LED design, for the ripple, theoretically and in fact are certain to exist. There are usually five ways to suppress or reduce it:
Increase inductance and output capacitance filtering
According to the formula of the LED drive power supply, the magnitude of the current fluctuation in the inductor is inversely proportional to the inductance value, and the output ripple is inversely proportional to the output capacitance. So increasing the inductance value and the output capacitance value can reduce the ripple.
Output ripple and output capacitance of the relationship: vripple = Imax / (Co × f). It can be seen that increasing the output capacitance can reduce the ripple.
The usual practice for the output capacitor, the use of aluminum electrolytic capacitors to achieve the purpose of large capacity. But the electrolytic capacitor in the suppression of high-frequency noise is not very good, and ESR is relatively large, so it will be in parallel with a ceramic capacitor, to make up for the lack of aluminum electrolytic capacitors.
At the same time, LED drive power supply work, the input voltage Vin unchanged, but the current is with the switch changes. At this time the input power supply does not provide a good current, usually near the current input (BucK type, for example, SWITcH near), parallel capacitor to provide current.
Secondary filter, that is, plus an LC filter
LC filter on the noise ripple suppression effect is obvious, according to the removal of the ripple frequency to select the appropriate inductance capacitance filter circuit, generally can reduce the ripple. However, in this case it is necessary to consider the sampling point of the feedback comparison voltage.
Sampling is done before the LC filter (Pa), and the output voltage is reduced. Because any inductor has a DC resistance, when there is current output, there will be a voltage drop in the inductor, resulting in reduced power supply output voltage. And this voltage drop is with the output current changes.
LED drive power output, then LDO filter
This is the most effective way to reduce ripple and noise, the output voltage is constant, do not need to change the original feedback system, but also the highest cost, the highest power consumption approach. Any LDO has an indicator: noise suppression ratio.
After LDO, the ripple is generally below 10mV.
On the diode and capacitor C or RC
When the diode is turned off at high speed, consider parasitic parameters. During diode reverse recovery, the equivalent inductance and equivalent capacitance become an RC oscillator that generates high frequency oscillations. In order to suppress this high frequency oscillation, the diode must be connected at both ends of the capacitor C or RC buffer network. Resistance generally take 10Ω-100Ω, the capacitor to take 4.7pf-2.2nf.
In the diode in parallel with the capacitance C or RC, the value of the test to be repeated to determine. If improper selection, but will cause more serious oscillation.
Diode after induction inductance (EMI filter)
This is also commonly used to suppress high-frequency noise. For the frequency of noise generation, select the appropriate inductance components, the same can effectively suppress the noise. It should be noted that the rated current of the inductor meets the actual requirements.
The above is on the LED design, reduce the output ripple of some common methods, although may not be full, but the general application is sufficient. On the noise suppression, the actual is not necessarily all applications, it is important according to their design requirements, such as product size, cost, development cycle, select the appropriate method.