is a type of DC stabilized power supply. In fact, it was a 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, , A series regulator usually has an OP amplifier and a Zener diode to form an error detection circuit, as shown in the figure below. In this 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). Since the amplified signal ΔVs is negative, the base voltage of the control transistor drops, so the output voltage decreases. Under normal circumstances, there must be Vref=Vs=Vout*R2/(R1 +R2), adjust the ratio of R1 and R2 to set the required output voltage value.

What is shown in the figure is only the basic principle of a three-terminal voltage regulator. In fact, depending on the load size, the transistor can be replaced by a Darlington tube, etc. The DC voltage stabilized power supply composed of this series voltage stabilizing circuit is improperly handled and extremely harmful. Easy to produce oscillation. Nowadays, engineers who do not have certain analog skills generally do not use this method. Instead, they directly use an integrated three-terminal voltage stabilizing circuit to use the DC/DC conversion circuit.

Rule 5: Commonly used design solutions for linear (analog) integrated voltage regulator circuits. The design solutions for linear voltage regulator circuits are mainly three-terminal integrated voltage regulators. There are two main types of three-terminal voltage regulators:

A kind of output voltage is fixed, which is called a fixed output three-terminal voltage regulator. The common products of three-terminal voltage regulators are 78 series (positive power supply) and 79 series (negative power supply). The output voltage is determined by the last two of the specific model. Each number represents 5V, 6V, 8V, 9V, 12V, 15V, 18V, 24V and other grades. The output current is distinguished by adding a letter after 78 (or 79). L means 0.1A, M means 0.5A, no letter means 1.5A, such as 78L05 table to find 5V 0.1A.

Another type of output voltage is an adjustable linear voltage regulator circuit, called an adjustable output three-terminal voltage regulator. Representatives of this type of chip are the LM317 (positive output) and LM337 (negative output) series. The input and output limit difference is 40V, the output voltage is continuously adjustable from 1.2V-35V (-1.2V--35V), the output current is 0.5-1.5A, the voltage between the output end and the adjustment end is 1.25V, and the adjustment end Quiescent current is 50uA.

The basic principles are the same, and they all use series voltage stabilizing circuits. Among linear integrated voltage regulators, since the three-terminal voltage regulator has only three lead-out terminals, it has the advantages of fewer external components, easy use, stable performance, and low price, it is widely used.

Rule 6: DC/DC conversion switch-type voltage stabilizing circuit design scheme. The above-mentioned DCDC conversion circuits are all series feedback voltage stabilizing circuits. In this working mode, the adjustment tube in the integrated voltage regulator works in a linear amplification state. Therefore, when the load current is large, the loss is relatively large, that is, the conversion efficiency is not high. Therefore, the power of power circuits using integrated voltage regulators is not very large, generally only 2-3W. This design scheme is only suitable for small power power circuits.

DCDC conversion circuits designed using switching power supply chips have high conversion efficiency and are suitable for larger power power supply circuits. Currently it has been widely used, and the commonly used ones are non-isolated switching power supply and isolated switching power supply circuits.

DCDC conversion switching voltage stabilizing circuit design scheme. The DCDC conversion circuit designed using switching power supply chip has high conversion efficiency and is suitable for larger power power supply circuits. Currently it has been widely used, and the commonly used ones are non-isolated switching power supply and isolated switching power supply circuits. Of course, the basic topologies of switching power supplies include buck, boost, buck-boost, flyback, forward, bridge changes, etc.

Non-isolated DCDC switching conversion circuit design scheme. Isolated DCDC switching conversion circuit design scheme.

Rule 7: Non-isolated DCDC switch conversion integrated circuit chip circuit design scheme DCDC switch conversion integrated circuit chip. The use of this type of chip is very similar to the LM317 in Article 6. Here is an example. Generally, a 50Hz power transformer is first used for AC -AC conversion, reduce ~220V to the input voltage range of the switching power supply integrated conversion chip, such as 1.2~34V, and the L4960 performs DC-DC conversion. At this time, the output voltage can be adjusted down to 5V and up to 40V, and the output current can be Up to 2.5A (it can also be connected to a high-power switching tube for current expansion), and it is equipped with complete protection functions, such as overcurrent protection, overheating protection, etc.

Rule 8: Isolated DC/DC switching power module circuit design. Commonly used isolated DC/DC conversions are mainly divided into three categories: 1. Flyback conversion. 2. Forward conversion. 3. The single-ended flyback DC/DC conversion circuit commonly used in bridge conversion, there are many models of this type of isolated control chip. A typical representative of the control chip is the commonly used UC3842 series. This kind of high-performance fixed-frequency current controller is mainly used to isolate AC/DC and DC/DC conversion circuits. Its main application principle is: the circuit consists of 4 parts: main circuit, control circuit, starting circuit and feedback circuit. The main circuit uses a single-ended flyback topology, which is composed of an isolation transformer after the evolution of a buck-boost chopper circuit. This circuit has the advantages of simple structure, high efficiency, and wide input voltage range. The control circuit is 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 dual-loop control, that is, adding peak current feedback control to the voltage closed-loop control system. This type of scheme can increase the power by selecting appropriate transformers and MOS tubes. Compared with the previous design schemes, the circuit structure is complex, the component parameters are difficult to determine, and the development cost is high. Therefore, this scheme can be given priority when needed. The cheaper DC/DC isolation module on the market.

Rule 9 DCDC switching integrated power module solution Many microprocessors and digital signal processors (DSP) require core power and an input/output (I/O) power supply, which must be sequenced at startup. Designers must consider the relative voltages and timing of the core and I/O voltage sources during power-up and power-down operations to meet manufacturer-specified performance specifications. Without proper power supply sequencing, latchup or excessive current draw may occur, which may result in microprocessor I/O ports or memory, programmable logic devices (PLDs), field programmable gate arrays (FPGAs), or data conversion The I/O port of a supporting device such as a processor is damaged. Core power and I/O power tracking are required to ensure that the core voltage is biased correctly before driving I/O loads. Nowadays, there are specialized power module companies that tailor-make some special switching power supply modules, mainly those that, in addition to conventional electrical performance indicators, are small in size, high in power density, high in conversion efficiency, low in heat, and long in average trouble-free working time. , DC/DC power module with good reliability, lower cost and higher performance. These modules combine most or all of the components required to implement a plug-and-play solution and can replace up to 40 different components. This simplifies integration and speeds design, while reducing the power management section's board space.

Traditional and common non-isolated DC/DC power modules are still single in-line (SiP) packages. These open framework solutions are indeed making progress in reducing design complexity. However, it is simple to use standard packaged components on a printed circuit board.

Rule 10: Things to note when choosing a DCDC power conversion solution. This golden rule is also the summary of this article and is very important. This article mainly briefly introduces several commonly used design methods and solutions for the three circuit modes of voltage regulator tube stabilization, linear (analog) voltage stabilization, and DCDC switching voltage stabilization for DCDC power conversion.

① It should be noted that the voltage regulator tube stabilizing circuit cannot be used as a power supply and can only be used to power chips with no power requirements; ② The linear voltage stabilizing circuit has a simple circuit structure, but due to low conversion efficiency, it can only be used for low-power stabilization. In voltage power supply; ③ The switching voltage stabilizing circuit has high conversion efficiency and can be used in high-power situations, but its limitation is that the circuit structure is relatively complex (especially high-power circuits), which is not conducive to miniaturization.

From the above ten rules, it can be seen that during the design process, appropriate design solutions can be selected according to actual needs.

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