Patent Publication Number: US-8981595-B2

Title: Method of forming a power supply controller and system therefor

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates, in general, to electronics, and more particularly, to power supply controllers including semiconductors devices for power supplies and methods of forming such semiconductor devices. 
     In the past, the electronics industry utilized various methods and structures to form an illumination system for image capturing devices such as still image cameras and motion video cameras. Advances in digital technology allowed merging of still image capturing and motion video capturing into a single image capturing device. For example, cellular telephones progressed to include the ability to capture a single image as a still picture or to capture continuous motion has a movie or motion video. In order to provide the ability to operate in low light situations, these image capturing devices also included a method of illuminating the subject to be captured. A xenon light source generally was utilized to provide a pulsed flash in order to capture a single still image. A xenon light source was used because the xenon light source provided a high intensity light. However, the xenon light source was not suitable for continuous illumination that was required for continuous motion video. As a result, a second light source, typically a light emitting diode (LED), was used to provide a continuous light source for the continuous motion video. 
     The xenon light source required a high voltage pulse, typically about two hundred fifty to three hundred twenty volts (250-320 V), in order to energize the xenon light source to produce a pulse or flash of light. Conversely, the LED utilized a lower voltage source that can be supplied for a longer period of time. Consequently, the still image section included a complete power supply to operate the xenon light source and the video section included another complete power supply to operate the LED. As a result, the illumination system usually included a complete power supply including a switching power supply controller and an inductor, such as a transformer, for the still image section and another complete power supply including another switching power supply controller and another inductor, such as another transformer, for the video section. Having two complete power supply systems with two power supply controllers and two inductors increased the cost of the image capturing device. 
     Accordingly is desirable to have a power supply system for the illumination section of an image capturing device that does not require two inductors or two transformers, that does not required two separate switching power supply controllers, and that has a lower cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates an embodiment of a typical prior art power supply system; 
         FIG. 2  schematically illustrates an exemplary embodiment of a portion of a power supply system for an illumination system of an image capturing device in accordance with the present invention; 
         FIG. 3  schematically illustrates a simplified block diagram of a switching power supply controller in accordance with the present invention; and 
         FIG. 4  illustrates an enlarged plan view of a semiconductor device that includes the power supply controller of  FIG. 3  in accordance with the present invention. 
     
    
    
     For simplicity and clarity of the illustration, elements in the figures are not necessarily to scale, and the same reference numbers in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or anode of a diode, and a control electrode means an element of the device that controls current through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain N-channel or P-Channel devices, or certain N-type or P-type doped regions, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with the present invention. It will be appreciated by those skilled in the art that the words during, while, and when as used herein relating to circuit operation are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action. The use of the word approximately or substantially means that a value of an element has a parameter that is expected to be very close to a stated value or position. However, as is well known in the art there are always minor variances that prevent the values or positions from being exactly as stated. It is well established in the art that variances of up to at least ten percent (10%) (and up to twenty percent (20%) for semiconductor doping concentrations) are reasonable variances from the ideal goal of exactly as described. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     An example of a typical prior art power supply for an illumination system of an image capturing device is illustrated in  FIG. 1 . The power supply generally includes a still image section that supplies power to generate a flash to capture a single image (PS 1 ) and also includes a video section that supplies power to illuminate the subject for continuous motion video capture. The still image section includes a switching power supply controller  17 , an inductor, such as a transformer  19 , a xenon light source (X 1 ), and a high voltage discharge circuit for the xenon light source (X 1 ) which includes a transformer T 2 , a capacitor C 1 , and an insulated gate bipolar transistor (IGBT) S 1 . Output stage  18  is utilized to form a regulated output voltage on an output  15 . Output stage  18  usually includes a rectifier, such as a diode D 1 , a storage element, such as an output capacitor C 2 , that stores the average value of the voltage from diode D 1 , and a feedback network, such as series connected resistors R 2  and R 3 . A photo sensor (PHS) may be used in the still image section to sense the light from the xenon light source (X 1 ), such as sensing light in order to determine the distance to a subject in order to set the focus of a camera. 
