Abstract:
This invention discloses a power converter with a secondary-side control, including an input circuit with one or more switches, an output circuit with an output end and a controller, and a transformer with a primary-side coil assembly connecting the switch(es) and a secondary-side coil assembly connecting the output circuit. The on/off state of the switch(es) is controlled by variations in voltage of primary-side coil assembly. The controller in the output circuit detects an output voltage and sends detected results to the primary-side coil assembly as a feedback for primary-side coil assembly to regulate the PWM or PFM action of the switch in a specific way to maintain voltage stability.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a switching power converter and more particularly, to a switching power converter that applies a secondary-side control to achieve system stability and durability. 
         [0003]    2. Description of the Prior Art 
         [0004]    Switching power converters have the advantages of high working efficiency and limited volume in size, and therefore, are widely used in a variety of electronic devices.  FIG. 11  shows the circuit of a conventional flyback converter, including mainly an input circuit  1 ′ (indicated by a dotted rectangle at the left), an output circuit  2 ′ (indicated by a dotted rectangle at the right), a transformer T 1 ′ (in between the above two dotted rectangles), and an optical coupler  3 ′. 
         [0005]    Referring to  FIG. 11 , an input circuit  1 ′ connects to an input voltage Vin and includes mainly two parts, a transistor Q 1 ′ for switching, and, a controller  11 ′ for regulating PWM (Pulse Width Modulation). The transistor Q 1 ′ connects to the primary-side coil assembly of the transformer T 1 ′ at one end, and also connects to the output end of the controller  11 ′ at the other. The input end FB of the controller  11 ′ connects to one end of the optical coupler  3 ′. 
         [0006]    At the system output side (indicated by a dotted rectangle at the right in  FIG. 11 ) an output circuit  2 ′ connects to a secondary-side coil assembly of the transformer T 1 ′. The output voltage Vout connects in parallel to one end of the optical coupler  3 ′, isolating the input circuit  1 ′ from the output circuit  2 ′, and conveying the output voltage Vout back to the controller  11 ′ (of the input circuit  1 ′). Consequently, the controller  11 ′ is able to output a more or less stable voltage in correspondence to an output by controlling the on-off states of the transistor T 1 ′. 
         [0007]    To sum up, the above-mentioned prior art (illustrated in  FIG. 11 ) makes use of the feed-back control function of an optical coupler to manage the output voltage; the physical characteristics of an optical coupler unavoidably effects the stability and durability of the system. For instance, the coupling efficiency of an optical coupler reflects the accuracy of an output voltage. Furthermore, extra electric elements are required to avoid or reduce the unstable performance of an optical coupler when it is used as a current-stabilizing current charger, adding more cost and bringing in more idleness and worn-out. 
         [0008]      FIG. 12  illustrates the electric circuits of another conventional flyback converter, including mainly an input circuit  5 ′ (indicated by a dotted rectangle on the left-hand side of  FIG. 12 ), a transformer T 2 ′, an output circuit  6 ′ (as indicated by a dotted rectangle on the right-hand side of  FIG. 12 ). Unlike a conventional converter shown in  FIG. 11 , the transformer T 2 ′ (shown in  FIG. 12 ) includes three coils: two coils on the primary-side coil assembly (i.e. a primary-side 1 st  coil assembly, and a primary-side 2 nd  coil assembly) and one on the secondary-side coil assembly (i.e. a secondary-side 3 rd  coil assembly). 
         [0009]    The input circuit  5 ′ connects to an input voltage Vin and includes two main parts: a transistor Q 2 ′ for switching, and a controller  51 ′ for regulating PWM. One end of the transistor Q 2 ′ connects to the primary-side 1 st  coil assembly N 1 ′ (of the transformer T 2 ′), and the other end, to the output end of the controller  51 ′. The input end of the controller  51 ′ connects to the primary-side 2 nd  coil assembly N 2 ′ (of the transformer T 2 ′). 
