Abstract:
It is presented a power supply for providing power to control a power switch for a high voltage application. The power supply comprises: a high voltage divider arranged to be connected to a first current terminal of the power switch; a step down DC/DC converter connected to an output of the high voltage divider, wherein the step down DC/DC converter is arrange to provide an output voltage for control of the at least one power switch to an output of the power supply; and a bypass control unit arranged to control the high voltage divider to short circuit an main input and a main output of the high voltage divider when the voltage across the power switch is lower than a threshold voltage.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a power supply for providing power to control a power switch for a high voltage application. 
       BACKGROUND 
       [0002]    Power switches such as insulated-gate bipolar transistors (IGBTs) can be used for many high voltage applications. The gates of the IGBTs can be driven by means of a respective gate unit. When the IGBTs are used e.g. as part of an inverter, by using pulse width modulation (PWM) the IGBTs are continually switched on and off to achieve a sinusoidal waveform. When the IGBTs are off (blocking), the voltage across the IGBT can be used to power the gate unit. Power can be stored between switches e.g. in capacitors to ensure the gate unit is continuously powered. 
         [0003]    However, when the IGBTs are used in an application where the IGBTs are conducting during a majority of the time, such as for a DC breaker, there is no reliable reoccurring time when the IGBTs are blocking, to charge up energy to power the gate unit. Nevertheless, it is useful if the power supply can be powered by the main circuit through the IGBTs, to reduce the need for any external power source. 
         [0004]    Consequently, there is a need to provide a power supply for providing power to control a power switch which does not require the power switch to be blocking to function. 
       SUMMARY 
       [0005]    An object is to alleviate or eliminate the problems described above. 
         [0006]    According to a first aspect, it is presented a power supply for providing power to control a power switch for a high voltage application. The power supply comprises: a high voltage divider arranged to be connected to a first current terminal of the power switch; a step down DC/DC converter connected to an output of the high voltage divider, wherein the step down DC/DC converter is arrange to provide an output voltage for control of the at least one power switch to an output of the power supply; and a bypass control unit arranged to control the high voltage divider to short circuit an main input and a main output of the high voltage divider when the voltage across the power switch is lower than a threshold voltage. This allows the power supply to provide a suitable output power for a great range of input voltages. Moreover, the power supply can be connected across the power switch, eliminating any need for an additional power source. 
         [0007]    The power supply may further comprise: a power limiter connected between an output of the step down DC/DC converter and the output of the power supply. In this way, a suitable output power for a gate unit is ensured. 
         [0008]    The power supply may further comprise: a voltage polarity dependent switch arranged to be connected to a second current terminal of the power switch and the output of the step down DC/DC converter; and a step up DC/DC converter connected to the voltage polarity dependent switch and the output of the power supply. The voltage polarity dependent switch allows the power supply to supply power even when a reverse voltage is applied to the power switch. Through the step up DC/DC converter, a suitable output voltage from the power supply is ensured, even if the input voltage to the power supply is low. 
         [0009]    The high voltage divider may comprise a plurality of serially connected resistors, selectively controlled to be part of a circuit between the first current terminal of the power switch and the first DC/DC converter. When the input voltage is high, the resistors are connected to lower the output voltage of the high voltage divider. 
         [0010]    The high voltage divider may comprise a plurality of serially connected capacitors, selectively controlled to be part of a circuit between the first current terminal of the power switch and the first DC/DC converter. 
         [0011]    The high voltage divider may comprise a plurality of serially connected transistors, each transistor being individually controlled by the bypass control unit. The transistors can be controlled to conduct when the input voltage is low, whereby the resistors of the high voltage divider are bypassed. 
         [0012]    The bypass controller may comprise a voltage divider, comprising a plurality of resistors, and diodes arranged to block any current from the high voltage divider. The diodes allow a voltage to be provided from the output of the power supply to the bypass controller to block the transistors for the low voltage case. On the other hand, the diodes prevent any reverse current from the high voltage divider to the output through the bypass controller. 
         [0013]    The voltage polarity dependent switch may comprise a diode bridge. 
         [0014]    The step down DC/DC converter may comprise a Zener diode to control its output voltage. 
         [0015]    The word ‘plurality’ in the description and claims is to be interpreted as meaning ‘more than one’. 