     Controller  17  may also include control logic that was responsive to control signal inputs such as an enable signal (EN), a trigger signal (TG), and a pre-flash (PREF) signal. When the enable (EN) signal is active, controller  17  operates an internal switch (SW) to control a current  16  through an inductor, such is the inductor of the primary side of transformer  19 , in order to regulate an output voltage at output  15  of the power supply. The operation of the switch (SW) is controlled in response to a feedback signal (FB) that is received at a sense signal input (SN) of controller  17 . The feedback network of output stage  18  is connected to output  15  in order to form the feedback signal at a node  14  to be representative of the value of the output voltage formed at output  15 . As illustrated in  FIG. 1 , the feedback network includes the resistor divider of resistors R 2  and R 3 . However, those skilled in the art will appreciate that the feedback network may be any other well-known circuitry used to provide a feedback signal that is representative of an output voltage. For the embodiment illustrated in  FIG. 1 , the inductor through which current  16  flows is a primary side inductor of transformer  19 . In other embodiments, the inductor may be a separate single inductor instead of a portion of a transformer. As is well known to those skilled in the art, switching power supply controller  17  controls the switching of the internal switch transistor and forms current  16  as required to maintain the voltage at output  15  to a target value within a range of values around the target value. For example, the target value may be fifteen volts (15V) and the range of values may be plus or minus five percent (5%) around the fifteen volts. 
     The power supply for the video section includes a switching power supply controller  13  and an inductor. For the embodiment that is illustrated in  FIG. 1 , the inductor is a primary inductor of a transformer T 3 . Controller  13  regulates an output voltage at node OV in response to another feedback signal that is representative of the output voltage at node OV. Controller  13  may be similar to controller  17 . A feedback network, for example a feedback network of resistors R 6  and R 7 , forms a feedback signal that is representative of the value of the output voltage. Controller  13  receives the feedback signal and controls current through the inductor in order to regulate the value of the output voltage. A rectifier, such as a diode D 2 , rectifies the voltage from the inductor and an average value of the voltage is stored on an output capacitor C 3 . A video enable (VE) control signal is used to enable a transistor S 2  and cause the LED to emit light. As seen in  FIG. 1 , controllers  17  and  13  operate independently of each other. 
       FIG. 2  schematically illustrates an exemplary embodiment of a portion of a power supply system  20  for an illumination system of an image capturing device. System  20  includes a still image section that is similar to the still image section of  FIG. 1 , however, the still image section of system  20  does not connect the feedback signal (FB) from node  14  directly to the sense signal (SN) of controller  17 . 
     System  20  also includes a video section  21 . Section  21  does not include a switching power supply controller or an inductor or transformer but reuses the switching power supply controller and inductor of the still image section. Section  21  includes a feedback selector  22  and an output stage  35 . As will be seen further hereinafter, output stage  35  has an input  36  connected to receive a voltage from the switched terminal of the inductor of the still image section, such as from the switched terminal of the primary inductor of transformer  19 . Output stage  35  receives the input voltage and forms a video output voltage, or second output voltage, on output  49  of section  21 . Input  36  receives the voltage from the switched terminal of the inductor as controller  17  switches the switch (SW) that is internal to controller  17 . The voltage on input  36  is rectified by a diode  37  and the resulting average voltage is stored on an output capacitor  38  as a stored voltage on a node  39 . Output stage  35  also includes an output voltage selector  41  that selectively couples the stored voltage from node  39  and capacitor  38  to output  49  as the video output voltage. A feedback network of section  21  includes a resistor  53  configured to form a second feedback signal or video feedback signal (VFB) that is representative of the video output voltage on output  49 . Those skilled in the art will realize that other well-known feedback circuits may also be used to form the feedback signal. 
     A feedback selector  22  of section  21  is configured to receive the first feedback signal from node  14  that is representative of the output voltage formed on output  15  and to also receive the video feedback signal (VFB) that is representative of the video output voltage formed on output  49 . Feedback selector  22  is configured to selectively couple the first feedback signal to the sense signal (SN) input of controller  17  in order to facilitate regulating the output voltage on output  15  to the target value for output  15  and to selectively couple the video feedback signal (VFB) to the sense signal (SN) input for regulating the video output voltage on output  49  to the target value for the video output voltage. 
