AN1212: Advanced Load Driving Considerations for the Si834x Isolated Smart Switch

PublishTime：2020-10-22

Output circuits in industrial programmable logic controllers(PLCs) can be routinely subjected to adverse load conditions. PLC outputs often drive relay coils or other inductive loads that can generate potentially dangerous voltage spikes and must dis-charge significant energy from their magnetic fields when switch-ed off. Loads such as lamp filaments may require amps of inrush current, and short-circuited outputs must also be tolerated. These adverse load conditions can damage the PLC output switch device, and damage may spread to the host controller interface, potentially putting the PLC controller and possibly system operators at risk, if adequate protection mechanisms are not in place.The Si834x Isolated Smart Switch devices incorporate a rich set of protection mechanisms and diagnostic reporting capabilities to help ensure long, safe, reliable operation of PLC systems. The Si834x devices offer galvanic isolation to insulate the low voltage PLC controller circuitry, and users, from transients and fault conditions that may be present on the PLC output circuits. An integrated “smart” multi-voltage demagnetization clamp limits voltage spikes and absorbs demagnetization energy from inductive loads,and protection mechanisms such as temperature sensing, dissipated energy estimation, current monitoring, and under voltage shutdown are included to help PLC systems handle adverse load conditions.
KEY POINTS:
• Galvanic isolation insulates low-voltage host interface from industrial loads
• Integrated “smart” multi-voltage demagnetization clamp
• Short-circuit and over-current protection
• Under voltage fault detection
• Open-circuit detection
• Diagnostic reporting
The Si834x Isolated Smart switch devices are designed for use in industrial applications such as Programmable Logic Controller (PLC)output circuits. PLC output circuits typically provide switching of 24 VDC supplies to control industrial loads like relays, motor actuators,or lamps. The Si834x isolated switches allow logic signals from low voltage processors to control switching of the higher-voltage PLC outputs while providing galvanic isolation between the logic inputs and the PLC outputs. Galvanic isolation provides protection for the processors and human operators on the low voltage side of the circuit from the higher voltages and possible transients or fault conditions that may arise on the output side of the PLC equipment. PLC outputs can be exposed to a wide variety of load conditions that can present many challenges for traditional switch circuits. For example, PLC outputs often drive relay coils or other inductive loads which store energy in their magnetic field while the switch is in the“on” state. The interruption of current when the switch turns off can often generate enough “inductive kick” or “flyback” voltage to dam-age the switch unless some form of voltage “clamping” is implemented. Care must be taken when implementing demagnetization clamp circuits to ensure that absorption of the coil's demagnetization energy does not cause overheating in the clamp. PLC output circuits should also be designed to protect themselves from overheating when the output is accidentally shorted. On the other hand, PLC out-put may also be used to illuminate light bulbs, which can draw amps of initial “inrush” current before the filament heats up, so these loads may initially look very much like a short circuit.The Si834x isolated smart switch devices incorporate a number of innovative protection and diagnostic capabilities which facilitate driving of difficult industrial loads, and which distinguish this family of switches from competing devices. The Si834x 4-channel switches deliver up to 700 mA (IO(ON)) continuously per channel, meeting and exceeding the IEC 61131-2 requirements for 500 mA drive capability. An innovative “smart” multi-voltage demagnetization clamp is included on-chip to protect against the over-voltage conditions and absorb the demagnetization energy that can result from driving inductive loads such as relays, solenoids and motors. This integrated demagnetization clamp dynamically switches between operation at 2.1 V (VCLMPL) or 17.5 V (VCLMPH), based on sensed operating cur-rent, temperature, and supply voltage. A special “Inrush” mode of operation accommodates up to 8 A (IOPCL) of peak inrush current for loads such as lamps. Inrush mode is complemented by a “Safe Start” mode of operation which uses an integrated power estimation technique to prevent excessive power dissipation or overheating when exposed to a short circuited output or other elevated current conditions for extended time periods. The Si834x on-chip diagnostics also detect and report open-circuit output conditions back to the host.

Brand of Product：	Silicon Labs
Part#：	Si834x More Part#
Product Category：	programmable logic controllers 、 Isolated Smart Switch devices More
Application：	More
Data Type：	Application note & Design Guide More
Safety & Environment Standards：	More
Operating Temperature：	Please see the document for details
Qualification Grade：	More
Qualification Approval：	More
Package：
Language：	English Chinese Chinese and English Japanese
Date of Creation：	2019/10/22
Version Number：	Rev. 0.1
Data Encoding：	AN1212
Data Size：	858 KB

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