Let’s briefly talk about the measurement of power supply ripples. How to suppress these ripples?

1. Generation of power supply ripple

Our common power supplies include linear power supplies and switching power supplies. The DC voltage they output is obtained by rectifying, filtering, and stabilizing the AC voltage. Because the filtering is not clean, clutter signals containing periodic and random components will be attached to the DC level, which produces ripples.

Under the rated output voltage and current, the peak value of the AC voltage in the output DC voltage is commonly referred to as the ripple voltage. Ripple is a complex clutter signal. It is a periodic signal that fluctuates up and down around the output DC voltage. However, the period and amplitude are not fixed values, but change with time, and the ripples of different power supplies The waveforms are also different.              

2. The harm of ripple

Generally speaking, ripple is harmful but not beneficial. The main hazards of ripple are as follows:

The ripple carried in the power supply will produce harmonics on the electrical appliances, reducing the efficiency of the power supply;

Higher ripple may produce surge voltage or current, causing electrical equipment to operate abnormally or accelerating equipment aging;

In digital circuits, ripple can interfere with circuit logic;

Ripple will also bring noise interference to communication, measurement and metering instruments and instruments, disrupt normal measurement and metering of signals, and even damage equipment.

Therefore, when making a power supply, we must consider reducing the ripple to less than a few percent. For equipment with high ripple requirements, we must consider reducing the ripple to an even smaller level. The measurement methods of power supply ripple are usually divided into two categories, one is the identification of a separate power supply, and the other is the debugging measurement of the product. When the power supply industry and power users conduct power supply appraisal, they are required to conduct it indoors (around 20°C). The humidity should be less than 80%. The surrounding mechanical vibration and electromagnetic interference that have an impact on the measurement should be minimal. The standard instrument and the power supply under test should be above Place it in the test environment for more than 24 hours. For pure power supplies, when measuring the power supply ripple, it is required to measure when loading, and the added load must make the output current greater than 80% of the rated output current. For low-noise pure resistive loads or electronic loads, the corresponding measurement standard must also be selected. Different standards will produce different measurement results. Ripple voltage can be expressed as an absolute quantity or as a relative quantity. Generally, the ratio of ripple voltage to DC output voltage is used to evaluate the filtering performance of the DC power supply, that is, the ripple coefficient. As an important indicator for evaluating DC power supply, the ripple coefficient is calculated as the percentage of the effective value of the ripple voltage and the DC output voltage.

3. Measurement of power supply ripple

Power supply ripple is generally measured with an oscilloscope. There are three commonly used measurement methods:

Close connection method: Use an oscilloscope probe with a ground loop, directly contact the probe with the positive output pin, and the wire loop directly contact with the negative output pin. This is because the loop should be kept as short as possible, so that it can be read from the oscilloscope. The peak value is the ripple and noise on the output line, as shown in the figure below.  

Direct method: Connect the ground loop directly to the negative output pin, and use the probe ground loop to test the output end.

Twisted connection method: The output pin is connected to a twisted pair and then connected to a capacitor, and an oscilloscope is used to measure both ends of the capacitor.  

When measuring ripple, you need to pay attention to: Be clear about the upper limit of the ripple bandwidth. Ripple is low-frequency noise, so generally use an oscilloscope that does not exceed the upper limit of the ripple bandwidth. When measuring, you must first turn on the bandwidth limiting function of the oscilloscope, limit the bandwidth to 20MHz, and directly connect the shielding ground of the probe to the output ground to reduce loop interference caused by too long ground wires. Connect a small ceramic capacitor and a small electrolytic capacitor in parallel at the probe access point to filter out external interference signals and prevent them from entering the oscilloscope.               

4. Ripple suppression methods

Power supply output ripple mainly comes from five aspects: low-frequency input ripple, high-frequency ripple, common-mode ripple noise caused by parasitic parameters, and ripple noise caused by closed-loop regulation control.

The usual methods to suppress these ripples are: increasing the capacitance in the filter circuit, using LC filter circuits, using multi-stage filter circuits, replacing switching power supplies with linear power supplies, reasonable wiring, etc. But according to its classification, targeted measures will often achieve twice the result with half the effort. Suppression of high-frequency ripple: High-frequency ripple noise mostly comes from high-frequency power conversion circuits. In high-frequency power conversion circuits, the input DC voltage is converted by high-frequency power devices and then rectified and filtered to achieve a regulated output, which generally contains high-frequency ripples with the same frequency as the switching operating frequency. Its impact on the external circuit is mainly related to the conversion frequency of the switching power supply and the structure and parameters of the output filter. In the design, the operating frequency of the power converter should be increased as much as possible to reduce the filtering requirements for high-frequency switching ripples. Suppression of low-frequency ripple: The size of low-frequency ripple is related to the size of the filter capacitor in the output circuit. The capacitance of the capacitor cannot be increased indefinitely, which will inevitably result in residual low-frequency ripple in the output. The AC ripple is attenuated by the DC/DC conversion circuit and then output. It belongs to the low-frequency noise range, and its size is determined by the gain of the control system and the DC/DC conversion circuit. Because the ripple suppression capabilities of current-mode and voltage-mode control DC/DC conversion circuits are relatively low and their output terminals have large low-frequency AC ripples. Therefore, filtering measures must be taken to reduce the low-frequency power supply ripple to achieve low-ripple output of the power supply. For some power supplies, the ripple suppression effect can be enhanced by increasing the closed-loop gain circuit of the DC/DC converter and using a pre-stage pre-stabilizer circuit. This can be achieved by changing the capacitance of the rectifier filter and adjusting the parameters of the feedback loop. Suppression of low frequency ripple. Suppression of common mode ripple: Common mode ripple noise generally appears in switching power supplies. When the rectangular wave voltage of the switching power supply acts on the power device, it is related to the parasitic capacitance between the power device and the radiator bottom plate and the primary and secondary sides of the transformer. There are parasitic inductance interactions in the wires, causing common mode ripple noise. Methods for common mode ripple noise suppression include:

Reduce the parasitic capacitance between the control power device, the transformer and the chassis ground, and add a common mode suppression inductor and capacitor at the output end;

The use of EMI filters can effectively suppress the interference of common mode ripple;

Reduce switching glitch amplitude.

Suppression of closed-loop control loop ripple: The cause of closed-loop control loop ripple is generally inappropriate parameter settings in the loop. When there is a certain fluctuation at the output end, the feedback network feeds back the fluctuating voltage at the output end to the regulator loop. This causes the regulator to produce a self-excited response, resulting in additional ripples. The main suppression methods include: reasonably selecting the loop amplification factor, suppressing the self-excited response of the regulator, regulating the stability of the regulator, and connecting the power output terminal with LDO filtering. This is the most effective method to reduce ripple and noise.