The details to be paid attention to when designing the switching power supply circuit

After selecting the power stage topology, the previous knowledge and steady state operating point selection can be used to optimize the power stage parameters such that:

--- Switching power devices have the least loss;

---The power transformer and filter inductor, filter capacitor, etc. have the smallest volume;

---The power supply has the highest power density;

--- Power level layout is the most reasonable, and so on.

Among these optimizations, the most important is the optimization of the power transformer. The ratio of the winding method directly affects the selection of other power components and the efficiency and power density of the entire power stage. Reasonable selection of power switching devices and their driver and sink circuits is also important for power stage performance.

After selecting the power level topology and control strategy, the previous knowledge can be utilized and the loop parameters can be optimized based on power level parameter optimization such that:

--- Minimize the closed-loop voltage audio isolation to reduce the PFC filter capacitor;

--- Minimize the closed-loop output impedance to reduce the DC output filter capacitor;

--- Try to increase the closed-loop response speed of the power supply to reduce the DC output filter capacitor;

In the loop optimization, the most important are the compensator parameters, the modulator parameters (such as the external ramp compensation content) and the optimization of the optocoupler circuit parameters. The PCB layout of the power supply has a great influence on the loop, only good. Under the PCB Layout, the optimization of the parameters of each part of the loop can make the bandwidth of the power loop gain as large as possible, thus achieving better dynamic performance and higher power density.

In switching power supplies with winding power supply, the quality of the auxiliary power supply must be optimized to:

--- Auxiliary power supply has the least impact on the steady state performance of the switching power supply;

--- Auxiliary power supply has the least impact on the dynamic performance of the switching power supply;

--- Auxiliary power supply will not affect the reliability of the switching power supply;

The quality of the output voltage of the auxiliary power supply using the transformer winding or the inductor winding is generally not good. By optimizing the auxiliary power supply, it is necessary to ensure that the performance of the power supply after self-power supply is minimal, and the reliability is not problematic.

--- Optimization of various protection circuits in the power supply;

--- EMI filter circuit optimization;

--- Optimization of the internal thermal environment of the power supply;

--- Optimization of other functional circuits of the power supply (such as current sharing, synchronization, hot swapping, remote compensation, etc.);

--- PCB Layout optimization, and so on.

1): compromise: a compromise between steady state performance and dynamic performance

Many power stage topologies, where steady state performance and dynamic performance are often difficult to balance. The steady state performance is good, the dynamic performance is poor, the dynamic performance is good, and the steady state performance is poor. There are many examples, so the topology must be selected according to the requirements and application occasions. Even in the same topology, when designing its power level parameters, we must consider the compromise between steady-state performance and dynamic performance. For example, the design of the output filter inductor is better for efficiency, but for dynamic performance, I hope that it is smaller, so I need to compromise when designing.

2): compromise 2: compromise between power density and reliability

Many topologies with higher power densities are more complex to implement, and often the topology itself has hidden risks. Therefore, when choosing a topology, you should also make a specific compromise based on reliability and performance. For example, some topologies that implement soft switching generally achieve higher switching frequencies and higher power density, but they are often less reliable in the products they implement.

3) A compromise between small signal performance and large signal performance

In a power supply, there are many performances that need to be met. Different control strategies and different compensation circuits can achieve different dynamic performance. Some control strategies or parameters have strong suppression of the disturbance at the input end, and some have strong suppression ability to the disturbance of the load end. Some parameters have good dynamic stability for small signals, but under large signals, and may not Stable, some parameters can meet the requirements of large signals, but the small signal will be worse, so the dynamic design of the size signal should be compromised.

4): compromise 4: design compromise at high and low temperatures

In a power supply, since various parameters are related to the temperature at work, it is necessary to find a set of parameters that can satisfy all performance indexes in the entire ambient temperature range, which requires many trade-offs.

5): compromise 5: a compromise between electrical and thermal performance

In a power supply, the requirements between electrical performance (such as electrical stress and EMI performance) and thermal performance are contradictory. In order to obtain good EMI and low electrical stress, it is desirable to make the loop formed by the power components as small as possible, but this will make the thermal influence between the components more severe, and the loss of each component will be greater; The increase of the loop between the two can reduce the thermal influence and improve the thermal design. However, due to the increase of parasitic parameters, the electrical stress of the device will increase, the efficiency will be low, and the EMI performance will deteriorate. Therefore, the heat and electricity in the power supply are both The design is very much in need of compromise.

6): compromise 6: compromise design of key components

In the switching power supply, there are some key components that need to be compromised in design. For example, the design of the power transformer, for the steady-state efficiency performance, after the ratio has been optimized, it is hoped that the leakage inductance is minimal, but the leakage is realized. At the same time, the distribution is often increased, which tends to increase the distributed capacitance between the windings. This usually increases the common mode EMI interference and reduces the safety requirements. In addition, if the driving ability is compromised, in order to reduce the switching loss of the power switching device (MOSFET), it is hoped. The switching process is as short as possible, which can be achieved by reducing the gate drive resistance, but as the switching speed is increased, the common mode EMI of the power supply is often increased, resulting in poor EMI characteristics.

7): Other compromises:

There are many other trade-offs to make a switching power supply. In short, the switching power supply is a power electronic product that meets the specifications within a certain boundary (a rectangular parallelepiped with input voltage, load current and ambient temperature). There are power processing and information processing, as well as heat treatment, so in order to do such a product, many and many compromises must be made. This requires developers to optimize on the basis of compromise, compromise on the basis of optimization, and make development The power products achieve the best price/performance ratio.