Control circuits and methods for switching mode power supplies

Abstract

A switching mode power supply (SMPS) includes a transformer having a primary winding, a secondary winding for providing an output voltage, and an auxiliary winding. The SMPS also includes a power switch coupled to the primary winding. A first control circuit is coupled to the secondary winding, and is configured to provide a first electrical signal to the secondary winding when the output voltage of the SMPS is less than a reference voltage during a discontinuous time, whereupon a second electrical signal is induced in the auxiliary winding. A second control circuit is coupled to the auxiliary winding and the power switch. The second control circuit is configured to regulate the output of the SMPS by controlling the power switch in response to a feedback voltage signal from the auxiliary winding, and is further configured to turn on the power switch in response to the second electrical signal.

What is claimed is: 
     
       1. A switching mode power supply (SMPS), comprising:
 a transformer having:
 a primary winding for coupling to an input power source; and 
 a secondary winding for providing an output voltage of the SMPS; and 
 an auxiliary winding; 
 
 a power switch coupled to the primary winding; 
 a first control circuit coupled to the secondary winding, the first control circuit being configured to provide a first electrical signal to the secondary winding when the output voltage of the SMPS is less than a reference voltage during a discontinuous time, whereupon a second electrical signal is induced on the auxiliary winding; and 
 a second control circuit coupled to the auxiliary winding and the power switch, the second control circuit being configured to regulate the output of the SMPS by controlling the power switch in response to a feedback voltage signal from the auxiliary winding, the second control circuit being further configured to turn on the power switch during the discontinuous time in response to the a second electrical signal when the output of the SMPS is less than the reference voltage; 
 wherein the first electrical signal causes a ringing voltage waveform in the secondary winding after a damping ringing waveform in the secondary winding that follows a secondary current flow induced by a primary current. 
 
     
     
       2. The switching mode power supply of  claim 1  wherein during the discontinuous time, substantially no energy is stored in the primary and secondary windings. 
     
     
       3. The switching mode power supply of  claim 1  wherein the first electrical signal comprises a current pulse in an opposite direction to a secondary current in normal operation of the SMPS. 
     
     
       4. The switching mode power supply of  claim 1  wherein the second control circuit is configured to detect the ringing waveform in the secondary winding through the auxiliary winding. 
     
     
       5. The switching mode power supply of  claim 1  wherein the second control circuit is configured to ignore the damping ringing waveform when detecting the ringing waveform in the secondary winding through the auxiliary winding. 
     
     
       6. The switching mode power supply of  claim 1  wherein the first control circuit comprises:
 a first terminal coupled to an output capacitor to receive the output voltage of the SMPS; and 
 a second terminal coupled to secondary winding; 
 wherein the first control circuit is configured to transfer electric charges from the output capacitor to the second winding when the output voltage is equal to or less than a reference voltage. 
 
     
     
       7. The switching mode power supply of  claim 1  wherein the first control circuit comprises:
 a voltage divider coupled to the output voltage and configured to generated an attenuated voltage; 
 a first comparator having:
 a first input terminal coupled to the attenuated voltage; 
 a second input terminal coupled to the reference voltage; and 
 an output terminal; 
 
 a pulse generator coupled to the comparator; and 
 a switch coupled to the pulse generator; 
 wherein the pulse generator is configured to turn on the switch when the attenuated voltage is less than the reference voltage. 
 
     
     
       8. The switching mode power supply of  claim 7  wherein the switch is a PMOS or an NMOS transistor. 
     
     
       9. The switching mode power supply of  claim 7  wherein the switch is connected between a terminal of the secondary winding and an output terminal of the SMPS. 
     
     
       10. The switching mode power supply of  claim 1  wherein the second control circuit comprises:
 a feedback terminal coupled to the auxiliary winding through a coupling network, the feedback terminal being configured to receive a feedback signal from the a coupling network; 
 a driver unit configured to drive the power switch; 
 a sense current input terminal coupled to a terminal of the power switch; 
 a control unit coupled to the sense current input terminal and the feedback terminal, the control unit configured to control the power switch for regulating the output of the SMPS; and 
 a dynamic event detection circuit coupled to the feedback terminal and configured to detect the second electrical signal. 
 
     
     
       11. The switching mode power supply of  claim 10  wherein the coupling network is a voltage divider network. 
     
     
       12. The switching mode power supply of  claim 10  wherein the dynamic event detection circuit is configured to activate the driver unit for turning on the power switch for a predetermined time period upon detecting the electrical signal. 
     
