What’s Getting Hot Inside the PSU?

(Last edited 5/14/2026)

During each stage of producing a new PSU, we apply thermistors to all of the components that either generate heat (such as a MOSFET), or are sensitive to heat (such as a capacitor).

To determine that a PSU is getting proper cooling, a number of thermistors are installed during the early engineering stages.

In many cases, we apply forty thermistors to the PSU. Thirty-seven are placed on internal components and three are placed on the outside. We keep an eye on the temperatures and make sure they stay at or below 80% of their rated junction temperature.

But why stop at 80 percent? Let’s start with capacitors and the “10°C rule.” For every 10°C drop below a capacitor’s maximum operating temperature, its expected life doubles. That sounds great until you remember that a typical 105°C electrolytic cap is only rated to last about 2000 hours.  That’s barely six weeks. 

While capacitors age through electrolyte evaporation, MOSFETs wear out by different mechanisms: bond-wire fatigue from thermal cycling, solder-joint or die-attach cracking, gate-oxide degradation, and electromigration in the metallization layers. Load profiles with frequent current bursts or high switching frequencies, especially in LLC or synchronous-rectifier stages, accelerate those failure modes, which is why GaN devices can offer a significant reliability advantage over MOS.

Worse yet, MOSFETs don’t benefit from a 10 °C rule.  They’re subject to essentially the opposite: thermal runaway. Their leakage current doubles with each 10°C rise, creating an exponential heat-generation curve while cooling capacity only increases roughly linearly.

Moving on to the inductors; whether in the PFC choke, main transformer, resonant choke, or output filters, these are also highly temperature-sensitive. As winding temperature climbs, copper resistance increases, undermining efficiency, reliability, magnetic performance, and safety margins. Those heat-driven shifts in inductance and core loss lead to higher output ripple (notably on 12V rails under GPU excursion loads), less stable regulation during fast transients, detuned resonant conditions in LLC converters, and increased EMI.

Even these passive components can experience thermal runaway. They dissipate a large share of the PSU’s heat; if airflow is inadequate or fan curves are poorly tuned, inductor temperatures rise, efficiency falls, heat generation accelerates and the cycle continues until core saturation sets in.

Saturation occurs when the magnetic core can no longer store additional flux. The inductor’s inductance collapses and it behaves like a short. As inductance vanishes, current spikes, risking MOSFET overstress and transformer overheating.