The switching power supply is possibly the well known noise generator. You should prevent these to conduct or the radiation noise returns input source, because this noise will give use identical input power source’s other components to create the major damage. The electromagnetic interference filter’s goal prevents these noises, and provides a returns noise source the low impedance way. This kind of noise is bigger, the filter design’s scale, the expense and the difficulty are bigger. The fixed frequency switching power supply produces the biggest electromagnetic interference radiation in this fixed base frequency, in addition, but also has in the turn-on frequency multiple produces the electromagnetic interference radiation, but its scope (intensity) has reduces. Shown in Figure 1 the simple electric circuit to enable the switch switch in many frequencies, but is not in the single frequency works, thus reduced the mean time which in any frequency works. This plan can reduce the peak value yield of radiation effectively.
Figure 1, pours into the RC pin the low frequency oscillator pitch voltage, modulates power source’s turn-on frequency.
Figure 1 the electric circuit is a self-starting oscillator, its oscilation frequency approximately is 500 Hz. When you add the electricity, C3 starts from 0V to charge, the TL331 comparator’s out-port is at the high impedance condition, this is because its non-opposition input end’s ratio-voltage opposition input end’s voltage is higher. When C3 charge, its voltage surpasses the R1-R6 divider’s reference voltage, the comparator out-port becomes the low impedance condition. Because R5 now and R6 is parallel, on the R6 voltage immediately drops to the low reference voltage level. Because at the same time R3 is parallel with C3, therefore C3 starts to discharge, its voltage drops to this new reference level. When the comparator out-port leads the way, the C3 electric discharge achieves on R6 the voltage. Hereafter, this circulation repeatedly carries on. You must choose various parts carefully, guaranteed that R6 two reference voltage condition is lower than the C3 high and low two charge condition. This electric circuit uses C3 to adjust the oscillator frequency, you choose the C3 capacitance value should be lower than C2. Oscillator’s frequency approximately was equal to
Capacitor C2 C3 pitch voltage alternating current coupling to UCC3813 oscillator pin. Pours into signal in its forward period (exchanged electrical signal) to increase the CT charging current, therefore raised controller’s operating frequency. Is pouring into the signal negative period, CT part charging current vanishing, therefore cuts controller’s operating frequency. Figure 2 showed has poured into the signal to the CT charge influence. R4 control injection current size. Reduces R4 the resistance number, may expand take its rating fixed frequency as the central operating frequency scope. Pours into the signal by the C3 hypothesis the oscilation frequency to control the frequency sweep speed.
Figure 2, the exterior oscillator changes fixed time capacitor’s charge situation.
Figure 3, the back feed switch’s electromagnetic interference in has the exterior modulation and in the exterior modulation situation has not been different.
After before Figure 3 difference electromagnetic interference electric current observed value (1 dBmV=1 dBmA) showed increased the frequency deviation oscillator, and the effect. This design may with ease reduce with the aid of 12 kHz scanning windows the electromagnetic interference 10 dBmA. A wider window may further reduce the electromagnetic interference, but the modulator frequency will possibly become in switch’s output ripple voltage very remarkable. Must cause the pitch voltage waveform which pours into is as far as possible linear, prevents the switching power supply to pause the long time in the limit switch frequency. The misalignment will produce one to have two entirely different frequency electromagnetic interference responses. You cannot let the electric circuit work by all means must in be lower than under the power source switch’s low frequency limit, will otherwise possibly have the magnetic saturation. This electric circuit had demonstrated one kind of low cost, the small area reduce the conduction electromagnetic interference radiation the method.
