Abstract:
In a switched mode power supply, and in accordance with a method for operating a switched mode power supply, the magnitude of each occurrence of a current flowing during operation of a power output switch is sensed for negative feedback control. A sense voltage is generated proportional to the primary supply voltage. Whenever the sense voltage exceeds a threshold, output power of the power supply is limited by increasing the sensed magnitude of each occurrence of the flowing currents by adding to each sensed magnitude a voltage proportional to the sense voltage. Threshold voltages can be selected by using Zener diodes having different breakdown voltages. Respective ranges within the overall range of the primary supply voltage in which supplemental power limiting occurs and does not occur can thus be controlled.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    A current mode controlled switched mode power supply according to the inventive arrangements limits the power output of the power supply over a wide over a wide input AC voltage range, in a manner that enables selection of a threshold, for example a voltage threshold, at which the power limiting begins. 
         [0002]    Switching power supplies that utilize current mode control provide a maximum power output that is proportional to the input voltage. In a power supply intended for use over a wide voltage input range, excessive power during overloads can be delivered at the high range of the input AC voltage, which can result in high operating temperatures for transformers, diodes and transistors in the power supply. 
         [0003]    If no power limiting is used in such a power supply, the components in the power supply must be designed to handle the maximum output power during overload, at the highest input voltage, and without overheating or failing. Utilizing more robust parts adds unnecessary cost and makes the power supply physically larger than if the parts were designed only for an intended or fixed power output in a narrower range of input AC voltage. 
         [0004]    An existing current mode controlled switched mode power supply teaches in part a circuit that generates an offset voltage that can be added to a voltage indicative of the operating current of the output switching transistor of the power supply. The offset voltage increases the magnitude of the feedback the voltage. Accordingly, power limiting begins at a lower input AC voltage than would otherwise occur absent the offset. 
         [0005]    The existing power supply solved a number of problems, but its operation presented an opportunity for further improvement. One such problem that was so raised is that the magnitude of the offset voltage was directly proportional to a DC voltage that was, in turn, directly proportional to the magnitude of the input AC voltage. Thus, there was no control over the input AC voltage level at which the power limiting was initiated. Power limiting always occurred even at lower output power levels when power limiting may not have been necessary, because it was not possible to establish a threshold within the input AC voltage range for initiating the power limiting. Establishing a threshold for initiating power limiting advantageously makes it possible to improve the design and operation of the power supply, for example based upon appropriate performance criteria. Such performance criteria can include, for example, optimizing: the input voltage range of the power supply; the maximum load capability of the power supply; the cost of the power supply; the size of the power supply; and, the efficiency of operation of the power supply. 
         [0006]    This problem and others are solved in accordance with the inventive arrangements taught herein by providing a selectable threshold within the input AC voltage range for initiating the power limiting. The selectable threshold enables choices to be made, for example, in combining more expensive, more robust components and less expensive, less robust components to satisfy design requirements associated with one or more of the performance criteria noted above. 
         [0007]    Moreover, further flexibility in design can be provided in accordance the the inventive arrangements taught herein by utilizing a negative voltage for establishing the threshold, as compared to the positive voltage utilized previously. 
         [0008]    A method in accordance with the inventive arrangements for operating a switched mode power supply can advantageously comprise the steps of: receiving a primary supply voltage; repetitively coupling the primary supply voltage to a load and uncoupling the primary supply voltage from the load; energizing a secondary voltage supply responsive to the coupling and uncoupling step; sensing a magnitude of each occurrence of a current flowing during the coupling and uncoupling step; controlling the coupling and uncoupling step at least in part responsive to the sensing step; generating a sense voltage proportional to the primary supply voltage; and, limiting output power of the power supply by increasing the sensed magnitude of each the occurrence of the flowing currents whenever the sense voltage exceeds a threshold. 
         [0009]    The method can advantageously further comprise one or more of the following steps: increasing the sensed magnitude of each the occurrence of the flowing currents by a factor proportional to the sense voltage; or, controlling the coupling and uncoupling step responsive to the sensing step and responsive to the energizing step. 