     In operation, negating the video enable (VE) control signal, for example a logic low, forces the output of an inverter  25  of selector  22  high which enables a transistor  27 . Enabling transistor  27  connects the feedback signal (FB) from node  14  to the sense signal input (SN) of controller  17 . This allows controller  17  to control the value of current  16  in order to regulate the value of the output voltage on output  15  to the target value for output  15 . The negated video enable (VE) control signal also disables a transistor  44  of selector  41  which allows resistor  47  to pull a base of a transistor  48  high thereby disabling transistor  48 . Since transistor  48  is disabled, the stored voltage on node  39  is not applied to output  49 . Therefore, selector  41  selectively inhibits the formation of an output voltage on output  49  responsively to a negated state of the VE signal. As a result, the value of the voltage on node  39  does not effect the operation of system  20  during this operating state. 
     Asserting the video enable (VE) control signal, for example a logic high, enables transistor  44  which pulls the base of transistor  48  low thereby enabling transistor  48 . Enabling transistor  48  couples output  49  to the storage element of capacitor  38  thereby coupling the voltage stored thereon to output  49  and forming the video output voltage on output  49 . The video output voltage on output  49  cause a current to flow through an LED  51  in order to generate light from LED  51 . Current flowing through LED  51  also flows through resistor  53  which forms the video feedback signal (VFB) at a feedback node  54 . The asserted video enable (VE) control signal also forces the output of inverter  25  low which disables transistor  27  and forces the output of buffer  24  high. The high from buffer  24  enables transistor  28  to selectively connect the video feedback signal (VFB) to the sense signal (SN) input of controller  17 . This allows controller  17  to control the value of current  16  in order to regulate the value of the video output voltage on output  49  to the target value for the video output voltage. 
     In order to facilitate this functionality for system  20 , node  14  is connected to a first feedback input of selector  22  and to a source of transistor  27 . A drain of transistor  27  is commonly connected to a drain of transistor  28 , an output of selector  22 , and to the sense signal (as n) input of controller  17 . Node  54  is connected to a second feedback input of selector  22  and to a source of transistor  28 . A control input  23  of selector  22  is connected to an input of buffer  24  and an input of inverter  25 . The output of buffer  24  is connected to the gate of transistor  28  and the output of inverter  25  is connected to the gate of transistor  27 . Input  36  of output stage  35  is connected to an anode of diode  37  and to a switched terminal of the inductor of the still image section. The cathode of diode  37  is commonly connected to a first terminal of capacitor  38 , the first terminal of resistor  47 , and to an emitter of transistor  48 . A second terminal of resistor  47  is commonly connected to the base of transistor  48  and a collector of transistor  44 . An emitter of transistor  44  is commonly connected to a terminal of resistor  53 , a second terminal of capacitor  38 , and to return  12 . The base of transistor  44  is connected to a first terminal of resistor  43  which has a second terminal connected to receive the video enable (VE) control signal on a video enable (VE) control signal terminal. The switching output of controller  17  is connected to the switched terminal of the primary inductor of transformer  19  and the non-switched terminal of the primary inductor is connected to receive power from a power input terminal  11 . A secondary inductor of transformer  19  has a switching terminal connected to an anode of diode D 1  which has a cathode connected to output  15 . A non-switched terminal of the secondary inductor of transformer  19  is commonly connected to return terminal  12 , the first terminal of capacitor C 2 , and a first terminal of resistor R 3 . A second terminal of capacitor C 2  is connected to output  15  and to a first terminal of resistor R 2 . A second terminal of resistor R 2  is connected to node  14  and to a second terminal of resistor R 3 . 
