Abstract:
A charge pump apparatus and a method for operating a charge pump power supply having an input and an output, the output coupled to a load, and the load having a bulk capacitor coupled thereto for providing a voltage source for the load. A source of varying voltage is provided as an input to the charge pump power supply. The output of the charge pump power supply is coupled to the load and to the bulk capacitor, and the charge pump power supply is operated in a first mode to provide current for charging the bulk capacitor when the voltage at the load is below a predetermined voltage, and alternatively in a second mode to cease the supply of current to the bulk capacitor when the voltage at the power supply is above a predetermined voltage. The charge pump comprises a capacitor-diode arrangement with a transistor switch operable to control the current flow through the charge pump and to the bulk capacitor.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to charge pump power supplies for use in providing power to circuits or systems, and more particularly to regulated charge pump power supplies.  
         BACKGROUND OF THE INVENTION  
         [0002]    Efficient power supplies are necessary for providing power to many types of electronic circuits. Switched mode power supplies (SMPS), for example, are used in many applications in consumer and other products including television receivers, personal computers, facsimile machines, video cassette recorders, and computer monitors. These SMPS may be based on flyback transformer technology, but other technologies may be used as well.  
           [0003]    Because of the heavy use of these power supplies in high volume consumer applications, standby power consumption is of great importance, since the devices are kept in standby mode most of the time. Additionally, industry standards recommend the permissible standby power consumption levels of such apparatus. Furthermore, again because of the high volume applications for these power supplies, low cost is also a significant factor.  
           [0004]    In the past, when SMPS were only operated in a power on mode or turned off, the SMPS controller, which may be a separate integrated circuit, was usually powered from a secondary winding of the output transformer of the SMPS. Later apparatus such as television receivers, computer monitors, and the like required a power supply which could be operated in a standby mode, usually controlled by a microcontroller which was used to sense, for example, an infrared signal from a remote device to turn on the main power supply by providing an enable signal to the SMPS controller.  
           [0005]    To achieve the low power consumption required in standby mode, a secondary reconfiguration technique is often used. Instead of disconnecting the loads from the power supply, which would be a very expensive solution because of the cost of the switches and other components needed to perform such an operation, all the secondary windings voltages must be drastically reduced. To do so, the voltage on one of the secondary windings of the power transformer must be pulled down to a level such that the microcontroller can be appropriately supplied. Since the voltage per turn of the output transformer is constant, however, the other output voltages are similarly reduced according to the same ratio. If the SMPS controller were also powered from the output transformer in standby mode as well as in power on mode, the winding voltage reduction in the transformer results in the SMPS controller no longer being properly supplied. To take advantage of the relatively inexpensive reconfiguration technique for standby operation, a cost effective way to supply the SMPS controller must be found.  
           [0006]    Presently the problem of supplying power to the microcontroller is solved by the use of a separate, stand alone, low power flyback SMPS dedicated to supplying operating voltage to the microcontroller. While this solution does not require the use of the cost effective reconfiguration technique because if the microcontroller is separately powered the entire power supply, including the SMPS controller, may be turned off awaiting a signal from the microcontroller to resume supplying power, it itself is a somewhat expensive solution because of the cost of the additional components used as well as the area taken up by the additional circuitry on the SMPS circuit board. Accordingly, it is an object of this invention to provide a low cost method and circuit for providing power to an electronic device including, for example, an SMPS controller during standby operation, by providing a charge pump apparatus and a method for operating a charge pump power supply so as to provide regulated power to the electronic device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 shows a schematic diagram of a switched mode power supply where the microcontroller power supply is a dedicated low power SMPS.  
         [0008]    [0008]FIG. 2 shows a schematic diagram of a switched mode power supply according to the instant invention.  
         [0009]    [0009]FIG. 3 is a schematic diagram of the circuitry used to generate the signal Vddreg.  
         [0010]    [0010]FIG. 4 is a timing diagram describing the operation of the circuit of FIG. 3.  
         [0011]    [0011]FIG. 5 shows an alternative arrangement of the components of the charge pump power supply according to the invention.  
