In the LED design, the ripple is theoretically and practically present. There are five ways to suppress or reduce it:
Increase inductance and output capacitance filtering
According to the formula of the LED driving power supply , 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 inductance value and output capacitance value can reduce the ripple.
The relationship between output ripple and output capacitance: vripple=Imax/(Co×f). It can be seen that increasing the output capacitor value can reduce the ripple.
As a general rule, for output capacitors, aluminum electrolytic capacitors are used to achieve high capacity. However, the electrolytic capacitor is not very effective in suppressing high-frequency noise, and the ESR is also relatively large, so a ceramic capacitor is connected in parallel with it to compensate for the shortage of the aluminum electrolytic capacitor.
At the same time, when the LED driving power supply is working, the voltage Vin at the input terminal does not change, but the current changes with the switch. At this time, the input power supply does not provide a good current, usually near the current input terminal (in the case of the BucK type, which is near SWITcH), and a parallel capacitor is used to supply current.
Secondary filtering is to add a first-order LC filter.
The LC filter suppresses the noise ripple significantly. Selecting the appropriate inductor and capacitor according to the ripple frequency to be removed constitutes the filter circuit, which can generally reduce the ripple. However, in this case, it is necessary to consider the sampling point of the feedback comparison voltage.
The sampling point is selected before the LC filter (Pa) and the output voltage is reduced. Because any inductor has a DC resistance, when there is a 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 is a function of the output current.
After the LED driver power output, connect to LDO filter
This is the most effective way to reduce ripple and noise. The output voltage is constant and there is no need to change the original feedback system, but it is also the most costly and power consuming method. Any LDO has an indicator: the noise suppression ratio.
After the LDO, the ripple is generally below 10mV.
On the diode and capacitor C or RC
Parasitic parameters should be considered when the diode is turned on and off at high speed. During 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 buffer network is connected in parallel across the diode. The resistance is generally 10Ω-100Ω, and the capacitance is 4.7pf-2.2nf.
The capacitance C or RC connected in parallel on the diode is determined by trial and error. If improperly chosen, it will cause more serious oscillations.
Diode followed by inductor (EMI filter)
This is also a common method of suppressing high frequency noise. Selecting a suitable inductive component for the frequency at which noise is generated can also effectively suppress noise. It should be noted that the rated current of the inductor should meet the actual requirements.
summary
The above is some common methods for reducing output ripple in LED design. Although it may not be comprehensive, it is sufficient for general applications. Regarding noise suppression, not all of them are actually applied. It is important to choose the appropriate method according to your own design requirements, such as product volume, cost, development cycle, etc.
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