         [0010]    At the system output side (indicated by a dotted rectangle at the right of  FIG. 12 ), the output circuit  6 ′ with an output voltage of Vout connects to the secondary-side 3 rd  coil assembly N 3 ′ (of the transformer T 2 ′). Variations in voltage is conveyed from secondary-side coil assembly (of the transistor T 2 ′) to primary-side coil assembly (of the transformer T 2 ′) and is detected by the controller  51 ′. As a result, the controller  51 ′ regulates the output voltage Vout by detecting variations in voltage, and then by controlling the switching on/off of the transistor Q 2 ′. 
         [0011]    The above-mentioned conventional circuit has the merit of simplicity in structure, which, nevertheless, relies completely upon the physical characteristics of the transformer T 2 ′ to detect variations in voltage and to regulate the output circuit accordingly. Furthermore, the voltage conveyed back is not continuous, affecting adversely the stability and durability in the electric system, the no-load output voltage, as well as in the efficiency of dynamic voltage management. 
       SUMMARY OF THE INVENTION  
       [0012]    The object of the invention is to control the switching on/off the transistor (located in the primary-side coil assembly) for more reliability and precision in the electric output system by way of a controller, located in the secondary-side coil assembly of a transformer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]      FIG. 1  is an illustration of the basic circuit structure of this invention. 
           [0014]      FIG. 2  is an illustration of the circuit configuration of the 1 st  embodiment of this invention. 
           [0015]      FIG. 3  is an illustration of the voltage and current points of the 1 st  embodiment of this invention. 
           [0016]      FIG. 4  is an illustration of the wave motions corresponding to  FIG. 3 . 
           [0017]      FIG. 5  illustrates the inactive mode of the controller when the output voltage goes above a preset threshold. 
           [0018]      FIG. 6  illustrates the re-start mode of the controller by the secondary-side coil assembly when the output voltage goes below a preset threshold. 
           [0019]      FIG. 7  illustrates the reaction of the controller when the output voltage goes under a pre-set threshold. 
           [0020]      FIG. 8  illustrates the wave forms corresponding to  FIG. 5 ,  6 , and  7 . 
           [0021]      FIG. 9  illustrates the wave forms of the second transistor of the 1 st  embodiment as a restrictive-current protective device. 
           [0022]      FIG. 10  is an illustration of the circuit configuration of the 2 nd  embodiment of this invention. 
           [0023]      FIG. 11  is an illustration in circuit configuration of a conventional flyback converter with an optical coupler. 
           [0024]      FIG. 12  is an illustration in circuit configuration of another conventional flyback converter. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    Referring to  FIG. 1 , the invention includes mainly an input circuit  1 , a transformer T, and an output circuit  2 . The input circuit  1  includes one ore more switching devices  11 , an input end, and an output end; the input end of input circuit  1  connects to an input voltage Vin, while the output end of input circuit  1  connects to transformer T. Switching device  11  could be a transistor, a MOSFET, a single device, or an assembly of devices with a switching function. 
         [0026]    Transformer T includes a primary-side coil assembly and a secondary-side coil assembly. The primary-side coil assembly (of transformer T) connects to switching device  11  (of input circuit  1 ) and controls the on/off states of switching device  11  in reaction to variations in voltage; while the secondary-side coil assembly (of the transformer T) connects to output circuit  2 . 
         [0027]    The input energy from input circuit  1  is conveyed by transformer T to the output end of the output circuit  2  as an output voltage Vout, as shown in  FIG. 2 . The output circuit  2  further includes a controller  21  to detect differences in voltage between output voltage (Vout) and a pre-set voltage threshold, and forward to the primary-side coil assembly as a decision-making feedback the voltage differences. The primary-side coil assembly (of the transformer) then reacts to voltages differences by controlling the on/off states of switching element  11  in PWM or PFM to stabilize output voltages. 
