Abstract:
The present invention provides a radiation-tolerant-solid-state-relay without radiation-hardened parts. A p-channel MOSFET provides power-switching functionality. In further detail, the solid-state-relay comprises a bias section, a control section, and a power-switch section. The bias section provides a voltage bias to the control section, the control section provides a control voltage to the power switch section as a function of the voltage bias, and the power switch section provides a switching voltage to the P-channel MOSFET as a function of the control voltage.

Description:
FIELD OF THE INVENTION 
   The present invention deals with electrical components and more specifically, a radiation-tolerant solid-state relay without radiation-hardened parts. 
   BACKGROUND OF THE INVENTION 
   Solid-state relays perform functions similar to electromagnetic relays, but are more reliable, since there are no moving parts. Since the turn on and turn off times of a solid-state relay are controllable, the solid-state relay also minimizes the generation of switching transients. 
   A preferred semiconductor device for power control in a solid-state relay is the insulated gate FET (Field Effect Transistor) because of its high power gain. FETs used for power switching use are usually enhancement mode types. This means that they are normally non-conducting. When a gate voltage above a threshold is applied, the FET becomes conducting. FETs are available in two gate polarities; N channel and P channel. 
   In an FET, current flows along a semiconductor path called the channel. At one end of the channel, there is a source electrode, and at the other end, a drain electrode. The physical diameter of the channel is fixed, but its effective electrical diameter is changed by applying voltage to a gate electrode. The conductivity of the FET depends, at any given time, on the electrical diameter of the channel. A small change in gate voltage can cause a large variation in current from the source to the drain. In this way, the FET switches current on or off. 
   Typically, FETs used for power switching are enhancement mode types, that is, they are normally non-conducting. When a gate voltage above a certain threshold is applied, the FET becomes conducting. Such FETs are used to control current flow and are available in two gate polarities; N channel and P channel. 
   Among many applications, solid-state relays are used in spacecraft, satellites and in high energy physics instrumentation where they are subjected to many forms of radiation damage. When electrical components are exposed to radiation, they behave differently. For example, when an N channel FET is exposed to relatively low radiation levels, the gate threshold voltage ultimately falls close to zero. In this condition, the FET conducts current with little or no applied gate voltage. In other words, the FET is uncontrollable because the current running through the channel cannot be shut off. 
   Solid-state relays designed for general purpose use are typically constructed with N channel FETs because, for any given die size transistor, the N channel FET has a lower on resistance than a correspondingly sized P channel FET. 
   To use electrical components in high radiation environments, they are radiation-hardened to withstand the damage caused by radiation. The radiation hardening process usually involves removing or adding some specific element or ions to the materials used for making the components. Being radiation hardened, the gate threshold voltage experiences minimal change after exposure to radiation. One method for chemically radiation hardening electronic components is disclosed in U.S. Pat. No. 3,836,836 to Cowett, Jr. (Cowett). 
   Radiation hardened components, however, have limited sources, are expensive and take a long time to produce, creating higher prices and longer delivery times for the radiation tolerant circuits that incorporate the hardened materials. It is desireable, therefore, to provide electrical components with ordinary (non-hardened) materials that can function when exposed to radiation. 
   SUMMARY OF THE INVENTION 
   The present invention provides a radiation-tolerant-solid-state-relay without radiation-hardened parts. A p-channel MOSFET provides power-switching functionality. In further detail, the solid-state-relay comprises a bias section, a control section, and a power-switch section. The bias section provides a voltage bias to the control section, the control section provides a control voltage to the power switch section as a function of the voltage bias, and the power switch section provides a switching voltage to the P-channel MOSFET as a function of the control voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE is a circuit diagram of a solid-state relay in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   In accordance with a preferred embodiment of the present invention, the gate drive signal should be high enough to saturate the drain to source channel. It should not, however, be so high that the gate to source breakdown voltage rating of the FET is exceeded. Preferably, the FET operates close to its maximum gate voltage signal. Higher signals can handle higher radiation levels, allowing the FET to function across a larger range of radiation exposure. 
   The FIGURE is a schematic diagram of a radiation-tolerant solid-state relay generally indicated by reference numeral  10  in accordance with a preferred embodiment of the present invention. This circuit example operates from a voltage source  102  to 50 VDC, but nominally 28 VDC. It may, however, be readily scaled for different bus voltages. 
   The circuit  10  includes three galvanically isolated sections, the bias section  200 , the control section  300  and the power switching section  100 . 
