TIDUF73 September   2024

 

  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 Design Theory
      2. 2.2.2 Resistor Selection
        1. 2.2.2.1 Transistor and Diode Selection
      3. 2.2.3 Overcurrent Detection – Short-Circuit Protection
    3. 2.3 Highlighted Products
      1. 2.3.1 TPSI3100-Q1
      2. 2.3.2 INA180-Q1
      3. 2.3.3 TPSI2140-Q1
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
      1. 3.1.1 External Hardware Requirements
    2. 3.2 Test Setup
    3. 3.3 Test Results
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Tools
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks
  11. 5About the Author

1 System Description

Precharge is a common circuit in Electric and Hybrid Electric Vehicles (EVs and HEVs) that prepares the high-voltage DC rails before the rails are connected to the battery. The positive and negative high-voltage rails are connected by the DC-Link capacitor, which helps stabilize the rails as loads are connected and disconnected during the vehicle operation. A precharge circuit charges the DC-link capacitor to the battery voltage, minimizing the inrush current caused when the main contactors close. For the health of the main contactors the inrush is minimized as too high of inrush can cause the contacts to weld together, rendering them defective.

TIDA-050080 Precharge Configurations Figure 1-1 Precharge Configurations

This design features passive precharge with solid-state relays. In passive precharge, the switch closes statically until the capacitor is charged. Figure 1-1 shows how precharge is often achieved with mechanical contractors or relays. The goal of this design is to replace the mechanical contactor with a solid-state relay, providing a more reliable design. The benefits of passive precharge are the low complexity and low switching noise emissions. This is a very common method of precharge in the industry because of the simplicity and the widespread availability and options of power resistors. However, as this design has less control logic, sizing components to withstand the power and protecting them from overcurrent is foremost of the design considerations.

TIDA-050080 Scope of Design Figure 1-2 Scope of Design

Outside of the high-power path, the control circuitry of this design is comprised of a FET driver and an overcurrent detection circuit. This design uses the TPSI3100-Q1 isolated switch driver which, when paired with a FET, creates a seamless solid-state relay design that replaces contactors such as the precharge contactor. Additionally, integrated in the TPSI3100-Q1 are fault and alarm comparators. The fault comparator disables the driver when tripped and sends a signal back across the barrier. The alarm comparator only sends a signal when tripped. Along with an INA180-Q1 current-sense amplifier, these comparators constitute the overcurrent detection circuit. The current-sense amplifier is powered through the internal secondary side supply of the TPSI3100-Q1, a nominal 5V rail generated from the VDDM pin.

The final element of this design is a discharge path for the voltage that is stored on the capacitor. In EVs, there are different types of discharge requirements. For safety-critical events, such as a crash, the capacitor must be discharged in under a few seconds, the exact time varying between manufacturers. For non-emergency cases, the discharge can be on the order of minutes. This design features a non-emergency discharge that is comprised of the isolated switch TPSI2140-Q1 and a power resistor. Once activated, the capacitor is discharged to below 60V in about 2 minutes from 1000V. This discharge circuit is also necessary for safe handling and testing of the design.