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Rebbeca Schulke
Power quality measurements, Current Harmonics, Input Capacitance, and Inrush Current are the four common sets of measurements made on the input section of a power supply, to analyze the effects of the power supply on the power line and evaluate the performance of the supply under various line conditions.
These measurements are optimized for line frequencies and are commonly performed at the AC line input of the power supply. They provide fast insight into the amount of power and the level of distortion at the input.
Any power supply with a non-linear device on its input (e.g. a rectifier) presents a nonlinear load to the AC line. Unless mitigated, excessive harmonic energy can affect the operation of other equipment connected to the power line and increase the cost of delivering the electric power. This has resulted in standards limiting harmonics generated by line-powered devices.
Advanced Power Measurement and Analysis includes test limits for the IEC61000-3-2, AM 14, and MIL- STD-1399 standards to help you perform pre-compliance testing before investing in official compliance testing. It presents up to 100 harmonics in graphical and tabular formats, and lets you easily traverse though the list to get details on any individual harmonic.
Inrush current, input surge current or switch-on surge is the maximum, instantaneous input current drawn by an electrical device when first turned on. Power converters have inrush current that is more than their steady state current due to the charging current of input capacitance. Measuring inrush current and input capacitance is important to ensure the design works effectively.
Although almost all components of a power supply contribute to energy losses, a significant portion of energy losses in a switch-mode power supply (SMPS) occur when the switching transistor transitions from a Turn-off (Toff) to a Turn-on (Ton) state and vice versa (Turn-off loss). By measuring the voltage drop across the switching device and the current flowing through the switching device, Advance Power Analysis automatically calculates switching loss measurement parameters for each cycle.
Until recently, taking switching measurements on the high side of half-bridge switching stages were almost impossible. Any measurement relative to the switching node, including high-side VDS and voltages across current shunts, suffered from distortion due to the significant common-mode voltage signal impinging on the differential signal. This problem is worse with wide bandgap devices, such as GaN and SiC transistors, as switching frequencies increase and the need to optimize new designs becomes imperative.
The 4/5/6 Series MSO is designed to work with IsoVu optically isolated probes, enabling designers to perform accurate switching measurements even in the presence of high common mode signals. Switching Loss shows power dissipation in a FET. Waveforms are annotated with color-coded markers showing the measurement regions for Ton, Toff, and Total cycle, corresponding to values in the results badge. Controls in the results badge let you easily traverse from cycle to cycle.
To get an overview of the switching loss for all captured cycles, you can use the trajectory plot. It automatically plots the voltage across the switch versus current through the switch during turn-on and turn-off, letting you judge the range of switching loss for all cycles at a glance.
The Safe Operating Area (SOA) plot is a graphical technique for evaluating a switching device to ensure that it is not being stressed beyond its maximum specifications. SOA testing can be used to validate performance over a range of operating conditions, including load variations, temperature changes, and variations in input voltages. Mask testing can also be used with SOA plots to automate validation.
This measurement characterizes the resistance of the switching device during the conduction cycle, when the device is ON and conducting current. The dynamic-on-resistance is the ratio of the voltage across the device when it is turned ON to the current flowing through the device. The software ensures that the minimum RDSon value in the acquisition is highlighted and zoomed in for easy viewing. In addition, the traverse capability helps to move from cycle to cycle to the respective RDSon values.
Magnetic components are an important part of any power supply system. Inductors and transformers are used as energy storage devices in both switch-mode and linear power supplies. Some power supplies also use Inductors in filters at their output stage. Given their important role in the system, it is essential to characterize these magnetic components to determine the stability and overall efficiency of the power supply.
Inductors exhibit increasing impedance as frequency increases, impeding higher frequencies more than lower frequencies. This behavior is known as inductance and is measured in units of Henries. The inductance can be measured automatically with Advanced Power Measurement and Analysis software.
An analysis of magnetic power losses is essential to accurately characterize the efficiency, reliability, and performance of a switching power supply. Advanced Power Measurement and Analysis software measures the inductive total magnetic power loss, as shown in the following figure.
