Dedicated oscilloscope and software to improve the efficiency of development and testing

In order to meet the efficiency and form factor requirements, switch-mode power supply manufacturers continue to adopt new semiconductor and circuit topologies, while also insisting on complying with higher power integrity standards. Therefore, designers must perform more complex measurements on switch-mode waveforms, and this will extend the test time and increase costs.
In order to shorten the test time, test equipment manufacturers have developed a variety of hardware and software, which can be used in conjunction to realize the automation of many functions and complex analysis. Teledyne LeCroy's HDO4104 oscilloscope and power analyzer software is a good example of the combination of hardware and software. This article will describe how they can be used in conjunction to perform critical analysis, thereby shortening the development and testing time of the power supply.
Efficiency, weight, size are the motivations driving the development of innovative design and testing methods
Switch-mode power conversion, whether in power supplies, inverters, DC-DC converters, or motor controllers, has the advantages of improving power efficiency and reducing size and weight at specific power levels. However, these features can also bring disadvantages, including increased complexity and increased component stress.
In order to solve this problem, manufacturers have begun to consider the use of new semiconductor types, more diverse circuit topologies, and higher power integrity standards. However, this requires detailed analysis of some key features, such as:
  Power loss
Safe Working Area (SOA)
Control loop margin
line power integrity
Power performance
To perform these types of analysis, it is necessary to improve the level of necessary development and test engineering technology, which in turn will increase the test time and increase the cost of the final product. HDO4104 and power analyzer software are designed to simplify these types of analysis.
HDO4104 is a high-resolution 12-bit oscilloscope with touch screen. The power analyzer software uses the oscilloscope's touch screen to create a dedicated user interface. With the automatic measurement function, we can use a combination of hardware and software to simplify key power switching device measurement, control loop modulation analysis, and line power harmonic testing. Teledyne LeCroy also provides a variety of accessories, including differential amplifiers, differential probes, current probes and anti-skew devices.
Using HDO4104 for device analysis
Switch mode power supplies usually use pulse width modulation (PWM) of the power supply device to control the output voltage. The voltage and current in the power device will switch between on and off states. The phase shift law of the waveform is that when the voltage is high, the current is low. Therefore, the power dissipated by the switching device is very small: only when switching between the on and off states, will the power be dissipated (Figure 1).


Picture of Teledyne LeCroy HDO4104 with power analysis function
Figure 1: In the process of using the Teledyne LeCroy HDO4104 with power analysis function to analyze the switch mode power supply, the voltage of the power transistor and the current flowing through the transistor will be collected. Then use these values to calculate and display the power dissipation. (Photo: Digi-Key Electronics)
The figure above shows the basic device measurement using power analysis software. The dialog box at the bottom of the figure shows the settings and how they are organized, so they can be operated from left to right. When the power analysis operation is enabled for the first time, the user selects the signal source. In this example, we used Teledyne LeCroy DA1855A differential amplifier and CP030 current probe.
The next step is to choose one of the four types of analysis. Later, we then select the specific tests associated with each type of analysis. In Figure 1, the analysis type we selected is "Device", and the test value displayed is the device loss. We use the switching device voltage (yellow trace at the top) and the current flowing through the switching device (red trace at the bottom) to calculate its power dissipation (yellow trace at the bottom). The interface uses a color overlay, which can automatically display the time period for the device to conduct and close.
The table below the graph lists the power dissipation of the device at each stage of the working cycle. These stages are on, conduction, off, and off states. Loss can be measured by power or energy loss. The energy loss shown is in Joules. Please note that this calculation is performed automatically based on the acquired waveform.
There are some other tabs on the 　　 dialog box, which are used to input settings, area recognition, and power trace scaling. The input setting is very important because it can be used to identify the voltage and current probes used by the oscilloscope. The important function is the anti-skew control of these probes, so that the current and voltage measurements are kept synchronized in time.
User Tip: Anti-torsion is very important for measuring power loss. If the power loss in the off state is close to zero, it means that the torsion resistance has been set correctly. In the example, the anti-skew setting is correct, and the power loss in the off state is less than one thousandth of the other loss readings.
Draw SOA diagram to confirm work margin
There are limits on the voltage, current and power of each switching device. These limits are specified by the device manufacturer and will be listed in its technical specifications. To ensure the reliability of the power supply, these limits must not be exceeded. The safe operating area (SOA) diagram helps confirm the working margin, including all aspects of circuit operation. The SOA graph is the X-Y axis graph of the device voltage and the current flowing through the device. Figure 2 shows the current and voltage waveforms of the power FET of the switch-mode power supply, as well as the SOA diagram for 30 power cycles.
Picture of 　　 Safe Work Area Diagram


