High Performance Synchronous Rectification in Discontinuous Current Mode Converters

Abstract

Disclosed herein are synchronous rectifier control techniques for Discontinuous Current Mode (DCM) converters. These techniques may be particularly advantageous where the current shape is triangular in nature with a fixed down-slope. Such converters may include DCM flyback converters and DCM buck converters. The proposed control techniques can reduce body diode conduction of the synchronous rectifier and optimize turn-off timing, while negating the effect of parasitic circuit elements. These techniques may also help simplify the control of synchronous rectifiers operated in parallel mode. Finally, such techniques may also help achieve higher performance in variable output voltage converters and converters that operate at high switching frequencies.

1 . A synchronous rectifier controller comprising:
 a voltage sensing terminal that receives a sensed voltage that appears across a synchronous rectifier switch coupled to an output of a power converter and a plurality of parasitic inductances associated with the synchronous rectifier switch;   an off threshold voltage offset terminal that receives a turn off voltage offset signal that is determined as a function of an output voltage of the power converter and the plurality of parasitic inductances;   a drive terminal that provides a drive signal to the synchronous rectifier switch; and   control circuitry that:
 compares the sensed voltage to a turn on threshold and turns on the synchronous rectifier switch when the sensed voltage exceeds the turn on threshold; and 
 compares the sensed voltage to the turn off voltage offset signal and turns off the synchronous rectifier switch when the sensed voltage falls below the turn off voltage offset signal. 
   
     
     
         2 . The synchronous rectifier controller of  claim 1 , wherein the control circuitry that compares the sensed voltage to a turn on threshold and turns on the synchronous rectifier switch when the sensed voltage exceeds the turn on threshold comprises a minimum on time timer circuit. 
     
     
         3 . The synchronous rectifier controller of  claim 2 , wherein the minimum on time timer circuit provides different minimum on times for different output voltages of the power converter. 
     
     
         4 . The synchronous rectifier controller of  claim 3 , wherein the minimum on time timer circuit provides a predetermined minimum on time at a maximum output voltage of the power converter and a minimum on time that is decreased by a factor inversely proportional to the output voltage of the power converter for output voltages less than the maximum output voltage. 
     
     
         5 . The synchronous rectifier controller of  claim 3 , further comprising at least one comparator configured to compare the sensed voltage to at least one threshold to determine whether the synchronous rectifier is turned on or off. 
     
     
         6 . The synchronous rectifier controller of  claim 5 , wherein the control logic is further configured to detect a light load condition by comparing a rectifier on time to a minimum rectifier on time. 
     
     
         7 . The synchronous rectifier controller of  claim 6 , wherein the minimum rectifier on time is a multiple of the minimum on time. 
     
     
         8 . The synchronous rectifier controller of  claim 6 , wherein the control logic is further configured to exit a light load operating mode responsive to a rectifier on time that is greater than a multiple of the minimum on time. 
     
     
         9 . The synchronous rectifier controller of  claim 1 , wherein the control logic that compares the sensed voltage to the turn off voltage offset signal and turns off the synchronous rectifier switch when the sensed voltage falls below the turn off voltage offset signal comprises a minimum off time timer circuit. 
     
     
         10 . The synchronous rectifier controller of  claim 9 , further comprising a differentiator based sensing circuit that selectively enables synchronous rectifier drive responsive to ringing of the sensed voltage. 
     
     
         11 . The synchronous rectifier controller of  claim 10 , wherein the differentiator based sensing circuit comprises:
 a differentiator that receives the sensed voltage and provides an output responsive to a rate of change of the sensed voltage;   a window comparator that receives the output of the differentiator, compares the output of the differentiator to one or more threshold voltage, and provides an output signal; and   a timer circuit that receives the output of the window comparator and, after a predetermined delay, allow a synchronous rectifier drive signal to be enabled.   
     
     
         12 . The synchronous rectifier controller of  claim 10 , further comprising a circuit that determines whether the sensed voltage is greater than a supply voltage of the synchronous rectifier switch and selectively allows a synchronous rectifier drive signal to be enabled responsive to the determination whether the sensed voltage is greater than a supply voltage of the synchronous rectifier device. 
     
     
         13 . A discontinuous current mode power converter comprising:
 an input configured to receive an input voltage;   an output configured to deliver an output voltage to a load;   a main switching device coupled to an inductive element, wherein the main switching device is operated to regulate the output voltage; and   a synchronous rectifier switch coupled to the output that switches on when a voltage across the synchronous rectifier device and a plurality of parasitic inductances associated with the synchronous rectifier switching device exceeds a first threshold and switches off when the voltage across the synchronous rectifier device and the plurality of parasitic inductances falls below a turn off voltage offset signal that is determined as a function of the output voltage and the plurality of parasitic inductances.   
     
     
         14 . The discontinuous current mode power converter of  claim 13 , wherein the power converter is a buck converter, and the inductive element is a buck inductor. 
     
     
         15 . The discontinuous current mode power converter of  claim 13 , wherein the power converter is a flyback converter, and the inductive element comprises a flyback transformer. 
     
     
         16 . The discontinuous current mode power converter of  claim 13 , wherein the plurality of parasitic inductances associated with the synchronous rectifier device comprise a plurality of parasitic inductances associated with a package of the synchronous rectifier device. 
     
     
         17 . The discontinuous current mode power converter of  claim 13 , wherein the plurality of parasitic inductances associated with the synchronous rectifier device comprise a plurality of parasitic inductances associated with a printed circuit board trace coupling the inductive element to the synchronous rectifier device. 
     
     
         18 . The discontinuous current mode power converter of  claim 13 , wherein the synchronous rectifier switch comprises two or more synchronous rectifier switches coupled in parallel. 
     
     
         19 . The discontinuous current mode power converter of  claim 13 , wherein the turn off voltage offset signal that is determined as a function of the output voltage and the plurality of parasitic inductances is produced by a voltage divider coupled across the output. 
     
     
         20 . The discontinuous current mode power converter of  claim 19 , wherein the voltage divider comprises first and second resistors having a resistance ratio the same as a ratio of the sum of the plurality of parasitic inductances to the inductance of the inductive element. 
     
     
         21 . A method of operating a synchronous rectifier of a discontinuous current mode power converter, the method comprising:
 sensing a voltage across a synchronous rectifier device and a plurality of parasitic inductances associated with the synchronous rectifier device;   comparing the sensed voltage to a turn off voltage offset signal that is determined as a function of an output voltage of the power converter and the plurality of parasitic inductances; and   turning off the synchronous rectifier device when the sensed voltage falls below the turn off voltage offset signal.   
     
     
         22 . The method of  claim 21 , wherein the power converter is a buck converter. 
     
     
         23 . The method of  claim 21 , wherein the power converter is a flyback converter. 
     
     
         24 . The method of  claim 21 , wherein the plurality of parasitic inductances associated with the synchronous rectifier device comprise parasitic inductances associated with a package of the synchronous rectifier device. 
     
     
         25 . The method of  claim 21 , wherein the plurality of parasitic inductances associated with the synchronous rectifier device comprise one or more parasitic inductances associated with a printed circuit board trace coupling the inductive element to the synchronous rectifier device.