Abstract:
A system and method thereof to regulate a current to a capacitive load from a power supply connected to the capacitive load. The system includes a first switch between the power supply and the capacitive load, a super-capacitor configured for charging by the power supply and powering the capacitive load, a current limiting circuit between the super-capacitor and the power supply, a second switch between the super-capacitor and the capacitive load, and a power control circuit configured to control opening and closing of the first switch and the second switch independently, sense a voltage of the power supply, and sense a voltage of the super-capacitor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/771,440, filed Mar. 1, 2013, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates in general to power supplies for electronic and computer systems employing backup power capability in the form of super-capacitors which allow the power supply to continue to function after main power has failed. 
         [0003]    As electronic and computerized systems are increasingly used in all aspects of daily life, so has the requirement for such systems to be available twenty-four hours a day and seven days a week, and to be able to continue to function in the event of other system failures such as interruptions or failures of national grid power. 
         [0004]    Large systems are often protected by backup power systems using battery power or diesel generators. Smaller systems such as computer servers may also protect against failure at a local level by using redundancy in the form of running twin identical servers and allowing seamless switching from one to the other (also called “failover”), or using redundant arrays of storage disks (termed “RAID”). 
         [0005]    With storage systems, the use of solid-state disks (SSDs) is becoming more prevalent due to their advantages in terms of speed and performance. SSDs employ a form of non-volatile memory, often in the form of SSD devices known as NAND flash memory (chips), and due to the requirement to present such memory as though it were a traditional hard disk drive, a memory controller is required to perform a translation of the hard disk protocols to instructions to read and write the memory. This operation requires a memory controller to use large translation and mapping tables and other metadata to assist in this process. For reasons of speed and performance, these tables are normally read from the flash memory at initial power-on, then stored in volatile RAM memory. In order not to affect performance, the data in RAM is only periodically flushed back to flash memory and when the system is properly shut down. 
         [0006]    An SSD is therefore exposed to a window of time wherein the user data and the translation and mapping tables and metadata are not consistent. If the power to the drive were to be suddenly removed from the drive during this time, the up-to-date translation/mapping tables and metadata in RAM may be lost and unrecoverable. The result may be that when the SSD next powers up, it may not be possible to reconcile the user data and translation/mapping tables and metadata stored in flash memory, and user data may be lost. 
         [0007]    To prevent data loss, it is now common to employ a backup power supply for an SSD, generally located on the same circuit board as the memory controller and flash memory devices. This severely limits the size and amount of power the backup supply can provide and batteries are generally too bulky. 
         [0008]    However, the main requirement is to supply enough power to keep the SSD running for enough time to allow it to flush all the unsaved data from RAM to the flash memory, so that the metadata can be brought into synchronization with the user data. This has allowed for the use of relatively small backup power components, for example, capacitors commonly known as super-capacitors. Super-capacitors have capacitance values up to 10,000 times that of normal electrolytic capacitors, but much less power capacity (e.g., about 10%) of a conventional backup battery. Super-capacitors do, however, have a much higher energy density than a conventional backup battery and can therefore satisfy the requirement for a short term backup supply in a small volume that can be readily fitted into an SSD package. 
         [0009]    Power circuits have therefore been developed for SSDs which merge super-capacitors with a DC to DC converter which supplies the various regulated DC voltages (such as 3.3V) required by the SSD, its controller and the flash memory, from the incoming main power supply voltages, which are typically 5V and 12V. 
         [0010]    These circuits generally employ sequencing of the application of power using a power management device such that the super-capacitors are powered up first, then the DC-DC converter, and finally the output regulated voltages to the components. 
         [0011]    A major consequence of adding super-capacitors is that extra inrush current limiting must be employed as the large values of capacitance and low internal resistance means that large currents will otherwise flow over a relatively long time as the super-capacitors charge up at power on. The current limiting may need only consist of a resistor and diode 
         [0012]    Without super-capacitors, some form of current limiting or slow start-up circuitry may still be required for the DC-DC converter, although while the input capacitance of the DC-DC converter will be less, the solution should be capable of supplying sufficient current for normal operation whilst not consuming power, which will involve a more sophisticated and expensive solution than a simple resistor. 
         [0013]    Current limiting is known in the prior art is for connecting a power supply to a DC-DC converter and may be based on using current limiting resistors in parallel with switches (mechanical and electronic) such as in U.S. Pat. No, 5,087,871 (Losel) and U.S. Pat. No. 6,646,842 (Pan et al). 
         [0014]    There is an ongoing need for systems and methods suitable for reducing an inrush current for a power supply with both backup super-capacitors and a DC-DC converter. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0015]    The present invention provides systems and methods suitable for limiting an initial rush of current into solid state drives (SSDs) that employ super-capacitors as a backup power supply to a DC-DC converter of the solid state drive. 