       FIG. 3  schematically illustrates a simplified block diagram of a switching power supply controller  60  that is similar to controller  17  of  FIG. 2  but that also includes feedback selector  22  that is explained in the description of  FIG. 2 . As is well known in the art, one example embodiment of a switching power supply controller usually includes an oscillator, a ramp generator, an error amplifier (EA) that receives the SN signal, a comparator that compares the error signal to the ramp signal, and a latch that is used to form a switching drive signal to operate the power switch and control the value of current  16 . The elements of controller  60  may be integrated onto a single semiconductor substrate. 
       FIG. 4  illustrates an enlarged plan view of a portion of an embodiment of a semiconductor device or integrated circuit  70  that is formed on a semiconductor die  71 . Controller  60  is formed on die  71 . Die  71  may also include other circuits that are not shown in  FIG. 4  for simplicity of the drawing. Controller  60  and device or integrated circuit  70  are formed on die  71  by semiconductor manufacturing techniques that are well known to those skilled in the art. 
     One skilled in the art can understand from the proceeding explanations that a power supply system for an illumination source of a video capturing device may be configured to include: a switching power supply controller, such as controller ( 17 ), coupled to control a current, such as a current  16 , through an inductor, for example an primary inductor of transformer  19 , to regulate a first output voltage on a first output, such as output  15 , to first desired value in response to a sense signal such a sense signal SN; a first output stage of the power supply system has a first storage element coupled to the first output to store the first output voltage, and also has a first feedback network configured to form a first feedback signal that is representative of the first output voltage; 
     a second output stage of the power supply system has an input coupled to receive a voltage from the inductor, a rectifier coupled to the input, a second output for forming a second output voltage, a second storage element coupled to receive a signal from the rectifier and store the second output voltage, and a second feedback network configured to form a second feedback signal that is representative of the second output voltage; and 
     a feedback selector of the power supply system is configured to selectively couple the first feedback signal to the sense signal for regulating the first output voltage to the first desired value and not regulating the second output voltage to the second desired value, and to selectively couple the second feedback signal to the sense signal for regulating the second output voltage to the second desired value and not regulating the first output voltage to the first desired value. 
     One skilled in the art can also understand from the proceeding explanations that a method of forming a power supply controller can include; forming a switching power supply control section to form a drive signal for switching operating a switch to control a current through an inductor in response to a sense signal; 
     configuring the power supply controller to receive a first feedback signal that is representative of a first output voltage and to receive a second feedback signal that is representative of a second output voltage; and 
     further can include forming a feedback selector of the power supply system to selectively couple the first feedback signal to the sense signal for operating the switch to control the current and regulate the first output voltage to a first desired value and not regulating the second output voltage, and to selectively couple the second feedback signal to the sense signal for operating the switch to control the current and regulate the second output voltage to a second desired value and not regulating the first output voltage to the first desired value. 
     In view of all of the above, it is evident that a novel device and method is disclosed. Included, among other features, is selectively coupling either a first feedback signal or a second feedback signal to a sense input of a switching power supply controller in order to regulate a respective first or second output voltage to a corresponding target value. Reusing a single power supply controller to regulate two different output voltages to two values including two different values reduces the number of switching power supply controllers and inductors that are required thereby reducing the cost. 
     While the subject matter of the invention is described with specific preferred embodiments, the foregoing drawings and descriptions thereof depict only typical and exemplary embodiments of the invention subject matter and are not therefore to be considered to be limiting of its scope, it is evident that many alternatives and variations will be apparent to those skilled in the art. System  20  and controller  60  are illustrated and explained as a leading edge fixed frequency switching buck power supply controller. However, the invention is applicable to other types of switching power supply systems including a boost system, a hysteretic system, a pulse frequency modulation system, and other well known switching control methods. Additionally, selector  22  is illustrated with a specific embodiment, however, other embodiments may also be used as long as the selector selects one feedback signal to be applied to controller from a plurality of feedback signals. Also, selector  35  is configured with a specific embodiment. The embodiment of selector  35  may be different as long as it forms an output voltage on output  49  responsively to the VFB signal being coupled to the switching power supply controller. Additionally, the word “connected” is used throughout for clarity of the description, however, it is intended to have the same meaning as the word “coupled”. Accordingly, “connected” should be interpreted as including either a direct connection or an indirect connection.