Abstract:
An inverter has a current sensor that senses a low side current before a high side of the inverter is permitted to power up. If an over-current situation is detected on the low side, powering up is prevented in order to avoid damage to the rectifier.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to an inverter with short circuit protection. 
         [0003]    2. Description of the Prior Art 
         [0004]    Industrial voltage source inverters generally consist of a DC voltage source such as a mains rectifier and smoothing capacitor connected to outputs by switching elements which are driven by Pulse Width Modulation control signals to generate the required output voltages. These circuits require short circuit protection to prevent damage to the components if the outputs are shorted. 
         [0005]    Short circuit protection is often provided by the use of current sensors which allow closed loop control of the output current. It is necessary to ensure that these sensors are powered and operational before the outputs of the rectifier are enabled. 
         [0006]    Inverters have upper and lower outputs and bootstrapping is used to provide power to gate drivers of the upper output. Bootstrapping involves the use of a diode between the gate drive power supply and a supply referenced to the negative DC link. This cannot be used however if phase current monitoring is employed since the current sensors will not be powered up until after the output devices are switched on. EPO431492 describes such a device. 
       SUMMARY OF THE INVENTION 
       [0007]    According to the invention there-is an inverter has a high and low side, and has low and high side switching elements, and a low side current sensor for sensing the current at the low side during at least a first phase of initiation of the inverter, an output current senor for, in use, monitoring the current provided by the inverter to a load, a control circuit to control low and high side switching elements and responsive to a signal from the output current sensor indicating that the sensor is operating to power up the high side switching element. 
         [0008]    Since power control is referenced to the low side the components used may be compact and inexpensive. The cost of the protected inverter is therefore less than conventional arrangements. 
         [0009]    The low side is powered up first and the high side powered up if the non-over current condition is determined on the low side. This is preferable since it the components used to do this may be more compact than would otherwise be the case. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows the circuitry concerned with one output phase of an inverter in accordance with an embodiment of the invention connected to a power supply unit and a load. 
           [0011]      FIGS. 2A and 2B  respectively show control waveform timings. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    As is shown in  FIG. 1 , an inverter  1  is connected to a power supply unit  2  and a load  3 . The inverter  1  uses well known pulse width modulation techniques to provide the required voltage to the load  3  from output  4 . 
         [0013]    The inverter  1  has a high side and a low side. The diagram shows only one output phase. 
         [0014]    The high side includes a high voltage rail  5  (a current sensor  7  shown in broken outline is a component which would have been present in the prior art but is omitted by the use of the invention) a gate drive  8  and a switch  9  providing an output  4  which is monitored by an output current sensor  10 . 
         [0015]    The low side includes a control circuit  11  referenced to the low side voltage rail  6 , a low side current sensor and trip  12 , a gate driver  13 , and a switch  14  to which the gate driver is coupled which provides an output to the output  4 . 
         [0016]    The control circuit  11  is microprocessor based and controls the operation of all components. It provides a control output to the gate driver  8  via a signal isolator  15  and receives inputs for the output current sensor  10  via isolator  16  and from the low side current sensor  12 . In the diagram it is shown as also receiving a signal from the current sensor  7  but this is to illustrate the advantage over the prior art arrangements which require such a circuit. 
         [0017]    The control circuit  11  is referenced to the low voltage side and provides a control signal via the isolator  15  to the gate drive  8  which then drives the switch  9 . The control circuit  11  is able to provide a control signal directly to the gate drive  13  because it is at the low side voltage to control the activation of the switch  14 . 
         [0018]    With the lower switch  14  on, the output to the load is monitored by the output current sensor  10 . The control circuit  11  is responsive to the signal of the output current sensor  10  indicating it is operational. The control  11  circuit is then able to switch the upper switch  9  on via a control signal to the gate driver  8 . 
         [0019]    The lower switch  14  is protected against a short circuit before the output current sensor  10  is fully operational by the lower current sensor and trip  12 . If the current is detected as being excessive then this will cause a trip signal to the controller circuit  11  which will then send a switch off signal to the gate drive which in turn will set the switch  14  to off. 
         [0020]    When the output current sensor  10  is operational if a current overload condition occurs then a signal is sent to the control circuit  11  which then responds by setting the switches  9 ,  14  to off by control signals to their respective drivers. 
         [0021]      FIGS. 2A and 2B  illustrate the difference between the prior art and the described embodiment. 
         [0022]    In  FIG. 2A , the prior art arrangement is shown which would utilize the components shown in broken outline. It will be seen that the upper two signals  20  and  21  to the switch drivers and hence the switches are on before the output sensor is functional as represented by the lower line  22 . 
         [0023]    In  FIG. 2B , the specific embodiment arrangement is shown and it will be seen that the upper switch control signal  23  remains off until the second pulse width modulated period this being after the output current sensor is operational as represented by the line  24 . The lower switch is activated at the start of the first pulse width modulated period but this is protected by the negative DC current sensor and trip  12  until the output current sensor is operational. 
         [0024]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his or her contribution to the art.