Abstract:
A switch mode power supply and method for operating the power supply and to the use of the same, wherein the switch mode power supply comprises a control for controlling a control element and is connected to a DC link of the converter, the switch mode power supply comprises a startup-delay such that control of the switch element is blocked until a predetermined start delay duration (T) has lapsed, where the start delay duration (T) is longer than the time span between DC link charging.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a U.S. national stage of application No. PCT/EP2012/059009 filed 15 May 2012. Priority is claimed on European Application No. 11175353.9 filed 26 Jul. 2011, the content of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a switch mode power supply comprising a control for controlling a switch element and connected to a DC link of a converter, and a method for operating the switch mode power supply. 
         [0004]    2. Description of the Related Art 
         [0005]    Converters are used to connect loads, such as variable-speed machines, to a three-phase supply. In this case, a DC link is connected via a rectifier to the three-phase supply and the load is controlled from the DC link via a suitable bridge circuit. It is usual for the DC link voltage to be charged via a resistor, i.e., a PTC resistor, and an auxiliary contactor. This series circuit, comprising the resistor and auxiliary contactor, is shunted via a main contactor at 95% of the charge. 
         [0006]    The charging of the DC link is usually checked by a monitoring circuit to detect the occurrence of a voltage sag. Such a voltage sag occurs if, for example, beyond a specific voltage, a load is erroneously short-circuited. The converter controller therefore switches into a fault mode as soon as a voltage sag is detected. 
         [0007]    Power supplies that are connected to such a DC link of a converter have to take these conditions into account. In conventional circuits, a representative arrangement includes a signaling device for signaling to the power supply that a DC link charging process is completed or more or less completed. Only then does the power supply switch on, and this prevents the monitoring circuit from detecting the switching-on of the power supply as a faulty voltage sag. However, the disadvantage here is the additional cable outlay and the fact that the input lines have to be suitably protected against interference. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore an object of the invention to provide an improved switch mode power supply and to provide a method for operating the switch mode power supply. 
         [0009]    These and other objects and advantages are achieved in accordance with the invention by providing a switch mode power supply with a start-up delay, so that the control of the switch element is blocked until a predetermined start delay period has elapsed. Here, the start delay period is longer than the time interval for charging the DC link. The DC link charging operations usually last approximately 3 to 6 seconds. The effect of a sufficiently long start delay period of 10 seconds, for example, is that the switch mode power supply always only begins to cycle if the DC link charging has ended or at least progressed until a minor voltage sag resulting from the start-up of the switch mode power supply is not detected as a malfunction. 
         [0010]    Provision is made in a further embodiment for the switch mode power supply to a have a switching circuit for detecting a voltage rise at the input side, and for a blocking circuit to block the control of the switch element during the start delay period. The desired function of a start delay of the switch mode power supply is therefore implemented in a simple manner. 
         [0011]    It is advantageous if the switch mode power supply includes a DIP switch for activating the start-up delay. A switch mode power supply configured in this way has universal application. In accordance with the given operating conditions, the start-up delay function may be activated or de-activated via DIP switches. 
         [0012]    It is also advantageous if the start delay period can be adjusted with the aid of operating means of the switch mode power supply. The switch mode power supply can be adapted in this way to different converter DC links. For converters that require a shorter period for charging the DC link, the start delay period is shortened so that the supply voltage delivered by the switch mode power supply is available quicker. 
         [0013]    This can be simply achieved by providing the switch mode power supply with a function switch for adjusting the start delay period. In an embodiment, the DIP switch comprise a function switch for adjusting the start delay period. 
         [0014]    More conveniently, the switch element also includes a signaling element for adjusting the start delay period. Here, a light element or a tone generator enables the set start delay period to be output in a simple manner. 
         [0015]    In accordance with the invention, provision is made for the switch mode power supply to be used to supply a control of the converter. It is generally advantageous if the switch mode power supply is used to supply the converter and/or its peripherals. 
         [0016]    It is also an object to provide a method for operating an appropriate switch mode power supply in which a threshold value of a DC link voltage is provided at the input-side of the switch mode power supply and detected so that the start delay period commences after the threshold value is reached, and so that control of a switch element is blocked until expiration of the start delay period. The switch mode power supply needs no further signals to detect the expiration of a DC link charging process. 
         [0017]    For the adjustment of a modified start delay period, it is advantageous if a programming mode is activated via a function switch, during which programming mode a signaling element outputs repeated signals to indicate the start delay period to be set, and if the start delay period is determined by renewed actuation of the function switch. Such an adjustment process can be realized with simple operating elements. 
         [0018]    Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention is explained below by way of examples with reference to the attached figures, in which: 
           [0020]      FIG. 1  a schematic block diagram of a converter circuit in accordance with the invention; 
           [0021]      FIG. 2  is a graphical plot of waveforms of a DC link voltage; and 
           [0022]      FIG. 3  is flowchart of the method in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    A converter circuit is illustrated in  FIG. 1 . Here, inventive switch mode power supply  9  is connected to a DC link voltage U which is applied to a DC link capacitor  5 . The DC link is fed via a rectifier circuit  3  that is connected to a three-phase power supply  1 . A three-phase output terminal  2  for connecting a three-phase machine is connected via a bridge circuit  4  to the DC link. 
