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

2.2.2 Design Challenge With Single Isolation

Figure 2-4 shows the configuration having SSR-1 and SSR-2 MOSFETs using a common ground for the gate drive at secondary of the isolator. Assume that SS-1 is turned on and SSR-2 is kept off. When the SSR-1 is turned on, the common ground at the gate drive causes circulating current through the common ground and the anti-parallel diode of the low-side MOSFET of SSR-2 during the negative half cycle of 24-V AC power supply. The unintentional half wave current flows through load (ILOAD2) connected through SSR-2 (despite being turned OFF), will turn on the SSR-2 load relay in the negative half cycle of input 24-V AC supply. Figure 2-4 shows the unintentional half wave rectified current waveform.

GUID-20210210-CA0I-XDB5-GDJ9-S9GRNNHWXXCJ-low.gif Figure 2-4 Unintentional Current Flow in Driving Multiple SSR With Common Ground Gate-Drive Supply
GUID-20210210-CA0I-BFHN-XDN0-WS3KB1G5RZZ8-low.png Figure 2-5 Half-Wave Rectified Current in Driving Multiple SSR With Common Ground Gate-Drive Supply