The initial change in output voltage of a power supply from turn-on until it reaches thermal equilibrium at nominal line, full load, and ambient temperature.

Power supplies take time to stabilize their outputs. After the warm up, the power supply output reaches its specified stability. And even though the supply is capable of working immediately it is turned on, it is necessary to wait for it to stabilize if it powers a sensitive load that requires a very stable supply. Allowing the supply to warm up will therefore give it time to reach its thermal equilibrium and stabilize the output.

The warm-up drift measures the change in the output of a power supply that results from the initial warming of the components over a certain period. This measurement is carried out by turning the supply at its rated full load and then determining the output after the supply has stabilized for between 10 and 20 seconds. The supply is then allowed to run for a period of between 15 and 60 minutes, after which the second reading is taken.

The warm-up drift is then calculated from the two readings using the formula:

Where:

Vta is the voltage at ambient temperatue, usually 25°C

Vtw is the voltage after the warm-up

D is the percentage drift of the supply output.

The warm-up drift is classified as either a short-term drift or a long-term drift. The difference between the two is the time the test is run; short-term drift is determined after a few minutes, while the long-term drift is after about one year.

Factors that affect the magnitude of the warm-up drift

The short term drift is influenced by:

• Inadequate circuit design
• Inferior components that do not meet specifications
• Poor circuit layout or packade design
• Thermal excursions in the environment
• Changes in load current beyond circuit limits

The long-term drift is affected by:

• Component aging
• Inadequate thermal design
• Inadequate physical layout and packaging
• Poor circuit design
• Unreliable components that do not meet specifications
• Thermal excursions in the environment

The warm-up drift occurs in a power supply due to the effects of the heat generated or power dissipated in the components and, in particular, those in the feedback network. These feedback resistors suffer from the self-heating effects due to the current through the feedback network, which results in the I²R losses.

The heat from other components in the supply also contributes to the rise in temperature of the feedback network. After the warm-up period, the supply reaches its thermal equilibrium and, hence, a stable output.

The time it takes to stabilise is influenced by the physical design, circuit design, internal heat transfer characteristics, thermal mass content, and the way the air flows in and out of the power enclosure.