Patent Publication Number: US-5298817-A

Title: High-frequency solid-state relay

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to relays generally and, more particularly, to solid-state relays which can switch high-frequency signals. 
     2. Description of the Prior Art 
     Electromechanical relays are the standard means for switching high frequency (e.g., &gt;1 MHz) signals, such as T1 or DS1 (1.544 Mbit/sec) carrier signals. Two advantages electromechanical relays have over solid-state relays are the lower ON resistance (when the relay contacts are closed) and higher isolation (when the contacts are open) at high frequencies. However, electromechanical relays are large, noisy, require relatively large power to operate, and are slow to operate, compared to solid-state relays. Thus, it is desirable to provide a solid-state relay having the capability of switching high-frequency signals. 
     SUMMARY OF THE INVENTION 
     A solid-state relay having first and second terminals and responding to a control signal is characterized by: a control circuit, responsive to the control signal; a first pass transistor, connecting between the first terminal and a common node and coupling to the control circuit, the conductivity thereof being substantially determined by the control circuit; a second pass transistor, connecting between the second terminal and the common node and coupling to the control circuit, the conductivity thereof being substantially determined by the control circuit; a first shunt transistor, connecting between the common node and a reference node and coupling to the control circuit, the conductivity thereof being substantially determined by the control circuit. In addition, the pass transistors have a zero bias conductivity opposite that of the shunt transistor and the conductivity of the pass transistors changes inversely to the conductivity of the first shunt transistor in response to the control circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following detailed description of the drawings, in which: 
     FIG. 1 is a simplified schematic drawing of one embodiment of the invention, a non-differential mode solid-state relay capable of switching high frequencies; and 
     FIG. 2 is a simplified schematic drawing of another embodiment of the invention, a differential mode solid-state relay capable of switching high frequencies. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the exemplary embodiment of the invention is described herein as a solid-state relay 1 having first and second terminals 2, 3 and responding to a control signal (optical, in this example) incident on a photodiode array 4. A control circuit 5 converts the control signal into an electrical signal to drive the gates of a first and second pass transistors 6, 7. The pass transistors 6, 7 connect between corresponding input terminals 2, 3 and a common node 8. A shunt transistor 9 is connected between the common node 8 and a reference node 10 (typically ground). The shunt transistor 9 is also controlled by the electrical signal from the control circuit 5. The pass transistors 6, 7 are metal-oxide-semiconductor field-effect transistors (MOSFETs) and the shunt transistor 9 is a junction field-effect transistor (JFET). In this example, the MOSFETs are enhancement mode transistors (the transistor is normally not conductive when a zero gate-to-source bias voltage is applied thereto) and the JFET is a depletion mode transistor (the transistor is normally conducting when a zero gate-to-source bias voltage is applied thereto). The electrical signal from the control circuit 5 controls (changes) the conductivity of the transistors 6, 7, 9 such that the change in conductivity of the pass transistors 6, 7 is the inverse of the change in conductivity of the shunt transistor 9. For example, when the pass transistors 6, 7 are conducting, the shunt transistor 9 is not conducting, and vice-versa. The shunt transistor provides a low-impedance path for signals leaking through the transistors 6, 7 when the transistors 6, 7 are not conducting, i.e., when the relay 1 is &#34;open&#34;. When the relay 1 is &#34;closed&#34;, the shunt transistors is not substantially conducting, while the pass transistors 6, 7 are conducting. This allows for high frequency signals to be switched with little feed-through when the relay 1 is &#34;open&#34;. 
     The optical control signal is, in this example, generated by a light-emitting-diode (LED) 11 which converts an electrical control signal applied to terminals 12 into the optical control signal. An array of LEDs may be used instead of a single LED 11 as shown. 
     The control circuit 4 includes the photodiode array 4, junction FET 13, resistor 14, and Zener diode 16. When the optical control signal is applied to the photodiode array 4, a voltage generated by the array 4 charges the gates of transistors 6, 7, turning them on. When the optical control signal is removed, the FET 13 and resistor 14 accelerates the discharge of the gate of transistors 6, 7 to turn them off. Zener diode 16 prevents the gate-to-source voltage of the pass transistors 6, 7 from reaching such a high voltage that the transistors 6, 7 are damaged. 
     In FIG. 2, an alternative embodiment of the invention is shown. The relay 20, a differential relay, is an elaboration on the relay 1 of FIG. 1, having a control circuit 26 (including a photodiode array 25), and shunt transistor 27 coupling between common nodes 31, 32. The differential relay 20 shown has two pairs of pass transistors 21, 22 and 23, 24, each pair in series with one path (leg) through the relay. In addition to shunt transistor 27, transistors 28 and resistor 29 assist in the discharge of gate voltage on shunt transistor 27. Diodes 33 allow for the photodiode array 25 to drive both sets of pass transistors 21-24 while providing isolation between the signal paths. Diodes 34 and 35 allow for faster operation of the relay 20 by providing current from the photodiode array 25 directly to the gates of the pass transistors 21-24 and to the shunt transistor 27 without having to first pass through resistors 29, 30. The diodes 34, 35 thus guaranties that the relay 20 will break-before-make. 
     The components of the differential relay 20 shown in FIG. 2 has been fabricated in a common semiconductor substrate using approximately 1030×480 micron enhancement mode MOSFET pass transistors 21-24 and transistors 27, 28, and 36 are depletion mode JFETs. Shunt transistor 27 has an exemplary ON resistance of approximately 5 Ω. Resistors 30 are an exemplary 20M Ω, and resistor 29 is an exemplary 10M Ω. There are typically fifty two photodiodes in the photodiode array 25, each producing about 1 μA at 0.5 volts when illuminated by LED 37. Alternatively, an array of LEDs may be used in place of the shown single LED 37. The relay 20 has an exemplary ON resistance (differential, each leg) of 5 Ω, and an exemplary input-to-output isolation of 63 dB at 1.544 MHz across a 50 Ω load. Without the shunt transistor 27, isolation is about 28 dB. 
     Having described the preferred embodiment of this invention, it will now be apparent to one of skill in the art that other embodiments incorporating its concept may be used. Therefore, this invention should not be limited to the disclosed embodiment, but rather should be limited only by the spirit and scope of the appended claims.