         [0016]    According to one aspect of the invention, a system to regulate a current to a capacitive load from a power supply connected to the capacitive load includes a first switch between the power supply and the capacitive load, electrically connecting the capacitive load to the power supply in a closed position thereof, and electrically disconnecting the capacitive load from the power supply in an open position thereof. A super-capacitor is configured for charging by the power supply and powering the capacitive load. A current limiting circuit between the super-capacitor and the power supply is provided, electrically connecting the super-capacitor to the power supply in a closed position thereof, and electrically disconnecting the super-capacitor from the power supply in an open position thereof. A second switch is provided between the super-capacitor and the capacitive load, electrically connecting the super-capacitor to the capacitive load in a closed position thereof, and electrically disconnecting the super-capacitor from the capacitive load in an open position thereof. A power control circuit is configured to control opening and closing of the first switch and the second switch independently, sense a voltage of the power supply, and sense a voltage of the super-capacitor. 
         [0017]    According to another aspect of the invention, a method of regulating a current from a power supply to a capacitive load includes electrically connecting the capacitive load to the power supply through a super-capacitor when power initially begins transferring from the power supply to the super-capacitor. The super-capacitor is electrically connected to the power supply by a current limiting circuit and is charged by the power supply. The super-capacitor is electrically disconnected from the capacitive load if the super-capacitor reaches a charged state, and the power supply is subsequently electrically connected to the capacitive load without the super-capacitor being electrically connected therebetween. 
         [0018]    Technical effects of the system and method described above preferably include the ability to limit the inrush of current in a solid state drive while incorporating super-capacitors capable of providing backup power to a DC-DC converter. In particular, it is believed that, by providing a switch which normally enables the super-capacitors to supply power to the DC-DC converter in the event of main power failure to also be employed at the time that the main power is switched on to charge the both the super-capacitors and input capacitors of the DC-DC converter, the current rush will be reduced when main power is applied to the DC-DC converter after the super-capacitors have been sufficiently charged. 
         [0019]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1   a  shows an inrush current limiter circuit using a switch and current limiting resistor according to the prior art. 
           [0021]      FIG. 1   b  shows an inrush current limiter circuit using a switch and current limiting circuit according to the prior art. 
           [0022]      FIG. 2  shows an inrush current limiter used with a super-capacitor, according to the prior art. 
           [0023]      FIG. 3  shows a schematic circuit of an inrush current limiter combined with a super-capacitor backup supply according to an aspect of present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The present invention is generally applicable to non-volatile solid state drive (SSD) mass storage devices. An aspect of the present invention is to be able to use a power management system to reduce an inrush current of a DC-DC power converter of an SSD storage device wherein the system also employs super-capacitors to provide a back-up power source to the DC-DC power converter. In particular, preferred embodiments of the present invention utilize the same circuitry for both the inrush current limitation and for the backup power source. 
         [0025]    Referring to  FIG. 1   a , a known configuration in the prior art comprises connecting a main power source  100  to a capacitive load  120  (e.g., a DC-DC converter connected to an SSD) via a main power switch  110 , where a current limiting resistor  130  is placed in parallel with the main power switch  110 . If the main power switch  110  were closed at the time the main power is connected, then a high inrush current would flow, which would decline as the capacitance in the load charges up. Instead, the main power switch  110  is open when the main power  100  is connected. A current still flows but is limited by the resistor  130 . The maximum current is given by the output voltage of the main power source  100  divided by the value in Ohms of the resistance  130 . The current will decline as the internal capacitance of the load  120  charges up and settles at a value given by the output voltage divided by the total value of the resistor  130  plus the internal resistance of the load  120 . There will be a small voltage drop across the current limiting resistor, but at this time the switch  110  can be closed whereupon there will be a small current surge due to the smaller voltage difference than when the main power is initially connected, but this will be greatly reduced compared to any voltage surge that would have occurred had the current limiting resistor  130  not been used. 
         [0026]    In  FIG. 1   b  an alternative schematic circuit is shown, where an electronic current limiting circuit  140  is used in series between the power switch  110  and the load  120 . In this arrangement, the current limit can be electronically controlled and, by using an active (semiconductor) controller device, can be arranged to gradually switch from presenting a current limiting resistance at initial turn on to almost no resistance in normal operating mode. 
         [0027]    In a conventional SSD, such sophisticated measures may not be necessary, as the inrush current may be well within the parameters of the main power supply. However, with the addition of a super-capacitor backup supply, some measures are likely needed as the super-capacitor will consume a large current over a longer period of time than a regular capacitor due to its low internal resistance and very large capacity measured in Farads rather than microFarads. This initial inrush current due to the super-capacitor can be comparable to short circuiting the main power supply. 
