“With the development of switching power supplies, more and more traditional linear power supplies have been replaced by their superior performance. However, the output ripple of switching power supplies has always been a headache for engineers, so what is power supply ripple and how does it occur? What? We know that the DC voltage output by the switching power supply is obtained by rectifying, filtering and stabilizing the AC voltage.
With the development of switching power supplies, more and more traditional linear power supplies have been replaced by their superior performance. However, the output ripple of switching power supplies has always been a headache for engineers, so what is power supply ripple and how does it occur? What? We know that the DC voltage output by the switching power supply is obtained by rectifying, filtering and stabilizing the AC voltage. Due to the unreasonable design of the filter circuit in the circuit, periodic and random clutter will be attached to the DC level, which will generate ripple. In the case of rated output voltage and current, the peak value of the AC voltage in the output DC voltage is the so-called ripple voltage. Simply put, ripple is the AC component superimposed on a stable DC output.
Figure 1 Main structure of switching power supply
Method for Testing Power Supply Ripple and Suppressing Power Supply Ripple
The ripple carried in the switching power supply will reduce the efficiency of the power supply. Higher ripple may also generate surge voltage or current, and in the digital circuit, the ripple will also interfere with the logic level relationship of the circuit, which is harmless. Advantageous; in the process of switching power supply design, we not only need to measure the ripple correctly, but also reduce the ripple as much as possible.
The correct way to measure ripple usually requires the following steps:
1. First of all, the probe should select the appropriate gear. Under normal circumstances, it is recommended to use the X1 gear to avoid unnecessary noise attenuation affecting the ripple measurement;
2. Select the channel coupling mode as AC coupling to limit the input of DC signals;
3. Turn on the “bandwidth limit” function of the oscilloscope and select “20MHz” bandwidth limit to filter out unnecessary high frequency noise;
4. In order to avoid electromagnetic radiation and other interference to the signal, it is recommended to use a “grounding spring” for grounding during measurement to avoid unnecessary interference caused by long grounding wires;
5. Adjust the horizontal time base, vertical scale and offset so that the ripple signal is displayed in the center of the screen (the blue box in Figure 2 shows the oscilloscope ripple test result).
Figure 2 Oscilloscope test ripple results
In order to effectively reduce the ripple, the circuit design can be improved from the following aspects:
1. Increase inductance or switching frequency
According to the formula of the switching power supply, the current fluctuation in the Inductor is inversely proportional to the inductance value, and the switching frequency can be increased to reduce the energy transferred each time, thereby reducing the ripple (but increasing the switching frequency will also increase the switching loss), or increase The large inductance value L that transmits energy reduces the sudden peak value of the current, thereby reducing the fluctuation range of the ripple.
Figure 3 Current waveform in the inductor
2. Increase the capacitance
Since the output ripple is inversely proportional to the output capacitor value, the output filter capacitor can be increased to reduce the ripple, and the capacitor cannot be increased indefinitely. Most switching power supply modules have the maximum capacitive load limit. It is also possible to combine capacitor C or RC on the diode and switch (as shown in Figure 4, D2, Q1). When the diode is turned on and off at high speed, during the 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, 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.
Figure 4 Switching power supply circuit
3. After switching power supply output, add voltage regulator
Adding a low dropout linear regulator (LDO) after the output of the switching power supply or module power supply can greatly reduce the output noise to meet the needs of circuits with special requirements for noise, and the output noise can reach μV level. Since the voltage drop of the LDO (the difference between the input and output voltage) is only a few hundred mV, the standard voltage can be output when the output of the switching power supply is a few hundred mV higher than the LDO, and its loss is not large.
4. Standardized PCB layout
There may be multiple module power supplies working together on the same PCB. If the module power supplies are unshielded and in close proximity, they may interfere with each other and increase the output noise voltage. In order to avoid this mutual interference, shielding measures can be adopted or it can be properly kept away to reduce the mutual interference. For example, two output capacitors can generally be used, one close to the rectifier tube and the other close to the output terminal. The effect of two small-capacity capacitors in parallel is better than that of one large-capacity capacitor. Multiple capacitors in parallel can improve the capacitance of the capacitor. frequency impedance characteristics.
In the power supply design, the above methods can be used to reduce the ripple, but more or less the advantages and disadvantages coexist. It is necessary to weigh the points that need to be improved to choose the appropriate solution. Of course, you can also choose the off-the-shelf stable power supply. , isolation parts, etc. have been highly integrated. Based on nearly 20 years of power supply design experience, Zhiyuan Electronics has independently developed and designed its own power supply IC to create a DC-DC power supply that is optimized for all working conditions to meet the needs of all working conditions and provide users with stable and high-quality power supply solutions. The P series isolation is based on the self-developed switching power supply chip ZLG1002. Compared with the traditional design, the ripple noise is as low as 40mV, creating a highly reliable power supply environment for users. It also achieves a quiescent current as low as 5mA, the standby power consumption is only 25mW, and the standby is as quiet as sleep, which can effectively reduce the energy loss during standby.