Refined PCB design of low-power DC-DC power module
In electronic devices, power modules are core components that convert input DC voltage into various DC voltages required by the device. DC-DC power converters, especially those based on switching mode, are widely used due to their high efficiency, small size and light weight. However, it is not easy to design an excellent DC-DC power converter. Layout, electromagnetic compatibility, current and heat dissipation management all require careful consideration and fine adjustment.
To optimize power supply performance, the switching power supply should be placed close to the chip to avoid voltage drop caused by too long output lines. To reduce electromagnetic interference, avoid placing sensitive components around the switching power supply. To enhance stability and reliability, factors such as wiring strategy, thickening of ground wires, and heat dissipation design need to be considered.
1.Introduction to DC-DC power supply applications
Introduction to DC-DC application types
DC-DC power supply is the core device for DC voltage conversion, including LDO and other types. Among them, devices implemented using switching methods are often called DC-DC converters. This type of power supply has the advantages of low power consumption, high efficiency, small size, light weight, high reliability, and anti-interference and wide range output, so it is widely used in the electronic field.
2.Working principle of DC-DC power supply
The working principle of DC-DC power supply involves voltage conversion and regulation. This process is mainly achieved through the converter of the switching power supply. The DC-DC converter is responsible for converting the input DC voltage into the required output DC voltage in the switching power supply. In general, the working principle of this converter can be divided into three main steps:
1) Filtering: When the mains power is turned on through the input switch, it first passes through the rectifier circuit to become pulsating DC power, and then passes through the filter circuit to filter out the ripple and become smooth DC power, which is supplied to the converter for conversion.
2) Conversion: The DC-DC converter generates a certain amount of pulse power through the input voltage through the power switch tube and the energy storage inductor, and then uses the pulse transformer and the rectifier filter circuit to obtain the corresponding output voltage. The voltage conversion is achieved by controlling the switching timing of the switch tube by PWM (pulse width modulation) or PFM (frequency modulation). In a switching power supply, the switching speed of the switch tube is very fast, generally completing the switching action within tens of microseconds, which is much faster than the adjustment speed of a traditional linear power supply.
3) Regulation: The stability of the output voltage is achieved through a feedback loop. When the output voltage increases, the sample signal obtained through the sampling resistor will increase, and the error signal obtained by comparing this signal with the reference voltage will increase. Then, the on-time of the switch tube is reduced by PWM or PFM control, thereby reducing the average power output of the converter and achieving output voltage stability.
2.Selection of key components
（1）Output inductor
The inductor stores magnetic energy to ensure stable current output. When selecting an inductor, you need to weigh the size. A large inductor has low loss but slow response, while a small inductor has fast response but high loss. Consider the saturation current to ensure the filtering effect.
（2）Voltage divider resistor
The voltage divider resistor forms a voltage divider network, feeds back the output voltage to the control circuit, accurately controls the PWM duty cycle, and stabilizes the output voltage value. Select high-precision resistors to ensure circuit accuracy.
（3）Input capacitor
When selecting input capacitor, equivalent inductance and self-resonant frequency should be considered. Large capacitance capacitors filter low-frequency noise, and small capacitance capacitors filter high-frequency noise. Combining parallel use can achieve excellent filtering effect, stabilize input voltage and filter AC components.
（4）Output capacitor
The output capacitor filters switching ripple to ensure pure output current. The larger the capacitance, the smaller the impedance, and the easier it is for ripple to flow through. Choosing a suitable output capacitor is crucial for the stable operation of the circuit.
When selecting these core components, we need to consider various factors comprehensively to achieve the best balance. For example, the selection of inductors is to find a balance between efficiency and response speed; the voltage divider resistor needs to accurately and stably feedback the voltage; the input and output capacitors need to be selected according to different operating frequency noise characteristics; the final output capacitor must comprehensively consider the capacitance and ESR to minimize ripple. Through careful selection, we can make the circuit work in an efficient and stable state, thereby maximizing the performance of the circuit.
3.DC-DC power supply layout and wiring suggestions
For the layout and wiring of the power module, you can download the chip datasheet in advance and design it according to the recommended layout and wiring.
4.Layout design
1) Chip power proximity principle: For the switching power supply that provides voltage to the chip, it should be placed as close to the chip as possible. This can avoid the low voltage output line being too long, thereby reducing the voltage drop and ensuring that the power supply performance is not affected.
2) Avoid electromagnetic interference: The switching power supply operates under high voltage and high current conditions, which may cause complex electromagnetic compatibility problems. Therefore, sensitive components should be avoided around the switching power supply to reduce the impact of electromagnetic interference on the operation of the components.
3) Layout with the power chip as the core: When laying out, the switching power supply chip should be organized as the core component. When laying out the input and output ends of the power filter, ensure sufficient distance to prevent noise from coupling from the input to the output. The components should be arranged neatly and compactly on the PCB to reduce and shorten the leads and connections between the components.
- Wiring design
1) Avoid parallel wires: The wires used for input and output should be avoided to be parallel to each other as much as possible to reduce unnecessary electromagnetic interference.
2) Thicken the ground wire: Thicken the ground wire between input and output to ensure current stability and reduce feedback coupling.
3) High current lead processing: High current leads, such as common ground wire, power input/output wire, etc., should be as thick as possible. This can reduce wiring resistance and voltage drop, thereby reducing self-excitation caused by parasitic coupling.
- Heat dissipation considerations
1) Heat dissipation ground area: Since the heat dissipation of the switching power supply is relatively large, the area of the heat dissipation ground (copper) should be increased as much as possible to ensure effective heat dissipation.
2) Large area copper plating and drilling: The input and output ends should be as large as possible copper plating and more vias should be drilled, which can not only meet the current requirements, but also help improve the heat dissipation effect.
Conclusion:
DC-DC power converters play a vital role in the world of electronic devices, and their design involves many delicate and critical links. From understanding its working principle, to carefully selecting core components, to optimizing layout and wiring design, every link requires us to think deeply and operate meticulously. This requires not only solid professional knowledge, but also innovative thinking and problem-solving skills. I hope this article can provide some useful references and help for everyone in designing DC-DC power converters.
