A new method based on summing of phase controlled AC voltages makes it possible to compare thermal voltage converters with a ratio of 2:1 without introducing any error due to level dependence. The method is used to determine the level dependence of thermal voltage converters up to 1 kV and 100 kHz. The level dependence is compared to the change of AC-DC transfer difference due to power dissipation in the range resistor and due to DC-effects in the thermal converter. The results for two investigated TVC's indicate that the power dissipation is the only cause of level dependence in the range resistor of a high voltage TVC.