Figure 2: The safe operating area diagram on the right records the current and voltage traces of multiple power cycles. The voltage point is at the right end of the graph. The current is at the top. Both points cross the test mask and are highlighted in red. (Photo: Digi-Key Electronics)
The vertical axis of the SOA graph represents current, and the horizontal axis represents voltage. The sample-by-sample product of these two signals is the instantaneous power dissipation of the device. At the point in the lower left corner of the SOA diagram, both current and voltage are zero. The point on the right is the voltage of the device. The vertical limit point is the current flowing through the device.
The SOA diagram will be displayed in the test mask, which can be described by the following four points:
  Voltage
current
Current at voltage
Voltage at current
The user enters these points in the dialog box shown in Figure 2, and the input value is the limit value of the device under test specified by the manufacturer. If the SOA diagram crosses the test mask, a red circle highlights the sample. This circle indicates that the mask test tolerance limit has been exceeded. The figure shows two examples of such intersections.
SOA graph mainly involves voltage, current and power value, while the measurement of FET dynamic on-resistance (Rds(ON)) requires detailed measurement of FET saturation voltage and channel current (Figure 3).


FET dynamic on-resistance picture
Figure 3: FET dynamic on-resistance (Rds(ON)) is calculated based on the ratio of saturation voltage to channel current. (Photo: Digi-Key Electronics)
This measurement may be difficult to perform, because in the presence of a voltage swing of several hundred volts, the oscilloscope will only find a signal level of a few volts.
A good differential amplifier, such as Teledyne LeCroy's DA1855A, can act as a signal conditioning preamplifier for simpler and more accurate measurements. DA1855A also has a fast overdrive recovery function. DA1855A uses a ÷ 100 probe, so it can stabilize to within 100 mV within 100 ns, using the input from the 400 volt input signal as the reference input. In this way, the signal of the MOSFET can be overdriven outside the screen, and the vertical scale can be used to scale the part of interest. Please note that trying this technique directly on the oscilloscope input will usually cause the instrument's front-end amplifier to saturate.
With the rapid overdrive recovery function of DA1855A, users can overdrive the signal without worrying about errors when the trace returns to the screen. Therefore, the user can obtain the details of the saturation voltage.
To calculate the channel on-resistance, it is necessary to collect the FET saturation voltage and its corresponding channel current. Here, simply apply Ohm's law to calculate the resistance value. During the conduction phase of the power cycle, the power analyzer software can isolate the FET voltage and automatically perform the measurement using the correct voltage and current segments. In the example shown in Figure 3, the result is 1.1 Ω.
Other tests related to the device include the time rate of change of voltage and current waveforms (dV/dt and dI/dt). It shows the time rate of change of voltage and current when the device is turned on and off.
Device testing can also be extended to magnetic measurement in the form of B-H curve analysis. The B-H curve can show the saturation state of magnetic devices such as transformers. It is a graph of magnetic flux density (B) and magnetic field strength (H).
Device Analysis Toolbox can provide a complete view of key issues related to switching devices and important magnetic components. The next measurement challenge is the switch-mode control loop.
Control loop analysis
Switch mode power converters use feedback to control the output voltage or current and keep the output within acceptable limits. Most switch-mode power supplies use PWM in their control loop. Increasing the width of the driving pulse can increase the output voltage, and decreasing the width can decrease the output voltage. Through control loop analysis, we can study this feedback loop up close, especially during transient conditions. However, to analyze the loop dynamics, we must be able to demodulate the PWM signal.
power analyzer software provides simple and easy-to-use modulation analysis function, can measure the width or duty cycle of each pulse, and can draw a graph of the pulse width or duty cycle over time. This tool is extremely convenient, allowing you to visually view the time domain response of the entire control loop, including any time constants added by the pulse width modulator in the control IC. To clarify this concept, Figure 4 shows the effect of a step load change on the pulse width of the gate drive signal of the switching FET.


The response picture of the control loop to the step change of the load
Figure 4: Shows the response of the control loop of the gate drive signal to the step change of the load. The control loop analysis diagram shows the controller response, which manages load changes by changing the duty cycle of the gate drive signal. (Photo: Digi-Key Electronics)
The load current, which is trace C2 (red) in the upper grid, has a positive step of 500 mA. The gate drive signal can be collected on the trace C1 (yellow), and the response of the controller to the load change will be displayed, which is represented by the width change of the gate drive pulse. The control analysis trace (blue) shows the dynamic response of the control loop to step changes in load current.
The trace in the center grid is the horizontal scaling of the gate drive signal. The zoomed part is highlighted on the gate drive trace in order to show its position relative to the load change. The user can view the pulse width change very clearly on the zoom trace.
The measurement parameter table below the trace grid shows the value, average, value, value and standard deviation of each parameter, including the frequency period, width and duty cycle of the gate drive signal. This table quantitatively describes the change of the gate drive signal before and after the load change. In this example, the key values are the value, value, and value of the duty cycle. The value is the initial duty cycle before the load change. At this point, the duty cycle is only 4.8%. At the top of the control analysis chart, the duty cycle is 15.5%, which is the value of the duty cycle parameter. The fixed value of the duty cycle in the collection history is the value of the duty cycle. It is the duty cycle value reading in the waveform, and its value is 8.9%.
The control loop analysis type can also be used to track the response of the PWM regulator during power up or shutdown. Figure 5 shows the startup of the power supply. The traces, from top to bottom, are drain-source voltage, drain current, FET gate drive voltage, and gate drive signal duty cycle. The trace at the bottom shows the change in the gate drive duty cycle during the entire start-up process.
The picture of monitoring the PWM duty cycle change during the power-on process