         [0012]    [0012]FIG. 6 shows another alternative arrangement of the components of the charge pump power supply according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]    As an example of a use of the regulated charge pump power supply of the instant invention, a preferred embodiment of the charge pump will be shown as it may be used to address a power supply problem relating to switched mode power supplies. FIG. 1 shows a schematic diagram of a switched mode power supply (SMPS) of the half bridge type where the microcontroller power supply is a dedicated low power SMPS. The input voltage from the power mains is applied to an input filter  10 , which is an RF filter designed to eliminate electromagnetic interference in accordance with the design standards for many, particularly consumer, products and in accordance with good engineering practice. The output of filter  10  is applied to a conventional bridge rectifier  12  consisting of four diodes in a bridge configuration. A filter capacitor  14  filters the output of the bridge rectifier  12  and the voltage on capacitor  14  is the rectified mains voltage of approximately 120 to 400 volts (depending upon the original mains voltage) which operates a half bridge structure  16  which is used, in conjunction with an SMPS controller  18 , to regulate the output voltage of the power supply. The SMPS controller  18  in this preferred embodiment may be any of several conventional SMPS controllers designed for use with half bridge structures.  
         [0014]    At the mid point of the half bridge structure  16  is produced a square wave which is filtered by a resonant filter  20  to produce a quasi-sine wave. The output of the resonant filter  20  is the input to the primary winding of a power transformer  22 , the secondary winding of which is coupled through a diode  24  and a capacitor  26  to an output load  27 . The secondary winding of transformer  22  is provided with a ratio of turns with respect to the primary to produce the desired load voltage for the particular apparatus for which the SMPS is to be used.  
         [0015]    In power on mode the voltage to power the SMPS controller  18  is provided by another secondary winding  22   s  of power transformer  22 . The voltage from the secondary winding is rectified and filtered by the diode  28  and bulk capacitor  30  and applied to the Vdd input pin of the SMPS controller  18 . A microcontroller  32  provides an enable signal (EN) to the controller  18  via an optical coupler  33  when it in turn receives a signal from, for example, an infrared remote device operated by the user of the apparatus that may be a television set or VCR. The optical coupler  33  is used to provide isolation between the power circuitry associated with the microcontroller  32  and the power supply controller  18 .  
         [0016]    Power to the microcontroller is supplied by a separate, dedicated switched mode power supply  34  that, in turn, is supplied from the power mains. The use of a separate power supply for the microcontroller  32  allows the controller  18 , via the enable input EN, to switch to a disable mode where it draws minimum power when the power supply is in standby mode.  
         [0017]    As previously mentioned, however, the provision of a separate SMPS power supply for the microcontroller  32  solves one problem of powering different portions of the SMPS power supply, but it is not itself a particularly cost effective overall solution.  
         [0018]    In operation in power on mode, the bridge rectifier  12  full wave rectifies the filtered mains voltage to produce the operating voltage for the half bridge  16 . The half bridge  16  is comprised of two diodes  36  and  38 , which respectively have connected across them two switches  40  and  42 , which may be MOS transistors. The square wave output at the midpoint of the half bridge is filtered to create a quasi-sine wave by resonant filter  20 . The output of resonant filter  20  is applied to transformer  22  that transforms the input voltage to the desired load voltage that is subsequently applied to the load  27 .  
         [0019]    Coupled to the load  27  through an appropriate resistor  34  is a feedback sense circuit that produces a sense signal applied as a Sense input to the SMPS controller  18 . The sense signal usually is provided by an optical coupling circuit to provide isolation between the output circuit and the controller  18 . The Sense input of the controller  18  operates an oscillator within the controller (not shown) that, in conjunction with an external (to the controller) resistor-capacitor circuit (not shown), controls the frequency of operation of the switches  40  and  42  of the half bridge thereby regulating the voltage at the load.  
         [0020]    The circuit of FIG. 1, however, still suffers from the problem of operation in the reconfiguration mode since although the microcontroller  32  now has a stable power supply when the SMPS is off, is it desirable to reduce the overall cost of the SMPS power supply.  
         [0021]    [0021]FIG. 2 shows a schematic diagram of a switched mode power supply according to the instant invention. Similar components of the circuit of FIG. 2 are numbered the same as their counterparts in FIG. 1. The overall structure and operation of the SMPS of FIG. 2 is similar to that of FIG. 1, the main difference being in the power sources for the microcontroller  32  and for the SMPS controller  18 . As noted previously, if an auxiliary power supply for the SMPS controller can be provided, the microcontroller  32  can be powered from a secondary winding of the power transformer  22  while the SMPS is operating in its on mode. Also, in standby mode the microcontroller is powered at a consistent voltage by means of a connection to a suitable power supply as will be discussed later.  
