TIDUEZ1 March   2021

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Multichannel SSR with Independent Isolation Between SSR Channels
      2. 2.2.2 Design Challenge With Single Isolation
      3. 2.2.3 Multichannel SSR Drive With Single Isolation Multichannel Digital Isolator
      4. 2.2.4 Need of High-Impedance Voltage Translator
      5. 2.2.5 Design to Minimize Cross-Coupling and MOSFET Gate Pick up Due to Other SSR Switching
      6. 2.2.6 Schematic: Design of Gate-Drive Circuit
        1. 2.2.6.1 Calculation of Gate-Driver Power Consumption
      7. 2.2.7 Schematic: Digital Isolator Circuit
      8. 2.2.8 Schematic: 3.3 V to 10V_ISO, 5V_ISO Power Supply
    3. 2.3 Highlighted Products
      1. 2.3.1 ISO7760
      2. 2.3.2 ISO7740
      3. 2.3.3 ISO7041
      4. 2.3.4 CSD19538Q2
      5. 2.3.5 CSD17382F4
      6. 2.3.6 TPL7407LA
      7. 2.3.7 TLV760
      8. 2.3.8 TLC555
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
      1. 3.1.1 Test Equipment Needed to Validate Board
      2. 3.1.2 Test Conditions
      3. 3.1.3 Test Procedure
    2. 3.2 Test Setup
    3. 3.3 Test Results
      1. 3.3.1 Functional Tests
      2. 3.3.2 Overcurrent Testing With External Fuse
      3. 3.3.3 Surge Testing
      4. 3.3.4 Multichannel SSR Driven From Two 24-VAC Transformers
      5. 3.3.5 Alternate SSR Topology for High Voltage
  9. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  10. 5About the Author

Design to Minimize Cross-Coupling and MOSFET Gate Pick up Due to Other SSR Switching

Referring to Figure 2-6, consider the scenario where SSR-6 is kept off and SSR-1 is turned on and off. When SSR-1 is switching, the voltage across the blocking diode D2 changes (the voltage change depends on the instantaneous AC voltage). The voltage change dv/dt across the SSR MOSFETs couples to the blocking diode of SSR6 (which is off) and then through the gate-driver power supply (10 V) low impedance path, couples to the gate-drive transistor circuit (Q2). This can cause momentary base current and resulting collector current in Q2, resulting a gate pick up at the MOSFET gate of SSR6 causing the SSR-6 to conduct. One way to reduce cross coupling is by controlling dv/dt during FET switching, by reducing switching turn on and turn off time by adjusting the gate resistor. The reference design uses decoupling capacitor at the base-emitter of transistor (Q2) to alleviate the unintentional turn on of the gate path transistor Q2, allowing faster MOSFET switching slew rate. The capacitor can be used at the gate to source the of MOSFET to further enhance the noise immunity. shows the simulation results confirming that the gate voltage pick up is negligibly small. The transistor remains off ensuring that there is negligible cross-coupling.

GUID-20210210-CA0I-BRXQ-ZTV5-DKLKLTZRHGN2-low.gifFigure 2-8 Simulation Results Illustrating Negligible Cross-Coupling

The performance comparison of the single isolation multichannel SSR topology against other independent isolation SSR topologies and electromechanical relays are listed in Table 2-1.

Table 2-1 Comparison of Single Isolation Topology Against Other SSR Topologies and EMR
ParameterElectromechanical RelaySSR: Optical Isolated DriveSSR: Independent Transformer IsolationSSR: Single Isolation With Multichannel SSR
Cost per relayLowHighMediumLow
PCB size per relayMediumLowMediumLow
LifeLowMediumHighHigh
Quiescent current per channelHighLow - MediumHighLow - Medium
Turn on, Turn off timeHighMediumMediumLow