PCB Circuit Board Power Supply Design Needs To Be Considered

PCB power supply design is a complex process with many factors to consider.

1. When selecting the power supply topology, it is necessary to comprehensively consider the input voltage range, output voltage, power level, efficiency requirements, ripple index, cost budget and other factors.

Take Buck circuit as an example, it is suitable for the application of high input voltage and low output voltage, and can achieve high efficiency Buck conversion, but when the output voltage is lower than about 35% of the input voltage, serious on-off loss will occur, and BUCK structure is not recommended at this time.

Another common Boost circuit is suitable for low-voltage boost applications, which can obtain a high boost ratio and efficiency, but there are working limitations such as too low input voltage and imbalance.

2. When selecting key devices, the power switch tube is one of the core devices.

For MOSFETs, it is necessary to comprehensively consider its voltage level, rated current, on-impedance Rdson, reverse recovery characteristics, actuable capability, switching speed, and power loss during operation.

For IGBT, it is more suitable for high voltage and high power occasions. In addition, the design of the transformer/inductor is also very critical, which needs to be reasonably designed according to the core size, material, winding length, turn ratio, coupling coefficient, scattering inductance and other parameters to ensure good energy transfer efficiency and working stability.

3. When designing output and input filter capacitors, electrolytic capacitors are usually required to suppress low-frequency ripple.

For the output filter capacitor, the selection needs to be combined with the expected service life, operating temperature, ripple current and other conditions to select the appropriate capacitance value and electrolyte model specifications.

Parallel ceramic capacitors can restrain AC ripple and overshoot peak well. For input filter capacitors, electrolytic capacitors are also required to filter out low-frequency ripple and have safety resistors in series at both ends, while ceramic capacitors are used to suppress high-frequency ripple.

4. Good thermal design is essential for the reliable operation of power products.

In the PCB layout, it is necessary to keep the heating power device away from the temperature-sensitive components, while retaining a good heat dissipation channel around the heat source element.

For small power circuits, the natural convection heat dissipation capacity can be improved by increasing the area of the external heat sink and increasing the area of the copper foil layer inside the PCB.

For high-power circuits, it is necessary to use fan forced air convection heat dissipation, and combined with a suitable radiator for heat dissipation design.

5. In order to meet EMI/EMC regulations and standards, corresponding protective measures need to be taken in the power supply design.

At the AC input, it is necessary to design a Pi or T filter network to filter out EMI at the AC input. At the DC output end, it is necessary to design LC filter circuit or add independent EMI filter to suppress EMI radiated to the load end.

At the PCB layout level, the high voltage and high frequency switching circuit should be reasonably isolated from the low voltage analog circuit, and the appropriate shield or isolation layer should be used. At the same time, it is necessary to ensure that each area is well grounded, and the copper foil wiring is as thick as possible at the high-frequency loop to reduce the high-frequency grounding impedance.

6. Design immunity requirements for electrical and electronic equipment used in commercial, public, light industrial or residential premises.

Switchgear and controls EN/IEC 60947-1 Low-voltage switchgear and control equipment regulating operating voltages in the range of 1500 V DC and 1000 V AC. Power Stations and substations IEC TS 61000-6-5 Sets the immunity levels of equipment used for generation, transmission and distribution. Process control and measurement EN/IEC 61326-1 specifies immunity and radiation levels of electrical equipment or devices with operating potentials less than 1000 Vac and 1500 V DC.

7. In terms of feedback control design, the classic control modes include peak current mode control and average current mode control.

The former has a simple structure and can effectively prevent the current from flying, but there is a coupling resistance effect and the feedback response is relatively slow. The average current mode is more suitable for high-current load applications, and the reaction time is short, the ripple and harmonic content is also low, but the cost is relatively high.

In recent years, the rise of digital control chips has brought new possibilities for power control algorithms, which can realize A variety of flexible and intelligent control algorithms, but the delay of its A/D sampling and digital processing also needs to be considered.

No matter what kind of control mode is used, the parameter design of the feedback compensation network is the key, which directly determines the dynamic response performance of the power supply.

8. In terms of protection circuit design, it is necessary to cover the normal working range of the entire power supply.

These include input undervoltage latch protection, overvoltage/undervoltage protection to detect output voltage, overcurrent/short circuit protection based on resistance sampling or peak current mode, soft start circuit to avoid the surge current during startup, and temperature detection protection circuit to prevent power overheating.

These protection measures need to be properly designed to ensure reliable protection against circuit damage under abnormal conditions.

9. A reasonable PCB layout is also very important for achieving high-performance power supply design.

When wiring, it is necessary to pay attention to minimize the area of the high-frequency and high-energy current path loop to reduce the high-frequency electromagnetic radiation. At the same time, devices such as magnetic components and power switches should be kept away from analog control circuits to avoid interference from magnetic fields and electric fields.

For high-frequency switch nodes, solder resistance holes or gap wiring can be used to reduce cross-coupling. For magnetic devices such as coupling inductors, a magnetic field shield or isolation layer can be set.

The whole power circuit should adopt a separate ground plane to reduce the ground impedance. Finally, the important role of modeling and simulation in modern power supply design cannot be ignored.

It is usually necessary to perform an idealized power-level circuit simulation to predict key performance indicators such as efficiency, ripple, steady-state characteristics, etc. Then parasitic parameters are added to simulate the whole circuit to analyze the transient response and EMI noise. Finally, the thermal model and EMI simulation model are added to comprehensively analyze the thermal effect and EMI performance.

Simulation not only helps to identify and solve design problems in advance, but also optimizes key parameters and shortens the actual design verification cycle.

PCB power supply design is a complex system project, the need to comprehensively control circuit topology, device selection, thermal design, EMI control, control algorithm, protection circuit, PCB wiring and simulation verification and many other key links, and in practice to accumulate experience, in order to finally design high-performance, highly reliable power supply products.