This measurement computes the properties of the magnetic components including magnetic flux density (B), magnetic field intensity strength (H) and various loss components including Hysteresis Loss and Total Loss. This measurement also supports multiple secondary source configuration based on the scope channel count.
The properties of magnetic materials are described by the magnetic flux density (B), magnetic field intensity strength (H), and the magnetic permeability of a material (μ). B-H plots are often used to verify the saturation (or lack thereof) of the magnetic elements in a switching supply and provide a measure of the energy lost per cycle in a unit volume of core material. Advanced Power Measurement and Analysis software measures the voltage across the magnetic element and the current flowing through it, and plots B versus H, as shown in the following figure. You can test multiple secondary windings of a transformer simultaneously, thereby ensuring faster validation/testing times leading to faster time to market.
The ultimate goal of a DC-output power supply is to transform input power into one or more DC output voltages. The most important output measurements for switching power supplies are line ripple and switching ripple.
The quality of a power supply's DC output should be clean, with minimal AC noise and ripple. Advanced Power Measurements and Analysis software measures ripple to help you isolate the cause. Line ripple measurements indicate the amount of AC signal related to the input line frequency (since the input is rectified, line ripple is usually twice the frequency of the AC line). Switching ripple measures the amount of AC signal related to the switching frequency.
It is very important for SMPS to operate at specified turn on and turn off time. If the delay between mains power and SMPS startup is not as per design (typically 1 ms) it can disrupt the operation of some sensitive loads. Most embedded systems use more than one power supply and many use multiple outputs.
The Control Loop Response analysis (Bode plots), Power Supply Rejection Ratio (PSRR) and Impedance measurement provide key measurements to ensure stable, low-noise power supply designs. While it is possible to perform this analysis with a vector network analyzer or dedicated frequency response analyzer, these instruments can require significant setup time and long learning curves. Advanced Power Measurement and Analysis enables frequency response analysis right on the 4/5/6 Series MSOs, taking advantage of the optional, built-in or external arbitrary/function generator.
These measurements leverage the built-in Spectrum View tool on the 4/5/6 Series MSOs to get a finer frequency resolution across the various user-configurable frequency bands, and improve the accuracy of test results by analyzing the measurements in the frequency domain.
Bode plots and gain/phase margin measurements enable designers to determine the stability of a power supply control loop. Unstable control loops lead to oscillations and inefficient performance. Filter designers also use amplitude and phase plots to test filter designs.
Automated Control Loop Response measurements use the built-in AFG to provide a single source to sweep through a specified frequency range, plotting amplitude and phase at each point. Signals are introduced into the control loop using an injection transformer, such as the J21xxA models from Picotest. The resulting gain and phase plots (Bode plots) are used to automatically calculate gain and phase margins. Cursors allow you to view gain and phase values at any frequency on the curves.
Control loop response measurement configuration allows user to set START and STOP frequencies, select constant/amplitude profile, impedance, phase wrap, and points per decade for better plot rendering.
The PSRR measurements enable designers of DC-DC converters and regulators to quantify the ability of devices to attenuate AC over a specified frequency range. The test uses the optional, built-in function generator of the 4/5/6 Series MSO or an external Tektronix AFG31000 function generator, along with an injection transformer (such as the Picotest J2120A Line Injector), to modulate the input to the regulator. The system automatically measures the AC voltage at both the modulated input and output. It calculates the rejection ratio as 20Log (Vin/Vout) at each frequency within the swept band, and plots the result.
The 2-port impedance measurement enables designers to verify the impedance of their Power Distribution Network (PDN) over a specified frequency range. The test uses the optional built-in function generator of 4/5/6 Series MSO or an external Tektronix AFG31000 Series function generator, along with an active splitter J2161A, and power supply J2170B, an injection transformer (such as the Picotest J2102B or J2113A line injector) to measure the impedance of the PDN network. The system automatically calculates the impedance at each frequency of the swept band, and plots the results. BNC or a direct SMA connection are recommended.
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