   The bias section  200  provides transformer isolated power, or a voltage bias, to the other two sections. Input voltage  102 , in the range of 5 to 35 VDC, is applied to the collector of NPN bipolar-emitter-follower transistor  31 , as well as a constant-current diode  21 , thus setting an operation point for programmable-shunt-regulator IC  61 . The base of transistor  31  is connected to the other end of the constant-current diode  21  as well as to the cathode of the regulator IC  61 . 
   A first regulating resistor  41  and second regulating resistor  42  provide a voltage divider across the reference terminal of the regulator IC  61 . The conduction of the regulator IC  61  seeks to maintain a nominal 2.5 VDC level. Therefore, the base voltage of the transistor  31  is controlled by the IC  61  so that the emitter voltage of the transistor  31  stabilizes at a voltage of approximately 4.6 VDC. 
   As the bias voltage varies over a range of 5 VDC to 35 VDC, the voltage on the transistor&#39;s  31  emitter is relatively stable at 4.6 VDC. This voltage is decoupled by an internal-bias-power-supply-filter capacitor  11 . 
   A quad comparator IC  62  has four comparator sections, but only three of its four sections are used. In this particular example, the first comparator section  62 A is connected as an astable multivibrator. The non-inverting terminal of the first comparator  62 A is connected to a resistor network with two equal value resistors, a first  43  and second  44  center-tapping resistor that center-tap the bias voltage applied to the first comparator  62 A. and a positive-feedback-resistor  45 . The first  43  and second  44  center-tapping resistors and the positive-feedback resistor  45  each preferably have the same resistance value. 
   The output of the first comparator  62 A is connected to one end of the resistor  45 . A collector-pull-up resistor  47  is included because the output of  62 A is an open collector configuration. Osciillator-frequency-timing resistor  46  is connected between the output of the first comparator  62 A and an internal-bias-power-supply-timing capacitor  14 . The timing capacitor  14  is also connected to the inverting input of the first section  62 A. 
   In operation, the voltage on the timing capacitor  14  is always “chasing” the voltage derived by the first  43  and second  44  center-tapping resistor  43 , to center-tap the bias voltage applied to the first comparator  62 A and the feedback resistor  45 . When the two voltages are equal, the comparator  62 A changes state. In the present example, the circuit oscillates at approximately 200 kHz. The duty cycle of the oscillator is between 20–70%, preferably between 30–60% and most preferably about 50%. 
   The inputs of the second  62 B and third  62 C comparators are connected in parallel with the input of the first comparator  62 A. In this particular example, the input pins of the second comparator  62 B and third comparator  62 C are connected in opposite polarity to each other, making their outputs out of phase, switching at about 200 kHz. 
   The comparator outputs drive the center tapped primary of a power-switch-section-voltage-isolating-bias transformer  51  (pins  511 ,  512  and  513 ). Spike-filtering capacitor  15  limits the spike voltage on the output of the second  62 B and third  62 C comparator sections. 
   The waveform across the primary winding (pins  511  and  513 ) of the transformer  51  is an approximate 200 kHz square wave with an amplitude of approximately 8 volts peak to peak. 
   The secondary winding of the transformer  51  appears at pins  514  and  515 . The amplitude of the secondary winding voltage is approximately 42 volts peak to peak. Pin  515  of the transformer of  51  is referenced to the power switching ground. 
   The control section  300  provides a control voltage to the power switching section  100  as a function of the voltage bias provided by the bias section  200 . 
   The primary winding of the control transformer  52  that isolates the control pins, bias supply and power switch section is connected across the primary winding of the bias transformer  51  through current-limiting resistor  413 . The secondary winding of the transformer  52  (pins  525  and  526 ) is connected to a diode bridge of four diodes  27 ,  28 ,  29  and  210 . The open circuit voltage of the diode bridge is filtered by a spike-filtering capacitor  17 . 
   The second winding of the control transformer  52  (pins  523  and  524 ) is connected to the emitter of an NPN-bipolar-control transistor  34 . A reverse-voltage-limiting diode  26  limits the reverse voltage applied to the transistor&#39;s  34  base emitter junction. 
   Pin  524  of the control transformer  52  is referenced to the power switch ground. Pins  521  and  522  are referenced to the bias ground. Pins  525  and  526  are floating, not galvanically connected to either ground. Therefore, the control transformer  52  is a magnetically coupled device that allows isolation between the three isolated ground sections. 
