Abstract:
A power supply for an RF transmitting tube while protecting the tube from a high voltage arc event. The supply includes a rectifier circuit configured to be connected to an AC voltage supply and having an output circuit that supplies a DC voltage to a load including the tube and wherein the DC voltage may have a ripple voltage thereon. A control circuit is located intermediate the output circuit and the load for reducing any ripple voltage. The control circuit includes a series solid state switch that is connected in series between the rectifier circuit and the load. The control circuit also includes a reference capacitor coupled to the output circuit and to the load for supplying a reference voltage to the switch so that a voltage at the output circuit that exceeds the reference voltage will be clipped to reduce the voltage that is applied to the load.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention is directed to the art of RF broadcast transmission systems and, more particularly, to improvements in controlling the linearity performance of an inductive output tube (IOT). 
   2. Description of the Prior Art 
   It is known that an inductive output tube (IOT) has particular application for use in television broadcasting wherein high kilowatt level RF power is required. An example of such an IOT is found in the U.S. Pat. No. 6,232,721 to D. Danielsons assigned to the same assignee as the invention herein and the disclosure of which is herein incorporated by reference. 
   Inductive Output Tubes or IOT, as they are commonly called, are high vacuum electron tubes, which allow an electron beam to travel from one end to another in a controlled way. There are four primary parts to an IOT: a cathode which emits electrons, an anode which accelerates the electrons, a collector which collects the electrons, and a grid for controlling the electron emission. The electrons are emitted from a spherical surface cathode consisting of a tungsten matrix heated from behind by a tungsten heater. A spherical pyrolytic carbon grid is positioned close to the cathode and controls the emissions of electrons from the cathode. The cathode is maintained at a relatively high potential (−35,000 volts for typical tubes) while the grid is at a relatively low potential (−50 to −250 volts for typical tubes) with respect to the cathode. If the grid is made less negative with respect to the cathode, then more electrons are emitted. The high electric field between the cathode and anode makes the emitted electrons travel toward the anode or collector. A magnetic field is used to focus the electrons into a beam. Emitted electrons are collected in the collector completing the circuit. 
   The Inductive Output Tube is used primarily as a high power UHF amplifier. One primary use is in UHF television transmitters operating in the frequency range of 470 MHz to 860 MHz. It is used both for analog television and digital television transmissions. In order to obtain good efficiency, the IOT is operated in a class A/B mode of operation. Due to the class A/B mode of operation, the amplifier draws current which is proportional to the modulation frequencies of the RF signal applied. For analog and digital television signals, these modulation frequencies cover the range of DC through 8 MHz and are commonly called video current. 
   In the construction of an IOT, the pyrolytic grid is extremely fragile. Due to the high acceleration voltages used, it is possible for the tube to arc from grid to anode. If an arc occurs, the high tension supply may destroy the grid. To overcome this problem, a crowbar or other current limiting device is placed between the IOT and the high tension supply. If an arc occurs, the crowbar directs the high tension supply current away from the IOT preventing the delicate grid from being damaged. Common crowbars use either a gas filled thyratron or a triggered spark gap. These crowbars use a controlled arc to divert the current from the high tension supply away from the IOT. Since the undesired arc in the IOT and the controlled arc in the crowbar have the same impedance, a series resistor must be placed between the crowbar and the IOT, thus forcing the high tension current through the crowbar and away from the IOT. 
   Reference is now made to  FIG. 1  that illustrates a prior art system employing a low pass filter in conjunction with a circuit for controlling an inductive output tube (IOT). The IOT and associated circuits is located in the block that is labeled “L”. The IOT employs an internal grid structure that is very fragile, as noted hereinbefore, and hence, the tube is easily damaged internally if exposed to high voltage arcing. It has been recommended that a transmitter employing an IOT should be able to protect it from damage in the event of such an internal arc. That is, the beam power must be removed from the IOT to limit the energy dissipated within it to less than about 20 joules. This may be achieved by employing the crow bar circuit (CB) shown in  FIG. 1 . 
   It has been proposed to employ a fast disconnect system that would be capable of protecting a 300 mm length of thin copper wire having a diameter on the order of 0.1143 to 0.127 mm at voltages above 5 kV. Calculations will show that the energy (I 2 T) to fuse the wire is approximately 13.6 joules, and this is lower than the 20 joules specified in the statement noted above. 