         [0022]    Accordingly, in this preferred embodiment of the invention, another secondary winding  22   s   2  for power transformer  22  is provided. The output from this winding is provided through diode  44  and filtered by capacitor  46  to the power input terminal of the microcontroller  32 . Also shown in FIG. 2 is the switching arrangement for reconfiguring the circuit upon entering standby mode. From the secondary winding of transformer  22  is added an additional diode  48  and a zener diode  50 . A switch  52 , which may be an MOS transistor, is coupled between diode  48  and zener diode  50 . The switch  52  is controlled by the microcontroller  32 . The operation of this segment of the circuitry is as follows. When switch  52  is open diode  48  and zener diode  50  are not actively connected in the circuit and the microcontroller  32  is powered from the secondary winding  22   s   2 . Upon the power supply entering standby mode, however, switch  52  is closed and the high voltage output of the main secondary of transformer  22  is pulled down to the breakdown voltage of the zener diode  50 . This can be done because the microcontroller has its operating voltage applied through a coupling between the cathode of the zener diode  50  and the cathode of diode  44  supplying the microcontroller  32 . By circuitry nor shown, as it forms no part of the instant invention, the voltage powering the microcontroller  32  is reduced upon the system entering reconfiguration mode to a nominal five volts. While the zener diode arrangement is shown here in a preferred embodiment, other ways to provide this result are possible.  
         [0023]    This use of the reconfiguration technique and the independent powering of the microcontroller  32  are made possible because the SMPS controller is now separately supplied in a cost effective and reliable manner. The supply for the SMPS controller  18  is a charge pump circuit  60  that takes advantage of the square wave at the mid point of the half bridge structure  16 .  
         [0024]    A charge pump usually is not a viable power supply for a SMPS because SMPSs have to accommodate a wide variation of mains voltages. The magnitude of the square wave applied to such a charge pump varies in accordance with the mains voltage. Also, the current generated by the charge pump is directly proportional to the mains voltage as follows:  
         
       I=C dv/dt  
     
         [0025]    The energy transferred from the charge pump capacitor to the Vdd EIcap is:  
           E= ½ CV   mains   2    
         [0026]    The line voltage of the mains may be from 80V to 280V so the transferred energy varies according to a ratio of 10:1. Additionally, the transferred energy is directly proportional to the operating frequency.  
         [0027]    The input voltage of the SMPS controller  18  must be limited to about 15 volts for proper operation, so the excess energy coming from the charge pump if operating in an environment where the square wave output of the half bridge is high, must be dissipated by circuitry within the controller  18  or externally to the controller, for example a zener diode. If an external zener diode were used, it may possibly be provided across capacitor  30 . Dissipating the excess power internally of the controller  18  is not a viable solution because of energy dissipation problems relating to packaging considerations. Dissipating the excess power externally is not a desirable solution either, because of the general need to reduce energy waste in consumer circuits and the cost associated with providing an additional device, the zener diode. Thus, a conventional charge pump circuit cannot be used.  
         [0028]    In the circuit of FIG. 2, in normal operation, the power to the controller  18  is provided by the secondary winding of the power transformer  22 . The current through the winding charges the capacitor  30  that is coupled to the Vdd power supply pin of controller  18 . When the SMPS reverts to standby mode, the reconfiguration switch  52  is closed and the current through secondary winding  22   s  collapses. Power to the controller  18  is then provided by the charging of capacitor  30  by charge pump  60 .  
         [0029]    The charge pump of the instant invention comprises a capacitor  62  and a pair of diodes  64  and  66 . In a preferred embodiment capacitor  62  is coupled to the midpoint of the half bridge  16  from which it receives a square wave signal. The capacitor is also coupled to diodes  64  and  66 . The output of diode  66  is coupled to the Vdd input terminal of SMPS controller  18 . Across diode  64  is a switch, which may be a MOSFET  68  that is gated by a signal Vddreg from the SMPS controller. Depending on the configuration of diodes and switch forming a part of the charge pump  60 , the body of the MOSFET may be used as a diode in place of its associated diode.  
