What is DESAT?
DESAT - is the desaturation (DESAT) protection feature that is present in automotive gate drivers. The following diagrams illustrate how DESAT protection works:
Figure 1: Typical application example
Figure 2: DESAT function: Diagram of principle
Figure 1 shows the application example using Infineon’s third-generation galvanic isolated EiceDRIVER™ driving an IGBT. The driver is connected to the microcontroller on the primary side with various pins and their connections.
Figure 2 shows the internal structure of the DESAT pin. When the voltage at VCE pin exceeds the threshold, the NFLT logic is set low and the signal is detected by the microcontroller. The driver initiates a safe turn-off procedure to protect the switch. The DESAT pin has an internal clamping, which clamps DESAT to VDESATL when TOUT= low (negative secondary supply). The clamp is released after the DESAT blanking time is exceeded.
When an IGBT or MOSFET is completely on, it is said to be saturated; when it exceeds the maximum current ratings, it is known as over saturate which will eventually cause device failure. A desaturation protection circuit provides protection for power semiconductor switches (IGBT or MOSFETs) against such events. The power switch collector-emitter voltage, VCESAT, the voltage drop across the component, is monitored by the DESAT pin of the gate driver while the device issues a PWM ON command.
When there is a short circuit in an application and a very high current flows through the power semiconductor switch, it will go into the desaturation mode; hence, its VCESAT voltage will rise. A fault is detected by the gate driver (while the switch is on) once this VCESAT voltage goes above the internal desaturation fault detection threshold voltage, which is typically 9 V for IGBT and 6 V for SiC MOSFETs.
Differences between SiC MOSFET and IGBT during DESAT detection
Table 1 Differences between SiC MOSFET and IGBT during DESAT detection
|
Sr. No. |
DESAT detection with SiC MOSFET |
DESAT detection with IGBT |
|
1. |
DESAT threshold voltage: 6 V |
DESAT threshold voltage: 9 V |
|
2. |
|
Transfer characteristic 
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|
3. |
SiC MOSFET works in the linear region during normal operation. SiC MOSFET does not desaturate virtually . The desaturation level of the SiC MOSFET is much higher than the level of an IGBT when assuming an identical chip size for both components. |
IGBT typically works in the saturation region during a normal on state and hits the current limit very early. A desaturation protection circuit is usually adopted to sense the VCE and the IGBT needs to be shut down when VCE exceeds the preset voltage ratio. |
External components of the DESAT circuit
The connection of the DESAT pin via three external components to the SiC/ IGBT module provides the following three functionalities:
- A resistor with a minimum value of 1 kΩ limits the current flowing through the DESAT diode if VCE is negative.
- A HV-diode protects the DESAT pin from high-voltages when the IGBT/ SiC MOSFET is turned off.
- A capacitor is used to define a filtering time together with the internal current source and to increase the robustness against false triggering of DESAT for short-term voltage peaks during runtime.
Related FAQs on DESAT
1.What is blanking time in the DESAT circuit?
Blanking time (DESATBT) is the time taken to ensure no unintended tripping of the DESAT protection happens while turning on the IGBT / SiC MOSFET after the DESAT circuitry has been disabled for a short time period such that the collector voltage of the IGBT / SIC MOSFET falls below the threshold level.
The blanking time, together with the external filter and DESAT reaction time defines the minimum time required for the gate driver to detect the fault condition and to activate the DESAT protection. The filter time is defined by the internal charge current of ICHG = 500 μA (typically), DESAT threshold VDESATth, and an external blanking capacitor, CDESAT.
tblanking = VDESATth × CDESAT/ICHG
The external filter time can be further reduced by connecting an external connecting source on the DESAT pin or by connecting an external resistor between VCC and the DESAT pin.
2. What is the minimum pulse required to trigger the DESAT function?
It depends strongly on external filtering components and the selected DESATBT. Generally, the DESAT detection will be triggered after passing VDESAT threshold followed by the DESAT blanking time.
3. Can a Schottky diode be used as a DESAT diode?
Yes, a fast recovery diode or Schottky diode can also be used as a DESAT diode.
Community Translation: 車載用ゲートドライバのDESAT保護– KBA236920
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