SLUAAY2 December   2024 ISO5451 , ISO5451-Q1 , ISO5452 , ISO5452-Q1 , ISO5851 , ISO5851-Q1 , ISO5852S , ISO5852S-EP , ISO5852S-Q1 , UCC21710 , UCC21710-Q1 , UCC21717-Q1 , UCC21732 , UCC21732-Q1 , UCC21736-Q1 , UCC21737-Q1 , UCC21738-Q1 , UCC21739-Q1 , UCC21750 , UCC21750-Q1 , UCC21755-Q1 , UCC21756-Q1 , UCC21759-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1 Introduction
  5. 2 SiC and IGBT Characteristics
  6. 3 Failure Modes
  7. 4 Short-Circuit Protection Approaches
    1. 4.1 Short-Circuit Current-Based Protection Implementation
    2. 4.2 Short Circuit Voltage-Based Protection Implementation
  8. 5 DESAT Circuitry Design
    1. 5.1 DESAT Circuit Component Selection
    2. 5.2 Effect of Parasitic Elements
    3. 5.3 Effect of Rlim on DESAT Noise
  9. 6 Safe Shutdown
    1. 6.1 Safe Shutdown Mechanisms
    2. 6.2 Safe Shutdown Considerations
  10. 7 Short-Circuit Test Setup and Data
    1. 7.1 Short-Circuit Bench Measurement Setup
    2. 7.2 SC Board Setup for Data Collection
    3. 7.3 Different Circuit Configurations for SC Testing
    4. 7.4 Bench Measurement Results
    5. 7.5 Overall Summary of SiC vs IGBT Power Module SC Observation
  11. 8 Key Consideration in Designing SC Protection Circuit
  12. 9 Summary
  13. 10References

5.1 DESAT Circuit Component Selection

To achieve proper short circuit and overcurrent detection, it is crucial to choose the right components for the DESAT circuitry. The DESAT circuitry usually includes a blanking capacitor (CBLK), a current-limiting resistor (RLIM), and high-voltage diode(s) (DHV(1,2..)). Depending on system conditions, usually protection diodes (DD1, DD2) and a zener diode in the DESAT path (DZ) are also used.

During normal operation of the power switch, the VDS or VCE is lower than the desired VDS/VCE threshold. Under this condition, the high-voltage diodes are forward biased, and the DESAT pin internal charging current (ICHG) flows through the high-voltage diodes into the collector or drain of the power switch. Thus, voltage on the DESAT pin is lower than the DESAT detection threshold, and DESAT is not triggered.

UCC21750Q1 DESAT Normal OperationFigure 5-1 DESAT Normal Operation

During a short-circuit or overcurrent event, the VDS or VCE is higher than the desired VDS/VCE threshold. Under this condition, the high-voltage diodes become reverse biased, and the DESAT pin internal charging current cannot flow through the diodes. As a result, ICHG starts charging the blanking capacitor, causing the voltage on the DESAT pin to rapidly rise up to above the DESAT detection threshold. After a deglitch period, DESAT is triggered in this case.

UCC21750Q1 DESAT Short-Circuit OperationFigure 5-2 DESAT Short-Circuit Operation

To correctly select the external circuitry values, first determine the VDS/VCE threshold above, which the system is considered short-circuited or overcurrent. Then, design the external circuitry values using Equation 1:

During normal operation:

Equation 1. VCBLK=(VRLIM+VZ+VHV 1,2,...n+VCE or VDS)<VDESAT

During short-circuit or overcurrent operation:

Equation 2. VCE or VDS>VDESAT-(VRLIM+VZ+VHV 1,2,...n)

Besides the voltage detection threshold, also pay attention to the blanking capacitor charging time (tBLK), since it contributes to the total DESAT detection and shutdown time. tBLK can be estimated using Equation 3:

Equation 3. tBLK=CBLK×VDESATICHG

Where Cblk is the size of the blanking capacitor, Vdesat is the DESAT detection threshold, and ICHG is the DESAT charging current.

After taking all elements into consideration, the total DESAT detection time can be calculated by using Equation 4:

Equation 4. ttotal_detection=tDESATLEB+tBLK+tDESATFIL

Where tDESATLEB is the leading edge blanking time, tBLK is the blanking capacitor charging time, and tDESATFIL is the DESAT filter time.