Self-oscillating switching power supply with output voltage regulated from the primary side

Abstract

Regulation of the output voltage of a power supply employing a flyback-type self-oscillating DC-DC converter employing a transformer. The primary winding circuit of the transformer senses a current recirculation loop for discharging the energy cyclically stored in an auxiliary winding of the self-oscillation loop of the converter such as to represent a replica of the circuit of the secondary winding of the transformer and by summing a signal representative of the level of the energy stored in the auxiliary winding with a drive signal on a control node of a driver of the power switch of the converter.

I claim:  
     
       1. A self-oscillating, DC—DC converter, comprising: 
       a transformer having a primary winding coupled to a primary circuit and a secondary winding coupled to a secondary circuit, said primary circuit including a first  switch, functionally connected in series with the primary winding, having a first terminal thereof coupled to an input node;  
       a sensing resistance functionally connected between said first  switch and a common potential node of the circuit, said first  switch being driven by a self-oscillation circuit composed of at least an auxiliary winding having a first and a second terminal magnetically coupled to said primary winding and a first capacitor connected between a control element of said first  switch and an intermediate connection node between said auxiliary winding and said first capacitor;  
       a second switch  device capable of shortcircuiting said control element of  deactivating said first  switch to said common potential node  when a current through the primary winding reaches a preestablished level;  
       at least a second capacitor connected between a second terminal of said auxiliary winding and said common potential node;  
       at least a diode having an anode coupled to said common potential node and a cathode coupled to said intermediate connection node; and  
       at least a zener diode connected between said second terminal of said auxiliary winding and a control element of said second switch  device.  
     
     
       2. The self-oscillating, DC—DC converter, according to  claim 1  wherein said first  switch is an isolated-gate, field effect device and said second switch  device is bipolar NPN transistor. 
     
     
       3. A DC—DC voltage regulating circuit, comprising: 
       an input voltage terminal;  
       a first  switch;  
       a primary winding serially coupled between said input voltage terminal and said first  switch;  
       a secondary winding magnetically coupled to said primary winding;  
       a sensing resistance serially coupled between said first  switch and a common potential node; and  
       a self-oscillation circuit coupled to a first control terminal of said first  switch, wherein said self-oscillation circuit comprises an auxiliary winding magnetically coupled to said primary winding and having a first node and a first intermediate node;  
       a first capacitive element coupled between said first node and said common potential node;  
       a first diode having an anode coupled to said common potential node and a cathode coupled to said first intermediate node;  
       a second capacitive element coupled between said first intermediate node and said first control terminal of said first  switch;  
       a first resistive element coupled between said input voltage terminal and said first control terminal of said first  switch;  
       a second switch  device coupled between said first control terminal of said first  switch and said common potential node and having a second control terminal coupled to a second intermediate node connecting said first  switch and said sensing resistance; and  
       a second diode coupled between said first node and said second control terminal.  
     
     
       4. The circuit of  claim 3  wherein said second switch  device is a bipolar NPN transistor. 
     
     
       5. The circuit of  claim 3  wherein said second diode is a zener diode. 
     
     
       6. The circuit of  claim 3  wherein said self-oscillation circuit further includes means for summing a signal representative of the level of the energy stored in said auxiliary winding with a control signal provided to said first control terminal of said first  switch. 
     
     
       7. The circuit of  claim 3 , further including a filtering capacitive element coupled between said input voltage terminal and said common potential node. 
     
     
       8. The circuit of  claim 3 , wherein said second switch shortcircuits said first control terminal to said common potential node  device deactivates said switch when a current through the primary winding reaches a predetermined level. 
     
     
       9. The circuit of  claim 3 , further including a third resistive element coupled between said second control terminal and said second intermediate node. 
     
     
       10. The circuit of  claim 3  wherein said first  switch is an isolated-gate, field effect device. 
     
     
       11. A power- supply circuit, comprising:    
       
         an output terminal;  
       
       
         a transformer having a primary winding, having an auxiliary winding, and having a secondary winding coupled to the output terminal and operable to generate an output voltage on the output terminal;  
       
       
         a device having a variable conductivity and operable to control a flow of current through the primary winding; and  
       
       
         a regulation circuit including the auxiliary winding and a sense element coupled to the device and operable to conduct the current the regulation circuit operable to maintain the output voltage at a constant or an approximately constant level by coupling a regulation signal to the sense element during a period in which the device has a high conductivity. 
       
     
     
       12. The power- supply circuit of    claim 11    wherein the primary and auxiliary windings are electrically isolated from the secondary winding.   
     
     
       13. The power- supply circuit of    claim 11   , further comprising an input terminal coupled to the primary winding and operable to receive an unregulated AC power signal.   
     
     
       14. The power- supply circuit of    claim 11    wherein the device comprises an N - channel MOS transistor.   
     
     
       15. The power- supply circuit of    claim 11    wherein the regulation circuit is operable to generate the regulation signal.   
     
     
       16. The power- supply circuit of    claim 11    wherein the regulation circuit controls the conductivity of the device by periodically varying the conductivity of the device from the high conductivity to a low conductivity at a frequency that is proportional to a flyback signal generated across the primary winding when the device has the low conductivity.   
     
     
       17. The power- supply circuit of    claim 11    wherein the regulation circuit controls the conductivity of the device by periodically varying the conductivity of the device from the high conductivity to a low conductivity at a frequency that is proportional to a flyback signal generated across the auxiliary winding when the device has the low conductivity.   
     
     
       18. The power- supply circuit of    claim 11    wherein:    
       
         the secondary winding is operable to provide an output current to the output terminal; and the regulation circuit controls the conductivity of the device by periodically varying the conductivity of the device from the high conductivity to a low conductivity at a frequency that is proportional to the output current. 
       
