《Switching Power Supply Simulation and Design》-Introduction to Power Converters

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《Switching Power Supply Simulation and Design》-Introduction to Power Converters [Copy link]

In electronic design, simulation technology has become an indispensable part. This chapter elaborates on the importance of simulation technology for switching power supply design. Through simulation, designers can verify and optimize the circuit without wasting actual resources and time. In addition, simulation also helps to deeply understand the working principle and performance characteristics of the circuit, providing strong support for actual design.

SPICE software is a successful simulation software with a friendly user interface. It can help designers quickly grasp the full meaning of the circuit, observe the circuit to be built and examine all parameters. It is useful for both power supply designers and beginners. Through simulation technology, you can avoid wasting time and money, quickly test and evaluate design ideas, conduct simulation tests before experiments, and conduct hypothetical experiments. SPICE allows designers to safely conduct various experiments and provide various answers.

This chapter introduces what we want to know, what is a converter? At the beginning, the important position of the power converter in the electronic system is made clear. As a bridge connecting the power supply and the load, it is responsible for converting one form of electrical energy into another form of electrical energy to meet the power supply needs of different circuits and equipment. Subsequently, the difference between switching power supply and linear power supply is introduced, and the advantages of switching power supply in conversion efficiency, power density and other aspects are emphasized.

In electronic design and application, facing the diverse circuit requirements, especially when these circuits have different and specific requirements for power supply voltage, how to effectively manage and distribute power becomes a key issue. In this case, the introduction of converters (also called voltage regulators or voltage converters) is particularly critical. A converter (Voltage Converter) is an electronic circuit or device whose main function is to convert one voltage level to another voltage level to meet the power supply requirements of different circuits or devices. This conversion can be boost (converting low voltage to high voltage) or buck (converting high voltage to low voltage), and even in some special cases, it can achieve voltage polarity conversion (such as converting positive voltage to negative voltage).

type

Resistor voltage divider

A resistor divider is a simple voltage conversion circuit that divides the input voltage through a series resistor network to obtain the desired output voltage. Its basic principle is based on Ohm's law, that is, when current passes through a resistor, a voltage drop will occur. The resistor divider is suitable for occasions where voltage accuracy is not required and the load current is small. However, since the resistor itself consumes power (that is, generates heat), its efficiency is low in occasions with high current or high efficiency requirements, and it is not suitable as the main power conversion method.

Linear Regulator

线性调压器是一种通过调整晶体管（如BJT或MOSFET）的工作状态来稳定输出电压的电路。它利用晶体管的线性放大区或饱和区特性，将输入电压的一部分能量转换为热能，从而实现对输出电压的精确控制。线性调压器具有电路简单、输出电压稳定、纹波小等优点。然而，由于其工作原理决定了它必须消耗额外的功率来维持输出电压的稳定，因此在输入电压与输出电压差值较大时，其效率会显著降低。线性调压器通常采用负反馈机制来提高输出电压的稳定性。通过比较输出电压与参考电压的差值，并调整晶体管的工作状态来减小这个差值，从而实现输出电压的精确控制。线性调压器并不适合做高频变换,除非Vout与Vin之间的电压减小到几百毫伏。不过线性调压器能很好地抑制纹波,可以在有较大噪声的输出线路上用做滤波整流器。它们对A/D变换器之类的噪声敏感电路供电是安全的。

Switching Power Supply

A switching power supply is a device that uses the on and off of a switching element to control the conversion of electric energy. It converts the input voltage into a high-frequency square wave (or pulse), and then converts the high-frequency square wave into the required DC voltage or AC voltage through a transformer, rectifier filter and other circuits. The core of the switching power supply is to adjust the output voltage by controlling the on and off time (i.e., duty cycle) of the switching element.

PWM control is one of the most commonly used control methods in switching power supplies. It controls the output voltage by adjusting the on and off time (i.e., pulse width) of the switching element. PWM control has the advantages of fast response speed and high control accuracy. PFM control adjusts the output voltage by changing the switching frequency of the switching element. Although PFM control has advantages in some applications (such as higher efficiency under light load), its control accuracy and stability are usually not as good as PWM control. In order to take into account the advantages of PWM control and PFM control, some advanced switching power supplies adopt a hybrid control method. That is, different control strategies are selected under different load conditions to achieve the best performance.

