Patent claim: A method of controlling a resonant power converter 900 comprising first and second series connected switches 905a 905b connected between a supply voltage line 906 and a ground line 907 and a resonance circuit comprising a capacitor 908b and an inductor Ls Lm the resonance circuit connected to a node connecting the first and second switches 905a 905b the method comprising the repeated sequential steps of closing 2702 the first switch 905a to start a conduction interval sampling 2703 a voltage Vcr across the capacitor 908b to obtain a sampled voltage level and opening 2704 2705 the first switch 905a to end the conduction interval when a voltage Vcr across the capacitor crosses a voltage level determined by addition of the sampled voltage level with a predetermined voltage difference deltaVcrh wherein controlling the predetermined voltage difference deltaVcrh determines a power output of the resonant power converter 900.

Related passage: Fig.1 describes a switch mode power supply 2 having a power input terminal 4 primarily for DCpower and output terminals 6 8 between which the power supply can deliver DCpower.Semiconductor switches 10 12 are connected so that if switch 10 is open switch 12 is closed.Hereby the voltage at the connection point 14 between the two semiconductor switches 10 12 changes from zero and up to the input DCvoltage.The point 14 is connected to a first coil 15 from where current is flowing to a coil 16 which is part of a transformer 18.The coil 16 is further connected to a capacitor 19.The transformer 18 contains further a coil 20 which is connected to rectifier means which can be formed as a bridge rectifier 20 having an input terminal 22.A rectified DC power is delivered at the output 24 towards the output terminals 68 between which a capacitor Cout and a resistor Rload are shown.A feedback signal 26 is connected to the output terminal 6.The feedback signal 26 is sent to an error amplifier 28.The now converted feedback signal 29 is led forwards to electrical isolation means 30 which in practice is in the form of an optocoupler.This optocoupler is connected to a pin 4 of the integrated circuit L6598.Inside the integrated circuit 4 is an internal power supply comprising a voltage reference connected to the pin 4.Outside the integrated circuit is the optocoupler connected to the pin 4 through a serial resistor.Also connected to the pin 4 a resistor is connected to the earth connection.In this way all currents between two levels can be generated to flow from the pin 4 depending on the collector voltage on the transistor in the optocoupler 30.In this way the feedback signal level defines the current.A current change in the pin 4 leads to a change in size of the current in the constant current generators 42 44.This leads to a change of the charging and the decharging speed of the capacitor 46.Hereby the oscillating frequency is over the switching means 10 12 and the coil 15.The coil 16 at the transformer and the capacitors 19 is changed according to the load.A switching means 41 defines which of the constant current generators 42 44 that are to be active.Both cannot be active at the same time.The common output from the two constantcurrent generators 42 44 is led through a pin 3 at the integrated circuit.Outside the pin 3 a capacitor 46 is connected.Charging a capacitor 46 by a constant current leads to a linear increase or decrease in voltage over the capacitor 46.In this way a threeangle voltage is generated at the pin 3 of the integrated circuit.This signal with a threeangle voltage is over a line 32 led to the input of two comparators and a flipflop.The output flipflop 36 is switching its output depending on the input of the terminal 32 and on a reference voltage.The output of the flipflop 36 is connected to driving means 38 40.Output terminals at the integrated circuit are pin 11 and pin 15.Pin 11 has the number 45 and pin 15 the number 43.These are connected to the input of the semiconductor switching means 10 12.