         [0030]    In the operation of the charge pump, the Vdd voltage is monitored by the controller  18 . As the Vdd voltage exceeds a given threshold, for example thirteen volts, the Vddreg control signal from the controller  18  is also set to about thirteen volts to turn the MOSFET switch  68  on. MOSFET  68  shorts diode  64  making the charge pump mechanism inefficient. Due to the power consumption of the controller  18 , the Vdd voltage of the controller decreases as capacitor  30  discharges and reaches a second threshold of perhaps 12 volts, at which time Vddreg is reset to zero volts and MOSFET switch  68  is opened and the free operation of the charge pump to recharge capacitor  30  is resumed. In the operation of the charge pump of the instant invention, then, excess energy need not be dissipated within the controller, and less energy waste is incurred.  
         [0031]    The signal Vddreg is provided by circuitry internal to the controller  18  as shown in FIG. 3, which can be better understood in conjunction with the timing diagram of FIG. 4.  
         [0032]    The power input pin Vdd of controller  18 , which receives the output of the charge pump  60  is connected internally of the controller  60  by means of a resistor bridge  70 ,  72  to a hysteretic comparator  74 . The two thresholds of the comparator  74  are adjusted by means of the resistor bridge formed by resistors  76 ,  78 ,  80 , in conjunction with transistor  82 . The ability of transistor  82  to short resistor  80  provides a voltage at the comparator input of either of two voltages sufficient to turn the comparator  74  on or off when Vdd reaches (in this example) twelve or thirteen volts, respectively. For other applications, of course, different thresholds could be selected by varying the values for resistors  76 ,  78 , and  80 . A voltage source  76  provides power to the comparator  74 . The voltage source  76  may be a band gap reference or other convenient source. The output of comparator  74  is applied to a voltage shifter  78  that adapts the output of the comparator  74  to the Vdd supply rail. A buffer  80  provides the correct current level to drive the external switch  68  used in conjunction with the charge pump  60 .  
         [0033]    In operation, when Vdd is rising from twelve volts to thirteen volts (meaning that the external capacitance  30  of FIG. 2 needs to be recharged) the Vddreg output is low, the switch  68  is off, and the charge pump  60  is activated allowing current flow through diode  66  to charge capacitor  30  as shown in FIG. 4. When Vdd reaches thirteen volts, the output of comparator  74  goes high causing Vddreg to go high thus turning on switch  68  destroying the charge pump efficiency and reducing or completely stopping the flow of current (depending on the diode/switch configuration) through diode  66  as is also seen in FIG. 4. During this time the capacitor  30  is allowed to discharge because of the current consumption of the controller  18  and overall switching activity. When Vdd reaches twelve volts the output of comparator  74  goes low causing Vddreg to go low, as in FIG. 4, thus turning off transistor  68 , restoring the action of the charge pump, and the cycle repeats.  
         [0034]    The preferred embodiment of the invention as shown in FIG. 2 shows only one of several possible arrangements of the diodes, capacitor and MOSFET switch which are possible. FIG. 5 shows an alternative arrangement of the capacitor, diode and switch components of the charge pump power supply in which the switch  68  that is controlled by Vddreg is across diode  66 .  
         [0035]    [0035]FIG. 6 likewise shows an alternative arrangement of the capacitor, diode and switch components of the charge pump power supply in which the switch  68  is in series with capacitor  62 . When open, switch  68  stops all flow of current through capacitor  62 . While FIG. 6 shows the switch on the charge pump side of capacitor  62 , it could as well be on the half bridge side. In the case of the arrangements of FIG. 2 and FIG. 5, the switch is closed to short diode  64  or diode  66 , respectively, which stops all current flow through capacitor  62  and stops all charge pump action.  
         [0036]    In any of the alternative diode/switch arrangements of FIG. 2, FIG. 5 or FIG. 6 it should be noted that the body of the MOSFET  68  itself could serve as one of the diodes, thus eliminating the need for one of the components.  
         [0037]    Although the regulated charge pump power supply has been described in a configuration relating to the solution to a power supply problem in switched mode power supplies, the invention has many other uses where a low cost regulated supply voltage is needed and a supply of varying voltage is available.