The DC/DC switch type power source converter’s designer wants to face one of challenges is, time control normal operation’s EMI (electromagnetic interference) radiates. If radiates is too strong, they through the power line dissemination, or radiate to system’s in other installments, harm system’s performance. The radiation peak value appears generally in converter’s foundation turn-on frequency place, on each higher harmonic’s radiation intensity gradually reduces, the majority of radiating capacity is only restricted in the fundamental wave and is inferior the overtone. Through to the power source converter operating frequency’s adjustment and the vibration, may proliferate EMI to a wider frequency, thus reduces the peak value radiation.
The majority modern PWM controller uses the external connection resistor to establish the operating frequency, generally reduces the resistance value to be possible to raise the frequency. For example, the LM5020 internal neterodyne (RT) provides one in its programming foot after constant voltage 2V, programs the resistor one to meet then establishes the electric current which to the RT foot RT flows out. The oscillator also provides for the internal fixed time capacitor to become the proportion the electric current (reference). Fixed time condenser voltage’s rise cycle has decided oscillator’s frequency.
Figure 1 the external connection vibration electric circuit uses the independent comparator’s simple oscillator including one, the operating frequency approximately is about 800 Hz. Comparator IC2 output condition when on electricity is the high level. R1, R2 and R3 establish the comparator the input, its initial setting value is 2.9V. On capacitor C3 voltage to threshold value climb.
When the comparator negative input end’s voltage achieves the threshold voltage, the comparator output cut is the low level, thus drops to the comparator input end’s threshold voltage 2.1V. Therefore, on the capacitor C3 voltage dips to the new threshold value, when arrives at the low threshold voltage, this cycle is redundant. On the C3 voltage is approximately a minimum value is 2.1V, the maximum is the 2.9V triangular wave.
For oscillator’s base frequency makes the vibration to LM5020 the controller PWM, produces the triangular wave modulation controller RT foot’s electric current with IC2. Resistor R5 hypothesis modulation vibration percentage. The R5 right margin fixed is the RT foot after the constant voltage 2V voltage, the low frequency triangular wave which comes from the IC2 coupling actuates R5 through capacitor C2 left side. Regarding 64.9 kΩ R5, passes resistor R5 the Feng Feng electric current approximately is 12mA. When has not connected the vibration electric circuit, RT flows out the steady current approximately is 121mA, therefore the 12mA p-p vibration electric current is equal to 10% total modulations.
PWM which controls with LM5020 back feed DC/DC converter IC1 appraisal vibration electric circuit’s effect. This electric circuit’s foundation operating frequency is 250kHz, establishes by controller’s RT resistor. Figure 2 the red line expressed that in the vibration electric circuit does not work in the situation, the electric circuit is inputting the radiation which on the power line conducts. The peak value radiation is limited narrowly, in 250 kHz bases inspire nearby the frequency, the base frequency place obtains oscillation amplitude for - 24dB.
Figure 2 the blue color pathline expressed that the result which after will vibrate the electric circuit to connect controller’s RT input end, produces. The base frequency nearby conduction radiation is proliferated, the maximum oscillation amplitude reduced - 34 dB, achieves 10dB.
