Voltage sensing of an active clamp switching power converter circuit

Abstract

An active clamp switching power converter circuit includes a sensing circuit that generates a sensed auxiliary winding voltage of an auxiliary winding around the core. A primary side controller controls switching of a power switch coupled to a primary winding to control current through the primary winding and controls switching of the active clamp switch based on the sensed voltage to control leakage energy when the power switch is turned off. The primary side controller regulates timing of the switching to achieve a zero voltage switching condition prior to turning on the power switch for power efficient operation.

What is claimed is: 
     
         1 . A switching power converter circuit comprising:
 a transformer coupled between an input terminal receiving an input voltage and an output terminal supplying an output voltage, the transformer including a core, a primary winding around the core coupled to the input terminal, and a secondary winding around the core coupled to the output terminal;   a power switch coupled to the primary winding, the power switch to control current through the primary winding;   an active clamp switch to control leakage energy of the switching power converter circuit while the power switch is turned off;   a sensing circuit to generate a detection signal, the sensing circuit comprising:
 an auxiliary winding around the core of the transformer, and 
 a detection circuit coupled to the auxiliary winding, the detection circuit to receive sensed voltage across the auxiliary winding and generate the detection signal based on the sensed voltage satisfying a predefined detection condition, wherein the sensed voltage satisfies the predefined detection condition when the sensed voltage decreases by at least a predefined amount over a predefined time interval while the power switch is turned off and the active clamp switch is turned on; and 
   a primary side controller coupled to the power switch and the active clamp switch, the primary side controller to receive the detection signal and to control switching of the power switch to regulate the output voltage and to turn on the active clamp switch following the power switch turning off and to turn off the active clamp switch based on a timing of the detection signal.   
     
     
         2 . The switching power converter circuit of  claim 1 , further comprising:
 a secondary side switch to control current through the secondary winding;   a current sensor to monitor current through the secondary winding and to generate a signal based on the current decreasing to substantially zero; and   a secondary side controller coupled to the secondary side switch, the secondary side controller to receive the signal and to turn off the secondary side switch based on the signal.   
     
     
         3 . The switching power converter circuit of  claim 2 , wherein the sensed voltage satisfies the predefined detection condition at a same time as the secondary side controller turns off the secondary side switch. 
     
     
         4 . The switching power converter circuit of  claim 3 , wherein a zero voltage switching condition of the power switch takes place at a substantially fixed time period after the sensed voltage satisfies the predefined detection condition and the secondary side controller turns off the secondary side switch. 
     
     
         5 . The switching power converter circuit of  claim 1 , wherein the timing of the detection signal is an amount of time between the primary side controller receiving the detection signal and the primary side controller turning off the active clamp switch. 
     
     
         6 . The switching power converter circuit of  claim 5 , wherein, in a first switching cycle the timing is greater than a target timing and, in a second switching cycle subsequent to the first switching cycle, the primary side controller turns off the active clamp switch earlier thereby decreasing an amount of on-time of the active clamp switch such that the timing in the second switching cycle is closer to the target timing than in the first switching cycle. 
     
     
         7 . The switching power converter circuit of  claim 5 , wherein, in a first switching cycle the timing is less than a target timing and, in a second switching cycle subsequent to the first switching cycle, the primary side controller turns off the active clamp switch later thereby increasing an amount of on-time of the active clamp switch such that the timing in the second switching cycle is closer to the target timing than in the first switching cycle. 
     
     
         8 . The switching power converter circuit of  claim 1 , wherein the timing of the detection signal is an amount of time between the primary side controller receiving the detection signal and the primary side controller turning on the power switch in a first switching cycle, and wherein the timing includes a first amount of time between the primary side controller receiving the detection signal and the primary side controller turning off the active clamp switch and a second amount of time between the primary side controller turning off the active clamp switch and the primary side controller turning on the power switch. 
     
     
         9 . The switching power converter circuit of  claim 8 , wherein, in the first switching cycle the second amount of time is greater than a target time period and, in a second switching cycle subsequent to the first switching cycle, the primary side controller turns off the active clamp switch later thereby increasing an amount of on-time of the active clamp switch such that the second amount of time in the second switching cycle is closer to the target time period than in the first switching cycle. 
     
     
         10 . The switching power converter circuit of  claim 8 , wherein, in the first switching cycle, the second amount of time is less than a target time period and, in a second switching cycle subsequent to the first switching cycle, the primary side controller turns off the active clamp switch earlier thereby decreasing an amount of on-time of the active clamp switch such that the second amount of time in the second switching cycle is closer to the target time period than in the first switching cycle. 
     