     
       13. The switching mode power supply of  claim 12  wherein the dynamic event detection circuit is configured to detect a ringing waveform in the auxiliary winding. 
     
     
       14. A switching mode power supply (SMPS), comprising:
 a transformer having:
 a primary winding for coupling to an input power source; 
 a secondary winding for providing an output voltage of the SMPS; and 
 an auxiliary winding; 
 
 a power switch coupled to the primary winding; 
 a first control circuit coupled to the secondary winding, the first control circuit being configured to provide a first electrical signal to the secondary winding when the output voltage of the SMPS is less than a reference voltage during a discontinuous time, whereupon a second electrical signal is induced in the auxiliary winding; and 
 a second control circuit coupled to the auxiliary winding and the power switch, the second control circuit being configured to regulate the output of the SMPS by controlling the power switch in response to a feedback voltage signal from the auxiliary winding, the second control circuit being further configured to turn on the power switch in response to the a second electrical signal when the output of the SMPS is less than the reference voltage, 
 wherein the first electrical signal causes a ringing voltage waveform in the secondary winding after a damping ringing waveform in the secondary winding that follows a secondary current flow induced by a primary current. 
 
     
     
       15. The switching mode power supply of  claim 14  wherein during the discontinuous time, substantially no energy is stored in the primary and secondary windings. 
     
     
       16. The switching mode power supply of  claim 14  wherein the second control circuit is configured to detect the ringing voltage waveform in the secondary winding through the auxiliary winding. 
     
     
       17. The switching mode power supply of  claim 14  wherein the second control circuit is configured to ignore the damping ringing waveform when detecting the ringing waveform in the secondary winding through the auxiliary winding. 
     
     
       18. A switching mode power supply (SMPS) comprising:
 a transformer;   a feedback terminal configured to receive, during a discontinuous time of the transformer, a pulse signal from a secondary side of the SMPS;   a detection circuit on a primary side of the SMPS configured to generate a signal in response to the pulse signal; and   a driver block coupled to the detection circuit for delivering a switching signal to a switch coupled to a primary winding of the transformer.   
     
     
       19. The switching mode power supply of claim 18, in which the discontinuous time follows a conduction time of a secondary winding of the transformer, which follows a conduction time of a primary winding of the transformer. 
     
     
       20. The switching mode power supply of claim 19, in which the switch is turned off during the discontinuous time and during the conduction time of the secondary winding. 
     
     
       21. The switching mode power supply of claim 20, in which the switch turns on in response to the switching signal. 
     
     
       22. The switching mode power supply of claim 18, in which the pulse signal is coupled to the detection circuit via an auxiliary winding of the transformer. 
     
     
       23. The switching mode power supply of claim 18, in which the pulse signal is generated in response to an output voltage of the power supply. 
     
     
       24. The switching mode power supply of claim 23, in which the response to an output voltage comprises the output voltage dropping in response to load transients. 
     
     
       25. The switching mode power supply of claim 23, in which the response to an output voltage comprises the output voltage dropping in response to dynamic load changes. 
     
     
       26. The switching mode power supply of claim 18, further comprising a pulse generator. 
     
     
       27. The switching mode power supply of claim 26, in which the pulse generator comprises a multi-vibrator oscillator. 
     
     
       28. The switching mode power supply of claim 18, in which the detection circuit is further configured to receive a first pulse signal followed by a second pulse signal during the discontinuous time of the transformer. 
     
     
       29. The switching mode power supply of claim 28, in which the detection circuit is further configured to generate the signal to the switch in response to the second pulse signal. 
     
     
       30. The switching mode power supply of claim 28, in which the first pulse signal comprises a damping ringing waveform. 
     
     
       31. The switching mode power supply of claim 28, in which the second pulse signal comprises a ringing waveform. 
     
     
       32. The switching mode power supply of claim 18, further comprising a MOS transistor on the secondary side of the SMPS. 
     
     
       33. The switching mode power supply of claim 32, in which the pulse signal is generated when the MOS transistor is off. 
     
     
       34. The switching mode power supply of claim 18, further comprising an IC on the secondary side of the SMPS. 
     
     
       35. The switching mode power supply of claim 34, in which the IC comprises a switch in the form of a transistor. 
     
     
       36. The switching mode power supply of claim 35, further comprising a control block configured to compare an output voltage and a predetermined voltage. 
     
     
       37. The switching mode power supply of claim 36, in which the switch turns on when the output voltage is lower than the predetermined voltage.