DC voltage fluctuations produce ripple. The component superimposed on DC is called ripple. In our daily applications, excessive ripple in the DC-DC output power supply may affect normally operating chips, and in severe cases, it may even cause the CPU to crash. For example, excessive VDD ripple of the onboard DDR chip may cause the CPU to make errors in reading and writing data to the DDR, and the CPU may access an illegal address space, causing the chip to crash.
The AC ripple of the power supply output can be regarded as a DC output superimposed with an AC component; as can be seen from the figure, the ripple includes two AC components: a superposition of a DC-DC output ripple signal and a high-frequency noise.
(1) Increase the output capacitor of BUCK:
Increasing the output capacitor capacity increases the energy stored in the power supply system. When the CPU requires a large current during loading, a larger capacitor on the power plane can provide the instantaneous energy needed by the CPU, minimizing voltage fluctuations. However, capacitor selection is crucial. For low-current power planes (load currents like 3A), adding a few ceramic capacitors may suffice, but for high-current power planes (load currents exceeding 100A), the increased capacitor capacity becomes significant, making ESR a critical factor. Typically, CPU core power supplies operate at low voltage and high current, generally choosing large-capacity, low-ESR polymer aluminum electrolytic capacitors rather than liquid aluminum electrolytic capacitors.
Power supply ripple is generally measured using an oscilloscope, and there are three common measurement methods:
1) Connection method
When using an oscilloscope probe with a ground loop, directly contact the probe to the positive output pin and the ground loop to the negative output pin. This is to minimize the loop length, so that the peak value read from the oscilloscope represents the ripple and noise on the output line. See the diagram below.
2) Direct method
Connect the grounding ring directly to the negative output pin and use the probe grounding ring to test the output.
3) Twisting method
Connect the output pin to a twisted pair cable and then to a capacitor. Use an oscilloscope to measure the distance between the two ends of the capacitor.
When measuring ripple, it is important to know the upper limit of the ripple bandwidth. Since ripple is low-frequency noise, an oscilloscope with a bandwidth not exceeding the upper limit of the ripple is generally used.
During measurement, the oscilloscope's bandwidth limiting function must be enabled first, limiting the bandwidth to 20MHz.
Connect the probe's shield ground and output ground directly to reduce loop interference caused by excessively long ground wires.
A small ceramic capacitor and a small electrolytic capacitor are connected in parallel at the probe access point to filter out external interference signals and prevent them from entering the oscilloscope.
IV. 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 and control.
Common methods to suppress these ripples include increasing the capacitance in the filter circuit, using an LC filter circuit, employing a multi-stage filter circuit, replacing the switching power supply with a linear power supply, and proper wiring. However, taking targeted measures based on their classification often yields better results.
1. Suppression of high-frequency ripple
High-frequency ripple noise often originates 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. This output typically contains high-frequency ripple with the same frequency as the switching operating frequency. The magnitude of its impact on external circuits depends mainly on the switching frequency of the power supply, the structure and parameters of the output filter. In the design, maximizing the operating frequency of the power converter can reduce the filtering requirements for high-frequency switching ripple.
2. Suppression of low-frequency ripple
The magnitude of low-frequency ripple is related to the size of the filter capacitor in the output circuit. The capacitance cannot be increased indefinitely, inevitably resulting in residual low-frequency ripple at the output. The AC ripple, after attenuation by the DC/DC converter circuit, falls within the low-frequency noise range, and its magnitude is determined by the gain of the control system and the DC/DC converter circuit. Since the ripple suppression capability of current-source and voltage-source controlled DC/DC converter circuits is relatively low, and their output low-frequency AC ripple is relatively large, filtering measures must be taken to achieve low-ripple output from 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. Low-frequency ripple can also be suppressed 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 typically occurs in switching power supplies. When a rectangular wave voltage from a switching power supply is applied to power devices, it interacts with the parasitic capacitance between the power devices and the heatsink base and the primary and secondary windings of the transformer, as well as the parasitic inductance in the wires, generating common-mode ripple noise. Methods for suppressing common-mode ripple noise include:
1) Reduce the parasitic capacitance between the control power devices, transformer, and chassis ground, and add a common-mode rejection inductor and capacitor at the output terminal;
2) EMI filters can effectively suppress common-mode ripple interference;
3) Reduce the amplitude of switching glitches.
