Common Problems with DC-DC Converters: Large Output Ripple DC-DC converters are becoming increasingly common in modern electronic products, but so are the problems they cause. Below, we will introduce common problems with DC-DC output. This is part of a series; today we will discuss the output voltage ripple of DC-DC converters and how to improve it. Ripple: AC output ripple can be considered as a DC output superimposed with an AC component. As shown in the diagram, the ripple includes two AC components: one is a sine wave with a frequency twice that of the power frequency input voltage, caused by the AC input rectifier circuit.
Power supply ripple is a crucial parameter for switching power supply modules or DC/DC converters. Power supply ripple can be understood as the fluctuation in the output voltage of the power module, including the VRM, unrelated to the complex power supply network; or, more specifically, the fluctuation in the voltage at the source of the power supply output. Power supply noise, on the other hand, refers to the voltage fluctuation at the chip pins when the power module operates in an actual product system, as the power is delivered to the chip pins via the power distribution network; or simply, the voltage fluctuation at the end of the power supply output. Power supply noise, after being transmitted from the output through the power supply network (PDN) to the chip pins, may introduce interference from other circuit components, such as clock crosstalk, and other noise generated during the circuit's operation, in addition to the inherent power supply ripple.
I. Introduction
In layman's terms, the fluctuations in the DC power supply output that are at the same frequency as the power switching frequency are called ripple, while high-frequency noise is called noise. As shown in the diagram below, the lower-frequency, regular fluctuations are ripple, and the spikes are noise.
II. Ripple Measurement
1. Probe selection and measurement location
Choose the appropriate probe setting. For high voltage or bandwidth requirements, use the X10 setting. Under normal circumstances, X1 is recommended. A 10:1 probe attenuates the measured signal by a factor of 10, which the oscilloscope then amplifies by a factor of 10. However, this also amplifies the oscilloscope's inherent noise floor. In most cases, use the X1 setting to avoid unnecessary noise attenuation affecting ripple measurement.
Low frequency ripple
Low-frequency ripple is related to the capacitance of the filter capacitor in the output circuit. Due to the size limitations of switching power supplies, the capacitance of electrolytic capacitors cannot be increased indefinitely, resulting in residual low-frequency ripple in the output. The frequency of this output ripple varies depending on the rectifier circuit configuration.
A typical switching power supply consists of two parts: an AC/DC converter and a DC/DC converter. The basic structure of the AC/DC converter is a rectifier and filter circuit. Its output DC voltage contains low-frequency AC ripple, the frequency of which is twice the frequency of the input AC power supply. The amplitude is related to the power supply output power and the capacitance of the filter capacitor, and is generally controlled within 10%. After being attenuated by the DC/DC converter, this AC ripple manifests as low-frequency noise at the output of the switching power supply. Its magnitude is determined by the turns ratio of the DC/DC converter and the gain of the control system.
(Low-frequency ripple)
For example, for a typical 24V power supply, the ripple rejection ratio of a voltage-source controlled DC/DC converter is generally 45–50 dB, and the effective value of its low-frequency AC ripple at the output is 60–120 mV. The ripple rejection ratio of a current-source controlled DC/DC converter is slightly improved, but its low-frequency AC ripple at the output is still relatively large. To achieve low-ripple output from a switching power supply, filtering measures must be taken to eliminate the low-frequency power ripple. This can be achieved by using a pre-regulator and increasing the closed-loop gain of the DC/DC converter.
2. Suppression of low-frequency ripple
a. Increase the inductor and capacitor parameters of the output low-frequency filter to reduce the low-frequency ripple to the required level.
b. Use feedforward control to reduce low-frequency ripple components.
3 High-frequency ripple
High-frequency ripple noise originates from high-frequency power switching converter circuits. In these circuits, the input DC voltage is switched and rectified by power devices before being filtered and regulated for output. The output of these circuits contains high-frequency ripple with the same frequency as the switching operation frequency. The magnitude of this ripple's impact on external circuits depends mainly on the switching frequency of the power supply and the structure and parameters of the output filter. Increasing the operating frequency of the power converter as much as possible during the design process can reduce the filtering requirements for high-frequency switching ripple.
(High-frequency ripple)
4. Suppression of high-frequency ripple
a. Increasing the operating frequency of the switching power supply can increase the high-frequency ripple frequency, which helps to suppress high-frequency output ripple.
b. Increasing the output high-frequency filter can suppress output high-frequency ripple.
c. Employ multi-stage filtering.
5 Common-mode ripple noise
Because of parasitic capacitance between power devices and the heat sink base plate and the primary and secondary sides of the transformer, and parasitic inductance in the wires, common-mode ripple noise will be generated at the output of the switching power supply when a rectangular wave voltage is applied to the power devices. Reducing and controlling the parasitic capacitance between the power devices, the transformer and the chassis ground, and adding a common-mode rejection inductor and capacitor on the output side can reduce the common-mode ripple noise at the output.
6 Common-mode ripple noise
a. The output uses a specially designed EMI filter.
b. Reduce switching glitches
7. Ultra-high frequency resonant noise
Ultra-high frequency resonant noise mainly originates from the resonance of the diode junction capacitance during reverse recovery of the high-frequency rectifier diode, the junction capacitance of the power device and the parasitic inductance of the line during switching of the power device, and the frequency is generally 1 to 10 MHz. Ultra-high frequency resonant noise can be reduced by selecting diodes with soft recovery characteristics, switching transistors with small junction capacitance and reducing wiring length.
(Ultra-high frequency resonant noise)
8. Suppression of UHF resonant noise
Ultra-high frequency resonant noise can be reduced by using diodes with soft recovery characteristics, switching transistors with small junction capacitance, and reducing wiring length.
9. Ripple noise caused by closed-loop regulation and control
Switching power supplies all require closed-loop control of the output voltage. Inappropriate regulator parameter design can also cause ripple. When the output fluctuates, it enters the regulator loop through the feedback network, potentially causing self-oscillation of the regulator and resulting in additional ripple. This ripple voltage generally does not have a fixed frequency.
(Ripple noise caused by closed-loop regulation and control)
10. Suppression of ripple noise caused by closed-loop regulation and control
In switching DC power supplies, improper selection of regulator parameters often leads to an increase in output ripple. This ripple can be suppressed by the following methods.
a. Adding a compensation network to ground at the regulator output can suppress the increase in ripple caused by regulator self-excitation.
b. Reasonably select the open-loop gain and parameters of the closed-loop regulator. An excessively large open-loop gain may cause the regulator to oscillate or self-excite, increasing the output ripple content. An excessively small open-loop gain will cause the output voltage stability to deteriorate and the ripple content to increase. Therefore, the open-loop gain and parameters of the closed-loop regulator should be reasonably selected and adjusted according to the load conditions during debugging.
c. By not adding a pure time delay filter in the feedback channel, the time delay is minimized, thereby increasing the speed and timeliness of closed-loop regulation, which is beneficial for suppressing output voltage ripple.