   The circuit illustrated in  FIG. 1  includes an AC line input voltage supply  10  that supplies an AC voltage to a transformer and rectifier circuit A 2  by way of a switch A 1 . The transformer and rectifier circuit A 2  provides a six pulse rectified supply. The ripple voltage may be on the order of −27.5 dB. If the ripple voltage is to be held to a level less than −60 dB, then an additional −32.5 dB of ripple attenuation is needed from the low pass filter F 1  which is comprised of inductor L 1  and resistor R 1  in series with capacitor C 1  to ground. This filter is somewhat bulky and stores a significant amount of energy. For example, a typical value for the capacitor C 1  is on the order of 8 uF. This capacitor alone at 36 kV has a storage energy on the order of 5,200 joules which is significantly more than what the load device can handle during an internal arcing event. 
   The filter F 1  uses a low pass filter to remove the AC hum and ripple frequency of the linear power supply provided by the transformer and rectifier circuit A 2 . The lower bandwidth filter provides more ripple rejection and, in turn, more stored energy. 
   The crowbar circuit CB is a high speed switch that connects the high voltage terminal to ground potential in the event of an internal arc in the load device L. The crowbar circuit is triggered and shorts the high voltage power supply to ground potential. This allows the energy to be dissipated in the circuit path of the crow bar circuit CB instead of the inductive output tube (IOT) load L. While the crowbar circuit is enabled, the input switch A 1  is switched off to remove AC power to the transformer and rectifier A 2 . 
   Reference is now made to  FIG. 2  which illustrates another prior art circuit for controlling and protecting an IOT located in a load L. Since  FIGS. 1 and 2  are very similar, like components in both figures are identified with like character references and only the difference are described below in detail.  FIG. 2  includes smaller output filter network which includes inductor L 1 , capacitor C 1  and a resistor R 2 . The energy stored in this filter is optimized so as not to exceed the manufactured recommended level before internal damage to the IOT can take place. This filter has lower energy storage than that in  FIG. 1 . Because  FIG. 2  employs a small, low pass filter, the crow bar circuit of  FIG. 1  is removed. 
   A disadvantage of using a low storage filter configuration as shown in  FIG. 2  is that more ripple voltage is evident and performance of the system will be compromised by the residue ripple on the power supply. The low pass filter system is sufficient for a DTV (digital television) signal. The DTV signal has a lower signal to noise (SNR) ripple requirement from the high voltage power supply. This is good for the DTV signal because the residue ripple is not adequate for the traditional analog television transmitter, wherein the analog television transmitter needs a ripple voltage around 60 dB. 
   In the circuit of  FIG. 2 , the series resistor R 2  provides a current limiting function. If a short circuit is detected by a protection circuit PC, a trigger signal is sent to the switch A 1  to open the switch and remove the supply  10 . 
   It is to be noted that in the prior art of  FIGS. 1 and 2 , low pass filter F 1  or F 2  is employed. These are relatively large and expensive components. The present invention contemplates achieving control of an IOT in the load  12  without employing such low pass filters. The control is directed toward minimizing the AC ripple voltage by obtaining a smooth DC voltage and without employing a large low pass filter. FIGS. 1 and 2 of the prior art are shown as FIGS. 1 and 2 in the U.S. patent to A. B. See et al. 6,724,153. 
   SUMMARY OF THE INVENTION 
   The present invention contemplates the provision of an IOT amplifier system that receives an RF input signal and provides an amplified RF output signal therefrom. In accordance with the present invention, there is provided a power supply for an RF transmitting tube while protecting the tube from a high voltage arc event. The supply includes a rectifier circuit that is configured to be connected to an AC voltage supply and has an output circuit that supplies a DC voltage to a load including such a tube and wherein the AC voltage supply may have a ripple voltage thereon. A control circuit is located intermediate the output circuit and the load for purposes of reducing any ripple voltage thereon. This control circuit includes a series solid state switch that is connected in series between the rectifier circuit and the load. The control circuit also includes a reference capacitor that is coupled between the output circuit and the load for purposes of supplying a reference voltage to the switch. Whenever the voltage from the output circuit of the supply is greater than the reference voltage then this is clipped so as to reduce that which is supplied to the load. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages of the invention will become more apparent to one skilled in the art to which the present invention relates upon consideration of the following description of the invention with reference to the accompanying drawings, wherein: 
       FIG. 1  is a schematic-block diagram of prior art useful in explaining the background of the present invention; 
       FIG. 2  is also a schematic-block diagram illustration of another prior art useful in explaining the background of the present invention; 
       FIG. 3  is a schematic-block diagram illustration of an embodiment of the invention herein; 
       FIG. 4  is a schematic-block diagram illustration of a second embodiment of the invention herein; 
       FIG. 5  is a schematic-block diagram illustration of another embodiment of the invention herein; 
       FIG. 6  is a schematic-block diagram illustration of a still further embodiment of the invention herein; 
       FIG. 7  is a schematic-block diagram illustration of another embodiment of the invention herein; 
       FIG. 8  is a schematic-block diagram illustration of a still further embodiment of the invention herein; 
       FIG. 9  is a schematic illustration of another embodiment of the invention herein; and 
       FIG. 10  is a schematic illustration showing ripple voltage with reference to time is useful in explaining the invention herein. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference is now made to  FIG. 3  which illustrates one embodiment of the present invention. It is to be noted that like components in  FIG. 3  with respect to  FIGS. 1 and 2  are identified with like character references to facilitate the description herein. 