         [0024]    An exemplary charging circuit for the DC link consists of a resistor  7  (e.g., a PTC resistor) and a series-connected auxiliary contactor  6 . As soon as a specific charge, such as 95% of the nominal DC link voltage U1, is reached, the resistor  7  and the auxiliary contactor  6  are shunted by a main contactor  8 . An isolating transformer, a step-down converter or an active rectifier for slow clocking rate can be used as alternate charging circuits, for example. 
         [0025]    The charging of a DC link is usually monitored via a monitoring circuit. The converter goes into a fault mode as soon as a negative voltage change (−du/dt) is detected during a charging phase. 
         [0026]    The switch mode power supply  9  is used to supply a converter controller  11 , which is connected in parallel to the switch mode power supply  9  and directly to the supply system  1  via a main power supply  10 . In normal operation, the supply is realized via the main power supply  10 . The switch mode power supply  9  connected to the DC link takes over the supply in the event of power failures. In this case, the energy stored in the DC link determines the duration of a power failure that is able to be bridged. 
         [0027]      FIG. 2  shows the voltage rise of a DC link voltage U during the charging phase. The voltage U rises to time t2 at which 95% the nominal DC link voltage is reached, for example. The resistor  7  and the auxiliary contactor  6  are then shunted by the main contactor  8 . A conventional switch mode power supply  9  would be active during the charging phase up to time t1, whereupon triggering of the monitoring circuit would result in a malfunction. 
         [0028]    The switch mode power supply  9  in accordance with the invention blocks the control of a switch element arranged in the switch mode power supply  9  until the DC link is charged up. A voltage drop caused by the inrush current of the switch mode power supply  9  is delayed, so that no converter malfunction occurs. 
         [0029]    The switch mode power supply  9  includes a DIP switch, for example, for activating this inventive start-up delay. Such a switch mode power supply  9  is suitable for connection to converter DC links and to other supply sources, without the described ramp-up problem. Upon activation of the start-up delay, a preset start delay period T (e.g., 10 seconds) starts to run as soon as the DC link voltage U reaches a specified threshold value Us at time t0. Here, the threshold value Us is, on the one hand, less than the ON threshold Ue at time t1 at which, without a start-up delay, the switch mode power supply  9  would usually begin to cycle. On the other hand, the threshold value Us is made sufficiently high so that charging of the DC link is reliably detected. 
         [0030]    In order to indicate the function of the switch mode power supply  9  to operating personnel, it is useful if the expiration of the start delay period T is signaled, such as via a flashing light-emitting diode. 
         [0031]    The duration of a charging phase usually depends on the size of the converter. Small converters having a relatively small DC link capacitance  5  reach the rated DC link voltage U1 even sooner. Here, the rise in the DC link voltage U′ is steeper than in larger converters. 
         [0032]    Advantageously, the start delay period T is adjustable to adapt the switch mode power supply  9  to the charging period of different converters. Smaller converters are then available sooner. A better range for the start delay period T is between two and ten seconds. 
         [0033]    Setting of the start delay period T is realized in a programming mode. The programming mode can be activated, for example, by actuating a function switch after the charging-up of the DC link voltage within a predefined activation time span. This function switch can also be the DIP switch for activating the start-up delay. The programming mode is then started by switching over the DIP switch to de-activation of the start-up delay function, following the charging phase. As soon as the device is in the programming mode, a signaling element, such as a light electrode, indicates the start delay period T to be set. A light-emitting diode flashes every second, for example. In order to set a six-second long start delay period T, for example, the light-emitting diode should flash six times before actuation of the function switch determines the start delay period T. If the function switch is formed by the DIP switch, the setting of the start delay period T is realized by switching over the DIP switch to activation of the start-up delay function. 
         [0034]    The programming mode is automatically terminated after the start delay period T has been set and the device is started. This can be indicated to the operating personnel, such as by rapid flashing of the light-emitting diode. At the initial starting cycle following a change to the start delay period T, for verification it is useful if the new start delay period T is indicated via a signaling element. For example, the light-emitting diode flashes every second according to the number of set seconds. 
         [0035]    It is better if the programming mode can also be activated when the switch mode power supply  9  is continuously operating, for example by switching over the function switch repeatedly. 
         [0036]      FIG. 3  is a flowchart of a method for operating a switch mode power supply ( 9 ). The method comprises detecting a threshold value (Us) of a DC link voltage (U, U′) applied to an input-side of the switch mode power supply ( 9 ) such that a predetermined start delay period (T) commences after the threshold value (Us) is reached, as indicated in step  310 . A control of a switch element is then blocked until a predetermined start delay period (T) has elapsed, as indicated in step  320 . 
         [0037]    While there have been shown, described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the methods described and the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.