         [0010]    The method can advantageously further comprise the following steps: receiving a primary alternating current (AC) supply voltage; generating a voltage signal related to each sensed magnitude of the flowing currents; generating the sense voltage responsive to negative portions of the AC supply voltage; and, whenever the sense voltage exceeds the threshold: increasing each the voltage signal in magnitude by a factor proportional to the sense voltage; and, controlling the coupling and uncoupling step at responsive to the increased magnitude voltage signals and responsive to the energizing step. 
         [0011]    A switched mode power supply according to the inventive arrangements can advantageously comprise: a source of a primary supply voltage; a switch for repetitively coupling the primary supply voltage to a transformer and uncoupling the primary supply voltage from the transformer; a secondary voltage supply coupled to the transformer and energized by operation of the switch; a resistance for sensing a magnitude of each occurrence of a current flowing during the operation of the switch; a controller for the switch forming part of a negative feedback loop and responsive at least in part to the sensed magnitudes; a sense voltage source proportional to the primary supply voltage; and, a circuit operable when the sense voltage exceeds a threshold for combining the sensed magnitude of each the occurrence of the flowing currents and a supplemental voltage proportional to the sense voltage, for limiting output power of the power supply. 
         [0012]    The switched mode power supply can further comprise the controller for the switch being responsive to the sensed magnitudes and being responsive to operation of the secondary voltage supply. 
         [0013]    In those cases where the primary supply voltage is an alternating current (AC) supply voltage, the sense voltage can be advantageously generated responsive to negative portions of the AC supply voltage. 
         [0014]    The switched mode power supply can advantageously further comprise: a first diode for rectifying the negative portions of the AC supply voltage; a capacitor responsive to the rectified voltage for developing the sense voltage; and, a Zener diode having a breakdown voltage establishing the threshold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a first section of a current mode controlled switched mode power supply according to the inventive arrangements. 
           [0016]      FIG. 1(   a ) illustrates the relationship between two grounds in the power supply, Z and M 1 . 
           [0017]      FIG. 1(   b ) illustrates the relationship between ground M 1  and two voltage supplies, +12V Standby (SB) and +12V. 
           [0018]      FIG. 2  is a second section of the power supply. 
           [0019]      FIG. 3  is a third section of the power supply. 
           [0020]      FIGS. 2 and 3  each include transformer LP 102  and opto-coupler DP 302  for purposes of establishing continuity between  FIGS. 2 and 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Generally, the magnitude of the current switched in a current mode controlled switched mode power supply is monitored by measuring the voltage developed across a current sensing resistor. When the sensed voltage reaches a threshold set by the control circuit or the power supply, the power switching device is turned off until the start of the next cycle. 
         [0022]    An offset voltage proportional to the AC input voltage can be added to the sensed voltage, which in effect lowers the threshold at which the control circuit turns off the switching device as the AC input voltage rises. 
         [0023]    In accordance with the inventive arrangements, further control and design flexibility is advantageously provided by enabling a threshold to be selected and established at which the power limiting is initiated, apart from the normal feedback operation of the control circuit responsive to the combination of the sensed voltage and the offset voltage. Component values and component power capacities of the power supply can be selected, for example, so that a constant overload power can be maintained independent of the AC input voltage. 
         [0024]    A power supply  1  is shown in  FIGS. 1 ,  1 ( a ),  1 ( b ),  2  and  3 . The manner in which the various sections of the power supply are divided between the Figures is based on convenience of illustration. 
         [0025]      FIG. 1  shows a first section  10  of a current mode controlled switched mode power supply  1  according to the inventive arrangements. Section  10  of the power supply generates two output voltages response to an AC voltage input mains supply, RAW B+ and LO B+. There are four grounds in the power supply, designated Z, M, M 1  and M 2 . In  FIG. 1(   a ) circuit  10 A illustrates the relationship between grounds Z and M 1 . In  FIG. 1(   b ) circuit  10 B illustrates the relationship between ground M 1  and two voltage supplies developed in the third section in  FIG. 3 , +12V Standby (SB) and +12V. 
         [0026]      FIG. 2  show a second portion  20  of the power supply  1 . Second portion  20  receives the LO B+ and RAW B+ voltages generated in first portion  10 . 