   It is to be noted from an inspection of  FIG. 3  that the transformer and rectifier circuit A 2  is not provided in its output circuit with a low pass filter as in the case of filters F 1  and F 2  in  FIGS. 1 and 2 . It is also to be noted that this circuit does not employ a crow bar circuit. Instead, this embodiment of the invention employs an active series switch A 3  located in the output circuit of the transformer and rectifier circuit A 2  for purposes of removing the inherent ripple on a simple rectified linear power supply provided by the AC input supply source  10  in conjunction with the rectifier circuit A 2 . A typical six pulse rectified AC power supply as provided herein has an inherent ripple on the order of −27.5 dB and a 12 pulse rectified power supply has a typical ripple on the order of −39 dB. A low ripple high voltage power supply is obtained herein by clipping off the ripple voltage riding on top of the DC power supply with the use of a peak detector which includes diode CR 1  and capacitor C 10 . 
   The purpose of the series solid state switch A 3  is to reference minimum voltage of the input. If the input voltage is higher than the minimum reference, the output is maintained constant and the ripple voltage will be impeded by the switch. Therefore, the output will be a smooth DC voltage and the AC ripple voltage of less than −60 dB is achievable without using a large, low pass filter. This high voltage power supply (HVPS) can be used for both DTV and traditional analog television transmitters. 
   The diode CR 1  and the capacitor C 10  form a peak detector which also provides the needed bias current for diode CR 2  and capacitor C 20 . The output ripple is controlled by the low pass filter set by resistor R 10  and capacitor C 20 . If the time constant of this filter is longer, the output ripple voltage will be smaller. 
   As in the prior art, the unfiltered DC power supply includes a transformer and rectifier circuit A 2  that produces a raw DC voltage with a ripple frequency at a multiple of the AC line frequency. This may be better appreciated by reference to the depiction of this in  FIG. 10  which illustrates ripple voltage with respect to time. This ripple voltage needs to be reduced to a level that is suitable to drive the load L. The prior art technique has been to smooth out this ripple by using a low pass filter, such as filters F 1  and F 2  in  FIGS. 1 and 2 . The function of the inductor and capacitor is to provide a low pass filter that limits the ripple component going to the load L. A low band width will achieve a better ripple reduction. 
   A switch A 1  serves as a disconnect switch and is sufficiently fast to disconnect and interrupt the input AC supply to the transformer and rectifier circuit A 2 . The required speed is determined by the circuit components and the total energy to the load device. 
   The solid state switch A 3  is a series solid state switch that consists of a single transistor or a series of transistors for higher breakdown voltage. Standard transient protection circuits are used to protect each transistor in the event of any transient condition. 
   A monitor and protection circuit A 4  detects any short circuit in the load L and generates a turn-off command to the input switch A 1 . The disconnect of the input switch is quick in order to interrupt the input AC voltage to make sure that the total energy does not exceed 13.6 joules, the lower of the two limits. 
   The transformer employed in the transformer and rectifier circuit A 2  has a typical series loss of approximately 3%. Thus, a short circuit current is increased by 33 times. If the normal operation of the load device current is 3 amps at 36 Kv, then the short circuit current will be on the order 100 A. The source resistance (R t ) of the transformer is on the order of 360 ohms. If the resistor R 20  is greater than 0 ohm, then the short circuit current will be less. The short circuit current I S  is defined as 
             I   s     =       V       R   t     +     R   20         .           
