TIDUF21A December   2022  – January 2023

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Description of Control Logic
      2. 2.2.2 Behavior Throughout Charge Cycle
      3. 2.2.3 Additional Design Recommendations
      4. 2.2.4 Simulation Results
    3. 2.3 Highlighted Products
      1. 2.3.1 TPSI3052-Q1
      2. 2.3.2 TLV7011
      3. 2.3.3 UCC27517A-Q1
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Setup
    3. 3.3 Test Results
  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 Authors
  11. 6Revision History

1 System Description

This reference design introduces an active precharge circuit which is essentially a buck converter topology to achieve the precharge for high voltage (HV) DC link capacitors. This reference design will talk through the reasons for adopting an active precharge approach instead of a traditional resistive implementation. The operation of the reference circuit will be explained and analyzed in detail.

Figure 1-1 Resistor Precharge Design Using SSR
Figure 1-2 Active Precharge Design

The benefits of an active precharge solution can be seen from a power analysis perspective. Many existing high voltage batteries for HEV and EV market use a 400-V architecture with typical system's capacitance in the order of milli-Farads. The only value missing to complete the equation is the charging time for the capacitors. Usually these charging times allow for up to 400-ms delay approximately. The following calculations show the peak and average power across the resistor for the mentioned values.

Equation 1. R=charging times5×C = 400 ms5×2 mF = 40 Ω

In equation (1) a factor of 5 time constants (5τ) is assumed for the charging time which provides that the voltage on the capacitor is 99% of the input voltage.

Equation 2. PPEAK=V2R = (400 V)240 Ω  = 4000 W
Equation 3. PAVG=Energytime (s) = C×V22× time = (2 mF)(400 V)22(400 ms) = 400 W

These calculations are taking the assumption that the system is a 400-V system. However, the HEV/EV industry has shown signs of planning to move to 800-V systems. This increase in voltage leads to lower currents, leading to lower DC conduction losses. By increasing the voltage, power can be supplied using less current. Less current allows to reduce the required size for the cables and required copper which effectively reduces the overall weight of the car and the cost for the system. Optimizing the weight allows to optimize the efficiency of the vehicle. In addition, 800-V systems allow for faster charging since the lower current reduces the overheating of the conductors and the battery. On the other hand, increasing the battery voltage to 800-V makes the DC link capacitor precharge more challenging. If the system needs to be precharged using the same charging time of 400-ms, the power requirements for the resistor can be described as resulting in four times higher power.

Equation 4. PPEAK=V2R = (800 V)240 Ω  = 16000 W
Equation 5. PAVG=Energytime (s) = C×V22× time = (2 mF)(400 V)22(400 ms) = 1600 W

Typical resistors that can meet these requirements are large, costly, heavy, and not as common or available as low-power resistors. In addition, using a resistor for the precharge is not very efficient and the increased power loss leads to more heat and increased temperature rise. The question that follows here, after describing the limitations for the resistor is simple. What other electrical component can be used to control current? Here is where the active precharge circuit topology comes as a buck converter topology configuration to precharge the DC link capacitor using an inductor to limit the current.

An inductor does not allow sudden current changes and stores the energy in the form of a magnetic field. An ideal inductor is essentially loss-less and the energy stored is not converter to heat. However, in reality, the quality factor of the inductor Energy Stored / Energy Dissipated can affect the efficiency of the system. Usually, the resistance of the inductor is very small and the power dissipated can be insignificant compared to a resistor precharge design.