Figure 5: Monitor the PWM duty cycle change during the power-on process. The trace at the bottom (dark blue) shows the change in the gate drive duty cycle during the entire start-up process. (Photo: Digi-Key Electronics)
When choosing an oscilloscope, the designer must understand that to analyze the power converter startup, an oscilloscope with long acquisition waveform memory is needed. This example shows a 10 millisecond start-up acquisition, using 2.5 megasamples of acquisition waveform memory. Teledyne LeCroy's HDO4000 series oscilloscopes can provide up to 50 megasamples of acquired waveform memory. The acquisition time depends on the memory length and sampling period. Collecting the same data at the required sampling rate provides a 10-second acquisition window for all four channels.
So far we have analyzed the output side of the power converter. The next two analysis functions are mainly aimed at the primary side of the switch-mode converter, that is, the line side.
Line power analysis
Line power analysis includes two test functions, namely power measurement and harmonic analysis. Power measurement can determine line voltage and current. They can also measure and display actual power, apparent power, and reactive power. In addition, users can choose to calculate and display power factor, phase angle and crest factor. The "Harmonic" test option is designed to measure line harmonics and verify whether they comply with international standards such as EN 61000-3-2. This compliance test is extremely important to ensure that excessive harmonic power is not injected back into the power supply from the switch-mode device.
line harmonic analysis is shown in Figure 6. In addition to the line voltage and current in channels C1 and C2, the first 20 odd harmonics of the 60 Hz line current are displayed in the spectrum display. The blue coverage area shows the compliance level of the EN 61000-3-2 standard. The table below the trace lists all these harmonics, including level, frequency, and compliance.
Line harmonic analysis to evaluate the picture of the harmonic content of the line current
Figure 6: Line harmonic analysis uses spectrum analysis method to evaluate the harmonic content of line current. According to the EN 61000-3-2 standard, the harmonic amplitude is tested against the built-in test mask. (Photo: Digi-Key Electronics)
After introducing the switching devices, control loops, power loss, and input power, one factor of power analysis that we will discuss is overall performance.
performance analysis
Performance analysis includes two important tests. The term is device efficiency. This item
The test measures the total input and output power of the switch-mode converter under test, and then uses this information to calculate device efficiency.
The second performance test is the output ripple. This is a very important power integrity measurement, because excessive ripple on the power bus may cause additional jitter in the clock and data signals (Figure 7).
Ripple analysis in time domain and frequency domain Output ripple picture
Figure 7: Output ripple is a very important power integrity measurement. Ripple analysis can display output ripple in the time domain and frequency domain. The trace on the left is the timing diagram of the ripple. The grid on the right contains the frequency spectrum of the ripple. The table on the left lists ten components of the ripple frequency. (Photo: Digi-Key Electronics)
Figure 7 shows the output current (traces in the upper left corner) and AC coupling voltage (middle left). The zoomed graph of the output voltage shows the extended ripple view in the time domain. We can clearly see that this ripple has a high periodicity. The basic period of cursor measurement is 450 kHz. The ripple parameter lists a peak-to-peak ripple voltage of 65.2 millivolts.
The trace in the grid on the right is the frequency spectrum of the ripple voltage. The upper left table lists the frequency and amplitude of ten frequency components of amplitude. They are also marked on the spectrum display. Understanding the frequency components of the ripple helps users determine possible signal sources.
  in conclusion
As various applications require devices with higher efficiency, smaller size, and lower cost, power measurement has become increasingly complex. As described in this article, Teledyne LeCroy's HDO4104 oscilloscope used in conjunction with the power analyzer software can significantly simplify the test and shorten the test time. The oscilloscope provides a dedicated user interface and automatic measurement function, so it can simplify the key measurement of power switching devices, PWM control loop analysis, line power efficiency and harmonic testing. In addition, Teledyne LeCroy also offers numerous probing solutions to set voltage and current easily and quickly.
Using appropriate equipment and software, power measurements can now be performed easily.