Inductance Selection and Layout and Wiring Principle of Switching Power Supply
Switched-Mode Power Supply (SMPS) is a very efficient power converter. Its theoretical value is close to 100%, and there are many kinds. According to the topological structure, there are Boost, Buck, Boost-Buck, Charge-pump and so on; according to the switch control mode, there are PWM, PFM; according to the switch tube category, there are BJT, FET, IGBT and so on. This discussion mainly focuses on PWM control Buck and Boost which are commonly used in power management of data card.
The main components of switching power supply include input source, switch tube, energy storage inductance, control circuit, diode, load and output capacitor. At present, most semiconductor manufacturers integrate switches, control circuits and diodes into a CMOS/Bipolar power management IC, which greatly simplifies the external circuit.
As a key component of switching power supply, energy storage inductor plays an important role in the performance of power supply. It is also the focus of product design engineers and debugging objects. With the development of consumer electronic devices such as mobile phones, PMP and data cards, the size of these devices is tending to be light, thin, compact and fashionable, which is in contradiction with the larger capacity and larger size inductance and capacitance required by the stronger product performance. Therefore, how to reduce the inductance size (occupied PCB area and height) of switching power supply is an important topic to be discussed in this paper, and the designer will have to compromise between circuit performance and inductance parameters.
Everything has two sides, switching power supply is no exception. Bad PCB layout and wiring design will not only reduce the performance of switching power supply, but also strengthen EMC, EMI, grounding, etc. In the layout and wiring of switching power supply, the problems that should be paid attention to and the principles that should be followed are another important proposition to be discussed in this paper.
Take Buck circuit as an example. Whether the switch is closed-open or open-closed, the transient part of the current is shown in figure (c), which produces a very rich rise or fall edge of the harmonic component. Popularly speaking, these transient current traces are called "AC current" and the rest are "DC current". Of course, the difference between AC and DC is not the definition in the traditional textbook, but the PWM frequency of the switch is only a component of the "AC" FFT transformation, while the harmonic component in the "DC" is very low, which can be ignored or forgotten. So it is not surprising that the energy storage inductance belongs to the "DC" type. After all, the inductance has the characteristic of preventing the current from transient. Therefore, in the layout of switching power supply, "AC" track is the most important and need to be carefully considered. This is also the only basic rule to keep in mind and applies to other rules and topologies. The following figure shows the current transient trace of Boost circuit. Note the difference between Boost circuit and Buck circuit.
The resistance of 1 inch long, 50 mm wide, 1.4 mil thick (1 ounce) copper conductor at room temperature is 2.5 mm_. If the overcurrent is 1A, the voltage drop is 2.5 mV, which will not adversely affect most ICs. However, the parasitic inductance of such a 1inch long conductor is 20nH. It can be seen from V=L*dI/dt that if the current changes rapidly, a large voltage drop may occur. Typical Buck power supply generates 1.2 times of the output current when switching on-off, and 0.8 times of the output current when switching on-off. The switching time of FET switch is 30ns and that of Bipolar switch is 75ns. Therefore, when the 1inch conductor of the "AC" part of the switching power supply flows through the 1A transient current, the voltage drop of 0.7V will occur. Compared with 2.5mV, 0.7V increases nearly 300 times, so the layout of high-speed switch is particularly important.
As close as possible to all peripheral devices beside the converter, reducing the length of the line would be the ideal layout, but limited to the extremely limited layout space, it is often impossible to do so, so it is necessary to proceed in priority order according to the severity of the transient pressure drop. For the Buck circuit, the input bypass capacitor should be placed as close as possible to the IC, followed by the input capacitor, and finally the diode, which connects one end to SW and the other end to the ground by a short and thick track. For Boost circuit layout, it is arranged according to the priority order of output bypass capacitance, output capacitance and diode.