Abstract:
A DC/DC converter features a programmable controller in a feedback control loop thereof. The programmable controller enables easy changes to operating parameters by reprogramming the controller&#39;s application software. Additionally, the converter features a hardware-implemented fault protection circuit which shuts down the converter&#39;s switching transistor upon controller failure.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention generally pertains to voltage converters. More particularly, the invention concerns DC/DC voltage converters with fault protection.  
           [0002]    Vacuum fluorescent displays (VFD&#39;s) used, for example, in automotive applications commonly require an input voltage that is greater than the DC voltage available from the vehicle battery or charging system. Typical vehicles provide approximately 14 to 15 volts DC, while it is not uncommon for VFD&#39;s used in radio, instrument cluster or other display device applications to require greater than 50 volts in order to operate properly. To provide such an increased voltage over that available from the automotive system in which the display is operating, a DC to DC boost converter is used. Such a converter “boosts” the voltage delivered to the display, such as a VFD, to a voltage greater than that of the vehicle supply.  
           [0003]    Prior approaches to implementing DC to DC converters feature two basic methods of generating the required increased voltage. In a first prior approach, circuitry consisting of a switching regulator controller integrated circuit, also known as a DC to DC controller “IC” which generated a pulse width modulated (PWM) wave form was used to switch the input supply current through an inductor or transformer and to use the energy stored in the inductor or transformer to increase voltage delivered to the display to a level greater than the input supply voltage. Such prior DC to DC controller ICs also provided a feedback mechanism for monitoring the converter output voltage and adjusting the PWM waveform so as to regulate the voltage supplied to the converter output while maintaining proper operating voltages for which the circuit was designed. This conventional type of IC was specifically designed for DC to DC power supplies and typically included an oscillator, PWM generation circuitry, a feedback comparator circuit, and, in some cases, a switching transistor through which the inductor or transformer current was switched. This prior method operated at a fixed oscillation frequency set by discrete components and generated only a preselected output voltage. Any change in operating frequency or output voltage requirements dictated a change in the discrete components.  
           [0004]    In a second prior approach, circuitry consisting of all discrete components with no integrated circuits generated a PWM waveform. The circuitry additionally implemented appropriate oscillator and feedback apparatus to control the switching of current through an inductor or transformer in such a way as to use the energy stored in the inductor or transformer to increase the voltage supplied to the converter output to a level greater than that of the supply voltage. This discrete circuitry additionally regulated the converter output voltage in accordance with design intents. This prior approach also operates at a fixed oscillator frequency and fixed output voltage, with any change in parametric operation requiring a change in the actual discrete circuit components.  
           [0005]    Therefore, there is seen to be a need in the art for a DC/DC converter arranged in such a way that modifications to the desired operating characteristics can be carried out in a more facile manner.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, a voltage converter comprises a direct current input voltage source, a voltage transforming element having an input coupled to the input voltage source and an output presenting a voltage signal thereon, a switching element coupled to the voltage transforming element for intermittently interrupting current flow from the input voltage source to the voltage transforming element, a converter output coupled to the output of the voltage transforming element via a rectifier and adapted to present a converter output voltage to a load, and a programmable controller having an input coupled to the converter output and an output coupled to the switching element, the programmable controller operative to control switching states of the switching element in accordance with preselected programmable operating parameters of the voltage converter.  