         [0028]      FIG. 2  shows a simple arrangement known in the prior art to provide current limiting for a super-capacitor acting as a backup supply. At switch on and during normal operation, the main power input  200  charges the super-capacitor  220  via a current limiting device  210  comprising a resistor  212  and a reverse protection diode  214 . The resistor  212  can be adjusted to a value that provides a compromise between changing time and power dissipated during initial charging, which may last only a number of seconds. During normal operation, the power supply compensates for any small leakage of charge from the super-capacitor as the discharge switch  230  is open and therefore very little current is drawn through the current limiting device  210  and a commensurately small amount of power is dissipated by the resistor  212 . 
         [0029]    The purpose of the super-capacitor  220  is to provide backup power to the backup power output terminal  240  when the main power in  200  fails. In this case, a power fail backup control device  250  senses a voltage drop via a sense line  252  and causes the discharge switch  230  to close via a control line  254  and the current flows through the discharge switch  230  to provide the backup power output terminal  240 . The main power input  200  which has failed will be at zero volts and the diode  214  prevents the super-capacitor discharging to ground through the resistor  212 . 
         [0030]    This simple and low cost solution works because, unlike a circuit to reduce the inrush to an always-active load, no extra switch is required in parallel to the current limiting resistor in order to reduce power dissipation by the resistor in normal operation and the initial inrush. In normal operation, very little current is drawn and the voltage on the super-capacitor asymptotically reaches the power supply voltage, minus a small voltage drop, generally of a few hundred millivolts, due to the diode  214 . 
         [0031]    Typically in an SSD, the main power is used to drive a DC-DC converter, where a current limiting arrangement such as in  FIG. 1   a  or  1   b  could be used. If a super-capacitor backup were added, additional current limiting would be necessary for the super-capacitor in a manner generally according to  FIG. 2 . Consequently, two separate forms of current limiting would be employed at the same time. 
         [0032]    According to an aspect of the present invention, when a super-capacitor based backup supply is used with an SSD, the current inrush limiter for the super-capacitor may be used to pre-charge the capacitors of the DC-DC converter&#39;s input capacitors and thereby reduce the current inrush when the main power is applied to the DC-DC input capacitors. Such as arrangement avoids having separate current limiting for the DC-DC converter and instead has a single common inrush current limiter circuit. 
         [0033]      FIG. 3  shows one possible arrangement of the present invention. An SSD  305  is electrically connected to the main host power  300  via a circuit board drive connector  310 . The circuit board drive connector  310  is electrically connected to the DC-DC converters  355  by a serially connected chain of a power filter  302 , a main power switch  320 , and the DC-DC converter input capacitors  350 . In the prior art, a current limiter would normally be electrically connected between the main power switch  320  and the DC-DC input capacitors  350  or incorporated into the main power switch  320 . Instead, in the present invention the backup power super-capacitor  350  may be electrically connected to the main power through a current limiter with reverse current protection  335  and to the DC-DC input-capacitors via a discharge switch  340 . 
         [0034]    During normal operation, the power control integrated circuit (IC)  360  may use the control signal  362  to keep the main power switch closed and the signal  364  to keep the discharge switch  340  open, and to control the DC-DC converters via control line  366 . When a failure of the host main power  300  occurs, sensed by line  362 , the power control IC  360  uses line  364  to close the discharge switch  340  and open the main power switch  320 , allowing the super-capacitor  330  to now supply power through the discharge switch  340  to the DC-DC input capacitors  350 , which in turn allows the DC-DC converters  355  to continue to function for a sufficient time for the memory controller and flash memory  370  to perform operations which can ensure the consistency of data on the SSD  305 . 
         [0035]    In addition to the above, the discharge switch  340  may be utilized for an alternative purpose, such that when the SSD  305  is first connected to the main host power  300  via the drive connector  310 , the power control IC  360  ensures the main power switch  320  is open by default, but the discharge switch  340  is close. Therefore, the super-capacitor  330  charges via its current limiter and reverse protection  335 , but as the discharge switch  340  also connects the DC-DC input capacitors  350  through the super-capacitor  330 , the DC-DC input capacitors  350  are charged at the same time. The DC-DC input capacitors  350  are relatively small in capacity and have higher internal resistance than the super-capacitor  330  and therefore are believed to have little effect on the time taken to charge the super-capacitor  330 . 
         [0036]    When the power control IC  360  senses that the super-capacitor  330  has reached its maximum voltage, the DC-DC input capacitors  350  will have reached the same value which will be close to the main power voltage. At this point, the discharge switch  340  may be opened and the main power switch  320  closed. Since the voltage of the DC-DC input capacitors  350  is already close to the main power voltage, any inrush current to the DC-DC input capacitors  350  is believed to be considerably reduced. Any residual inrush can be further reduced by connecting an RC circuit  325  comprising a resistor and capacitor in serial to the control line  362  of the main power switch  320 , which applies a short time constant between the main power switch  320  being fully open to fully closed. 
         [0037]    While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the components could differ from that shown, and the assembly could differ in appearance and construction from the embodiments shown in the Figures. Therefore, the scope of the invention is to be limited only by the following claims.