Abstract:
A low voltage power supply is disclosed. The power supply comprises a transformer having a primary winding, secondary and auxiliary windings wherein an interrupted current in said primary winding induces a voltage in each of the secondary and auxiliary windings, a first driver circuit and a second driver circuit connected in parallel to said primary winding, a first controller receiving said voltage from said auxiliary winding and connected to said first driver circuit and a second controller connected to said second driver circuit, providing a control signal to said second driver circuit when said auxiliary is below a predetermined value.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    Flyback Switch Mode Power Supplies (SMPS) are well-known in the art to provide a controlled and well-regulated output. Switching power supplies use a low-side driver connected to the primary side winding of a voltage transformer to switch current at a set frequency through the primary winding. The current is related to the input voltage connected to the high-side of the primary winding. The transformer transfers energy to the secondary side winding to produce an output voltage that is related to the voltage applied across the primary winding. In some Flyback SMPS applications, an ASIC (Application Specific Integrated Circuit) is used to control a FET (Field Effect Transistor) by applying a known switching frequency and duty cycle. For example, the ASIC may provide a pulse width modulation (PWM) signal to the gate of the FET to turn on/off the FET at a known rate and for a known time based on a monitored reference voltage. 
         [0002]    To maintain the output voltage across the secondary winding within a set range, an Auxiliary winding that shares the same core as the secondary output winding is used to maintain an in-direct close-loop control of the secondary winding output voltage. The Auxiliary winding output voltage can be used to power the ASIC once the Auxiliary winding voltage reaches a certain level. Typically, the SMPS ASIC has a minimum lower operating voltage in the range of 7 to 8.4 volts. 
         [0003]    This lower operating voltage range is not suitable for low voltage applications of shunt trip and under voltage release (UVR) devices, e.g., 12 volt shunt trip and UVR units which are employed to monitor and respond to voltages well below 7 to 8 volts. 
         [0004]    One attempt to overcome this deficiency involves the use of a boost circuit that boosts the voltage level to the ASIC. However, use of a boost circuit creates the requirement of different hardware configurations and topologies for different voltage levels that, in turn, increases the cost of manufacturing the SMPS. 
         [0005]    Hence, there is a need in the industry for a low voltage SMPS configuration that does not significantly increase the manufacturing cost. 
       SUMMARY OF THE INVENTION  
       [0006]    One aspect of the present invention relates to a device for engaging a switching power supply at a lower start voltage. The power supply comprises a transformer having a primary winding, secondary and auxiliary windings wherein an interrupted current in said primary winding induces a voltage in each of the secondary and auxiliary windings, a first driver circuit and a second driver circuit connected in parallel to said primary winding, a first controller receiving said voltage from said auxiliary winding and connected to said first driver circuit and a second controller connected to said second driver circuit, providing a control signal to said second driver circuit when said auxiliary is below a predetermined value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The embodiments shown in the accompanying drawings, and described in the accompanying detailed description, are to be used as illustrative embodiments and should not be construed as the only manner of practicing the invention. Also, the same reference numerals, possibly supplemented with reference characters where appropriate, have been used to identify similar elements. 
           [0008]      FIG. 1  illustrates an exemplary display device according to an aspect of the present invention. 
       
    
    
       [0009]    It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity many other elements found in conventional systems of the type described herein. Those of ordinary skill of the art may recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well-known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. 
       DETAILED DESCRIPTION  
       [0010]      FIG. 1  illustrates a Flyback Switch Mode Power Supply  100  according to an embodiment of the invention. As illustrated, an input signal  105  is applied to input terminals  106 ,  107  of supply  100 . Although full bridge rectification is not needed when the applied voltage is a DC (direct current), one reason for inclusion of such rectifier is to eliminate product versions between products with AC (alternating current) and DC inputs. Since having a bridge rectifier for DC will not hurt the signal (need to account for small voltage loss across rectifier diode), it is included to keep the PCBA (printed circuit board assembly) identical for both AC and DC cases. The applied input signal  105  is next applied to a full bridge rectifier  110  that converts the input signal  105  to create a rectified voltage  112 . The rectified voltage  112  is next stepped-down by step-down transformer or circuit  115 . Although, a step down transformer would work in the circuit illustrated, circuit  115  refers to a transistor controlled voltage regulator circuit that regulates in the range of +16 Vdc to power the ASIC  130  and +5V to power to the microcontroller  195 . DC/DC step-down transformation is well-known in the art and need not be discussed in detail herein. The stepped down voltage is next applied to diode  120 . Diode  120  serves as a blocking diode that is able to provide current to the driver circuits when the AUX winding in unable to do so (e.g., when there is not enough input voltage). The voltage at point  125  is applied to the PWR port  132  of a first controller such as an ASIC  130 . The output  134  of ASIC  130  is applied to a gate port  182  of switch  180 . In one aspect of the invention ASIC  130  may be a UC3843BVD current mode controller available from the Motorola Corporation. 
