Welcome to the first part of EEWorld’s two-part “virtual roundtable” discussion on ac/dc power supply testing and qualification. Today, we will be focused on various aspects of power supply efficiency, standby power, and static and dynamic output regulation testing. Our panelists are: Bill Griffith (BG), Power Products Marketing, Keysight Technologies: Johnnie Hancock (JH), Oscilloscope Product Manager, Keysight Technologies; and Yogesh Pai (YP), Tektronix Product and Marketing Manager for Power.
JS: What are the challenges in measuring power supply efficiency and standby power?
BG & JH: One of the biggest challenges in measuring standby power is finding a current probe with enough sensitivity to measure low-level standby currents in the range of microamps while also having the dynamic range to measure much higher levels of wake-up currents. With Keysight’s N2820A high sensitivity AC/DC current probe connected to an InfiniiVision or Infiniium oscilloscope, you can monitor dynamic changes of low-level standby currents on one channel. Also, at the same time, watch dynamics of wake-up and steady-state currents on another channel of the oscilloscope. The figure below shows a two-channel oscilloscope probe designed to measure both idle and higher wake-up currents.
The screen image below shows a high-sensitivity current measurement on channel-1 (yellow trace) scaled at 2 mA/div, while also measuring much larger wake-up spikes of current on channel-2 (green trace) scaled at 20 mA/div. Both channels are required to capture the current’s dynamic range as channel 2 captures the peak current, and channel 1 captures the standby current.
YP: Power supply designers attempt to improve the efficiency of their designs while maintaining specified performance over a range of input and load conditions, while also complying with the demanding international regulations for efficiency, standby power, current harmonics, and EMC. Tektronix offers different solutions and instruments to support these key measurements.
Efficiency is calculated as a ratio of the output power to AC input power at a specific loading condition. This is expressed as a percentage value. In the regular power converters, the bigger challenge is access to probing points due to highly dense electronics packaged within a small form factor. With the newer designs, multiple outputs are very common, as it becomes all the more important to measure efficiency at each output and total system efficiencies.
Another challenge is the topology of power supply designs and what kind of loads are used for different applications. The design should be able to maintain stable efficiency with varying load conditions, which requires good feedback circuit design. The power consumption testing at no-load condition is generally known as standby power. All power supplies are required to be tested for no-load condition (standby power) and be within the stated no-load power limits as published in the efficiency standard, known as IEC 62301.
Active or passive load is required to set the output current of the DUT while testing efficiency. The electronic loads can provide significant benefits over passive loads due to their versatility and programmability. The Keithley 2380 series Electronic Load features a 0.05% reading accuracy, 0.1mV/0.01mA voltage, and a current resolution to provide a perfect solution for AC-DC efficiency testing applications. High accuracy guarantees accurate loading at all required voltage outputs on the AC-DC power supply. There are 3 models: 200W, 250W, and 750W, which enable testing of a wide range of AC-DC power supplies at different power levels. Along with constant current (CC) mode that is required by the efficiency standards, 2380 Series Electronic Loads also support constant voltage (CV), constant resistance (CR), and constant power (CP) modes to test various load configurations on the DUT. The 2380 Series Electronic Loads make automating efficiency testing easy by using ‘List’ mode, allowing users to program the load with various load currents for specific turn on and turn off times.
In the case of 3-phase motors and drive, the challenge in measuring electrical efficiency is the availability of the number of analog channels in scope, with the maximum channels available being eight. This means one cannot setup and measure 3V3I wiring, which needs twelve channels. The other way is the 2V2I wiring method. This will use Line to Line voltage probing and the two-watt meter method, ideal for measuring drive input/output efficiency since this needs eight analog input channels.
Efficiency, standby power, and all other critical power-related measurements on power supplies that require sophisticated and accurate instrumentation to ensure that the power supply performs to its specification
In summary, power supply designers are faced with the challenges of always increasing efficiency and lower standby power requirements. The efficiency of the power converter increases with the accuracy of the power measurements being performed. Otherwise, it will be impossible to measure if small power losses are due to the power lost in high-efficiency conversion or if they are caused by measurement error.
JS: How important is the Titanium level of efficiency and the measurement of efficiency at 10% load?
YP: The Titanium level of efficiency relates to achieving the highest possible efficiency for power supply designs. There’s a lot of work and research currently happening with wide bandgap materials on how to achieve titanium grade level efficiency, including experiments with GaN MOSFETs.
It’s important to achieve Titanium grade since this helps to design power supplies with smaller size, so we get more compact designs, quicker sequencing times such as turn on time and turn off time, and very small heat sinks. All these factors contribute to reducing overall system costs and more electronics.
