Generation of Power Supply Ripple Common power supplies we use include linear power supplies and switching power supplies. The DC voltage they output is obtained by rectifying, filtering, and stabilizing the AC voltage. Due to incomplete filtering, clutter signals containing periodic and random components will be attached to the DC level, thus generating ripple. Under the conditions of rated output voltage and current, the peak value of the AC voltage in the output DC voltage is what is commonly referred to as ripple voltage. Ripple is a complex clutter signal. It is a periodic signal that fluctuates up and down around the output DC voltage, but its period and amplitude are not fixed; instead, they change over time, and the ripple waveforms of different power supplies also vary. Hazards of Ripple Generally speaking, ripple is harmful and has no benefits. The main hazards of ripple are as follows: - Ripple carried in the power supply will generate harmonics in electrical appliances, reducing the efficiency of the power supply; - Higher ripple may generate surge voltage or current, which may cause electrical equipment to operate abnormally or accelerate equipment aging; - In digital circuits, ripple will interfere with circuit logic relationships; - Ripple will also bring noise interference to communication, measurement, and metering instruments, disrupting the normal measurement and metering of signals, and even damaging equipment. Therefore, when manufacturing power supplies, we must consider reducing the ripple to below a few percent, and for equipment with high requirements on ripple, we need to reduce the ripple to a smaller level. The measurement methods of power supply ripple are usually divided into two categories: one is the identification of a single power supply, and the other is the debugging measurement of products. When the power supply industry and power supply users identify the power supply, it is required to carry out the test indoors (around 20℃), with humidity less than 80%, and minimize the mechanical vibration and electromagnetic interference that affect the measurement. The standard instruments and the power supply to be inspected should be placed in the above test environment for more than 24 hours. For a pure power supply, when measuring the power supply ripple, it is required to measure under load, and the applied load should make the output current greater than 80% of the rated output current. For low-noise pure resistive loads or electronic loads, the corresponding measurement standards should also be selected. Different standards will lead to different measurement results. Ripple voltage can be expressed in absolute terms or relative terms. Generally, the ratio of ripple voltage to DC output voltage is used to evaluate the filtering performance of DC power supplies, that is, the ripple coefficient. As an important indicator for evaluating DC power supplies, the ripple coefficient is calculated as the percentage of the effective value of ripple voltage to the DC output voltage. Common Mistakes How to correctly use an oscilloscope to measure ripple often troubles some junior engineers. They often make the following mistakes during measurement: (1) Using an oscilloscope probe with a long grounding lead; (2) Placing both the loop formed by the probe and the grounding lead near the power transformer and switching components; (3) Allowing excess inductance to exist between the oscilloscope probe and the output capacitor. Expression Methods of Ripple and Ripple Coefficient They can be expressed by effective value or peak value, or by absolute quantity or relative quantity; The unit is usually: mV For example: A power supply works in a regulated state, with an output of 12V5A, and the measured effective value of ripple is 10mV. This 10mV is the absolute quantity of ripple. The relative quantity, that is, the ripple coefficient = ripple voltage / output voltage = 10mV / 12V = 0.12%. Measurement of Power Supply Ripple Oscilloscopes are generally used to measure power supply ripple, and there are three common measurement methods: 1. Direct Connection Method Use an oscilloscope probe with a ground loop, directly contact the probe with the positive output pin, and the loop directly contact the negative output pin. This is to minimize the loop, so that the peak value read from the oscilloscope is the ripple and noise on the output line, as shown in the figure below: 2. Direct Method Connect the ground loop directly to the negative output pin, and use the probe's grounding loop to test the output terminal. 3. Twisted Connection Method Connect the output pin to a twisted pair and then to a capacitor, and measure across the capacitor with an oscilloscope. When measuring ripple, it should be noted that the upper limit of the ripple bandwidth should be clear. Ripple is low-frequency noise, so an oscilloscope that does not exceed the upper limit of the ripple bandwidth too much is generally used. During measurement, first turn on the bandwidth limit function of the oscilloscope, limit the bandwidth to 20MHz, and directly connect the shielded ground of the probe to the output ground to reduce loop interference caused by too long a ground wire. 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. Methods for Suppressing Ripple The output ripple of the power supply 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. Common methods to suppress these ripples are: increasing the capacitance of the capacitor in the filter circuit, using an LC filter circuit, using a multi-stage filter circuit, replacing the switching power supply with a linear power supply, and reasonable wiring. However, according to its classification, taking targeted measures can often achieve twice the result with half the effort. 1. 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 ripple with the same frequency as the switching frequency. The impact on external circuits is mainly related to the switching frequency of the switching power supply, the structure and parameters of the output filter. In design, increasing the operating frequency of the power converter as much as possible can reduce the filtering requirements for high-frequency switching ripple. 2. 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 cause residual output low-frequency ripple. The AC ripple is output after being attenuated by the DC/DC conversion circuit, which belongs to the range of low-frequency noise, and its size is determined by the gain of the control system and the DC/DC conversion circuit. Since the ripple suppression capabilities of current-mode and voltage-mode controlled DC/DC conversion circuits are relatively low and their output low-frequency AC ripple is large, it is necessary to take filtering measures for low-frequency power supply ripple to achieve low ripple output of the power supply. For some power supplies, increasing the closed-loop gain circuit of the DC/DC converter and using a pre-regulator circuit can enhance the ripple suppression effect, and the suppression of low-frequency ripple can be achieved by changing the capacitance of the rectifier filter and adjusting the parameters of the feedback loop. 3. 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 interacts with the parasitic capacitance between the power device and the radiator base, between the primary and secondary sides of the transformer, and the parasitic inductance in the wires, generating common-mode ripple noise. Methods for suppressing common-mode ripple noise are: - Reduce the parasitic capacitance between the control power device, transformer, and the chassis ground, and add a common-mode suppression inductor and capacitor at the output end; - Using an EMI filter can effectively suppress the interference of common-mode ripple; - Reduce the amplitude of switching burrs. 4. Suppression of Closed-Loop Control Loop Ripple The ripple in the closed-loop control loop is generally caused by inappropriate parameter settings in the loop. When there is a certain fluctuation at the output end, the feedback network feeds back the fluctuation voltage at the output end to the regulator loop, causing the regulator to generate a self-oscillating response, thereby generating additional ripple. The main suppression methods are: suppressing the self-oscillating response of the regulator, reasonably selecting the amplification factor of the loop, ensuring the stability of the regulator, and connecting an LDO filter at the output end of the power supply, which is the most effective method to reduce ripple and noise. Sources: Jidecheng, LiNUS Engineer Notes, Electricians and Coders with AI Ambitions