           [0007]    In another aspect of the invention, a DC/DC voltage converter comprises a direct current input voltage source, a transformer having primary and secondary windings, the primary winding being coupled to the input voltage source, a switching transistor coupled to the primary winding and operative in a first switching state to allow current flow from the input voltage source through the primary winding and operative in a second switching state to inhibit said current flow, a converter output coupled to the transformer secondary winding via a rectification circuit and adapted to present a converter output voltage to a load, and a microprocessor-based controller having an input coupled to the rectification circuit and an output coupled to the switching transistor, the controller operative to selectively place the switching transistor in its first and second switching states in accordance with preselected, programmable operating parameters of the voltage converter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The objects and features of the invention will become apparent from a reading of a detailed description taken in conjunction with the drawing, in which:  
         [0009]    [0009]FIG. 1 is a block diagram of a converter arranged in accordance with the principles of the invention;  
         [0010]    [0010]FIG. 2 is a circuit schematic of the converter of FIG. 1; and  
         [0011]    [0011]FIG. 3 is a flow chart describing the programmable regulation routine of the converter output by the microprocessor-based controller arranged in accordance with the principles of the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]    With reference to FIG. 1, a microprocessor controlled DC to DC converter with fault protection especially useful with automotive VFD&#39;s requires no application specific integrated circuit such as used in some prior approaches and allows the modification of both the operating frequency and the output voltage by changing only the microprocessor&#39;s application program. Most prior approaches to DC/DC conversion require discrete component changes to make such adjustments. With the present invention, it is not necessary to change discrete components to adjust operating frequency or voltage output. The invention also features a hardware fault prevention circuit that protects the converter from destruction due to a microprocessor malfunction. Additionally, the programmable controller incorporates software fault prevention to protect against catastrophic failure due to short circuits at the converter load.  
         [0013]    With reference to FIG. 1, DC/DC converter  100  has an input coupled to an input direct current voltage source  122 . The converter input is additionally coupled via filter network  124  to input  141  of inductor or transformer  120 .  
         [0014]    Another input to inductor or transformer  120  is coupled to the collector of switching transistor  130  which has its emitter coupled to ground potential. A programmable controller, such as a microprocessor-based controller  102 , has a pulse width modulated output  140  coupled via resistor  142  to the base of switching transistor  130 . Situated in parallel with resistor  142  is a fault prevention circuit  128 .  
         [0015]    Microprocessor  102  includes an oscillator  104 , a pulse width modulation generator  106 , microprocessor code  108  and an analog to digital converter  110  which has an input for receipt of an analog signal at controller input  147 . Microprocessor  102  forms the basis of a feedback loop running from a rectified output of the inductor or transformer  120  via rectifying diode  136  and path  145  through a protection circuit  134  to input  147  of microprocessor controller  102 .  
         [0016]    A second output of the inductor or transformer  120  is coupled via a rectifying diode  138  to path  143  which presents a converter output V 01  at lead  144  via filtering network  126 . A second converter output voltage V 02  is presented at output  146  via filter network  132  which is coupled to path  145  at the output of rectifier  136 .  
         [0017]    With reference to FIG. 2, circuitry details of many of the blocks set forth in FIG. 1 are presented. Components in FIG. 2 identical to those in the block diagram of FIG. 1 bear identical reference numerals. As seen from FIG. 2, in the specific example presented, the transforming element  120  of FIG. 1 utilizes a transformer  120  having a primary coil  224  and a secondary coil  222 . Filter  124  of FIG. 1 is comprised, as shown in FIG. 2, of inductor  202  and capacitor  204 . Fault prevention circuit  128  of FIG. 1 is comprised of resistors  226 ,  228  and  232 , along with capacitor  230  and shunting transistor  200 .  
         [0018]    The microprocessor protection circuitry  134  of FIG. 1 is comprised as seen in FIG. 2 of Zener diode  238  and resistors  234  and  236 . Smoothing filter  126  of FIG. 1 is comprised of inductor  214 , and capacitors  216  and  218 . Finally, smoothing filter  132  of FIG. 1 is comprised of inductor  208  and capacitors  206 ,  210  and  212 .  
         [0019]    With further reference to FIG. 2, a first converter output voltage V 01  and a second converter output voltage V 02  are provided from the secondary winding of transformer  120  by utilizing a secondary coil tap  220  in conjunction with rectifying elements  136  and  138 . In the specific embodiment shown in FIG. 2, the V 02  output is a positive voltage that is greater than the voltage of power supply  122 , while converter output V 01  is a negative voltage output. Two such outputs of differing polarity may optionally be provided by converters designed in accordance with the invention for those applications where the VFD&#39;s of differing specification are used, some requiring a positive voltage and some requiring a negative drive voltage.  
