Why power supply evaluation reports are useful
TDK-Lambda has for many years posted graphical and detailed test reports on the website. So why are they useful when the product specification gives all of the parameter limits?
- Test methods – A specification or datasheet does not tell the reader how the product was actually tested. Where are the voltage and current meters situated? How is the output ripple and noise measured? Is the earth leakage current recorded with a regular ammeter?
- Design margin – Often limits are described as “typical”. True test data can eliminate uncertainty (or the need to retest by the customer) and show what kind of margin the product has.
- What really happens to the power supply during the test? – Load and line regulation changes are given for a variety of inputs and load conditions. This can be used to avoid any surprises when exporting a product to a country that operates on a different input voltage.
- Over load – When overcurrent is specified it is quite often just stated as “>120%”. A plot will show if the power supply output current folds back, folds forward or remains constant through to a full short circuit. This helps determining cable sizes and pcb trace widths. As the current is often sensed on the primary side of the converter, seeing how the product behaves at different input voltages is also important.
- Start-up and shutdown characteristics – This will indicate if the power supply has monotonic rise and fall times and how long the product takes to start up. Products that have glitches during start-up can cause issues with certain load types.
- Actual hold-up times – Important if the power supply does not have active power factor correction, as hold-up times at 115Vac inputs are half those at 230Vac. It is informative to see what the hold-up performance is at different loads as some circuit topologies can have very small hold up times at light loads. Customers who need extended hold up can also review the data for a higher wattage model to see if that will meet their needs.
- Dynamic load response – This is a good indication of how stable the power supply is. If the output voltage oscillates after a load change, then the control circuit or output filter may be marginally designed.
- Leakage current – This data shows how the current leakage current not only changes with AC input, but also with power supply output load.
- EMI performance – Knowing what design margin the power supply has can determine if an external filter is required. A product with a low EMI signature can result in cost savings by eliminating a system filter.
Purchasing a power supply without such detailed data can result in having to requalify another power supply if last minute issues do show up, leading to launch schedule delays and loss of revenue. For customers that do perform extensive power supply testing, it allows those tests to be simplified, again saving money.
Immunity and reliability data is also available for many models on the TDK-Lambda website: