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

3.3.4 Multichannel SSR Driven From Two 24-VAC Transformers

Figure 3-19 shows a 6-channel SSR topology powered from two transformers, for example from the cooling transformer (RC) and heating transformer (RH) in an HVAC thermostat. The common wires (C – wire) of both the transformers are connected together. The single-isolation gate-drive circuit in the reference design can be used to drive all the six SSR channels. However, in this case the turn on PNP transistor (Q1, Q2 in Figure 2-6) in the gate-drive circuit and the blocking diodes (D1, D2 in Figure 2-6) has to be rated for two times the peak AC voltage plus the gate-drive voltage (this is by considering a use case where the transformers are mis-wired to create opposite polarity). For example, with a 24-VAC ±20% AC supply, having a peak instantaneous value of 41 V, the gate-drive PNP transistors and the blocking diodes has to be rated for more than 100 V.

GUID-20210210-CA0I-X229-FKH8-JZBXNWNGPNKX-low.gif Figure 3-19 Circuit Topology in Driving Multiple SSRs With two AC Transformers