How to Test Switch Mode Power Supplies
2019-10-17 | 12 min read
A Quick Overview on Power Supplies
The primary purpose of a power supply is to efficiently produce well-regulated and low-noise DC power from an input power rail.
Linear power supplies used to be the standard. The advantages of linear power supplies are that they are low noise and don’t require much filtering. However, the downside is that they can only be used to step-down power. They are inefficient and costly. They can also be large, bulky, and generate a lot of heat.
The industry trends of smaller form-factors, packing more functionality into our devices, trying to decrease thermals, and bring down costs, have driven the market towards using switch mode power supplies. Switch mode power supplies are more efficient than linear supplies. You can get more power in smaller packages. They are also more versatile in that they can step-down or step-up power and be used for AC-DC and DC-DC conversions. The downside is they have relatively high noise or output ripple.
Depending on the part of the design you are working on, you might be looking at the switch mode power supply holistically, or only be concerned about one portion of the power supply – the input, the switching, or the output.
The input side converts and filters your input voltage (that’s typically an AC line voltage of 110V in the US or 220V in other regions). If you are primarily concerned with the input side of your supply, you’ll likely be focused on power quality, current harmonics, and inrush current. Figure 1 shows an example of how you’d connect your oscilloscope and probes if you were testing the input side of your supply.
Figure 1. How to probe the input side of your SMPS for a power quality test
After the input, your signal goes into the main part of the supply which is the switching transistor. This regulates the voltage with the duty cycle, or the amount of time it’s on vs off. If this is the part of the power supply you are focused on, you’ll be concerned about power losses, modulation analysis, slew rate, and safe operating area. Figure 2 shows an example of how you’d connect your oscilloscope and probes if you were testing the switching portion of your supply.
Figure 2. How to probe the switching transistor of your SMPS for switch loss and slew rate analysis
After the switching transistor, the signal gets filtered again and rectified so you get a stepped DC output which is then used to run power through the rest of your device. If you are focused on the output side of the power supply, you’ll want to focus on output ripple, turn-on and turn-off time, transient response, power supply rejection ratio, and efficiency. Figure 3 shows an example of how you’d connect your oscilloscope and probes if you were testing the output of your supply.
Figure 3. How to probe the output of your SMPS for an output ripple test
An oscilloscope is the most common tool for making power supply measurements since you can hook up both a voltage probe and a current probe to calculate power.
P = IV
Another reason why oscilloscopes are a great tool for characterizing power supplies is the analysis applications that can run on them, making testing much more efficient.
Step-by-Step Input Power Measurements - Power Quality Analysis
To follow along with your InfiniiVision oscilloscope, download a free trial of the power application bundle https://connectlp.keysight.com/Free-SW-Trial-Oscilloscopes
Follow these steps to analyze the power quality at the input of your SMPS:
- It’s always a good idea to clear any existing settings from previous tests by selecting Default Setup.
- Press Analyze and select the Power application. You can now see the full list of power applications supported by the InfiniiVision power application bundle. Choose power quality.
- Connect your probes
- Connect a differential voltage probe and current probe to the input side of your supply. See Figure 1
- Choose the Signals menu. Since the scope will calculate power from your current and voltage measurements, double-check that the channels are assigned on the scope to match how you’ve connected the probes
- At this point, you can also choose how many cycles you want to look at. I usually choose somewhere between 5 and 20, depending on what I need to analyze
- Now, simply select AutoSetup and view the power quality results by pressing Apply
Figure 5. Input power quality measurement
Since I’m using a power application, the scope automatically scales the signal properly to take advantage of the full bits on the oscilloscope for accurate measurements and sets up the waveform math – in this case, it’s V*I to get instantaneous power.
The application will also calculate the other power parameters, such as apparent power (S), reactive power (Q), power factor (PF), and phase angle (ø).
The primary advantage of having an automated power measurement application is that these are all calculated for you with the press of a button (Apply), so you don’t waste time working out the math with pencil or paper or having to extract the voltage and current measurement results to calculate power quality on a computer.
Step-by-Step Switch Phase Measurement – Switching Loss Measurement
You’ll lose energy primarily during the switching phases of the transistor when it turns on and off, and during the conduction phase when voltage is at the transistor’s saturated minimum and current flows.
To check if the losses are acceptable, follow these steps to make a switching loss measurement:
- It’s always a good idea to clear any existing settings from previous tests by selecting Default Setup
- Press Analyze and select the Power application. You can now see the full list of power applications supported by the InfiniiVision power application bundle. Choose the Switching Loss measurement
- Connect Your probes
- Connect a differential voltage probe and current probe around your pulse width modulator/transistor. See Figure 2
- Choose the Signals menu. Double-check that the channels are assigned on the scope to match how you’ve connected the probes. Also, choose the number of cycles you’d like to view – for this example, 2
- Press AutoSetup and Apply
Figure 6. Power loss measurement
You can see two switching cycles in the top view, and a zoomed-in view of one cycle on the bottom view. You’ll also get all the power loss measurements calculated and displayed on the right.
You can zoom in to look at each phase, the conduction, the switching, and the non-conduction phase.
The purple waveform is instantaneous power.
Step-by-Step Output Power Measurement – Output Ripple Analysis
Output ripple, or power rail integrity measurements, are particularly important if you are driving a high-speed digital device. The noise on the power rail could cause jitter and timing uncertainty and affect the validity of your digital signal transmission. Output ripple is typically dominated by switching noise but can also include other random noise and signal coupling from various sources in your system. This test measures the quality of the power supply’s voltage regulation and filtering to reject switching noise, as well as other noise/interference sources.
To set up this test, follow similar setups steps as before:
- Clear any existing settings from previous tests by selecting Default Setup
- Press Analyze and select the Power application. Choose Output Ripple
- Connect your probes
- Press AutoSetup and Apply
Figure 8. Output ripple measurement
In addition to the three measurements I covered step-by-step in this blog, you can use the InfiniiVision power application to test inrush current, effective resistance of your switching transistor, modulation, and slew rate. You can also measure transient response, efficiency, power supply rejection ratio, and control loop response.
You can use these measurements to quantify and optimize your designs on the input side, switching, and output side of your supply. Learn more about measuring switch mode power supplies in this webcast. Download a free trial of the power application bundle to try it out yourself: https://connectlp.keysight.com/Free-SW-Trial-Oscilloscopes