Abstract:
A switching “power ( 100 ) includes a single, power switching Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) (Q 1 ) that without any additional transistor provides both self-oscillation and over-current protection. A transformer (Tr 1 ) that is included in a positive feedback path of the MOSFET has a tapped (intermediate terminal  103 ) auto-transformer winding. A source terminal ( 102 ) of the MOSFET is coupled via a current limiting resistor (R 2 ) to a junction terminal between first (n 1 )and second (n 2 ) windings of the tapped auto-transformer (Tr 1 ). The first winding forms the primary winding of the transformer and the second winding is coupled to a gate terminal of the MOSFET to form a regenerative feedback path. The second winding is direct-current (DC) coupled to the gate terminal to avoid the need for any discrete capacitor in the positive feedback path.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a switching power supply. 
       BACKGROUND OF THE INVENTION 
       [0002]    Switching power supplies using discrete components are usually constructed with at least two transistors. One transistor is used for switching current in a primary winding of a transformer coupled in series with a main current path of the transistor. The other one transistor is used for providing over-current protection in the first transistor. It may be desirable to form a switching power supply with a power switching Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) as an active element such that, without any additional active element, both self-oscillation and over-current protection are provided by the MOSFET. 
         [0003]    In carrying out an inventive feature, a transformer that is included in a positive feedback path of the MOSFET has a tapped auto-transformer winding. A source terminal of the MOSFET is coupled via a current limiting or sampling resistor to a junction terminal between first and second windings of the tapped auto-transformer winding. The first winding forms the primary winding of the transformer and the second winding is coupled to a gate terminal of the MOSFET to form a regenerative feedback path. 
         [0004]    In carrying out another inventive feature, the second winding is direct-current (DC) coupled to the gate terminal to avoid the need for any discrete capacitor in the positive feedback path. Thereby, advantageously, the power supply is simplified. 
       SUMMARY OF THE INVENTION 
       [0005]    A power supply, embodying an aspect of the invention, includes a source of input supply voltage. A transformer is coupled to a load for energizing the load. A switching, power metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) has a source terminal coupled to the transformer and has a drain terminal coupled to the input supply voltage. A ramping current that stores magnetic energy in the transformer is produced. The ramping current develops a voltage at the source terminal of the MOSFET. The source terminal voltage is coupled to a gate terminal of the MOSFET via the transformer by an auto-transformer action in a regenerative feedback manner, during a first portion of a switching cycle of the MOSFET, when the MOSFET is conductive. A current sampling resistor is coupled in a current path of the ramping current for developing a ramping, degenerative voltage in the resistor voltage to reduce the conductivity of the MOSFET in accordance with the resistor voltage until a turn off threshold voltage of the MOSFET is reached. When the turn off threshold voltage of the MOSFET is reached, the source terminal voltage changes in a degenerative feedback manner. The change in the source terminal voltage continues until the MOSFET becomes non-conductive at an end of the cycle portion. The MOSFET remains non-conductive until an oscillatory resonant voltage produced from the stored magnetic energy renders the MOSFET conductive again, at an end of a following, second portion of the switching cycle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  illustrates a first embodiment of the invention; 
           [0007]      FIGS. 2   a ,  2   b ,  2   c  and  2   d  provide corresponding waveforms in the arrangement of  FIG. 1 ; and 
           [0008]      FIG. 3  illustrates a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0009]      FIG. 1  depicts a switching power supply  100 . A switching, power Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) Q 1  has a drain terminal  101  coupled to an input supply voltage Vin. Supply voltage Vin is filtered or decoupled by a filter capacitor C 1 . A source terminal  102  of MOSFET Q 1  is coupled via a current limiting or sampling resistor R 2  to a transformer tap or intermediate terminal  103  coupled between a winding n 1  and a winding n 2  of a transformer Tr 1 . Windings n 1  and n 2  provide an auto-transformer action. Transformer Tr 1  is formed by a core E16-Type with an air gap of 0.1 mm. Winding n 1  has 130 windings and an inductance of 190 mH. Winding n 2  has 16 windings and an inductance of 2.8 mH. A winding n 3  of transformer Tr 1  has 8 windings and an inductance of 0.7 mH. 
         [0010]    A main current path of a drain current I 1  includes drain terminal  101 , source terminal  102  and current limiting resistor R 2 . Current I 1  flows when MOSFET Q 1  is conductive for storing magnetic energy in transformer Tr 1 . When MOSFET Q 1  is conductive, a source voltage Vs is coupled via resistor R 2  to terminal  103  for producing a voltage V(n 2 ) in winding n 2  by an auto-transformer action of windings n 1  and n 2 . 
         [0011]    In carrying out an inventive feature, source terminal  102  is direct-current (DC) coupled to a gate terminal  105  of MOSFET Q 1  via winding n 2  and a zener diode D 1 . Zener diode D 1  operates in a breakdown mode to provide DC voltage level shifting. Thereby, advantageously, there is no need for a discrete coupling capacitor in a signal path between terminals  102  and  105 . 
         [0012]      FIGS. 2   a ,  2   b ,  2   c  and  2   d  provide corresponding waveforms useful for explaining power supply  100  of  FIG. 1 . Similar symbols and numerals in  FIGS. 2   a ,  2   b ,  2   c ,  2   d  and  1  indicate similar items or functions. 