     
         11 . A control circuit for a switching power converter circuit, the control circuit comprising:
 a detection circuit coupled to an auxiliary winding around a core of a transformer of the switching power converter circuit, the detection circuit to receive sensed voltage across the auxiliary winding and generate a detection signal based on the sensed voltage satisfying a predefined detection condition, wherein the sensed voltage satisfies the predefined detection condition when the sensed voltage decreases by at least a predefined amount over a predefined time interval while a power switch is turned off and an active clamp switch is turned on; and   a primary side controller coupled to the power switch and the active clamp switch, the primary side controller to receive the detection signal and to control switching of the power switch and to turn on the active clamp switch following the power switch turning off and to turn off the active clamp switch based on a timing of the detection signal.   
     
     
         12 . The control circuit of  claim 11 , wherein the timing of the detection signal is an amount of time between the primary side controller receiving the detection signal and the primary side controller turning off the active clamp switch. 
     
     
         13 . The control circuit of  claim 12 , wherein, in a first switching cycle the timing is greater than a target timing and, in a second switching cycle subsequent to the first switching cycle, the primary side controller turns off the active clamp switch earlier thereby decreasing an amount of on-time of the active clamp switch such that the timing in the second switching cycle is closer to the target timing than in the first switching cycle. 
     
     
         14 . The control circuit of  claim 12 , wherein, in a first switching cycle the timing is less than a target timing and, in a second switching cycle subsequent to the first switching cycle, the primary side controller turns off the active clamp switch later thereby increasing an amount of on-time of the active clamp switch such that the timing in the second switching cycle is closer to the target timing than in the first switching cycle. 
     
     
         15 . The control circuit of  claim 11 , wherein the timing of the detection signal is an amount of time between the primary side controller receiving the detection signal and the primary side controller turning on the power switch in a first switching cycle, and wherein the timing includes a first amount of time between the primary side controller receiving the detection signal and the primary side controller turning off the active clamp switch and a second amount of time between the primary side controller turning off the active clamp switch and the primary side controller turning on the power switch. 
     
     
         16 . The control circuit of  claim 15 , wherein, in the first switching cycle the second amount of time is greater than a target time period and, in a second switching cycle subsequent to the first switching cycle, the primary side controller turns off the active clamp switch later thereby increasing an amount of on-time of the active clamp switch such that the second amount of time in the second switching cycle is closer to the target time period than in the first switching cycle. 
     
     
         17 . The control circuit of  claim 15 , wherein, in the first switching cycle, the second amount of time is less than a target time period and, in a second switching cycle subsequent to the first switching cycle, the primary side controller turns off the active clamp switch earlier thereby decreasing an amount of on-time of the active clamp switch such that the second amount of time in the second switching cycle is closer to the target time period than in the first switching cycle. 
     
     
         18 . A method for controlling a switching power converter circuit, the switching power converter circuit including a transformer coupled between an input terminal receiving an input voltage and an output terminal supplying an output voltage, the transformer including a core, a primary winding around the core coupled to the input terminal, a secondary winding around the core coupled to the output terminal, and an auxiliary winding around the core of the transformer, a power switch coupled to control current through the primary winding, an active clamp switch coupled to control leakage energy of the switching power converter circuit, a detection circuit coupled to the auxiliary winding, and a primary side controller to control switching of the power switch and the active clamp switch, the method comprising:
 receiving, by the detection circuit, sensed voltage across the auxiliary winding;   generating, by the detection circuit, a detection signal based on the sensed voltage satisfying a predefined detection condition, wherein the sensed voltage satisfies the predefined detection condition when the sensed voltage decreases by at least a predefined amount over a predefined time interval while the power switch is turned off and the active clamp switch is turned on;   receiving, by the primary side controller, the detection signal; and   controlling, by the primary side controller, a turn-off time of the active clamp switch based on a timing of the detection signal.   
     
     
         19 . The method of  claim 18 , wherein the timing of the detection signal is an amount of time between the primary side controller receiving the detection signal and the primary side controller turning off the active clamp switch. 
     
     
         20 . The method of  claim 18 , wherein the timing of the detection signal is an amount of time between the primary side controller receiving the detection signal and the primary side controller turning on the power switch, and wherein the timing includes a first amount of time between the primary side controller receiving the detection signal and the primary side controller turning off the active clamp switch and a second amount of time between the primary side controller turning off the active clamp switch and the primary side controller turning on the power switch.