Welcome to the second part of EEWorld’s two-part “virtual roundtable” discussion on ac/dc power supply testing and qualification. Today, we will be focused on power factor correction, leakage currents, and electromagnetic compliance assurance testing. Our panelists are: Yogesh Pai (YP), Tektronix Product and Marketing Manager for Power; Bill Griffith (BG), Power Products Marketing, with Keysight Technologies; and Johnnie Hancock (JH), Oscilloscope Product Manager, with Keysight Technologies.
JS: What are the common errors made when measuring power factor?
YP: When power designers perform power quality and harmonics measurements on the input analysis, one of the most important parameters is the power factor, commonly known as PF. Power factor is the ratio of true power to apparent power, where apparent power is the product of the RMs voltage and current. Another way to look at power factor is simply the cosine of the phase difference between the voltage and current. An energy-consuming device with a low PF draws high current than a device with a higher PF. Power analysis needs both voltage and current signals. It is also important to note that a very small time difference can result in significant errors in the measurement.
Proper vertical dynamic range is also important in the measurement. Tektronix’s MSO 5/6 series scopes provide ADC resolution up to 15 bits in HiRes mode, reducing errors in the measurements. Another common source of errors are DC offsets during voltage and current probing. The user needs to compensate for all these errors before doing the measurement.
In summary, engineers attempt to design their boards with high PF and then measure the power factor and harmonic content to verify their designs. This means they cannot afford to take measurement errors during the validation process.
BG & JH: The figure below shows the voltage and current drawn from a switching power supply. The high peaks of the current lead to a high crest factor, making it a complicated waveform to measure. Keysight’s AC sources digitize the voltage and current simultaneously. Calculations determine real, apparent power, and power factor.
A dedicated AC power analyzer like the PA2201A provides additional accuracy with continuous waveform analysis (CWA), plus digital filtering. The analyzer displays voltage, current, and power waveforms in real-time, allowing the user to see the filters’ effects and the consistent measurements from using CWA to measure over multiple periods. A power analyzer also provides harmonic analysis enabling further insights into complex waveforms.
Accurately detecting the current is also challenging as shunts and transformers can cause additional loading errors. A zero-flux sensor can characterize current through a wire with little to no interference.
JS: When is leakage current testing important, and what are the challenges?
BG & JH: When a battery-powered device is off, small currents can run down a battery leading to customer dissatisfaction. Tradeoffs between battery life and convenience such as instant-on and always-connected are often tricky. Consumers can configure more sophisticated devices to their needs. It can be helpful to create personas and test different use models and understand current usage. Single-use devices can be simple and run for long periods. TPMS sensors in tires can last up to ten years. A power supply with a built-in data logger can measure current usage over a period of time.
YP: Leakage current can be an indicator of the effectiveness of insulation on conductors in the power supply designs. The high levels of leakage current may be present in circuits where electronic equipment with filters is used, and can cause voltages that disrupt normal operation of equipment. This problem is common in power supplies. The challenge is finding the source of leakage, which requires specialized instruments and setup to perform this. It is possible to locate the source of leakage current by using a low current leakage current clamp to take methodical measurements.
JS: What considerations determine when EMC compliance should be done in-house and when an outside lab should perform it?
BG & JH: Full EMC compliance by a registered laboratory is time-consuming and expensive. Pre-compliance testing reduces risk to project schedules and budget. For AC to DC converters, emissions tests are more complicated than susceptibility tests. Regarding emission tests, radiated emissions are more difficult to solve than conducted tests (choke) and can typically take care of conducted emissions. It is essential to perform pre-compliance testing for emissions. Waiting until the last minute of a project for EMI testing is a recipe for a delayed introduction. Keysight utilizes the X-Series spectrum analyzer with the N6141C EMI measurement application for pre-compliance testing. Pre-compliance testing catches problems earlier and allows for the most cost-effective solutions. Keysight offers a complimentary application note that lists the various standards along with the test methodology. Keysight also provides EMC testing as a service in several locations.
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 first part of this virtual roundtable that focused on various aspects of power supply efficiency, standby power, and static and dynamic output regulation testing.