The series resistor R 20  adds dissipation and reduces the overall efficiency of the circuit. The benefit of resistor R 20  is to lower the short circuit current and provide a longer time before the interruption to the AC input voltage is needed. The relationship of energy (E) and time (t) is simply defined as:
 
           t   =       E     I   s   2       .           
In this formula, E represents the total energy delivered. For example, assuming the loss in the transformer is 3% at 3 A and 36 KV, and that the resistor R 20 =0. The short circuit current is 100 A. Consequently, t (time) is equal to 0.00136 seconds. This is the time needed to interrupt the input AC voltage before the total energy exceeds 13.6 joules.
 
   In another example, assume all the conditions as described above, except that resistor R 20  is equal to 150 ohms. The short circuit current is 70.6 A. Consequently, t=0.0027 seconds. The time is doubled by adding a resistor in series. 
   With reference to  FIG. 10 , it is to be noted that the ripple voltage is reduced by the series solid state switch without using a large low pass filter. The output ripple level is set by the bias reference voltage as determined by resistor R 10  and capacitor C 10 . 
   Reference is now made to  FIG. 4  which illustrates another embodiment of the invention. This embodiment is similar to that of  FIG. 3 , and consequently, like components are identified with like character references and only the changes are described below. It will be noted that  FIG. 4  illustrates the basic circuit configuration with the series resistor R 20  of  FIG. 3  removed. Consequently, the short circuit current is slightly higher, and a faster interruption to the AC input supply should be made. 
   Reference is now made to  FIG. 5  which shows another embodiment of the invention which is similar to that as illustrated in  FIGS. 3 and 4 , and consequently, like components are identified with like character references and only the differences will be described below. It is to be noted in  FIG. 5  that no resistor R 20  is provided. However, the embodiment of  FIG. 5  employs an inductor L 1  in the output circuit of the transformer and rectifier circuit A 2 . The purpose of this inductor is to provide a slower rate of change of the short circuit current to the load L. The benefit of using a series inductor is that under a short circuit condition, the short circuit current is not changing instantaneously. This allows the monitor and protection circuit A 4  additional time to trigger the switch A 1  and interrupt the AC input supply. 
   Reference is now made to  FIG. 6  which illustrates a circuit configuration similar to that as described hereinabove with reference to  FIGS. 3-5  and wherein like components are identified with like character references and only the differences are be described herein with reference to  FIG. 6 . As shown in  FIG. 6 , the resistor R 10 , diode CR 2  and inductor L 1  have been removed. A biasing power supply A 5  has been added to provide a negative bias voltage reference to the voltage and the capacitor C 10 . Diode CR 1  and capacitor C 10  form a positive peak detector with reference to the peak of the ripple voltage. Voltage at capacitor C 20  is equal to the peak detected voltage on capacitor C 10  minus the biasing power supply. Consequently, the output is equal to the reference voltage on capacitor C 20 . 
   Reference is now made to  FIG. 7  which illustrates an embodiment similar to that of  FIG. 5 , with the exception that capacitor C 10  has been removed and that resistor R 20  has been added. 
   Reference is now made to  FIG. 8 , which illustrates an embodiment similar to that of  FIG. 7  with like components being identified with like character references and only the differences being described below. In this figure it will be noted that capacitor C 10  has been added and that a high frequency decoupling capacitor C 3  has been added. The value of capacitor C 3  is small and only filters higher frequency ripples and not for purposes of reducing AC from noise. The energy in capacitor C 10  is negligible in comparison to the total energy of the short circuit event. 
   Reference is now made to  FIG. 9  which illustrates an embodiment similar to that of  FIG. 5  with like components being identified with like character references and only the differences being described below. This embodiment includes a MOV transient suppression device X 1  located in parallel with inductor L 1 . Under normal operating conditions, the breakdown voltage of the MOV is much higher than the ripple voltage across the inductor L 1 . Therefore, X 1  is not affecting the normal function of the circuit. Under a short circuit condition at the load device, the voltage across inductor L 1  is limited by the breakdown voltage of X 1  and the majority of the current flow will be going through X 1 . Since X 1  is a transient suppression device and is also a dissipative device, this will offer two benefits to the circuit, including limiting the short current and dissipating energy while current is flowing through X 1  during a short circuit condition. 
   Although the foregoing has been described in conjunction with the preferred embodiment, it is to be appreciated that various modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.