The core of the loss of switching devices lies in conduction loss and switching loss. Conduction loss, as the name implies, is the energy consumption of the semiconductor in a stable on or off state, and the calculation is relatively straightforward. Switching loss, on the other hand, is much more complicated. It involves the energy loss of the semiconductor when it switches between the on and off states, a process also known as turn-on or turn-off loss. As the state transition time is affected by the complex influence of the driving impedance and the internal parasitic elements of the device (such as inductance and capacitance), accurate analysis of switching loss remains a major challenge. Therefore, actual measurement of prototype circuits has become a key means of evaluating switching losses. Compared with switching loss, the calculation of conduction loss is relatively straightforward in simple structures such as buck converters. The key is to determine the root mean square current when the device is on, while ignoring the leakage current in the off state (note that this assumption is not applicable in certain situations, such as Schottky diodes). In buck, boost or buck-boost converter systems, the dynamic resistance of active and passive components is particularly sensitive to the root mean square current changes of inductors, power switches, output capacitors and freewheeling diodes.

Boost Converter

The boost converter, as a member of the indirect energy transfer converter, cleverly combines the energy storage and release in its power supply mechanism. When the switch is closed, the inductor takes on the responsibility of storing energy, while the output capacitor independently supplies power to the load to ensure the stable flow of current. When the switch is opened, the energy stored in the inductor works together with the input power supply to inject strong power into the output end. However, this mechanism is accompanied by an inherent conversion lag, especially when facing a sudden demand for output power, the converter needs to increase the energy storage capacity of the inductor in advance by extending the conduction period to cope with the upcoming peak energy demand. If the power demand increases slowly, the inductor current is given enough time to build up, and the output voltage can remain stable even if the current decays during the switch off period. On the contrary, if the power demand increases suddenly, the inductor current cannot quickly climb to the ideal peak value, and the output voltage will fluctuate. This process, that is, the accumulation and release of energy, brings challenges to the design and simulation of switching power supplies, especially in the small signal transfer function, which significantly affects the dynamic response of the circuit in the form of the right half plane zero (RHPZ). It is worth noting that the boost converter draws current from the power supply every time the switch is opened and closed, which is different from the buck converter that only draws power when the switch is closed. When the switch is open, the inductor releases energy through the output network. This unique circuit structure gives the CCM boost converter a non-pulse input current characteristic, greatly reducing the ripple of the input current and achieving a smoother current transmission.

The principle of the boost-buck converter is basically similar, with some small changes in the structure:

Switching mode converters are inherently noisy and can interfere with devices that share the same power supply. This is particularly noticeable in automotive applications, long distance communication equipment, and measurement devices. A filter circuit, in most cases an electromagnetic interference (EMI) filter, needs to be inserted between the power supply output and the converter input. In a closed-loop system, the feedback loop always tries to keep the output power of the circuit constant. By connecting an ammeter in series with the converter, it can be observed that when the input voltage increases, the input current decreases, and when the input voltage decreases, the input current increases. This shows that the power supply tries to keep the output power constant, and the converter appears as a negative resistance at the input. In the same converter operating in open loop, there is no negative input impedance.

This chapter explores several key points in switching power supply design. First, the efficiency limitations of linear regulators are emphasized, and it is pointed out that their efficiency can be improved by maintaining a low input-output voltage difference. Then, the basic principles of switching regulators are explained in detail, including the single-switch structure, the filtering requirements of square-wave outputs, and the importance of current continuity. In addition, this chapter analyzes the behavior of inductor current in different modes, especially the characteristics of continuous conduction mode (CCM), discontinuous conduction mode (DCM), and boundary conduction mode (BCM/CRM). Finally, the three basic converter structures of buck, boost, and buck-boost are summarized and extended to more complex topologies. At the same time, the inherent noise problems of switching mode converters and the challenges of filter design are pointed out, including the resonance conflict between the filter and the converter and the strategy of oscillation suppression through damping elements.

SongXL

A technical post about switching power supply. There are still many things I don’t understand. I still need to learn more. This is very good. Thumbs up!

qzgiky

Power supply is the basis of electronic appliances. Anything that needs electricity cannot be separated from power supply. Learn more and improve more.