         [0027]      FIG. 3  shows a third portion  30  of the power supply  1 , in which a plurality of secondary voltages are generated. Both transformer LP 102  and opto-coupler DP 302  are shown in portions  20  and  30  to facilitate the description of the power supply. 
         [0028]    Portion  30  supplies a first feedback control signal to integrated control circuit IP 101  in portion  20  through opto-coupler DP 302 . A second opto-coupler DP 106  in portion  20  supplies a second feedback control signal to control circuit IP 101 . 
         [0029]    Isolation barriers are created by transformer LP 102 , opto-coupler DP 302  and opto-coupler DP 106 . The circuitry  22  in portion  20  is isolated from the rest of the circuitry in portion  20  and isolated from the circuitry in portions  10  and  20 . Ground for that part  22  of the circuitry is designated M 2 . 
         [0030]    With reference to  FIG. 2  and portion  20  of the mode current mode controlled switching power supply  1 , the input voltage Raw B+ generated in portion  10  provides the main source of energy for the power supply. Transformer LP 102  provides output voltages that are rectified and filtered in portion  30  to generate a plurality of low voltage outputs. Integrated circuit IP 101  is a control IC that provides a pulse-width modulated output signal used to switch the power device TP 102 . In the presently preferred embodiment, IP 101  is an ON Semiconductor® NCP120 7 B current mode modulator manufactured by Semiconductor Components Industries, LLC. A Data Sheet is available as Publication Order Number: NCP1207A/D, October, 2006—Rev. 3. 
         [0031]    The pins of the NCP1207B current mode modulator are associated with the following functions. The DMG pin  1  receives an auxiliary flyback signal that ensures discontinuous operation and offers a fixed over-voltage detection level of 7.2V. When FB pin  2  is connected to an opto-coupler the peak current set point is adjusted according to the output power demand. Bringing pin  2  below an internal skip level shuts off the device. The CS pin  3  senses the primary current and routes it to an internal comparator. Inserting a resistor in series with pin  3  enables control of the level at which the skip operation takes place. The GND pin  4  is ground. The DRV pin  5  is the driver&#39;s output to an external MOSFET. The Vcc pin  6  is connected to an external bulk capacitor. The NC pin  7  is unconnected. The HV pin  8  is connected to the high voltage rail and injects a constant current into the VCC bulk capacitor. 
         [0032]    Resistor RP 108  is a current sense resistor that provides a voltage V SENSE  proportional to the current flowing through transformer LP 102  and transistor TP 102 . The sensed voltage V sense  is coupled to pin  3  (CS) of the control IC IP 101 . During normal operation, when this voltage reaches a given threshold at the input to the control IC, the output of IP 101  on pin  5  (DRV) goes low and turns off transistor TP 102  until the next cycle begins. If the system requires more power, the threshold voltage level in IP 101  is increased so that more energy is stored in LP 102  (higher peak current) and more power is delivered to the output windings. The threshold voltage has a maximum value that limits the maximum output power that is available. Component LP 103  is a ferrite bead. 
