Document Number For technical questi ons contact optocoupl eranswersvishay - PDF document

Document Number: 83858For technical questions, contact: optocoupleranswers@vishay.com www.vishay.comRev. 1.3, 28-Apr-09 VISHAY SEMICONDUCTORS Application Note 56 Solid-State Relays INTRODUCTIONVishay offers a full line of miniature MOSFET solid-staterelays (SSRs) for use in telecommunication, industrialcontrol, security, and instrumentation applications. MOSFET SSRs feature an optocoupler construction, buthave a pair of MOSFETs on the output instead of aphototransistor. A pair of source-coupled MOSFETs emulatean electromechanical relay by providing bidirectional switchcapability and a linear contact. No output power supply isrequired.Fig. 1 - SSR Internal ViewThe advantages of the MOSFET contacts are solid-statereliability and long life as well as very low thermal switchoffset, extremely high off-resistance, and lack of contactbounce. Thermal switch offset is actually a misnomer forSSRs. Any contact offset voltage is photo induced. Thisphoto-induced voltage is extremely low and typically runsabout 0.1 µV. These attributes make SSRs a significantcontender in applications historically served by reed relays.In some designs, however, the user must consider thecontact on-resistance and capacitance. Because MOSFETon-resistance is dominated by the bulk resistivity in the n-driftregion and there is no bipolar junction, no diode offset existsin the MOSFET SSR I-V characteristics and theon-resistance is extremely linear. The contact capacitance ofa MOSFET SSR is higher than an open contact of anelectromechanical relay.The majority of Vishay SSRs have LED inputs and monolithicswitch outputs (figure 1). The switch is built using BiCMOStechnology. Individual components are fabricated indielectrically isolated tubs. A fully integrated die has manyadvantages. Higher reliability is achieved due to a reductionin the number of wire bonds. Finer control over circuitoperating parameters is realized for higher-performancecircuits like current limiting.These miniature SSRs are offered in 6 pin packages, DIP orsurface mount, with single pole, normally open (1 form A) ornormally closed (1 form B) contacts. They also come in 8 pinDIP or surface-mount packages with two normally open (dualor 2 form A) or normally closed (dual form B) contacts. SomeSSRs are also available in a low-profile, small-outlinepackage (SOP).FUNCTIONAL DESCRIPTIONThe infrared light emitted by a gallium-aluminum-arsenide(GaAlAs) LED within the relay, controls the switch output.The LED is placed over the oulight downward onto a stack of photodiodes.Both input and output silicon are fully encapsulated in atranslucent inner-mold compound that passes light whileproviding a reliable, sustaining dielectric barrier in thethousands of volts. A dark outer mold compound, with amatched thermal expansion coefficient to the inner-moldcompound, is then called an over under double moldeddesign.In a basic schematic for an optically coupled MOSFET SSR,the photodiode array acts as a floating power source for theMOSFET switches (figure 2). Each diode is fabricated in itsown dielectrically isolated tub. Current-transfer ratios aresmall, and dielectric isolation provides optimum lightreception with no leakage to the substrate. Each diode actsas a 0.6 V battery when illuminated by the LED. With 20 to30 diodes, ample voltage is generated to turn on theMOSFET pair, even at high operating temperatures whereLED and photodiode output drop.Fig. 2 - SSR Functional DiagramTo turn-on the relay, current is applied to the LED. The LEDemits light, illuminating the inner mold and the photodiodearray. The amount of light emitted is dependent upon the 17279 5 mALED+-PDA Current- limitinguitry MOSFET MOSFETRSH 17280 Solid-State Relayswww.vishay.comFor technical questions, contact: optocoupleranswers@vishay.com Document Number: 83858Rev. 1.3, 28-Apr-09 Application Note 56Vishay Semiconductors APPLICATION NOTEamount of forward current applied. For high-temperature orhigh load current operation, more LED current is required.The photodiode voltage biases the normally open,enhancement-mode MOSFET gates positive with respect totheir sources. For a normally closed depletion-modeMOSFET, the photodiode array would be wired to bias thegates negative with respect to their sources. Figure 2 portrays a current-limiting circuit. This circuit is aunique feature on many of Vishay form A SSRs. Whencurrent through the MOSFETs becomes greater than theSSRs‘ rated value, the integrated current-limit circuit isactivated. This circuit increases the impedance of theMOSFET switches thereby regulating the amount of currentflowing through the SSR. When LED current is removed, thegate-to-source shunt resistance (R) turns the MOSFETswitches off by providing a discharge path for the gatecharge. Vishay SSRs use either a JFET or MOS circuit forthe gate-to-source shunt resistance in order to achieve fastturn-off. This control circuitry ensures a smooth “click free”turn-on and a slower turn-on than turn-off. This feature cansometimes be used to achieve break-before-make operationwhen using multiple relays. OUTPUT OPERATION Figure 3 shows the bidirectional or AC/DC I-V characteristicsof a current-limited SSR. Figure 4 shows the bidirectional I-Vcharacteristics of an SSR without current limiting. Inoperation, the SSR is exceptionally linear up to the kneecurrent (I). This linearity provides a distortion-free contact,making it ideal for small-signal applications such as V.34modems. Beyond I, the incremental resistance decreases,by approximately 35 %, thereby minimizing internal powerdissipation.For SSRs with current limiting, overload currents areclamped at ILMT by internal circuitry. The current-limitingcircuitry exhibits a negative temperature coefficient, therebyreducing the current-limit value when relay temperature isincreased. An extended clamp condition, which increasesrelay temperature, decreases the current-limit value,resulting in a current-fold back characteristic. When theoverload current is removed, the relay immediately resumesits normal I-V characteristics.Most 6 pin SSRs can be used in a unidirectional or DC mode.In this mode, on-resistance is reduced by 75 % and loadcurrents are doubled. For unidirectional applications, pins 4and 6 become the positive output of the relay and pin 5becomes the negative output of the relay. Only the LH1510provides current limiting in this configuration.Figure 5 shows the unidirectional I-V characteristics of theLH1510 SSR. Figure 6 shows the unidirectionalcharacteristics of the SSRs without DC current limiting. Herethe SSRs are exceptionally linear up to and beyond theirFig. 3 - Typical AC/DC On-Characteristics of a Current Limited SSR-LH1540Fig. 4 - Typical AC/DC On-Characteristics of a SSR without Current LimitingFig. 5 - Typical DC Characteristics of the LH1510, Pins 4 and 6 ShortedFig. 6 - Typical DC Characteristics of an SSR without DC Current Limiting, Pins 4 and 6 Shorted-LH1540 KIK- V+ I I + V17281-I 82 IL(max.)+ I- IIK- V+ VIL(max.)IK IL(max.)8 V720 mA350 mA- 320 mA Off-state 2.5 On-state + V17283 IL(max.)- I Off-state On-state - V+ V17284