Noise is annoying. PWM regulated power supply can help you suppress noise.

PWM type switching power supply has many advantages, such as high efficiency, small size, etc. Therefore, PWM is widely used as a power supply device in many fields. However, the short duration of the working state conversion of the switching transistor and the wide spectrum of the spike interference are its fatal weaknesses. It not only affects the switching power supply itself, but also interferes with other nearby electronic devices. When the switching power supply is working, the switching transistor and the freewheeling diode (or another switching transistor) are always turned on or off alternately. KQ and KD in Figure 1 are not ideal devices. The conversion of the two states requires a certain amount of time, which produces spike interference. During the state transition process, the on switch is not fully turned on, and the off switch is not turned off at the moment, so there is a direct path from the power supply to the ground, generating a transient current Is. This current is related to the difference between the current Imax when the switching transistor is turned on and the current Icmin when it is turned off, and the duration of the simultaneous conduction of switches KQ and KD. Due to the influence of the circuit distributed parameters, ringing oscillation appears on the waveform.

Figure 1 Transient current

The switching time of the transistor is inversely proportional to the cut-off frequency. The shorter the switching time, the faster the speed. The duration of simultaneous conduction depends on the switching speed of the devices used by KQ and KD. Comparing the switching devices with different speeds, the faster the switching device, the shorter the duration of simultaneous conduction, the narrower the width and the larger the amplitude of the spike interference. The larger the leakage inductance of the transformer, the higher the voltage spike, and the greater the RF interference. Especially after the transformer is shielded, due to the poor coupling, the leakage inductance is also larger. Generally speaking, the leakage inductance of the transformer wound with a toroidal core is smaller than that of the E type. In addition, the winding process is also very important. The better winding method is to first wind half of the total number of primary turns, then wind all the secondary turns, and finally wind the remaining half of the primary, that is, the secondary coil is in the middle of the primary coil. In this way, the primary coil maintains good coupling, so that the transformer has a smaller leakage inductance. The squareness of the switching waveform Usr(t) affects the spike interference. The rate at which the harmonic amplitude of a rectangular wave decreases with increasing frequency is 20dB per decade, while that of a trapezoidal wave is 40dB per decade. Consciously changing the steepness of the rectangular wave and the passivation of the two corners can suppress high-frequency components and reduce spike interference. Therefore, the switching speed of the switching transistor and the freewheeling diode should be reasonably selected. For the switching transistor, there are two ways to reduce spike interference, namely, increasing the rise time of Vce and reducing the fall time of Ic. In the circuit of Figure 2, after determining KQ, it can be seen from Figure 3 that increasing the turn-on time of KD and reducing the turn-off time can reduce spike interference.

Figure 2 Switching speed KD The RC snubber circuit connected in parallel between the CE of the switching transistor or at both ends of the freewheeling diode can significantly reduce the peak interference. In Figure 3, when the transistor T is turned off, the collector voltage rises, and C is charged through D and R1, so that its rising rate slows down. The value of the charging constant CR1 can control the rising rate. When T is turned on, D is cut off, and C discharges to R1 and R2, limiting the peak current at the moment of conduction. This snubber circuit changes the shape of the load line and reduces the loss of the switching transistor. It is also effective to connect an RC circuit at both ends of the freewheeling diode. In Figure 3, when 3DD11 and 2CK120C are used, a capacitor of about 0.022LF (f=2kHz) can be connected in parallel. The capacitance of this capacitor has an optimal value, and its effect can be seen from Figure 4. Figure 4 (a) is the case without C, and it is enlarged on the time axis to Figure 4 (b). After the snubber capacitor is connected, see Figures 4 (c) and (d) respectively.

Figure 3

The switch in the switching power supply is turned on and off quickly, and the didt is very large, which produces a large transient voltage drop on the leakage inductance of the power supply system, causing the input voltage source to drop for a very short time, destroying the normal waveform of the power grid and causing interference. The interference in the input power supply will also affect the switching power supply. The input filter has a certain isolation effect, and usually uses a P-type LC balanced filter, which can attenuate pulsating interference by 20dB and peak interference by as much as 6dB. The calculation formula for inductance is:

Where Epeak is the peak interference voltage (Vp-p), and fpeak is the frequency of the peak interference (Hz). The DC current value flowing through the inductor should also be considered to avoid saturation.

