Engineers' National Day "travel" guide: start the prize-winning journey of power supply ripple measurement

Beijing, China, September 30, 2021 – China’s 72nd birthday also marks the 75th anniversary of Tektronix. As China undergoes 72 changes and maintains its ever-changing vitality, Tektronix oscilloscopes have also undergone “72 changes”, adhering to the concept of “born for engineers” and continuously evolving.

What did an oscilloscope look like in 1974? Look at the picture below. Model 556 is its name. What does an oscilloscope look like in 2021? Look at the picture above. He is also the protagonist of our power supply ripple measurement journey. Who is he?

Power ripple is one of the important indicators of power quality. In addition to engineers, ordinary users are also concerned about the size of the ripple. Usually in the laboratory, an oscilloscope is used to measure power ripple, but the specific operation process has problems such as large arbitrariness and low reproducibility. Starting from the basic concepts, this article takes Raspberry Pi Pico as an example to introduce the basic process of power ripple measurement. This article uses actual measurement demonstrations to describe the skills and key points that need to be mastered in ripple measurement, helping users achieve the goal of fast and accurate ripple measurement.

The concept of power ripple

With the development and progress of integrated circuits, the power supply voltage of electrical equipment is getting lower and lower. For example, the power supply voltage of mainstream microprocessors is as low as about 1V, and the LP-DDR series memory used in mobile devices has a maximum power supply voltage of no more than 1.8V. These electrical equipments that are very close to the silicon threshold voltage have also put forward higher and higher requirements for the quality of power supply. In addition to power engineers who pay attention to the quality of power supply, ordinary users will also pay attention to and improve the quality of power supply through different means after suffering from low-quality power supply. For example, in the circle of high-fidelity (Hi-Fi) audio enthusiasts, there are jokes such as "thermal power is large, hydropower has high resolution, and the hydropower effect of the Yarlung Zangbo River is good." There is no shortage of people who spend a lot of money to buy an expensive power cable in order to improve the quality of power supply. Although some of the views and behaviors of Hi-Fi enthusiasts lack sufficient scientific basis, they also reflect from one side that the impact of power supply on electrical equipment is crucial.

One of the more important indicators of power quality is power ripple. Power ripple is generally considered to be an unnecessary AC component superimposed on the DC component in the DC power output. These AC components are usually generated during the AC-to-DC conversion process because the circuit limitations cannot completely filter out the unnecessary frequency components. It is worth noting that although the voltage generated by the battery is fixed in a short period of time and there is no ripple in principle, we still need to pay attention to the ripple of the power supply for battery-powered devices. On the one hand, as the battery capacity is consumed, the voltage will gradually decrease. In order to ensure that the input voltage of the power-consuming device is constant, the battery voltage will be converted through the DCDC converter, which will introduce additional AC components. On the other hand, the voltage requirements of power-consuming devices are inconsistent. A consumer-grade device will require multiple different power rails, so multiple voltage converters will be introduced, which will generate different AC components.

Basic Power Supply Ripple Measurements

Here, we take the power module of a common Raspberry Pi Pico development board as an example to introduce the basic process of power ripple measurement.

2.1 Introduction to Raspberry Pi Pico Power Supply

Raspberry Pi Pico is a small and practical MCU board. It is powered by a RT6150B from RICHTEK, with an output voltage of 3.3V. The circuit is shown in Figure 1. RT6150B is a Buck-Boost converter, so the input voltage can be higher or lower than 3.3V. The board is powered by 5V from the USB interface, which implements a step-down conversion. It is worth noting that RT6150B has a Power Save Mode (PSM). When the chip's 7th pin (PS) is pulled low, PSM is enabled, and the chip works in PFM mode, which is more efficient, but the ripple is also higher. When PS is pulled high, PSM is disabled, and the chip works in PWM mode, which reduces efficiency at light loads, but the ripple is also lower.

Figure 1: Raspberry Pi Pico power supply circuit

In actual measurement, we use software to control PS to pull down or pull up, so as to switch the power supply mode between PFM and PWM, and then compare the difference between the two. In terms of measurement points, there is a capacitor C2 at the power supply output, and we can measure the voltage across C2 to measure the ripple.

2.2 Oscilloscope Settings

Probe: Ripple is an AC voltage component superimposed on the DC component of the power supply, so it is similar to the measurement of ordinary voltage signals. You can choose a passive voltage probe. If the probe can be set to attenuation, such as 1X and 10X, it needs to be set to no attenuation, that is, 1X.

Probe ground wire: unplug. That's right, remove all ground extension wires from the probe, including the most commonly used ground clip. Use a ground spring to ground the probe. The ground spring is a standard accessory for passive probes and can be connected to the ground wire on the board using the shortest path.

Vertical channel: set to AC coupling; bandwidth limit set to 20 MHz; in line with the principle of coarse first and fine later, the vertical scale can be set to a larger value, such as 50mV/div; check and confirm that the probe attenuation is correctly set to 1X. Figure 2 is an example of the setting of an oscilloscope vertical channel.

Figure 2: Oscilloscope vertical channel settings

Time scale: Based on the principle of coarse first and fine later, the time scale can be set larger at first, such as 1ms/div, and then zoom in to view the details after observing the signal later.

Trigger system: Since AC coupling is used, the trigger level can be set to 0V and edge triggering can be used.

2.3 Measurement waveform

Using the above configuration, the AC voltage across the output capacitor can be measured as shown in Figure 3 and Figure 4. For easy comparison, the vertical scale of the two figures is uniformly set to 5mV/div.

It is not difficult to find that the power ripple in PFM mode is significantly larger than that in PWM mode, which is consistent with the description in the datasheet.

Figure 3: PFM mode ripple

Figure 4: Ripple in PWM mode

The specific ripple value can be obtained by counting grids, cursors or the automatic measurement function of the oscilloscope.

About Tektronix

Headquartered in Beaverton, Oregon, USA, Tektronix is ​​committed to providing innovative, accurate and easy-to-use test, measurement and monitoring solutions to solve various problems, release insights and promote innovation. For more than 70 years, Tektronix has been at the forefront of the digital age.