BG & JH: 80+ certification helps data centers reduce cost as they pay for inefficiencies twice. An inefficient power supply is converting energy to heat, which requires additional energy to keep the data center cool. A typical Titanium level power supply has a more sophisticated design to ensure efficiency across an extensive range of power levels.
For most applications, the greatest efficiencies come from selecting the right size power supply. Using the proper size power supply reduces excessive heat and reduces the noise as they use a smaller fan. A multiple output power supply can power your device and determine its maximum power for each voltage rail. Finally, remember to obtain a sample of power supplies, as their regulation can vary based on the current draw and have variations from unit to unit. The figure below shows the output voltage of a converter dropping two volts based on current. Other power supply designs may have better regulation, but you will need to make tradeoffs. The graph in Figure 3 is made using the Keysight EL30000 Series bench electronic load, using an input LIST simplifies testing a batch of converters at several current levels.
JS: How can remote sensing or “adaptive/fast charging” impact power supply efficiency?
BG & JH: An adaptive power supply can be efficient at multiple operating voltages. One of the most significant advantages is in the cabling. USB cables use tiny wires that have high resistance and turn current into heat. Using a higher voltage reduces the need for higher currents in the wires. A 3000 mAh (3 Ah) battery is not uncommon in a new phone. Higher voltages are necessary as the cables simply cannot handle the high charging currents.
YP: Fast charging drastically lowers the lifetime of batteries. The charge-per-minute efficiency of a system may not be ready to handle high-speed charging since it depends on the type of battery used. The fast charge happens due to low internal resistance that generates low heat and hence can get higher thermal efficiency. It also depends on different charging patterns performed by the user.
When a battery is charged with a fast charger, it is most stressed after the 75 percent charge level. This is when the high voltage current is not absorbed by the battery as efficiently as when it is charging from zero to 50 percent. Lithium-ion batteries absorb the charge efficiently during the initial phase and begin to drop-off after 70-80 percent charge level.
Most batteries are commonly tested using discharge-and-charge cycling. The discharge characteristics of a battery provide important indications of the battery’s capacity and life. In production testing, a discharge/charge cycle is often run to verify battery quality and to ensure it is not short-circuited. Instruments like Keithley’s 2460 and 2461 High Current Graphical Source Measure Units (SMU) eliminate the need for separate programmable power supplies, electronic loads, voltmeters, and ammeters. Since the SMU can source and measure both current and voltage, only one instrument is needed for cycle testing, which reduces the amount of rack space required and minimizes programming time. The SMU can also be used to create a model for the battery used in the product. A battery model-generating script operates the SMU instrument as a constant current load and derives the model parameters.
The Keithley 2281S Battery simulator enables the user to gauge the battery life and how the DUT performs at different stages of battery discharge. The 2281S makes it easy to simulate any type of battery required, so testing prototype devices can be done efficiently and with high repeatability at any battery state, as well as estimating battery life effectively.
JS: What are the factors that impact static and dynamic output regulation testing?
BG & JH: Most IoT devices pull current dynamically using small idle currents and larger currents in active states to gain efficiencies. Keysight’s new EL30000 Series bench dc electronic load includes a LIST mode to apply a dynamic load simulating an IoT device. A dynamic load can characterize chargers and batteries. It is best to match battery chemistries to the application. For example, lithium-ion batteries with magnesium (LMO, NMC) tend to have lower internal resistance and handle higher burst currents often found in power tools.
YP: There are many different kinds and sizes of power supplies, from traditional linear types to high-efficiency switch-mode power supplies. All face a complex, dynamic operating environment. Device loads and demands can change dramatically from one instant to the next.
A regulator has to guarantee that its output, voltage or current, accomplishes certain static and dynamic requirements, which may change depending on the type of application. Different tests are needed to measure the overall static and dynamic performance of a power supply board providing voltage or current regulation.
Dynamic regulation condition is the ability of a power supply to return to a steady-state of operation after major disturbances. For the static regulation part, it can return to a steady-state after small disturbances, as it’s important to monitor both parameters. The dynamic load changes can quickly discharge output capacitance, causing the output voltage to drop out of static regulation. Even if the load draws current that is within the rated current of the power supply, there may be some droop in the output voltage. This droop is sensed by the voltage feedback divider, which in turn causes the voltage loop through the feedback circuit of the power supply to increase the output voltage and bring it back to the level within static voltage regulation specification.
These are some of the factors that could impact during regulation in the designs. Designers can use an oscilloscope to observe turn-on time and turn off time with a load connected and also output overshoot and undershoot surges caused by the load and line variations; these are common in load transient tests.
JS: Thank you to our Virtual Roundtable participants for sharing their experience and insights into ac/dc power supply testing and qualification! You might also be interested in reading the second part of this virtual roundtable that will focus on power factor correction, leakage currents, and electromagnetic compliance assurance testing.