Isr is the maximum input DC current (A) of the switching power supply, and Usr is the input DC voltage (V). For a power supply system powered by AC power, the filter should be installed in a small sealed aluminum box. The small box should be placed in the chassis, next to the power cord entry hole, so that the power cord enters the chassis and reaches the filter box, and then leads to the power switch and rectifier. If a transformer is used before the rectifier, isolation should be added to its primary and secondary. The output capacitance of the switching power supply is large, and electrolytic capacitors are required. Ordinary electrolytic capacitors have poor high-frequency characteristics, and there are large equivalent inductance and resistance, so the impedance is large and the peak noise is also large. High-frequency electrolytic capacitors are low-inductance devices with excellent high-frequency characteristics. They provide good isolation from the ground loop for the pulse source and output voltage, and provide good noise filtering. There are three types of high-frequency electrolytic capacitors. One is a four-terminal capacitor, which has good high-frequency characteristics, but the load current flows through the capacitor to make it hot, so the current should be limited to less than 10A; the second is a large high-frequency filter electrolytic capacitor, which has the ability to withstand large currents but the high-frequency characteristics are not as good as the former; the third is a high-frequency filter electrolytic capacitor, which has the advantage of small size. Without changing other parameters of the circuit, if the ordinary electrolytic capacitor is used, the peak interference is 150mVp-p, while the four-terminal capacitor is 50mVp-p. Using a certain capacity of polycarbonate capacitor or high-frequency ceramic capacitor in parallel with the output electrolytic capacitor can further reduce the peak interference. The switching power supply radiates interference into space when working. The radiation noise level is inversely proportional to the distance from the radiation source. Generally, it is sufficient to wire 5cm away. If the structure does not allow it, it should be shielded. A strong electromagnetic field is generated around the power input line. In order to reduce the electromagnetic coupling between the input line and the output line, the two must be kept away. The wires flowing through the switch with large current should be as short as possible and not connected to other lines. The ground terminal of the freewheeling diode of the buck switching power supply or the boost switching transistor should be directly connected to the ground terminal of the output capacitor with the shortest lead. Figure 5 shows the influence of the wiring. The peak interference radiates through the loop I, so the inductive coupling of each section of the connection that constitutes this loop must be minimized, and the lead of the capacitor should be short to reduce the lead inductance.

To reduce the loss and spike interference on the output line, the connection line from the output to the load should be short. Figure 6 shows the interference waveform measured at the load end for three different lengths of output lines when the load current is 8A. The longer the output line, the larger the amplitude and width of the spike interference. For output lines of the same length, the thicker the wire diameter, the larger the amplitude and width of the spike interference. Using twisted pair can effectively attenuate the electromagnetic induction potential. Table 1 shows that the value of the attenuation of the induction potential increases as the torque becomes shorter.

The connection method of the output line has a great influence on the peak interference. Using twisted pair to output directly from the high and low ends of the output capacitor can offset the positive and reverse interference currents. Otherwise, the interference current flows through the output line and generates a large peak interference voltage on the line. When the switching voltage-stabilized power supply is used for several loads, it is better to feed each load with twisted pair from the output capacitor. In practical applications, the feed line is very long, and the switching voltage-stabilized power supply with a long output line has a large peak interference. For this reason, an LC low-pass balanced filter can be added at the output end or at the input end of the load. For example, a switching voltage-stabilized power supply outputs a 2m long feed line to the load, and the peak interference at the load end is 3Vp-p. If an LC filter is added on the load side, it will drop to 100mVp-p. If the inductor L is not connected and only the capacitor is added, the peak interference is 1Vp-p, which shows that a small inductor L is necessary.

From the above explanation, we can know that the spectrum of noise generated by pulse width modulation switching power supply is very wide, ranging from a few hertz to tens of megahertz. According to the factors analyzed in this article and the methods discussed above, the peak interference in these noises can be effectively suppressed. For individual circuits, a combination of several of them can often work. At present, there are still many difficulties in making the noise of switching power supply reach the millivolt peak-to-peak level. However, with the progress of components, the improvement of process level and the deepening of understanding of noise problems, the output noise of switching power supply is likely to reach a new level.