Detailed EMI and PCB design and switching frequency

Under the guidance of market applications, switching power supply products are increasingly demanding small, lightweight, high efficiency, low radiation, low cost and other characteristics to meet a variety of electronic terminal equipment. In order to meet the current portable electronic terminal equipment, the switching power supply must be small. The light weight features, therefore, increasing the operating frequency of the switching power supply has become a problem that designers are paying more and more attention to. However, what are the factors that limit the frequency increase of the switching power supply? In fact, it mainly includes three aspects, switching tube, transformer and EMI and PCB design.

First, the switch tube and switching frequency

Switching tube is the core device of switching power supply module. Its switching speed and switching loss directly affect the limit of switching frequency. The following is an analysis of everyone.

1, switching speed

The loss of the MOS transistor is composed of switching loss and driving loss, as shown in Fig. 1: the turn-on delay time td(on), the rise time tr, the turn-off delay time td(off), and the fall time tf.

Figure 1 Schematic diagram of MOS tube switch

Take FAIRCHILD's MOS as an example, as shown in Figure 2: FDD8880 switching time characteristics table.

Figure 2 FDD8880 switching time characteristics table

For this MOS tube, its limit switching frequency is: fs = 1 / (td (on) + tr + td (off) + tf) Hz = 1 / (8 ns + 91 ns + 38 ns + 32 ns) = 5.9 MHz, in practice In the design, since the control switch duty cycle realizes voltage regulation, the conduction and cut-off of the switch tube cannot be completed instantaneously, that is, the actual limit switching frequency of the switch is much smaller than 5.9 MHz, so the switching speed of the switch tube itself limits the switching frequency. .

2, switching loss

The corresponding waveform diagram when the switch is turned on is shown in Fig. 3(A). The corresponding waveform diagram when the switch is turned off is shown in Fig. 3(B). It can be seen that the VDS voltage of the switching transistor and the flow through the switch tube are turned on and off each time the switch tube is turned on and off. The current ID has overlapping time (yellow shaded position in the figure), resulting in loss P1, then the total loss PS=P1 *fs in the operating state of the switching frequency fs, that is, the number of times the switch is turned on and off when the switching frequency is increased The more the loss, the greater the loss, as shown in Figure 3 below.

Figure 3 Schematic diagram of switching tube loss

Second, transformer iron loss and switching frequency

The iron loss of the transformer is mainly caused by the eddy current loss of the transformer, as shown in Figure 4.

When a high-frequency current is applied to the coil, a varying magnetic field is generated in the conductor and outside the conductor perpendicular to the current direction (1→2→3 and 4→5→6 in the figure). According to the law of electromagnetic induction, a changing magnetic field generates an induced electromotive force inside the conductor, and this electromotive force generates eddy currents in the entire length direction (L side and N side) of the conductor (a→b→c→a and d→e→f→d). Then, the main current and the eddy current are strengthened on the surface of the conductor, and the current tends to the surface. Then, the effective AC cross-sectional area of ​​the wire is reduced, resulting in an increase in the AC resistance (eddy current loss coefficient) of the conductor and an increase in loss.

Figure 4 Schematic diagram of transformer eddy current

As shown in Fig. 5, the transformer iron loss is proportional to the kf power of the switching frequency, and is related to the magnetic temperature limitation. Therefore, as the switching frequency increases, the high-frequency current flows in the coil to generate a severe high-frequency effect. Thereby reducing the conversion efficiency of the transformer, resulting in an increase in the temperature of the transformer, thereby limiting the increase in the switching frequency.

Figure 5 Diagram of transformer iron loss and switching frequency

Third, EMI and PCB design and switching frequency

Assuming that the power device loss described above is solved, the real high frequency also needs to solve a series of engineering problems, because at high frequencies, the inductor is not the inductor we are familiar with, the capacitor is not the capacitor we know, all the parasitic parameters. Corresponding parasitic effects will be generated, which will seriously affect the performance of the power supply, such as the parasitic capacitance of the transformer's primary side, the leakage inductance of the transformer, the parasitic inductance and parasitic capacitance between the PCB wiring, which will cause a series of voltage and current waveform oscillations and EMI problems. The voltage stress of the switch tube is also a test.

Fourth, summary

To improve the power density of switching power supply products, the first consideration is to increase the switching frequency, which can effectively reduce the volume of transformers, filter inductors and capacitors, but it is faced with losses caused by switching frequency, which makes the design of temperature rise heat dissipation difficult. The increase in frequency will also lead to a series of engineering problems such as driving and EMI.

ZLG Zhiyuan Electronics has independently developed and produced isolated power modules for nearly 20 years. Currently, it adopts a new solution to achieve the same type of products with the smallest volume. For example, the E_UHBDD-10W module has been reduced by half compared with the previous generation ZY_UHBD-10W. Figure 6 shows.

Figure 6 E_UHBDD-10WN and ZY_UHBD-10W specifications comparison

At the same time, ZLG Zhiyuan Electronics has built a first-class testing laboratory in the industry to ensure the performance of power products. It is equipped with the most advanced and complete testing equipment. The full range of isolated DC-DC power supplies pass the complete EMC test, and the electrostatic immunity is up to 4KV, surge. With an immunity of up to 2KV, it can be applied to most complex and harsh industrial sites, providing users with a stable and reliable power isolation solution.

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