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Must-read for power supply engineers! Switching stabilized power supply spike interference

by:Shenmao     2021-04-29
01-Introduction PWM (PulseWidthmodulation) switching power supply has the advantages of small size and high efficiency. As a power supply device, it has been widely used in many fields. However, the short duration of the working state transition of the switching transistor and the peak interference with a wide frequency spectrum 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. The KQ and KD in Figure 1 are not ideal devices, and the transition between the two states takes a certain amount of time. , Which produces spike interference. In the state transition process, when the turned-on switch is not completely turned on, and the turned-off switch is not turned off, there is a direct path from the power supply to the ground, and a transient current Is is generated. The current is related to factors such as 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 simultaneous conduction of the switches KQ and KD. Due to the influence of circuit distribution parameters, ringing and oscillation appear on the waveform. 02-The effect of the duration of instantaneous turn-on of the power switch on the spike interference The turn-on and turn-off time of the transistor is inversely proportional to its cut-off frequency. The shorter the turn-on and turn-off time, the faster the switching speed. The duration of simultaneous conduction depends on the switching speed of the devices used by KQ and KD. Comparing with switching devices of different speeds, the faster the switching device is, and the shorter the duration of conduction at the same time, the narrower the width and the larger the amplitude of the spike interference. 03-Reduce the voltage spike caused by the leakage inductance of the transformer The larger the leakage inductance of the transformer, the higher the voltage spike, and the greater the radio frequency interference. Especially after the transformer is shielded, the leakage inductance is correspondingly larger due to poor coupling. Generally speaking, the leakage inductance of a transformer wound with a toroidal core is smaller than that of the E type. In addition, the winding process is also very important. A better winding method is to first wind half of the total number of turns of the primary, then wind all the turns of the secondary, and finally wind the remaining half of the primary, that is, the secondary coil is in the primary coil. intermediate. In this way, the primary coil maintains a better coupling, so that the transformer has a smaller leakage inductance. 04-The influence of the switching waveform of the power tube on the spike interference and the suppression of the squareness of the switching waveform Usr(t) affect the spike interference. The rate at which the harmonic amplitude of a rectangular wave decreases with increasing frequency is 20dB ten octave, while that of a trapezoidal wave is 40dB? Decade. Intentionally changing the steepness of the rectangular wave and the dullness 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 selected reasonably. For the switching transistor, there are two ways to reduce the spike interference, that is, increase the rise time of Vce and reduce the fall time of Ic. In the circuit of Fig. 2, after determining KQ, it can be seen from Fig. 3 that increasing the turn-on time of KD and reducing the turn-off time can reduce spike interference. Between the CEs of the switching transistors, or the parallel RC snubber circuit at both ends of the freewheeling diode, the spike interference can be significantly reduced. In Figure 3, when the transistor T is turned off, the collector voltage rises, and C is charged through D and R1 to slow down the rise rate. The value of the charging constant CR1 can be selected to control the rise rate. When T turns on, D turns off, and C discharges R1 and R2, limiting the peak current at the moment of turning on. The snubber circuit changes the shape of the load line and reduces the loss of the switching transistor. Connecting the RC circuit at both ends of the freewheeling diode is also effective. In Figure 3, when using 3DD11 and 2CK120C, a capacitor of about 0.022LF (fu003d2kHz) can be combined, and the capacitance of this capacitor has an optimal value. Its function can be seen from Figure 4. Figure 4(a) is the case without C, which is enlarged on the time axis as Figure 4(b). Figure 4 (c) and (d) respectively after connecting the buffer capacitors. 05-In the input power grid, the noise suppression method of the switching power supply is fast on and off, the didt is very large, and a large transient voltage drop is generated on the leakage inductance of the power supply system, so that the input voltage source has a time A very short instantaneous drop will destroy the normal waveform of the power grid and cause interference. Interference in the input power supply will also affect the switching regulated power supply. The input filter has a certain isolation effect, usually a P-type LC balanced filter, which can attenuate pulsation interference by 20dB, and spike interference can also attenuate as much as 6dB. Inductance calculation formula: where E spike is the spike interference voltage (Vp-p), and f spike is the frequency of spike interference (Hz). The value of DC current flowing through the inductor should also be considered to avoid saturation.
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