         [0013]    At a time T 1  of a switching cycle time T of  FIGS. 2   a - 2   d , a resonant voltage Vgt shown in  FIG. 2   b , developed between gate terminal  105  of  FIG. 1  and transformer intermediate terminal  103  reaches a turn-on threshold level of MOSFET Q 1  to initiate conduction in MOSFET Q 1 , as described below. Consequently, a voltage Vs is developed at source terminal  102  from voltage Vin. Voltage Vs is transformer coupled in a regenerative feedback manner by an auto-transformer action via winding n 1  to winding n 2  to produce voltage V(n 2 ) in winding n 2 . Voltage V(n 2 ) enhances the conductivity of MOSFET Q 1  to render MOSFET Q 1  fully conductive. Consequently, ramping-up current I 1  is produced, during an interval T 1 -T 2  of cycle time T of  FIG. 2   c.    
         [0014]    Current sampling resistor R 2  of  FIG. 1  develops a corresponding ramping-up, degenerative voltage V(R 2 ) in current limiting R 2  that varies a voltage Vgs in a gate-source capacitance Cgs that is formed between gate terminal  105  and source terminal  102  of  FIG. 1 . Voltage V(R 2 ) reduces the conductivity of MOSFET Q 1  in a progressive manner as current I 1  of  FIG. 2   c  further increases, during an interval T 1 -T 2 . At time T 2   FIG. 2   b  a conductivity threshold of MOSFET Q 1  of  FIG. 1 , determined by the value of resistor R 2  and current I 1  is reached. Consequently, voltage Vs in  FIG. 2   a  begins decreasing. 
         [0015]    In carrying out another inventive feature, the decrease in voltage Vs causes a corresponding decrease in voltage V(n 2 ) in winding n 2  of  FIG. 1  and also in voltage Vgt in a degenerative feedback manner until MOSFET Q 1  becomes non-conductive at time T 3  of  FIG. 2   c . Thus, advantageously, MOSFET Q 1  forms the sole active element in the over-current protection signal path to provide both self-oscillations and over-current protection. A conventional snubber network  110  is coupled to terminal  103  to perform a snubber network function, beginning at time T 2  of  FIG. 2   a.    
         [0016]    As a result of current I 1  ceasing to flow in winding n 1 , the stored magnetic energy in transformer Tr 1  of  FIG. 1  produces a flyback voltage V(n 3 ) of  FIG. 1  in secondary winding n 3  of transformer Tr 1  in a polarity and magnitude that causes the turn-on of a rectifier diode D 2  and the generation of an output current  12 . Current  12  of  FIG. 2   d  charges an output filter capacitor C 2  of  FIG. 1  to develop an output supply voltage Vout that is isolated by transformer Tr 1  from windings n 1  and n 2  with respect to electrical shock hazard. Voltage Vout is applied to energize a load  104 . 
         [0017]    MOSFET Q 1  remains non-conductive, during a following interval, T 3 -T 5 , of cycle time T of  FIG. 2   c . At a time T 4  of  FIG. 2   d , diode D 2  of  FIG. 1  becomes non-conductive. Consequently, the remaining stored energy in transformer Tr 1  produces an oscillatory resonant voltage portion  106  in voltage Vgt of  FIG. 2   b . At time T 5  of  FIG. 2   b , rising voltage Vgt causes gate-source voltage Vgs to exceed the threshold of MOSFET Q 1  of  FIG. 1  that renders MOSFET Q 1  conductive, at an end of interval T 3 -T 5  of  FIG. 2   c  for beginning the next cycle. 
         [0018]    At start-up, a start-up resistor R 1  of  FIG. 1  that is coupled to gate terminal  105  produces a current that charges parasitic gate capacitance Cgs until MOSFET Q 1  turns on. Resistor R 1  is only needed for start-up and the value can be in the mega-ohm range. When current I 1  starts flowing into winding n 1  of transformer Tr 1 , voltage V(n 2 ) develops in winding n 2 . Voltage V(n 2 ) causes an increase in voltage Vgt and supports the turn on of MOSFET Q 1  in a regenerative feedback manner, in a way similar to that explained before. 
         [0019]    A series arrangement that includes a resistor R 3 , a reference voltage producing zener diode D 3  and a light emitting element of an opto-coupler U 1  that is coupled to voltage Vout provides a secondary side regulation. Diode D 3  starts conducting when voltage Vout is above a threshold voltage of 6V. Consequently, opto-coupler U 1  clamps voltage Vgs such that MOSFET Q 1  is prevented from being turned on until voltage Vout is reduced to a level below 6V. The efficiency is 60% and thus within the range of other power supplies with an output power of 1 Watt or less. 
         [0020]      FIG. 3  depicts a power supply  100 ′ forming a second embodiment of the invention that is a step down converter. Similar symbols and numerals in  FIGS. 3 and 1  indicate similar items or functions. A free-wheel or catch diode D 4 ′ is coupled to the anode of diode D 1 ′. Winding n 2 ′ has an end terminal  111 ′. End terminal  111 ′ forms a junction terminal to diodes D 1 ′ and D 4 . Winding n 1 ′ has an end terminal  112 ′. End terminal  112 ′ forms a junction terminal between capacitor C 2 ′ and resistor R 3 . Single transistor Q 1 ′ oscillates autonomously as described with respect to  FIG. 1 . Transformer Tr 1 ′ is formed by a core having a straight cylindrical rod. Winding n 1 ′ has 70 windings and an inductance of 50 mH. Winding n 2 ′ has 15 windings and an inductance of 2.5 mH.