         [0011]    Voltage  112  is further applied to primary winding  142  of transformer  140 . Primary winding  142  is further connected to low side driver  180 . Typically, low side driver  180  is an N-Channel FET transistor that operates to interrupt the current flow through winding  142 . However, it would be recognized that the low side driver may be another type of switching device. Output  134  of ASIC  130  is applied to a control node  182  (e.g., gate node of an FET) to control the interruption of the current flow according to a known frequency and duty cycle. 
         [0012]    As known in transformer technology, the interruption of the current flow in primary winding  142  induces an energy transfer into secondary windings  144  to create a secondary voltage. The secondary voltage, in this case, is rectified and filtered by the combination of diode  146  and capacitor  148  to produce output voltage  150 . 
         [0013]    The interruption of the current in primary winding  142  further induces a voltage in auxiliary winding  160 , in a manner similar to that occurring in secondary winding  144 . As with secondary winding  144 , the voltage induced in the auxiliary winding  160  is rectified and filtered by diode  162  and capacitor  164  to produce a substantially constant voltage  170 , with a desired imposed ripple voltage. Voltage  170  is then applied to power port  132  of ASIC  130 . 
         [0014]    As previously discussed, when voltage  170  is of a sufficient magnitude (e.g., 7 volts), ASIC  130  is activated and provides a control signal to low side driver  180  to cause an interruption in the current flowing through primary winding  142 . 
         [0015]    Also shown is a second controller such as a microprocessor  195  that receives a power signal at port  191  via 5 volt regulator  136 . Regulator  136  maintains a substantially constant voltage, independent of the voltage at point  125 , so as to maintain microprocessor  195  in a power-on state prior to the operating threshold of ASIC  130 . An output of microprocessor  195  is provided to a control port  192  of second low side driver  190 . Port  194  of second low side driver  190  is connected in common with port  184  of first low side driver  180  to primary winding  142 . Microprocessor  195  provides a control signal to second low side driver  190  to interrupt the flow of current in primary winding  142 , in a manner similar to that described with regard to ASIC  130 . In this case, the interruption of the current flowing in primary winding  142  by second low side driver  190  begins at a voltage level less that nominally required when ASIC  130  is alone. 
         [0016]    Further illustrated is a variable resistor  188  for monitoring the voltage at point  125  and providing the monitored voltage to microprocessor  195 . In this case, when the monitored voltage remains below a predetermined limit the control signal to the second low side driver  190  is activated and when the monitored voltage exceeds the predetermined limit, the control signal to second low side driver  190  is deactivated. Thereafter, ASIC  130  takes control of the system operation and applies a control frequency and duty cycle sufficient to maintain voltage  170  within a desired range. As an example, the variable resistor  188  may be set such that a voltage of 5 volts applied to port  196  indicates the auxiliary voltage is above a minimum voltage to cause ASIC  130  to be activated. Although a variable resistor  188 , it would be recognized by those skilled in the art that any suitable means or circuit for monitoring the voltage may be employed such as a switch, e.g., transistor, that is configured to provide a signal to processor  195  when the voltage at point  125  exceeds a known value. 
         [0017]    As would be recognized by those skilled in the art, the frequency and duty cycle of the control signal applied to second low side driver  190  may be similar to, or different from, the control signal applied to low side driver  180 . In one aspect, the frequency and duty cycle applied to the second controller may be such to enable voltage  170  to quickly rise to a desired level. 
         [0018]    Accordingly, when the ASIC  130  power input is below its minimum operating voltage range, the microprocessor  195  switches the gate of the second driver  190 , at a known frequency and duty cycle, until the auxiliary winding voltage  170  achieves a voltage to activate the ASIC  130 . Once the auxiliary voltage is above the minimum operating range of the ASIC  130 , the ASIC  130  will turn-on and provide a known control signal to first low side driver  180 . ASIC  130  will maintain the voltages of the secondary windings  150  and auxiliary windings  170  thereafter. 
         [0019]    While there has been shown, described, and pointed out fundamental novel features of the present invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the apparatus described, in the form and details of the devices disclosed, and in their operation, may be made by those skilled in the art without departing from the spirit of the present invention. It is expressly intended that all combinations of those elements that perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. For example, although the invention has been described with regard to an ASIC and a microprocessor, it would be recognized that other type of control circuit or controller, hardware (e.g., FPGA) or software/hardware elements may be incorporated into the circuit shown in  FIG. 1  without altering the scope of the invention.