         [0033]    The selectable threshold for initiating power limiting in accordance with the presently preferred embodiment of the inventive arrangements is explained with further reference to portion  20  in  FIG. 2 . Pin  6  of transformer LP 102  provides an AC voltage that is coupled through resistor RP 118  to a number of destinations. Diode DP 109  couples the voltage to capacitors CP 108  and CP 110 , which become charged and form a supply voltage coupled to the Vcc input pin  6  of IC IP 101 . This voltage is further coupled to resistors RP 102 , RP 122 , RP 123  and RP 124  to bias the operation of transistor TP 101 . The positive portion of the voltage from pin  6  of transformer LP 102  becomes the regulated voltage as coupled to the DMG input pin  1  of IC IP 101 . The negative portion of the AC voltage from pin  6  of transformer LP 102  is coupled to the cathode of diode DP 103 . This negative portion of the AC voltage is unregulated and is proportional to the input voltage Raw B+. The anode of diode DP 103 ′is coupled to the anode of Zener diode DP 104  and to capacitor CP 118 , the other end of which is coupled to ground. Diode DP 103  rectifies the negative portion of voltage from pin  6  of transformer LP 102  and creates a negative supply voltage V TH , which is filtered by capacitor CP 118  and which varies with the Raw B+ voltage. When the negative supply voltage V TH  reaches a value that causes Zener diode DP 104  to conduct, a voltage is developed at the base of transistor TP 101  that causes current to flow in the collector. This collector current in turn flows through resistors RP 125  and RP 103  causing a voltage V OFFSET  to be developed across resistor RP 103  that artificially increases the current sense voltage. In other words, less current is allowed to flow through sense resistor RP 108  for a given threshold voltage set in IP 101 . Resistor RP 122  controls the gain of the circuit and resistor RP 125  determines the maximum reduction of current that is possible. The breakdown voltage of Zener diode DP 104  determines the voltage level, that is, the voltage threshold, at which the power limiting begins. In the presently preferred embodiment illustrated in the drawings, the power supply has been optimized to provide a combination of sufficient load capacity, reduced cost, reduced size and efficiency in operation. The presently preferred embodiment is suitable, inter alia, for a digital set top box, in which the Zener breakdown voltage is 24 volts. The voltage threshold can be advantageously and easily adjusted by substituting a Zener diode with a different breakdown voltage. Consequently, the power supply can be advantageously redesigned, in terms of balancing the performance factors, such as load capacity, reduced cost, reduced size and efficiency in operation as noted above. 
         [0034]    Most of the circuitry shown in portion  30  of  FIG. 3  generates secondary low voltage power supplies of +5V, +6.5V, +12V, +12V SB (Standby) and +5V Ref (Reference). The +5V, +6.5V and +12V supplies are routed from the power supply though connector BP 201 . One section of portion  30  supplies the input to opto-coupler DP 302 , the operation of which is explained below. 
         [0035]    There are additional feedback and control signals supplied to IC IP 101 . A feedback signal FB LOAD  is generated as the output of opto-coupler DP 106 . A control signal RS CONTROL  is generated as the output of opto-coupler DP 106 . The outputs of the two opto-couplers are coupled to one another at the junction J 1  of pin  4  of opto-coupler DP 302 , pin  4  of opto-coupler DP 106 , capacitor CP 113  and FB pin  2  of control IC IP 101 . 
         [0036]    The feedback signal FB LOAD  is related to the output power demand of the power supply loads. Circuit  22  monitors the +6.5V voltage supply. Device IP 102  in circuit  22  is a reference amplifier used to control the output voltage of the power supply. Reference amplifier IP 102  contains a fixed 2.5V reference and an amplifier that causes increased current to flow in the cathode of the device as the input voltage increases above the reference voltage. When current flows through the cathode of the device, the current also flows in series with the diode part of opto-coupler DP 106 . Current flowing in the diode part of opto-coupler DP 106  causes a current to flow in the transistor side of opto-coupler DP 106 , thus causing the control IC IP 101  to reduce the power supplied to the transformer. 
         [0037]    A peak current set point in control IC IP 101  is responsive to the feedback signal FB LOAD  and the peak current is thus controlled according to the output power demand. During normal operation, control IC IP 101  responds to signals on DMG pin  1  and CS pin  3 . Output power is limited outside the control of IC IP 101  only when threshold voltage V TH  is exceeded, during which the signal on CS pin  3  is increased by offset voltage V OFFSET . 
         [0038]    The control signal C CONTROL  is used to shut down the power supply as part of a reset function. Control signal C CONTROL  is used as an on/off signal, as compared to feedback signal FB LOAD , which is a voltage feedback used for regulation purposes. When the diode in opto-coupler DP 302  conducts, the control signal RS CONTROL  pulls down the common junction J 1  to or toward ground, interrupting the feedback signal FB LOAD . When the voltage at FB pin  2  of control IC IP 101  is pulled below an internal skip level inside the control IC IP 101 , the switching device transistor TP 102  is turned off until opto-coupler DP 302  turns off and the voltage at junction J 1  is again responsive to feedback signal FB LOAD . 
         [0039]    It can be seen that the normal operation and safety protocols built into the control IC are advantageously not adversely affected by the threshold initiated power limiting in accordance with the inventive arrangements.