         [0020]    With continued reference to FIGS. 1 and 2, the converter of the invention, operates as follows. When microprocessor  102  is powered up, a pre-programmed frequency and duty cycle for the pulse width modulator  106  is read from microprocessor code memory  108 . This results in microprocessor  102  outputting a pulse width modulated waveform at controller output  140  to a node formed by the junction of resistors  142  and  226 . Resistors  142  and  228  form a voltage divider to reduce the voltage output from the microprocessor  102  (typically 5 volts or 3.3 volts depending on the microprocessor design technology) to a proper level required for acceptable base drive to transistor  130 .  
         [0021]    Transistor  130  is a switching element which switches the current and transformer  120  primary  224  at a frequency and duty cycle matching the microprocessor&#39;s PWM output  140 .  
         [0022]    The input of transformer  120  is connected to the vehicle&#39;s power supply  122  (typically 8 volts to 16 volts DC) through filter components, inductor  202  and capacitor  204 .  
         [0023]    As the current through the primary  224  is switched by transistor  130 , voltage amplification occurs across the secondary  222  at an amplitude relative to the turns ratio of the transformer  120  and to the frequency and duty cycle of the PWM output signal on lead  140  from microprocessor  102 . The positive output voltage from transformer  120  is rectified by diode  136  and smoothed by the filter network consisting of capacitors  206 ,  210  and  212  and inductor  208  arranged as shown in FIG. 2. The converter output V o2  is fed, for example, or adapted to be coupled to, a VFD such as those requiring a positive DC voltage V o2  greater than the vehicle supply voltage  122 .  
         [0024]    If a negative DC supply voltage is required for the VFD in use or other converter load, as is the case for many automotive displays, such a negative output voltage V 01  is generated by grounding the secondary  222  of transformer  120  at an appropriate tap point  220 , such that the turns ratio gives the desired negative voltage. While possible, the generation of a negative voltage is not necessary for the operation of the invention. If the negative voltage V 01  is required, rectifier  138  provides rectification for the negative voltage and filter components comprising inductor  214  and capacitors  216  and  218  provide smoothing for the negative voltage output V 01 .  
         [0025]    The positive rectified output voltage V 02  from diode  136  also feeds a voltage divider formed by resistors  234  and  236  and is then fed to the analog to digital input  147  of the microprocessor  102  as the feedback voltage for the DC to DC converter. This feedback voltage varies linearly in proportion to changes in the converter&#39;s DC output voltage. Zener diode  238  is selected to provide a voltage clamp to insure that the input voltage to the microprocessor&#39;s analog to digital input  147  never exceeds a safe operating voltage limit of microprocessor  102 .  
         [0026]    The complete feedback loop for the converter  100  consists of resistors  234 ,  236 , diode  238  and analog to digital converter  110 . Control code of microprocessor  102  as stored in code memory  108  and the PWM output  140  of microprocessor  102  complete the feedback loop. Such utilization of a programmable controller such as microprocessor  102  in the DC to DC converter  100  feedback loop provides marked flexibility when compared to prior art approaches.  
         [0027]    The circuitry comprising transistor  200  along with resistors  226 ,  232  and capacitor  230  implements a hardware fault prevention circuit  128  (FIG. 1) which protects the converter  100  from a microprocessor malfunction. Without this circuit in place, if microprocessor  102  malfunctions and causes the PWM output  140  to be held in a constant “high” state, transistor  130  would be constantly conducting and providing a path to ground from the vehicle supply voltage through transformer  120 . Such constant conduction would eventually cause either transistor  130  or transformer  120  to be overstressed and to catastrophically fail. With the hardware fault protection circuitry  128  in place, if microprocessor PWM output  140  pulse rate starts decreasing below a rate determined by the time constant formed by resistor  226 ,  232  and capacitor  230 , then shunting transistor  200  will begin to conduct and will reduce the base current drive delivered to transistor  130 . This, in turn, will decrease the collector current of transistor  130  and will prevent failure of transistor  130  or transformer  120 .  
         [0028]    If the PWM output  140  happens to go to a constant “high” state, shunting transistor  200  would turn completely on and cause transistor  130  base current to drop to a level that would cause transistor  130  to go into cutoff or nonconduction from its collector to emitter circuitry. Hence, no current would flow in either transistor  130  or transformer  120  and both would be protected.  