         [0028]    Still referring to  FIG. 2 , a preferred embodiment of the invention is a self-excited flyback converter. The primary-side coil assembly (of the transformer T) includes a 1 st  coil assembly N 1  and a 2 nd  coil assembly N 2 , both of which are of the same polarity. The secondary-side coil assembly (of the transformer T) includes a 3 rd  coil assembly with a polarity opposite to that of 1 st  coil assembly N 1  and that of 2 nd  coil assembly N 2 . The input end of input circuit  1  connects with an input voltage Vin, which in turn connects to the input end of a start-up circuit  12  and to one end of 1 st  coil assembly N 1  (of transformer T). 
         [0029]    Switching device  11  in this embodiment includes a transistor Q 1  and a transistor Q 2 . The output end of the start-up circuit  12  connects to Base terminal (B) of 1 st  transistor Q 1 ; while Collector terminal (C) of 1 st  transistor Q 1  connects to one end of 1 st  coil assembly N 1 . One end of 2 nd  coil assembly N 2  (of primary-side coil assembly of transformer T) connects to a current-restrictive resistor and then to terminal Base (B) of 1 st  transistor Q 1 , while the other end of 2 nd  coil assembly N 2  is grounded. Terminal Emitter (E) of 1 st  transistor Q 1  connects to a grounding resistor RS. 1 st  transistor Q 1  connects to a current-restrictive protective circuit, including a 2 nd  transistor Q 2 , of which terminal Base (B) connects to terminal Emitter (E) of 1 st  transistor Q 1 , terminal Collector (C) connects to terminal Base (B) of the 1 st  transistor, and terminal Emitter is grounded. 
         [0030]    Still referring to  FIG. 2 , the output circuit  2  includes a rectifying Diode D 1  with the anode (i.e. more positive terminal) connects to one end of 3 rd  coil assembly N 3  (of transformer T); while the other end of 3 rd  coil assembly N 3  is grounded. The cathode (i.e. more negative terminal) of recetifying Diode D 1  connects to the output end thereof, with the output end of D 1  connects in parallel to a grounding capacitor C 1 . 
         [0031]    The output circuit  2  in  FIG. 2  includes a controller  21  with a VDD voltage and a grounding point GND. Voltage VDD (of controller  21 ) connects to an output voltage Vout. The input feedback end FB of controller  21  connects to a voltage-divider point (of two resistors in series), and also includes a deviation amplifier with a reference voltage (not showin in  FIG. 2 ) for calculating deviations or variations of output voltages. The two control ends of controller  21 , the 1 st  CTL 1  and 2 nd  control end CTL 2 , connect to the two ends of Diode D 1 , respectively. Three states in terms of circuit configuration that may appear between CTL 1  and CTL 2  are regulated by controller  21 , including a resistor load connection, a short circuit, and an open circuit. 
         [0032]    Referring to  FIG. 3 and 4 , when in activation, a working voltage Vin is sent to the input end of input circuit  1 , making start-up circuit  12  generate a pluse a to turn on 1 st  transistor Q 1  with a collector current Ic. The input current flows through 1 st  coil assembly N 1  (of transformer T), 1 st  transistor Q 1 , and resistor RS before grounding; part of the current also flows through 2 nd  coil assembly N 2  (of transformer T), 1 st  transistor Q 1  and resistor RS before grounding. When all this happens, the upper polarity of 1 st  and 2 nd  coil assembly is both positive, while that of 3 rd  coil assembly is negative. In the mean time, the voltage increases when 1 st  transistor Q 1  connects in serial to resistor RS, turning on 2 nd  transistor Q 2  and, at the same time, turning off 1 st  transistor Q; energy from the primary-side coil assembly is therefore transferred to the secondary-side coil assembly, and the polarity of 3 rd  coil assembly N 3  is reversed for current to go through Diode D 1  to generate an output voltage Vout and current Id. When the energy in 3 rd  coil assembly is completely released, the polarity of transistor T changes, and the energy stored in the parasitic inductance in 2 nd  coil assembly N 2  (of transformer T) will again activate 1 st  transistor Q 1 . The invention is hence capable of making an automatic soft-start and a self-excitation by ringing, and operates in a non-continuous mode. Start-up circuit  12  is idle without consuming further energy after the first start-up; start-up pluses are sent out either when the system reaches the pre-set time limit for idleness or when reset conditions are met. 