         [0016]    Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention is now described, by way of example, with reference to the accompanying drawings, in which: 
           [0018]      FIG. 1  is a schematic diagram illustrating an environment where embodiments presented herein can be applied; 
           [0019]      FIG. 2  is a schematic diagram illustrating a power supply and connected components of  FIG. 1 ; 
           [0020]      FIG. 3  is a schematic diagram illustrating an embodiment of a high voltage divider of  FIG. 2 ; 
           [0021]      FIG. 4  is a schematic diagram illustrating an embodiment of a bypass control unit of  FIG. 2 ; 
           [0022]      FIG. 5  is a schematic diagram illustrating an embodiment of a step down DC/DC converter of  FIG. 2 ; 
           [0023]      FIG. 6  is a schematic diagram illustrating an embodiment of a voltage polarity dependent switch of  FIG. 2 ; and 
           [0024]      FIG. 7  shows two graphs illustrating the function of the power supply of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. 
         [0026]    This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description. 
         [0027]      FIG. 1  is a schematic diagram illustrating an environment where embodiments presented herein can be applied. In this example, an application is shown where three power switches  2   a - c  are used to control a main current, to thereby act as a DC breaker. While it is here presented three power switches  2 - c , any suitable number of switches can be selected to achieve desired high voltage capacity across the power switches. 
         [0028]    The power switches  2   a - c  can be any suitable controllable power switch, such as an insulated-gate bipolar transistor (IGBT). A controller  20  provides a signal to three gate units  3   a - c , respectively connected to the three power switches  2   a - c . The gate units  3   a - c  are respectively powered by three power supplies  1   a - c.    
         [0029]    In the DC breaker application, the power switches  2   a - c  are normally in a conducting state. Due to this, conventional power supplies for gate units, relying on a voltage difference over the power switch, can not be used. Moreover, there is a great variation in input voltage for the power supply, which conventional power supplies are unable to handle. 
         [0030]    Instead, the power supplies  1   a - c  are structured to be able to supply power to the respective gate units  3   a - c  under a variety of electrical conditions, which is explained in more detail below. 
         [0031]      FIG. 2  is a schematic diagram illustrating a power supply  1  and connected components of  FIG. 1 . The power switch  2  can be any one of power switches  2   a - c  of  FIG. 1 . Analogously, the gate unit  3  can be any one of the gate units  3   a - c  and the power supply can be any one of the power supplies  1   a - c . The controller of  FIG. 1  is omitted in  FIG. 2  in order not to obscure the disclosed embodiment. The power switch  2 , in this case illustrated as an IGBT, comprises interface points in a gate G, and two current terminals being the emitter E and the collector C. 
         [0032]    The power supply  1  comprises a high voltage divider  6  connected to the collector C of the power switch  2 . A bypass control unit  5  is connected to several points of the high voltage divider  6  to control the high voltage divider to output a suitable voltage to a step down DC/DC converter  8 . A power limiter  7  conditions the signal provided on an output  4  to the gate unit, such that the supplied voltage and/or current is not too high. 
         [0033]    Furthermore, a voltage polarity dependent switch  10  is connected to the emitter E of the power switch  2  and the output of the step down DC/DC converter  8 . An output of the voltage polarity dependent switch  10  is connected to a step up DC/DC converter  9 . An output of the step up DC/DC converter  9  is connected to the output  4  of the power supply  1 . 
         [0034]    In this way, the voltage polarity switch will provide a positive voltage to the step up DC/DC converter  10  regardless if the voltage between the emitter E and collector C is positive or negative. 
         [0035]      FIG. 3  is a schematic diagram illustrating an embodiment of the high voltage divider  6  of  FIG. 2 . The high voltage divider comprises a string of a plurality of m serially connected transistors  14   a - m . The transistors can e.g. be MOSFETs (metal-oxide-semiconductor field-effect transistors), since the voltage drop across these is low. 
         [0036]    In parallel with the string of transistors  14   a - m , a string of n serially connected resistors  13   a - n  and a string of n serially connected capacitors  12   a - n  are provided. The capacitors  12   a - n  are used for any necessary AC biasing. The number of transistors, resistors and capacitors can be any suitable number. 
         [0037]    The gates of the transistors  14   a - n  are connected to a respective input  30   a - m . All inputs  30   a - m  are connected to the bypass controller unit  5 . 
         [0038]    When the voltage across a connected power switch is low, the bypass controller short circuits the transistors, whereby there is negligible voltage drop through the high voltage divider  6 . On the other hand, when the voltage across the connected power switch is high, the bypass controller controls the transistors  14   a - m  such that at least part of the current goes through the resistors  13 - n . In this way, the high voltage input drops through the high voltage divider  6 . 
         [0039]      FIG. 4  is a schematic diagram illustrating an embodiment of the bypass control unit  5  of  FIG. 2 . The bypass control unit  5  comprises a number of outputs  32   a - m  respectively connected to the inputs  30   a - m  of the high voltage divider  6 . The input  34  of the bypass controller unit  5  is connected to the output  4  of the whole power supply  1  ( FIG. 2 ). 
