A method of operating an electrical power supply device having a primary side and a secondary side is provided. The method includes transmitting with an optocoupler arranged between the primary side and the secondary side a wide band control signal and a numerical information signal. The method further includes transmitting the control signal and the information signal over a rectangular-wave signal modulated in combination with frequency modulation and pulse width modulation. The control signal and the information signal are the modulating signals for frequency modulation and pulse width modulation of said rectangular-wave signal.