Switched Mode Power Supply Including a Flyback Converter with Primary Side Control

Abstract

A method and apparatus for controlling a flyback converter are presented. The flyback converter includes a transformer, a semiconductor switch coupled to a primary winding of the transformer, a current measurement circuit coupled to the semiconductor switch, a diode coupled in series to a secondary winding of the transformer, and a controller. The controller is configured to receive a feedback voltage, a reference signal, and the measured primary current and generate a control signal for the semiconductor switch dependent on the feedback voltage, the reference signal, and the measured primary current. The semiconductor switch switches on and off cyclically in CCM operation.

What is claimed is: 
     
         1 . A method for controlling a switching converter, which is operated in continuous conduction mode (CCM) to convert an input primary current passing through a primary winding into a secondary current passing through a secondary winding, wherein the method comprises:
 regularly interrupting a switching operation of the switching converter such that the secondary current drops to zero at a first time instant while the primary current is switched off;   sampling a feedback voltage at or close to the first time instant to obtain a first sampled value, wherein the feedback voltage is generated at an auxiliary winding that is magnetically coupled to the primary and secondary winding; and   resuming the switching operation in CCM.   
     
     
         2 . The method of  claim 1 , wherein, while operating in CCM, the method further comprises:
 sampling the feedback voltage at a second time instant shortly before the primary current is switched on to obtain a second sampled value.   
     
     
         3 . The method of  claim 2 , wherein, while operating in CCM, the method further comprises:
 sampling the primary current at a third time instant at which the primary current is switched on to obtain a third sampled value.   
     
     
         4 . The method of  claim 3 , wherein, while operating in CCM, the method further comprises:
 adjusting a reference signal, which is used to generate a control signal to switch the primary current on and off, dependent on the first sample value, the second sample value and the third sampled values.   
     
     
         5 . The method of  claim 1  wherein the switching converter comprises:
 a transformer composed by the primary winding the secondary winding and the auxiliary winding, the primary winding operably carrying the primary current, the secondary winding operably carrying the secondary current, and the auxiliary winding operably providing the feedback voltage; 
 a semiconductor switch coupled in series to the primary winding and configured to switch the primary current on and off in accordance with a control signal, 
 a current measurement circuit coupled to the semiconductor switch or the transformer and configured to measure the primary current, 
 a diode coupled in series with the secondary winding and configured to rectify the secondary current; and 
 a controller coupled to receive the feedback voltage, a reference signal and the measured primary current and configured to generate the control signal for the semiconductor switch dependent on the feedback voltage, the reference signal, and the measured primary current, wherein the semiconductor switch switches on and off cyclically in CCM operation. 
 
     
     
         6 . The method of  claim 1  wherein the first sampled value is representative of an output voltage of the switching converter. 
     
     
         7 . The method of  claim 1 , wherein resuming the operation of a semiconductor switch comprises:
 detecting when a voltage drop across the semiconductor switch assumes a minimum; and   switching on the semiconductor switch at the time instant the voltage drop across the semiconductor switch assumes the minimum.   
     
     
         8 . The method of  claim 1 , wherein interrupting the switching operation comprises blanking a feedback signal received by the controller. 
     
     
         9 . The method of  claim 8 , wherein blanking the feedback signal includes using a semiconductor switch to interrupt a signal path of the feedback signal to the controller. 
     
     
         10 . The method of  claim 1 , wherein resuming the operation of a semiconductor switch comprises providing a feedback signal to the controller. 
     
     
         11 . The method of  claim 1 , wherein the controller is configured to generate a control signal for a semiconductor switch such that the switching converter operates in continuous conduction mode (CCM). 
     
     
         12 . The method of  claim 1 , wherein the switching converter is a flyback converter. 
     
     
         13 . A switched-mode power supply (SMPS) circuit comprising:
 a transformer having a primary winding, a secondary winding, and an auxiliary winding, the primary winding operably carrying a primary current, the secondary winding operably carrying a secondary current, and the auxiliary winding operably providing a feedback voltage;   a semiconductor switch coupled in series to the primary winding for switching the primary current on and off in accordance with a control signal;   a controller configured to switch the semiconductor switch on and off cyclically in continuous conduction mode (CCM) operation; and   a compensation circuit configured to regularly interrupt operation of the semiconductor switch such that the secondary current drops to zero while the semiconductor switch is off, and sample the feedback voltage at a first time instant to obtain a first sampled value, wherein the first time instant occurs when the secondary current reaches zero; and resume the operation of a semiconductor switch in CCM.   
     
     
         14 . The SMPS circuit of  claim 13 , wherein the compensation circuit is further configured to sample the feedback voltage at a second time instant to obtain a second sampled value, wherein the second time instant occurs when the control signal indicates to switch the semiconductor switch on. 
     
     
         15 . The SMPS circuit of  claim 14 , wherein the compensation circuit is further configured to sample a measured primary current at a third time instant to obtain a third sampled value, wherein the third time instant occurs when the semiconductor switch has switched on. 
     
     
         16 . The SMPS circuit of  claim 15 , wherein the compensation circuit is further configured to adjust a reference signal dependent on the first, the second, and the third sampled values upstream of the controller. 
     
     
         17 . The SMPS circuit of  claim 13 , wherein the first sampled value is representative of an output voltage of the SMPS. 
     
     
         18 . The SMPS circuit of  claim 13 , wherein the SMPS is a flyback converter. 
     
     
         19 . The SMPS circuit of  claim 13 , further comprising a current measurement circuit coupled to the semiconductor switch or the transformer for measuring the primary current. 
     
     
         20 . The SMPS circuit of  claim 13 , further comprising a diode coupled in series to the secondary winding for rectifying the secondary current. 
     
     
         21 . The SMPS circuit of  claim 13 , wherein the controller is further configured to receive the feedback voltage, a reference signal, and a measured primary current and to generate the control signal for the semiconductor switch dependent on the feedback voltage, the reference signal, and the measured primary current. 
     
     
         22 . The SMPS circuit of  claim 13 , wherein, in resuming the operation of a semiconductor switch, the compensation circuit is configured to detect when a voltage drop across the semiconductor switch assumes a minimum value and switch on the semiconductor switch when the voltage drop across the semiconductor switch assumes the minimum value. 
     
     
         23 . The SMPS circuit of  claim 13 , wherein the compensation circuit is further configured to blank a feedback signal received by the controller to interrupt the operation of a semiconductor switch. 
     
     
         24 . The SMPS circuit of  claim 23 , wherein the compensation circuit comprises a semiconductor switch configured to interrupt a signal path of the feedback signal to the controller. 
     
     
         25 . The SMPS circuit of  claim 24 , wherein the controller is configured to generate the control signal for the semiconductor switch such that the SMPS circuit operates in CCM.