         [0029]    The operation of the microprocessor control software held in code memory  108  is presented in the flow chart diagram in FIG. 3. With reference to FIG. 3, upon initial power application at block  300 , microprocessor  102  sets the PWM output  140  frequency to FHz, the operation frequency for which transformer  120  has been designed. Microprocessor  102  sets the PWM output  140  duty cycle to X %, an experimentally determined duty cycle that produces an output voltage close to the desired regulated output voltage for a given converter load.  
         [0030]    The feedback voltage applied to the analog to digital input  147  is derived from the converter output voltage and varies linearly with changes in the output voltage. After initialization, microprocessor  102  reads the feedback voltage on the analog to digital input  147  at block  302 .  
         [0031]    At decision block  304 , if the feedback voltage is lower than a minimum voltage threshold V MIN  it is an indication that the output of converter  100  is being loaded too heavily. The PWM output duty cycle is then reduced to 0% at block  306 , which reduces the converter output voltage to 0 volts, thereby preventing damage to transformer  120  or switching transistor  130 .  
         [0032]    If the feedback voltage is greater than a maximum voltage threshold, V MAX , it is an indication of either a duty cycle error or insufficient output loading at the converter output. The PWM output  147  is then reduced to 0% preventing damage to the VFD module or other devices being driven by the converter output V 01  or V 02 . The routine then returns to block  302 .  
         [0033]    If the feedback voltage is determined to be within the safe operation limits, V MIN  and V MAX  at block  304 , microprocessor  102  then determines if the feedback voltage is within the regulation limits, V LOW  and V HIGH  at decision blocks  308  and  314 , respectively. If the feedback voltage is within the regulation limits, no converter output voltage adjustment is necessary, and therefore, microprocessor  102  returns to the analog to digital input read for a new feedback voltage reading at block  302 .  
         [0034]    If the feedback voltage is less than the lower regulation limit, V LOW , it is an indication that the converter output voltage is too low. Microprocessor  102  then compares the PWM output duty cycle, X, to the maximum allowable duty cycle, X MAX  at decision block  310 . If the duty cycle percentage is less than X MAX , microprocessor  102  increases the PWM duty cycle by Y % at block  312 , causing a slight increase in the converter output voltage. The microprocessor  102  then returns to the analog to digital input read for a new feedback voltage reading at block  302 .  
         [0035]    If the duty cycle is greater than X MAX , it is an indication that the converter output current is being limited by the energy transfer characteristics of the transforming element  120 , and also indicates that increasing the duty cycle further will not produce a proportional increase in the converter output voltage. Therefore, the microprocessor  102  returns to the analog to digital input read at block  302  with no change to the PWM duty cycle.  
         [0036]    If the feedback voltage is greater than the upper regulation limit, V HIGH , as determined at decision block  314 , it is an indication that the converter output voltage is too high. Microprocessor  102  then compares the PWM output duty cycle, X, to the minimum allowable duty cycle, X MIN  at decision block  316 . If the duty cycle percentage is greater than X MIN , microprocessor  102  decreases the PWM duty cycle by Y % at block  318  causing a slight decrease in the converter output voltage. Microprocessor  102  then returns to the analog to digital read  302  for a new feedback voltage reading. If the duty cycle is less than X MIN , it is an indication that further reduction of the duty cycle will produce a 0% duty cycle, reducing the converter output voltage to 0. Microprocessor  102  therefore returns to the analog to digital input read function at block  302 .  
         [0037]    Hence, it is seen that the invention provides a marked advantage over the prior art in offering ease of design flexibility by simple changes to microprocessor control code. The converter output voltage may be disabled by turning off the pulse width modulated output of the programmable controller. The frequency of operation may be varied within the limits of the transformer design by changing the frequency variable, F. The regulated output voltage may be varied to cover a range of applications, such as different VFD&#39;s, by changing the initial duty cycle setting, X, and the regulation limits, V LOW  and V HIGH . The resolution of the output voltage correction may also be varied by changing the duty cycle increment/decrement variable, Y.  
         [0038]    The invention has been described with reference to a specific embodiment which is to be taken for the sake of example only. The scope and spirit of the invention is to be determined from a proper interpretation of the appended claims.