How to Prevent Ripple in Switching Power Supplies
Our goal is to reduce output ripple to an acceptable level. The fundamental solution to achieve this is to minimize ripple generation as much as possible. First, it is essential to understand the types of ripple in switching power supplies and their causes.
As the switch operates, the current in inductor L fluctuates around the RMS value of the output current. As a result, ripple at the same frequency as the switching signal appears at the output. This is generally what is referred to as output ripple. It is related to the capacitance and ESR of the output capacitor. The frequency of this ripple matches the switching frequency of the power supply, typically ranging from tens to hundreds of kHz.
In addition, switches usually use bipolar transistors or MOSFETs. Both have a rise time and fall time during turn-on and turn-off transitions. This introduces noise at frequencies corresponding to or at odd harmonics of the rise/fall time, generally in the tens of MHz range. Similarly, during the reverse recovery transient of diode D, its equivalent circuit forms an RLC series circuit, which can cause resonance and generate noise also in the tens of MHz range. These two types of noise are commonly known as high-frequency noise, and their amplitudes are usually much larger than those of ordinary ripple.
For AC/DC converters, in addition to the above two types of ripple (noise), there is also AC line noise at the input AC frequency, approximately 50–60 Hz. There is also common-mode noise caused by the equivalent capacitance formed when power devices in many switching supplies use the chassis as a heat sink. Since I work in automotive electronics R&D, I have less exposure to these latter two types of noise, so they are not considered here.
Measurement of Switching Power Supply Ripple
Basic requirements: Use oscilloscope AC coupling, 20 MHz bandwidth limit, and remove the probe's ground lead.
AC coupling removes the superimposed DC voltage to obtain an accurate waveform.
Enabling the 20 MHz bandwidth limit blocks high-frequency noise interference and prevents erroneous measurement results. Since high-frequency components have large amplitudes, they should be excluded during measurement.
Removing the oscilloscope probe's ground clip and using a ground ring reduces interference. Many setups do not have a ground ring; if the error tolerance allows, the probe's ground clip can be used directly. However, this factor must be considered when judging compliance.
Another point is the use of 50 Ω termination. Yokogawa oscilloscope documentation states that a 50 Ω module removes the DC component and measures the AC component. However, few oscilloscopes are equipped with such specialized probes. In most cases, standard probes with 100 kΩ to 10 MΩ input impedance are used, and their impact is not yet clear.
The above are basic precautions for measuring switching ripple. If the oscilloscope probe is not directly connected to the output node, measurements should be performed using twisted-pair wires or 50 Ω coaxial cable.
When measuring high-frequency noise, use the oscilloscope's full bandwidth, typically ranging from several hundred MHz to the GHz range. All other settings remain the same as above. Different companies may employ different test methods. Ultimately, you must clearly understand your own measurement results and ensure they are acceptable to the customer.