     
     
       19. The power- supply circuit of    claim 11    wherein the regulation signal comprises a regulation current.   
     
     
       20. A power- supply circuit, comprising:    
       
         an input terminal;  
       
       
         an output terminal operable to provide an output voltage;  
       
       
         a primary transformer winding coupled to the input terminal;  
       
       
         a secondary transformer winding coupled to the output terminal, the secondary transformer winding being electrically isolated from and magnetically coupled to the primary transformer winding;  
       
       
         a switching device having a first drive terminal coupled to the primary transformer winding, having a second drive terminal, and having a control terminal; and  
       
       
         a regulation circuit comprising,  
       
       
         an auxiliary transformer winding that is electrically isolated from the secondary transformer winding and that is magnetically coupled to the primary and secondary transformer windings,  
       
       
         a sense element coupled to the second drive terminal of the switching device, and  
       
       
         wherein the regulation circuit is operable to regulate the output voltage by,  
       
       
         generating a regulation signal, and  
       
       
         coupling the regulation signal to the sense element while energy is being stored in the primary transformer winding. 
       
     
     
       21. The power- supply circuit of    claim 20   , further comprising a common core upon which the primary, secondary, and auxiliary transformer windings are wound.   
     
     
       22. The power- supply circuit of    claim 20    wherein the device comprises an N - channel power transistor.   
     
     
       23. The power- supply circuit of    claim 20    wherein:    
       
         the regulation signal comprises a regulation current; and  
       
       
         the regulation circuit is operable to cause the regulation current to flow through the sense element. 
       
     
     
       24. A method for regulating an output voltage, the method comprising: 
       
         storing flyback energy by allowing a charging current to flow through a primary transformer winding and through a sense element during a charging period;  
       
       
         controlling the duration of the charging period by coupling a regulating signal to the sense element during the charging period;  
       
       
         generating a primary flyback voltage across the primary transformer winding after the charging period;  
       
       
         generating an auxiliary flyback voltage across an auxiliary transformer winding in response to the primary flyback voltage;  
       
       
         generating the regulating signal from the auxiliary flyback voltage;  
       
       
         generating a secondary flyback voltage across a secondary transformer winding in response to the primary flyback voltage; and  
       
       
         generating the output voltage from the secondary flyback voltage. 
       
     
     
       25. The method of  claim 24  wherein: 
       
         the regulating signal comprises a regulating current;  
       
       
         the sense element comprises an input terminal; and  
       
       
         controlling the duration of the charging current comprises summing the regulating and charging currents at the input terminal of the sense element. 
       
     
     
       26. The method of  claim 24  wherein: 
       
         the regulating signal comprises a regulating current; and  
       
       
         controlling the duration of the charging current comprises causing the regulating current to flow through the sense element. 
       
     
     
       27. The method of  claim 24 , wherein: 
       
         the regulating signal comprises a regulating current; and  
       
       
         controlling the duration of the charging period comprises causing the regulating current to flow through the sense element. 
       
     
     
       28. A power- supply circuit, comprising:    
       
         an input terminal;  
       
       
         a supply terminal;  
       
       
         a regulated output terminal operable to provide an output voltage;  
       
       
         a primary transformer winding coupled to the input terminal;  
       
       
         a secondary transformer winding coupled to the regulated output terminal, the secondary transformer winding being electrically isolated from and magnetically coupled to the primary transformer winding;  
       
       
         a switching device having a first drive terminal coupled to the primary transformer winding, having a second drive terminal, and having a control terminal; and  
       
       
         a regulation circuit comprising,  
       
       
         an auxiliary transformer winding that has first and second terminals, that is electrically isolated from the secondary transformer winding, and that is magnetically coupled to the primary and secondary transformer windings,  
       
       
         a sense element coupled between the supply terminal and the second drive terminal of the switching device,  
       
       
         a diode coupled between the supply terminal and the first terminal of the auxiliary winding,  
       
       
         a capacitor coupled between the supply terminal and the second terminal of the auxiliary winding,  
       
       
         a transistor having a first drive terminal coupled to the supply terminal, a second drive terminal coupled to the control terminal of the switching device, and  
       
       
         a control terminal coupled to the second drive terminal of the switching device, and  
       
       
         a zener diode coupled between the second terminal of the auxiliary transformer winding and the control terminal of the transistor. 
       
     
     
       29. A method for generating a regulated output voltage, the method comprising: 
       
         storing flyback energy by allowing a charging current to flow through a primary transformer winding during a charging period;  
       
       
         controlling the duration of the charging period by combining a regulating current with the charging current during the charging period;  
       
       
         generating a primary flyback voltage across the primary transformer winding after the charging period;  
       
       
         generating an auxiliary flyback voltage across an auxiliary transformer winding in response to the primary flyback voltage;  
       
       
         generating the regulating current from the auxiliary flyback voltage;  
       
       
         generating a secondary flyback voltage across a secondary transformer winding in response to the primary flyback voltage; and  
       
       
         generating the regulated output voltage from the secondary flyback voltage. 
       
     
     
       30. The method of  claim 29  wherein: 
       
         storing the flyback energy comprises reducing the impedance of a switching device coupled in series with the primary transformer winding; and  
       
       
         generating the primary flyback voltage comprises increasing the impedance of the switching device in response to the combination of the charging and regulating currents. 
       
     
     
       31. The method of  claim 29  wherein controlling the duration of the charging period comprises summing the charging and regulating currents. 
     
     
       32. The method of  claim 29  wherein: 
       
         controlling the duration of the charging period comprises summing the charging and regulating currents; and  
       
       
         generating the primary flyback voltage comprises increasing the impedance of a switching device coupled in series with the primary transformer winding when the sum of the charging and regulating currents equals or exceeds a predetermined value.