Frequently the current rating, Io, of a rectifier assembly contains the following info:

Three Phase Bridge with metal heatsink

A typical rating might include –

  • Io, 70°C in oil
  • Io, 55°C heat sink
  • Io, 25°C no heat sink

The current rating in oil assumes 70°C is the temperature of the oil, and that the oil is circulating.

A temperature of 55°C assumes that is the temperature measured at the bottom of the heat sink, opposite the site of attachment to the assembly.

An ambient temperature of 25°C assumes that is the temperature of the air, not the case temperature.