TIDUF55 November   2023

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Power Tree and Wakeup
      2. 2.2.2 Insulation Requirement for Isolated Interface
      3. 2.2.3 Robust Relay Driver
      4. 2.2.4 Stackable Daisy Chain Communication
    3. 2.3 Highlighted Products
      1. 2.3.1  TMDSCNCD263
      2. 2.3.2  LMR51440
      3. 2.3.3  TPS7A16
      4. 2.3.4  TPS7B81
      5. 2.3.5  TPS62913
      6. 2.3.6  TPS4H160-Q1
      7. 2.3.7  ULN2803C
      8. 2.3.8  ISO1042
      9. 2.3.9  UCC12050
      10. 2.3.10 ISO1410
      11. 2.3.11 SN6505B
      12. 2.3.12 BQ32002
      13. 2.3.13 HDC3020
      14. 2.3.14 TPS3823
      15. 2.3.15 DP83826E
      16. 2.3.16 TPS763
      17. 2.3.17 LM74701-Q1
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Software Requirements
    3. 3.3 Test Setup
    4. 3.4 Test Results
      1. 3.4.1 Power Supply Testing
      2. 3.4.2 Daisy Chain Signal Quality
      3. 3.4.3 Relay Driving
      4. 3.4.4 Isolated CAN Transceiver Operation
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Tools and Software
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks
  11. 5About the Author

3.4.3 Relay Driving

The coil driving circuit uses one channel of the smart high-side switch TPS4H160-Q1 and ULN2803 to implement a unidirectional drive, as Figure 3-17 shows. For testing this circuit, a high-voltage relay with economizer is used as the load, EVC500 is described in Table 3-19.

Table 3-19 Coil Data
OPERATING VOLTAGE RANGE / VDC INRUSH CURRENT
AT 23°C/A		 MAXIMUM INRUSH TIME/ms NOMINAL FREQUENCY/kHz NOMINAL DUTY CYCLE/%
9 – 36 3.8 130 19.9 20

Figure 3-17 shows an oscilloscope plot of the signals during current pickup. Channel 1 shows the voltage on the relay coil; Channel 2 shows the shunt (1.35 Ω) voltage which corresponds to the shunt current through the high-side switch to the relay coil and economizer. Initially the current reaches a peak when the economizer operates in 100% duty cycle, and thus has only a small back electromotive force voltage. After the relay contacts close, the economizer operates in a nominal duty cycle. In the current pick-up process, the shunt voltage signal reaches peak of about 2.034 V. The scale factor for the shunt voltage signal is about 1.35 V/A of shunt current, so this signal indicates the peak current is about 1.507 A.

GUID-20231026-SS0I-1G2G-C2DT-Z9X0S4HLNKX8-low.png Figure 3-9 Current Pickup Phase

Figure 3-17 shows an oscilloscope plot of the current signals. Channel 1 shows the current sensing (CS) signal of TPS4H160B which corresponds to the current through the high-side switch to the relay coil and economizer. Channel 2 shows the shunt voltage. The shunt voltage signal is followed closely by the CS signal. The CS signal setting time from IN falling and the rising edge of TPS4H160 is a maximum 150 μs, which causes the CS signal to lose the PWM portion of the real shunt current.

GUID-20231026-SS0I-JGPT-1VJW-GHHPKJ2FWWXT-low.png Figure 3-10 Current Measuring Performance

Figure 3-17 shows an oscilloscope plot of the signals during the current fast decay phase of the relay. Channel 1 shows the voltage on the relay coil; Channel 2 shows the shunt voltage. The voltage on Channel 1 reduces and the coil current begins to reduce. Due to the inductance of the coil, the coil current does not immediately drop to zero, but decays as the energy stored in the coil dissipates. The fast decay process lasts 3.14 ms.

GUID-20231026-SS0I-BVJZ-WQLD-HFK352PVPCFM-low.png Figure 3-11 Current Fast Decay Phase

The TPS4H160-Q1 specifies an absolute maximum of 40 mJ for the Inductive load switch-off energy dissipation, single pulse, single channel, so this energy that must be dissipated each time the relay coil is turned off is significantly less than the specified value. Designers can use the TPS4H160-Q1 for this application without the need for protection diodes in parallel with relay coil, unless the coil is significantly larger than the model used for testing.