One afternoon, we received a package containing six damaged transformers from an industrial power supply manufacturer in Italy. Their email was short and straightforward:
"Everything works perfectly in the laboratory, but after three or four months in the field, customers begin reporting failures. Can you help us find the reason?"
When we dismantled the returned units at Wuxi Huipu Electronics Co., Ltd., none of the transformers looked dramatically damaged. There were no burnt windings, no cracked ferrite cores, and no obvious signs of overload. Yet something was clearly wrong. After comparing the failed samples with newly manufactured units, the answer slowly emerged. The insulation between several winding layers had begun to deteriorate after prolonged exposure to elevated temperatures. That small change increased leakage current, created additional heat, and eventually damaged the switching MOSFETs. What appeared to be a semiconductor failure actually started inside the transformer months earlier.
This experience isn't unusual. One of the biggest misconceptions in power supply design is that transformers either work or they don't. In reality, switching power supply transformers almost always give warning signs long before complete failure occurs. The challenge is recognizing those signs early enough to prevent expensive field repairs.
Among all the problems we encounter, excessive temperature rise is by far the most common. During prototype testing, engineers often operate the power supply for thirty minutes, record acceptable temperatures, and move directly into production. Unfortunately, industrial equipment rarely operates for only thirty minutes. Many systems run continuously for thousands of hours. Small increases in copper loss or core loss gradually accumulate until internal temperatures exceed the design limits of insulation materials. By the time customers notice unstable output or unexpected shutdowns, the damage has already been done. This is why we always recommend evaluating transformers under realistic long-duration operating conditions rather than relying solely on short laboratory tests.
Another frequent source of failure is core saturation. Unlike overheating, saturation can appear suddenly and without much warning. The power supply may function normally under light load but begin drawing excessive current once operating conditions change. We've seen this happen after customers modified switching frequencies or expanded input voltage ranges without redesigning the transformer itself. The magnetic core simply reached its limit earlier than expected. Preventing saturation isn't complicated, but it requires conservative magnetic calculations and enough design margin to handle real operating conditions rather than ideal ones.
Leakage inductance is another issue that often hides behind other failures. Engineers usually discover burnt switching devices first because they are easier to identify. However, replacing MOSFETs rarely solves the problem if excessive leakage inductance remains inside the transformer. Poor winding arrangement creates voltage spikes during every switching cycle. Those spikes may remain within safe limits during laboratory testing but gradually stress semiconductors over months of operation. We've helped several OEM customers reduce switching losses significantly simply by redesigning the winding structure while leaving the rest of the circuit unchanged.
Electromagnetic interference tells a similar story. Many people think EMI is something to solve with larger filters or additional shielding after the design is complete. Our experience suggests otherwise. In most cases, unwanted noise begins inside the transformer itself. The way windings are layered, how closely primary and secondary circuits are coupled, and even the position of insulation tapes all influence conducted and radiated emissions. A transformer designed without considering EMI from the beginning often forces engineers to spend far more time modifying surrounding circuitry later.
Mechanical reliability is another factor that's easy to overlook because transformers appear to have no moving parts. In reality, high-frequency magnetic fields produce tiny vibrations inside both the ferrite core and the windings. Over thousands of operating hours, those microscopic movements can gradually wear insulation, loosen winding structures, or create the buzzing sound that many users mistakenly attribute to poor power quality. Proper winding tension, secure core assembly, and suitable impregnation techniques dramatically improve long-term stability, particularly in industrial environments where vibration already exists.
Insulation failure remains one of the most serious concerns, especially in medical, communication, and industrial control equipment where electrical isolation directly affects safety. Choosing insulation materials based only on voltage rating isn't enough. Creepage distance, clearance, thermal aging, humidity, and manufacturing consistency all contribute to long-term reliability. We routinely perform Hi-Pot testing and insulation verification because electrical safety isn't something customers can visually inspect after installation.
One interesting pattern we've noticed over the years is that transformers themselves are rarely manufactured incorrectly. More often, they are simply expected to do something they were never designed to do. A transformer selected only according to power rating may operate outside its thermal window. Another chosen purely by physical dimensions may create excessive EMI. Yet another copied from an earlier project may no longer suit a higher switching frequency. None of these transformers are defective-they're simply mismatched to the application.
That's why our engineering discussions with customers almost never begin by asking, "How many watts does your transformer need?" Instead, we ask how the equipment will actually be used. Will it operate continuously or intermittently? Is it installed inside a sealed cabinet or exposed to airflow? What ambient temperatures will it experience? Which switching topology is being used? Only after understanding the complete application do we begin optimizing the transformer design.
After working with switching power supplies for many years, we've reached a simple conclusion. Most transformer failures are not manufacturing failures; they're design failures that only become visible after products leave the factory. Preventing them doesn't usually require more expensive materials or larger transformers. It requires understanding the application, designing with adequate engineering margin, and treating the transformer as the heart of the power supply rather than just another component on the bill of materials.
The most reliable switching power supplies we've seen all share one thing in common: the transformer was never considered an afterthought. It was designed as part of the entire system from the very beginning.