         [0033]    Referring to  FIG. 3 ,  5 ,  7 , and  8 , controller  21  operates in accordance with the detected variations in voltage Vout: when output voltage Vout is higher than the pre-set voltage, controller  21  controls the two control ends CTL 1  and CTL 2  so that it can connect to a negative resistor load from the moment when energy in transformer T is completely transferred to the output end until the instant that 1 st  transistor Q 1  re-starts; consequently, when Id is equal to zero, the resistor load consumes the residual energy from leakage inductance of transformer T, and the current of which flows as indicated by an arrow in  FIG. 5 , stopping 1 st  transistor Q 1  from being activated and also reversing back to an open circuit (as shown in  FIG. 7 ) to avoid voltage Vout from keeping rising and hence to stabilize output voltages. 
         [0034]    Referring to  FIG. 6 and 8 , when output voltage Vout is lower than the pre-set voltage, controller  12  first creates a transient short circuit between CTL 1  and CTL 2 , and in the mean time, transfers energy in capacitor C 1  (of output circuit  2 ) to 3 rd  coil assembly N 3 , while current ID goes down accordingly. Afterwards, controller  21  makes an open circuit between CTRL  1  and CTRL  2 , as shown in  FIG. 7 and 8 , making voltage Vsw drop; it is now the energy in 2 nd  coil assembly N 2  that re-starts and turns on 1 st  transistor Q 1 , and the circuit is again back to the state of self-excited conversion, while energy in input circuit  1  is transferred quickly to output circuit  2  and hence raises output voltage Vout to maintain voltage stability. 
         [0035]    To sum up, by way of controller  21  (of the secondary-side coil assembly of transformer T) in this invention, 1 st  transistor Q 1  (of the primary-side coil assembly of transformer T) adequately performs the action of PWM or PFM to stablize output voltage Vout. Since controller  21  detects directly the output voltage Vout, deviations or variations are reduced and higher precision is achieved with another desired effect of a synchronous rectifier. Furthermore, absence of optical couplers contributes to system durability and reliability. 
         [0036]    Referring to  FIG. 3 and 9 , when output circuit  2  of the secondary-side coil assembly of transformer T shorts or overloads (for instance, due to malfunction of Diode D 1 ), current Ic of 1 st  coil assembly N 1  (of the primary-side coil assembly of transformer T) will quickly increase, and the voltage Vsense of resistor RS of output circuit  1  will turn on 2 nd  transistor Q 2  while 1 st  transistor Q 1  is off for the system to stop functioning. System will be re-activated by start-up circuit  12  when short circuit or overload disappears. In other words, 2 nd  transistor functions by delimiting currents to protect the system. 
         [0037]    When output circuit  2  has a short circuit, controller  21  will idolize both CTL 1  and CTL 2  for a period of substantial time for start-up circuit to do reset until the phase of short circuit is over. Both low manufacturing cost and high working efficiency of the start-up circuit help to contribute to the empirical rating of the invention. 
         [0038]    Referring to  FIG. 10 , the 2 nd  embodiment of the invention is very similar to the 1 st  embodiment discussed above. The difference between the two embodiments lies in the location of the rectifying Diode D 1 , which is now positioned beneath 3 rd  coil assembly and the grounding position, with a polarity opposite to that of the 1 st  embodiment but with the same function of self-excitation for conversion as the 1 st  embodiment. Furthermore, the value of control threshold of CTL 1  and CTL 2  of controller  21  can be adjusted in reaction to voltage variations for the same controlling effect. The function of synchronous rectifier can also be added onto controller  21  for more working efficiency. 
         [0039]    Although two preferred embodiments in accordance with the present invention have been provided in this application, it is to be understood that many other possible modification and variations can be made without departing from the scope of the present invention hereafter claimed.