Abstract:
A radiation-tolerant inrush limiter is provided. The inrush limiter does not incorporate radiation-hardened parts, but instead uses an ordinary (non-hardened) p-channel FET to provide switching functionality.

Description:
FIELD OF THE INVENTION 
     The present invention deals with inrush limiters and more specifically, a radiation-tolerant inrush limiter with non-hardened parts. 
     BACKGROUND OF THE INVENTION 
     Inrush limiting circuits are commonly used in conjunction with DC-DC converters. DC-DC converters usually incorporate large value capacitors in parallel with their input terminals for filtering purposes. If input voltage is suddenly applied to the power supply, inrush currents flow from the power source to charge the input capacitance. Inrush current limiters are circuits that control the rise time of the applied input voltage, thereby limiting the inrush current that flow upon application of power to tolerable levels. 
     In many applications, it is also desirable or mandated that no output should be produced by the power supply until and unless the input voltage is sufficiently high. Since DC-DC converters are regulating devices, for a constant output load, input current increases inversely proportional to input voltage. Therefore, by inhibiting production of output power from the power supply until the input voltage is above a sufficient level, the input current magnitude at low input voltages is controlled. This often prevents a damaging condition. 
     DC/DC converters are electronic devices that use switching components, such as field effect transistors (FETs) to transform voltage from one level to another. Typically, the output voltage is regulated and protected against short circuits. In many cases, the input and output potentials are galvanically isolated from each other. 
     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, inrush limiters 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. 
     Inrush limiters 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 DC/DC converters 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 desirable, therefore, to provide electrical components with ordinary (non-hardened) materials that can function when exposed to radiation. 
     SUMMARY OF THE INVENTION 
     A radiation-tolerant inrush limiter is provided. The inrush limiter does not incorporate radiation-hardened parts, but instead uses an ordinary (non-hardened) p-channel FET to provide switching functionality. 
     In further detail, the present invention provides a radiation-tolerant electrical component for limiting inrush currents in radiation-intensive applications. The component comprises a common line, a load input line, a positive input line, a voltage lockout circuit, and a non-hardened p-channel FET having a drain, a gate, and a source. The common line is operably connected to the gate, the load input line is operably connected to the drain, the positive input line is operably connected to the source, and the voltage lockout circuit is operably connected to the gate. 
     In accordance with further aspects of the preferred embodiment, the voltage lockout circuit further comprises a current-limiting resistor, a first voltage divider resistor, a second voltage divider resistor, a first and second pull-up resistor, and a bias current resistor, all connected to the common line. In addition, an under-voltage-lockout-PNP transistor has a first PNP connection that is connected to the second collector pull-up resistor, a second PNP connection that is connected to the bias-current resistor and an under-voltage-lockout-reference-zener diode, and a third PNP connection that is connected to a first voltage divider resistor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic wiring diagram of a radiation-tolerant inrush limiter in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Field-effect transistors exist in two major classifications, the junction FET (JFET) and the metal-oxide-semiconductor FET (MOSFET). A MOSFET is a special type of FET that works by electronically varying the width of a channel along which charge carriers (electrons or holes) flow. Wider channels provide better conductivity. The charge carriers enter the channel at the source, and exit via the drain. The width of the channel is controlled by the voltage on an electrode called the gate, which is located physically between the source and the drain and is insulated from the channel by an extremely thin layer of metal oxide. 
     There are two ways in which a MOSFET can function. The first is known as depletion mode. When there is no voltage on the gate, the channel exhibits its maximum conductance. As the voltage on the gate increases (either positively or negatively, depending on whether the channel is made of P-type or N-type semiconductor material), the channel conductivity decreases. The second mode of MOSFET operation is called enhancement mode. When there is no voltage on the gate, there is in effect no channel, and the device does not conduct. A channel is produced by the application of a voltage to the gate. Increasing gate voltage increases conductivity and thus, current flow. 
     The MOSFET has certain advantages over the conventional junction FET, or JFET because the gate is insulated electrically from the channel. No current flows between the gate and the channel, regardless of the gate voltage (as long as it does not become so great that it causes physical breakdown of the metallic oxide layer). Thus, the MOSFET has practically infinite impedance. 
     When conventional non-radiation hardened N Channels FETs are used in applications where radiation is present, the FETs become uncontrollable at relatively low radiation levels because the gate threshold voltage of the N channel FET experiences a negative shift, and ultimately falls close to zero. At that point, the N channel FET conducts current with little or no gate voltage applied making it uncontrollable, like a flood gate that cannot be closed. 
     The gate threshold voltage of a conventional, non-radiation hardened P channel FET also shifts negatively with radiation exposure. However, the initial threshold voltage of an ordinary P channel FET is negative to begin with. In the presence of radiation, therefore, the gate threshold voltage does not approach zero and therefore will not become uncontrollable. The gate threshold voltage does change, but from a negative value to a more negative value. Conventional P channel FETs, therefore, are more robust to total radiation dose effects as compared to conventional N channel FETs when the proper gate drive signal is provided. 
     In accordance with an 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. 
       FIG. 1  is a schematic diagram for a circuit according to a preferred embodiment of the present invention. This particular example operates from a voltage source of 22 VDC to 50 VDC, but nominally 28 VDC. It may, however, be readily scaled for different bus voltages. The circuit consists of an under voltage lockout portion indicated by dashed line  100 , and an inrush limiter portion indicated by dashed line  200 . A common line  300 , a load input line  400 , and a positive input line  500  are all operably connected to the circuit. The common line  300  carries the output of the circuit and the other lines  100 ,  200  carry the input to the circuit. 
     For the under voltage lockout portion  100 , under-voltage-lockout-voltage reference-zener diode  21  acts as a voltage reference for the under voltage lockout. The diode  21  and the base emitter junction of the under-voltage-lockout-PNP-bipolar transistor  31  form a voltage reference. Bias-current-resistor  41  supplies a minimum, or bias current through the reference-diode  21  to stabilize its zener operation. The diode  22  is operably connected to a filter capacitor  11 . 
     A first-voltage-divider-resistor  44  and second-voltage-divider-resistor  45  form a voltage divider that senses the bus voltage and determines the operating point of the under voltage lockout function. 
     A first-collector-pull-up-resistor  42  and second-collector-pull-up-resistor  43  act as the collector pull up for the transistor  31 . The common connection of resistors  42  and  43  drives the base of the under-voltage-lockout-circuit-NPN-bipolar transistor  32 . 
     Feeding-current-limiting-resistor  412  limits the current fed to the drive-circuit-stabilizing-zener diode  22  that establishes a stabilized internal voltage with reference to the positive bus voltage. 
     A first-collector-load-resistor  49  is a collector pull-up for the NPN transistor  32 . A hysteresis resistor  46  provides a hysteresis action to the operation of the transistor  31 , to prevent intermediate state operation. 
     When the bus voltage exceeds the desired minimum (approximately 22 VDC in this example), the under-voltage-lockout-NPN-bipolar transistor  32  conducts and applies voltage to one end of a second-collector-load-resistor  410 . This allows the inrush limiter portion  200  to operate. 
     The inrush limiter portion  200  consists of a common-base-amplifier-PNP-bipolar transistor  33  and a gate-drive-voltage-buffer-PNP-bipolar transistor  34  as well as a P-channel FET  35 . The common-base-amplifier-PNP-bipolar transistor  33  is used as a common base amplifier and the gate-drive-voltage-buffer-PNP-bipolar transistor  34  is used to buffer the gate drive voltage to the P channel MOSFET  35 . 
     A first-reference-voltage diode  23  and a second-reference-voltage diode  24  establish a reference voltage for the common-base-amplifier-PNP-bipolar transistor  33 . The established reference voltage is slightly more negative than the input bus. Diodes  23  and  24  are connected to the return leg of the input bus through bias-current resistor  48 . The purpose of these components is to establish a bias voltage for the base of the common-base-amplifier-PNP-bipolar transistor  33  that should be two diode drops more negative than the positive bus voltage. 
     The common-base-amplifier-PNP-bipolar transistor  33  provides a non-inverting stage of voltage gain. The collector load resistor for the transistor  33  is the second-collector-load-resistor  410 . 
     A reverse-voltage-limiting diode  25  limits the reverse voltage across the base emitter terminal of the common-base-amplifier-PNP-bipolar transistor  33 . 
     A gate-drive-voltage-buffer transistor  34  is a PNP bipolar transistor that acts as an emitter follower, lowering the impedance of the signal on the collector of the common-base-amplifier-PNP-bipolar transistor  33 . 
     A gate-to-source-filter resistor  411  and a current-limiting-resistor  412  act as filter components across the gate-source terminals of the MOSFET  35 . A noise-filter-decoupling capacitor  12  is connected across the filter resistor  411 . 
     When bus power is applied, the drain voltage of the MOSFET  35  is initially at ground potential. As the MOSFET  35  conducts and its drain voltage rises, current flows through a timing capacitor  13 . Most of this current flows through a timing resistor  47 , however a small current flows into the emitter of the MOSFET  33 . 
     Since the emitter voltage of the MOSFET  33  should be constant at approximately 1.8 VDC below the positive rail, the current through the timing resistor  47  is relatively constant during the turn-on interval. Therefore, the current through the timing capacitor  13  should be relatively constant as well. The relatively constant current through the timing capacitor  13  establishes a relatively linear ramp voltage across it. 
     Therefore, the output voltage at the drain terminal of the MOSFET  35  rises from zero to the positive bus voltage in a linear manner. By controlling the turn on rise time of the MOSFET  35 , inrush current flowing into downstream capacitors is determined and controlled. 
     This circuit provides a relatively constant power rise time to the load, essentially independent of the gate to source threshold voltage of the P channel FET  35 . Since the gate to source threshold voltage of the FET  35  changes considerably as a result of exposure to ionizing radiation, this circuit produces a relatively uniform power application rise time to the load, despite the parametric changes due to radiation. 
     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.