LED injection molding modules all need a special power supply to provide them with electrical energy, but in the design of LED injection molding modules, the ripple, both theoretically and practically, must exist. How do we deal with ripples?

Increase inductance and output capacitor filtering

According to the formula of the drive power supply for the injection molding module, the current fluctuation in the inductor is inversely proportional to the inductance value, and the output ripple is inversely proportional to the output capacitance value. Therefore, increasing the inductor value and output capacitor value can reduce the ripple.

The relationship between output ripple and output capacitance: vripple=Imax/(Co×f). It can be seen that increasing the value of the output capacitor can reduce the ripple.

The usual practice, for the output capacitor, use aluminum electrolytic capacitors to achieve the purpose of large capacity. However, electrolytic capacitors are not very effective in suppressing high-frequency noise, and the ESR is relatively large, so a ceramic capacitor is connected in parallel next to it to make up for the lack of aluminum electrolytic capacitors.

Two-stage filtering is to add a second-stage LC filter

The LC filter has obvious inhibitory effect on noise and ripple. According to the frequency of the ripple to be removed, the appropriate inductor and capacitor are selected to form a filter circuit, which can generally reduce the ripple well. However, in this case, the sampling point of the feedback comparison voltage needs to be considered.

If the sampling point is selected before the LC filter (Pa), the output voltage will decrease. Because any inductor has a DC resistance, when there is current output, there will be a voltage drop across the inductor, causing the output voltage of the power supply to drop. And this voltage drop varies with the output current.

After the injection molding module drives the power output, connect the LDO filter

This is the most effective way to reduce ripple and noise, the output voltage is constant, and the original feedback system does not need to be changed, but it is also the method with the highest cost and the highest power consumption. Any LDO has one indicator: the noise rejection ratio. After the LDO, the ripple is generally below 10mV.

on the diode and capacitor C or RC

When the diode is turned on and off at high speed, parasitic parameters must be considered. During the diode reverse recovery, the equivalent inductance and equivalent capacitance become an RC oscillator, producing high frequency oscillations. In order to suppress this high frequency oscillation, a capacitor C or RC snubber network should be connected in parallel across the diode. The resistance is generally 10Ω-100Ω, and the capacitor is 4.7pf-2.2nf.

The value of the capacitor C or RC connected in parallel with the diode can only be determined by trial and error. If it is not selected properly, it will cause more serious oscillation.

Diode followed by inductor (EMI filtering)

This is also a commonly used method to suppress high-frequency noise. According to the frequency that generates noise, selecting an appropriate inductive element can also effectively suppress noise. It should be noted that the rated current of the inductor must meet the actual requirements.