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

2.2.7 Schematic: Digital Isolator Circuit

Figure 2-11 shows the digital isolator circuit for the 10-channel SSR. To get ten SSR channels, a 6-channel (ISO7760) and a 4-channel (ISO7740) digital isolator is used. For any other number of SSR channels different combinations of digital isolators can be used. For example an 8-channel SSR circuit can be created using two 4-channel digital isolators (ISO7740). A 9-channel SSR can be realized using one 6-channel digital isolator (ISO7760) and one 3-channel digital isolator (ISO7730). The IN1-IN10 are the relay turn on and turn off control signal from the microcontroller. For example, to turn on the relay connected to SSR channel 1 (COIL-1), keep IN1 in the logic high state. To turn the relay off, keep IN1 in logic low state.

In the case where there is a requirement to route a diagnosis signal from the relay side to the MCU side, a digital isolator with one reverse channel can be used. For example, an ISO7761 device provides five forward-direction channels and one reverse-direction channel.

If the input power or signal is lost, the default output is high for ISO7760 devices without suffix F and low for devices with suffix F. This reference design uses the device with suffix F (ISO7760F and ISO7740F) to make sure that in the absence of input-side powers supply, all the SSR are turned off. The 100-kΩ pulldown resistors at the input of the digital isolator is optional and may not be needed in a system-level board design. The resistor at the EN pin of ISO7740 is not populated, so as to enable the outputs of ISO7740. The 0-Ω resistors at the VCC terminals of digital isolator is a placeholder for current consumption measurement, if needed.

The reference design uses the TPL7407LA device as the low-side switch for 7 channels, connected at the output of the digital isolator. The remaining 3 channels of SSR use a separate NPN transistor.

GUID-20210223-CA0I-KMXX-TWMW-NKKH2GD51BZN-low.gif Figure 2-11 Digital Isolator Interface for Ten Channel SSR