   In operation, when the external control pins are open circuit, current from the control transformer  52  pins  523  and  524  cause the control transistor  34  to conduct on each half cycle. When the external control pins are shorted, current flowing into the transistor&#39;s  34  emitter is reduced. This reflected action controls the power switching stage. 
   The power switching section  100  provides a switching voltage to a p-channel MOSFET  35  as a function of the switching voltage from the control section  300 . 
   Power to operate the power switching section is derived from the secondary winding of the transformer  51 , pins  514  and  515 . The voltage at pin  514  is rectified by a first  22  and second  23  peak-rectifier diode that establishes a bias voltage for the power switch section, filtered by a first  12  and second  13  power-switch-section-peak-filtering capacitor. The resultant voltage across the first and second power-switch-section-peak-filtering capacitor  12  and  13  is approximately +21VDC and −21 VDC respectively. 
   The active devices of the power switching section consists of a first  32 , second  33 , and third  34  PNP-bipolar-common-base-amplifier transistor and a P-Channel enhancement MOSFET  35 . 
   The first-common-base-amplifier transistor  32  is a common base amplifier that provides a non-inverting stage of voltage gain. The collector load resistor for the transistor  32  is a collector-pull-up resistor  48 . The base of the transistor  32  is connected to the power switching ground. 
   A reverse-voltage-limiting diode  25  limits the reverse voltage across the base emitter terminal of the transistor  32 . A gate-to-source-voltage-limiting zener diode  24  for the MOSFET  35  limits the collector voltage of the transistor  32 . In turn, the maximum gate voltage applied to the MOSFET  35  is also limited. 
   A gate-drive-voltage-buffer transistor  33  operates an emitter follower, lowering the impedance of the signal on the collector of the transistor  32 . 
   A gate-to-source filter resistor  410  for transistor  35  is connected across the gate-source terminals of the P-Channel enhancement MOSFET  35  to lower gate impedance. 
   The P-Channel enhancement MOSFET  35  acts as an output switch transistor controlled by the action of PNP-bipolar transistor  34 . When bias power is applied, the oscillator consisting of the first  62 A, second  62 B, and third  62 C comparator sections and related components, starts. The resultant AC waveform is transmitted through the control transformer  52 . Current-limiting resistor  414  and turn-on-bias-resistor  412  attenuate the AC voltage. Reverse-voltage-limiting diode  26  limits the reverse base-emitter voltage on the transistor  34 . On half cycles, the base current flow to the transistor  34  saturates the collector-emitter terminals. Turn-on-rise-time capacitor  18  filters any high frequency currents at this point. 
   When the transistor  34  turns on, current flows from the positive bias at the anode of diode  23  through turn-on-bias-resistor  411  to the junction of output-timing capacitor  16 , turn-off-bias resistor  49 , turn-on-bias-resistor  411 , reverse-voltage-limiting diode  25  and the emitter of the common-base-amplifier-PNP-bipolar transistor  32  to switch the transistor  32  off. This causes a forward bias of the transistor  32 . 
   The resulting current flow through turn-on-bias-resistor  411  and the current flow through turn-off-bias resistor  49  are in the same direction. However, when the transistor  34  is on, the net current flow into the emitter of the transistor  32  causes it to saturate. Through the gate-drive-voltage-buffer transistor  33 , the gate bias is removed from the MOSFET  35 , in effect, turning the switch off. 
   Output-timing capacitor  16  is a timing capacitor which controls the rise and fall time of  35 . 
   When the external control pins are shorted, the short circuit is reflected through the full-wave-diode bridge with a first  27 , second  28 , third  29 , and fourth  210  full-wave diode through the control transformer  52  to the emitter of the PNP bipolar transistor  34 . This causes the transistor  34  to turn off, removes the current flow from the turn-on-bias-resistor  411 , and allows an increased effect for the current flow through turn-off-bias resistor  49  to switch transistor  32  off. The transistor&#39;s  32  collector is pulled negative through a collector-pull-up resistor  48 . The transistor&#39;s  32  collector voltage is limited by a gate-to-source-voltage-limiting zener diode  24  which protects the gate of the p-channel MOSFET  35  against excessive gate-source voltage. The transistor  32  collector voltage is fed through the emitter follower of the gate-drive-voltage-buffer transistor  33  to the gate of the MOSFET  35 . This high amplitude gate voltage causes the MOSFET  35  to turn on. 
   Therefore, the solid-state relay has a normally off function, and turns on when the control pins are shorted to each other. 
   In the preceding specification, the invention has been described with reference to specific exemplary embodiments thereof. It will however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense.