Abstract:
A method for controlling a power supply, the method comprising: forming a front-end to the power supply, the front-end comprising a plurality of digital-to-analog circuits under control of a controller; and programming the controller to control the digital-to-analog circuits to send a user-defined output reference voltage and a user-defined over voltage protection reference voltage to the power supply.

Description:
FIELD OF THE INVENTION 
   This invention relates to programmable power supplies. In particular, the invention relates to a method and apparatus for controlling a programmable power supply. 
   BACKGROUND 
   Programmable power supplies refer to a class of power supplies whose output can be controlled or programmed via a control mechanism. Typically, the control mechanism comprises a controller which is operatively connected to the power supply via a general purpose interface bus (GPIB) or an analog input control port. The GPIB carries control signals from the controller to the programmable power supply. The control signals include voltage set point changes to control an output of the programmable power supply. 
   Sometimes there can be a delay of up to 400 milliseconds from transmission of a control signal over the GPIB to when a programmable power supply actually changes its output voltage in response to the control signal. This delay may be too long as there may be uses for programmable power supplies in which a faster change in the output voltage of the power supply is required. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of an apparatus for controlling a programmable power supply, in accordance with one embodiment of the invention; 
       FIG. 2  shows a block diagram of another embodiment of the apparatus of  FIG. 1 ; and 
       FIG. 3  shows a block diagram of a system in accordance with one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. In other instances, structures and devices are shown in block diagram form in order to avoid obscuring the invention. 
   Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments. 
   Referring to  FIG. 1  of the drawings, reference numeral  100  generally indicates an apparatus in the form of a control unit to control a programmable power supply  102 . The control unit  100  includes a microcontroller  104  which is connected to digital-to-analog (DAC) circuits  106 ,  108 , and  110 . The microcontroller  104  is also connected to an analog-to-digital circuit (ADC)  112 . 
   The digital-to-analog circuit  106  sends an output reference voltage signal  114  to the power supply  102 . The digital-to-analog circuit  108  sends an over voltage protection (OVP) reference voltage signal  116  to the programmable power supply  102 . The digital-to-analog circuit  110  sends an over current protection (OCP) reference current signal  118  to the programmable power supply  102 . The analog-to-digital circuit  112  receives an output voltage feedback signal  120  from the programmable power supply  102 , converts the signal to a digital signal and sends the signal to the microcontroller  104 . 
   In operation, the programmable power supply  102  uses the output reference voltage signal  114  and outputs a voltage signal  122  which has a voltage that corresponds to the output reference voltage signal  114 . Typically, the voltage output signal  122  is sent to a device which requires power, for example a device under test. 
   The function of the digital-to-analog circuit  106  is to convert a digital value for an output reference voltage into the analog output reference voltage signal  114 . The digital-to-analog circuit  106  receives the digital value for the output reference voltage from the microcontroller  104 . In particular, the microcontroller  104  includes a register to store a digital value corresponding to the output reference voltage. 
   The control unit  100  also includes a first interface  124  which defines a mechanism to input data into the microcontroller  104 . In particular, the interface  124  is used to program the register of the microcontroller  104  with the digital value for the output reference voltage. In one embodiment, the first interface  124  may be an RS232 interface which is configured to receive RS232 input signals from a master controller such as a computer terminal. 
   In one embodiment, the microcontroller  104  includes a plurality of registers, each to store a different value for the output reference voltage. Each of these registers is programmable via the first interface  124 . 
   The control unit  100  also includes a second interface  126  which defines a mechanism to input digital control signals to the microcontroller  104 . Typically, the digital control signal is sent from a control terminal or personal computer (pc) (not shown). In response to the digital control signal input into the microcontroller via interface  126 , the microcontroller sends the digital values stored within a register indicated by the control signal to the digital-to-analog circuit  106  for conversion to the analog output reference voltage signal  114 . In one embodiment, the microcontroller  104  sends the lowest value for the reference output voltage stored in one of its registers to the digital-to-analog circuit  106 . Thereafter, upon each receipt of a digital control signal via interface  126 , the microcontroller selects a next highest value for the output reference voltage from one of its registers and sends this value to the digital-to-analog circuit  106 . It will be appreciated, that with this embodiment, the programmable power supply  102  can be made to output progressively different values for the voltage output signal  122 . 
   Using the interface  124 , the microcontroller  104  may be programmed with a value for an Over Voltage Protection (OVP) reference voltage. Typically, this value is stored in one of the registers within the microcontroller  104 . In use, the microcontroller  104  sends the value for the over voltage protection reference voltage to the digital-to-analog circuit  108  which converts the value to the analog over voltage protection reference voltage signal  116 . The programmable power supply  102  ensures that the output voltage output signal  122  does not exceed the OVP reference voltage signal  116 . 
   One of the registers within the microcontroller  104  may be programmed with a value for an over current protection OCP reference voltage via the first interface  124 . In use, the microcontroller  104  sends the value for the OCP reference voltage to the digital-to-analog circuit  110  which converts it to the OCP reference voltage signal  118 . The programmable power supply  102  uses the OCP reference voltage signal  118  as a benchmark to limit the current output by the power supply  102 . 
   The output voltage feedback signal  120  which is converted to a digital value by the analog-to-digital circuit  112  and fed to the microcontroller  104  as described above, is used by the microcontroller  104  to determine whether the output reference voltage signal  114  is to be increased to compensate for transmission losses. For example, if the microcontroller  104  determines that the digital value corresponding to the output voltage feedback signal  120  is less than the digital value corresponding to the current output reference voltage signal  114 , then the microcontroller  104  increases the output reference voltage signal  114  to compensate for transmission losses in the output reference voltage signal  114 . The net effect of this compensation is that the programmable power supply  102  outputs a voltage signal  122  which corresponds to the digital value for the output reference voltage signal  114  that is currently being output by the digital-to-analog circuit  106 . 
   The components of control unit  100  downstream of the microcontroller  104  are referenced to a ground for the power supply  102  and can derive power from the power supply side. Interfaces  124  and  126  are referenced to a ground for microcontroller  104  and derives power from the master controller side. 
   In one embodiment, the digital-to-analog circuit  106  provides a 0–5 volt output. 
   In one embodiment, the control unit  100  comprises a single circuit board embedded within a protective enclosure. The control unit  100  may be mounted directly to the programmable power supply  102 . In one embodiment, the overall dimensions of the control unit  100  may be approximately 5″×3″×0.5″. 
   Referring now to  FIG. 2  of the drawings, reference numeral  200  shows a high-level block diagram of another embodiment of a control unit for controlling a power supply. The control unit  200  is similar to the control unit  100 , and accordingly the same reference numbers have been used, wherever possible. One difference between the control unit  200  and the control unit  100  is that the control unit  200  includes isolation barriers  202  and  204  disposed between the first interface  124  and the microcontroller  104 , and the second interface  126  and the microcontroller  104 , respectively. The function of the isolation barriers  202  and  204  is to isolate the digital signals received from the interfaces  124 , and  126 , respectively from the microcontroller  104 . This ensures that the microcontroller  104  receives a clean signal with minimal interference. In one embodiment, the isolation barriers  202 , and  204  may operate to optically isolate the digital signals received from the interfaces  124 , and  126 , respectively, from the microcontroller  104 . 
   In one embodiment, the controllers  100  and  200  of  FIGS. 1 and 2 , respectively may be connected to an input or control unit  300 , as can be seen in  FIG. 3  of the drawings. Typically, the input unit  300  may be a personal computer (pc) which provides a mechanism to program the microcontroller  104  via the interfaces  124 , and  126 , as described above. Thus, the input unit  300 , and the controllers  100 , and  200  form a system, in accordance with one embodiment of the invention. Also shown in  FIG. 3  of the drawings, is a device under test  302  which is representative of a device which is required to be powered by the programmable power supply  102 . 
   As will be seen from the above description, the control unit  100 , and  200  form a front-end to the power supply  102 , which front-end comprises a number of digital-to-analog circuits under control of the microcontroller  104 . Further, as described above, the microcontroller  104  may be programmed via the input unit  300  to send a user-defined output reference voltage and a user-defined over voltage protection reference voltage to the power supply  102 . 
   As noted above, with GPIB busses, latencies of up to 400 milliseconds from a voltage change command to when the voltage output change occurs may be observed. In one embodiment, the control units  100 , and  200  reduce these latencies to less than 7 milliseconds. 
   Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that the various modification and changes can be made to these embodiments without departing from the broader spirit of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than in a restrictive sense.