Control of a MOS transistor as a rectifying element

Abstract

A circuit for controlling a switch to be controlled in unidirectional fashion while the voltage present there across is an A.C. voltage, including circuitry for delaying the switch turning-on with respect to a zero crossing of the voltage there across, and circuitry for triggering the switch turning-off after its turning on, at the end of a predetermined time interval plus or minus an error time controlled by the duty cycle of the A.C. voltage across the switch, in one or several previous periods. The control circuit applies to the forming of a rectifying circuit by the switch.

1 . A circuit for controlling a switch to be controlled in a unidirectional fashion while the voltage present there across is an A.C. voltage, comprising: 
 means for delaying the switch turning-on with respect to a zero crossing of the voltage there across; and    means for triggering the switch turning-off after its turning on, at the end of a predetermined time interval plus or minus an error time controlled by the duty cycle of the A.C. voltage across the switch, in one or several previous periods.    
   
   
       2 . The circuit of  claim 1 , wherein said predetermined duration is selected according to the maximum expected variations of the duty cycle.  
   
   
       3 . The circuit of  claim 1 , wherein a capacitor and a diode are series-connected between the terminals of the switch to provide a supply voltage to the control circuit, the capacitor being charged when the switch is off.  
   
   
       4 . The circuit of  claim 1 , comprising: 
 a circuit for detecting the sign of the voltage across the capacitor;    a ramp generator reset on each sign switching of the voltage across the switch in a direction in which it must become conductive, said generator being controlled by the detection circuit;    a means for causing the turning-on of the switch after detection of a sign switching of the voltage there across; and    a circuit for controlling the duration of the on state with a predetermined value.    
   
   
       5 . The circuit of  claim 4 , wherein a first delay element brings a minimum delay to a turn-off order of the switch which follows its turning-on.  
   
   
       6 . The circuit of  claim 5 , wherein a second delay element brings a delay to the turning-on of the switch with respect to the inversion of the voltage there across.  
   
   
       7 . The circuit of  claim 1 , wherein an output amplifier providing the control signal of the switch is controlled by means for detecting an inversion of the voltage direction, to cause the turning-off of the switch in case of an incidental voltage inversion.  
   
   
       8 . The circuit of  claim 1 , wherein the switch is a MOS transistor.  
   
   
       9 . The circuit of  claim 1 , wherein a diode in parallel on the switch is used for the starting.  
   
   
       10 . A rectifying circuit with a low threshold voltage comprising a switch in parallel with a diode preferably formed of its parasitic diode, and the control circuit of  claim 1  associated with a supply circuit drawing its power supply directly from across the controlled transistor when said transistor is on.  
   
   
       11 . The circuit of  claim 10 , exclusively comprising two external connection terminals.