4. Suppression of closed-loop control circuit ripple
The cause of closed-loop control loop ripple is generally due to inappropriate parameter settings in the loop. When there is a certain fluctuation at the output, the feedback network feeds the fluctuating voltage at the output back to the regulator loop, causing the regulator to generate a self-excited response, thereby generating additional ripple.
The main suppression methods include: suppressing the regulator's self-oscillation response, appropriately selecting the loop amplification factor, improving regulator stability, and connecting an LDO filter to the power supply output. These are the most effective methods for reducing ripple and noise.
Suppression of switching power supply ripple
Switching ripple is both theoretically and practically guaranteed to exist. There are generally five ways to suppress or reduce it:
1. Increase the inductance and output capacitor for filtering.
According to the formula for switching power supplies, the magnitude of current fluctuation within 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 and output capacitance values can reduce ripple.
Similarly, the relationship between output ripple and output capacitance is: vripple = Imax / (Co × f). It can be seen that increasing the output capacitance value can reduce ripple.
The usual practice is to use aluminum electrolytic capacitors for output capacitors to achieve a large capacitance. However, electrolytic capacitors are not very effective at suppressing high-frequency noise and have a relatively high ESR. Therefore, a ceramic capacitor is connected in parallel with it to compensate for the shortcomings of aluminum electrolytic capacitors.
Meanwhile, when the switching power supply is working, the input voltage Vin remains constant, but the current changes with the switch. In this case, the input power supply will not provide a good current supply, so a capacitor is usually connected in parallel near the current input terminal (near the SWITCH terminal in the BucK type, for example) to provide the current.
The above approach has limited effectiveness in reducing ripple. Due to size limitations, the inductor cannot be made very large; increasing the output capacitor to a certain extent no longer has a significant effect on reducing ripple; increasing the switching frequency will increase switching losses. Therefore, this method is not ideal when requirements are stringent. For more information on the principles of switching power supplies, please refer to various switching power supply design manuals.
2. Second-stage filtering, which is to add another LC filter.
LC filters are effective at suppressing noise and ripple. By selecting appropriate inductors and capacitors to form a filter circuit based on the ripple frequency to be removed, the ripple can generally be reduced effectively.
If the sampling point is selected before the LC filter (Pa), the output voltage will decrease. This is because any inductor has a DC resistance, and when there is current output, a voltage drop will be generated across the inductor, causing the power supply's output voltage to decrease. Moreover, this voltage drop varies with the output current.
The sampling point is chosen after the LC filter (Pb), so the output voltage is the desired voltage. However, this introduces an inductor and a capacitor into the power supply system, which may cause system instability. Many resources discuss system stability, so I won't go into detail here.
3. After the switching power supply output, connect an LDO filter.
This is the most effective way to reduce ripple and noise, maintaining a constant output voltage and requiring no changes to the existing feedback system. However, it is also the most expensive and power-consuming method. Every LDO has a key performance indicator: noise rejection ratio (RDR), which is represented by a frequency-dB curve.
To reduce ripple, the PCB layout of the switching power supply is also crucial, and this is a very challenging issue. There are dedicated switching power supply PCB engineers. For high-frequency noise, due to the high frequency and large amplitude, while subsequent filtering has some effect, the result is not significant. There is specialized research in this area; a simple approach is to connect a capacitor C or RC in parallel with the diode, or to connect an inductor in series.
4. Connect a capacitor C or RC in parallel with the diode.
When a diode is in high-speed conduction and cutoff, parasitic parameters must be considered. During the diode's reverse recovery period, the equivalent inductance and equivalent capacitance form an RC oscillator, generating high-frequency oscillations. To suppress these high-frequency oscillations, a capacitor C or an RC snubber network needs to be connected in parallel across the diode. The resistor is typically 10Ω-100Ω, and the capacitor is 4.7pF-2.2nF.
The value of the capacitor C or RC connected in parallel with the diode must be determined through repeated experiments. If an inappropriate capacitor is selected, it may cause more severe oscillations.
For applications requiring strict control over high-frequency noise, soft-switching technology can be used. Many books specifically discuss soft-switching.
5. Diode followed by inductor (EMI filtering)
This is also a commonly used method for suppressing high-frequency noise. Selecting an appropriate inductor for the frequency that generates the noise can also effectively suppress it. It's important to note that the inductor's rated current must meet the actual requirements. This is a relatively simple approach and will not be explained in detail.