         [0040]    The bypass control unit  5  optionally comprises a voltage divider  24  comprising a number of resistors  23   a - m , where each level in the voltage divider  24  is connected to a respective output  32   a - m , via respective a diode  22   a - m  and output resistor  21   a - m.    
         [0041]    The bypass control unit  5  thus provides a bias gate voltage for each transistor  14   a - m  in the high-voltage divider  6  to force a conducting state. In this way, the transistors  14   a - m  shortcuts the voltage across the power switch through the high voltage divider  6  only if the primary voltage is within low voltage levels. 
         [0042]    Furthermore, the bypass control unit  5  provides a bias voltage for the step down DC/DC converter  8 . 
         [0043]    With reference also to  FIGS. 2 and 3 , the output voltage at the output  4  is applied to the bypass controller  5  which provides the voltage to the gates of the transistors  14   a - m . When the gate voltage of the transistors  14   a - m  is higher than the voltage on the output  4 , e.g. when the power switch is in a blocking state, and the voltage across the power switch  2  is high, the diodes  22   a - m  block reverse current from the high voltage divider  6  to the output  4 . Conversely, when the voltage across the power switch  2  is low, the gate voltages on the transistors  14   a - m  is low, whereby a voltage is fed from the output  4  to the gates of the transistors  14   a - m  to make then conduct and let the controlled current bypass the resistors  13   a - n . The resistors  13   a - n  of the high voltage divider can be selected to tune the output voltage of the high voltage divider  6  accordingly. 
         [0044]      FIG. 5  is a schematic diagram illustrating an embodiment of the step down DC/DC converter  8  of  FIG. 2 . In this embodiment, the step down DC/DC converter  8  comprises one or more Zener diodes  30  to control the output voltage to a suitable level. Other structures which provide a controlled voltage output are equally possible. 
         [0045]      FIG. 6  is a schematic diagram illustrating an embodiment of the voltage polarity dependent switch  10  of  FIG. 2 . There are two inputs  35   a - b  to the voltage dependent switch  10 , where a first input  35   a  is connected to the output of the step down DC/DC converter  8  and the second input is connected to the emitter E of the power switch  2  (see  FIG. 2 ). The voltage polarity dependent switch  10  comprises four diodes  25   a - d  making up a diode bridge, whereby an output  34  of the voltage polarity dependent switch  10  always provides a positive voltage, regardless of the voltage sign across the collector C and emitter E of the power switch  2  ( FIG. 2 ). 
         [0046]      FIG. 7  shows two graphs illustrating the function of the power supply of  FIG. 2 . In the upper part of  FIG. 7 , a main voltage, defined as a voltage between the emitter E and collector C of the power switch  2  is shown over time. In the lower part of  FIG. 7 , power output of the power supply  1  is shown over time using the same time scale as the upper part. It is to be noted that the vertical scales of the two graphs are not proportional. 
         [0047]    The graphs will now be explained in conjunction with references to  FIG. 2 . 
         [0048]    At a time  50 , the power switch  2  is controlled to be in a blocking state, whereby v main  increases to v 1 . The power supply here supplies a power P 1 . 
         [0049]    At time  51 , the power switch  2  is controlled to be in a conductive state, whereby v main  decreases to v 3 , which is the voltage drop over the power switch  2 , e.g. 3 volts. There is a delay t 1  between the voltage drop at time  51  and time  52 , at which point the high voltage divider has altered the states of its transistors  14   a - m , under the control of the bypass control unit  5 , to all be conducting. In this way, the resistors  13   a - n  of the high voltage divider are all bypassed, such that the voltage output of the high voltage divider  6  is the voltage of the collector C, less any voltage drop over the transistors  14   a - m . The current is then routed to the voltage polarity dependent switch  10  and to the step up DC/DC converter  9  to provide an output voltage to the gate unit  3  which is suitable for being able to control the power switch  2 . Here, an output power P 2  is supplied to the connected gate unit. 
         [0050]    At time  53 , the power switch  2  is again controlled to be in a blocking state, whereby the bypass controller blocks the transistors of the high voltage divider  6 , to force the current through the resistors of the high voltage divider. 
         [0051]    At time  54 , the power switch  2  is reverse biased. The current then goes in upwards through the power switch  2  of  FIG. 2 . This results in a reverse voltage of v 4 , which is the voltage drop through the freewheel diode of the power switch  2 . In one example, the reverse voltage v 4  is −3 volts. 
         [0052]    Time  55  corresponds to time  53  and time  56  corresponds to time  51 . Analogously, time  57  corresponds to time  53  and time  58  corresponds to time  51 . 
         [0053]    The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.