top of page

DESAT (Desaturation) Protection For IGBT MOSFET Switching

Instagram: @lentark

IGBT DESAT Protection Blog Banner
DESAT Protection Circuit

IGBTs (Insulated Gate Bipolar Transistors) and MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are switching elements used in power electronics applications, and there are many protection methods to ensure these components are not damaged. DESAT (desaturation) protection is just one of these methods and it detects overcurrent/short-circuit conditions of the switching element, allowing the driver to turn off the switching element before any damage occurs. Here are detailed information on DESAT protection for those interested.


In this blog post, we will focus on DESAT (Desaturation) Protection for IGBT MOSFET Switching. You can download the post as a File:

DESAT_Protection[EN]
.pdf
Download PDF • 747KB

1.Introduction to DESAT (Desaturation) Protection For IGBT MOSFET Switching

In power electronics applications, semiconductor switching devices such as IGBTs and MOSFETs play an indispensable role. These components are critical for energy conversion and control and are essential in high-power applications. However, for these components to operate reliably and efficiently, various protective mechanisms need to be integrated. One such protective strategy is the DESAT (desaturation) protection.


As the name suggests, DESAT protection monitors the saturation state of the IGBT or MOSFET, guarding the device against risks that might arise from this state. Saturation refers to a condition where the IGBT or MOSFET, operating outside its nominal conditions, might get damaged due to excessive current flow. This situation is visualized on the output characteristic graph of an IGBT in Figure 1 and on a SiC MOSFET in Figure 2.


Demonstration of desaturation on a sample IGBT output characteristic graph.
Figure 1: Demonstration of desaturation on a sample IGBT output characteristic graph. [1]

Desaturation represents a state where the IGBT or MOSFET is not operating correctly. It increases the internal resistance of the device, leads to overheating, and eventually causes permanent damage. In contrast, during excessive current situations, the DESAT protection circuit activates, swiftly set the driver signal to LOW state and protects the component from potential harm.


The use of DESAT protection with SiC MOSFETs is possible, but it is not the best protection option for SiC MOSFETs. SiC MOSFETs have a shorter short-circuit withstand time (SCWT) compared to IGBTs. Their rapid switching capabilities lead to quick changes in the Id current, making timing critical to detect a potential short-circuit issue. Monitoring the current with a shunt resistor is the best method for SiC MOSFETs.[2]


In SiC MOSFET, the current continues to increase as the Vds increases, which causes the device to fail more quickly due to the high power dissipation and the resultant heat. Also, SiC MOSFETs reach the point of maximum power distribution earlier than IGBTs because they are switched much faster. Thus, for SiC MOSFETs, it's crucial to factor in their unique traits and choose quicker protection methods to ensure device longevity and safety.


Demonstrating desaturation using the output characteristic graph of a sample SiC MOSFET.
Figure 2: Demonstrating desaturation using the output characteristic graph of a sample SiC MOSFET. [2]

Short-circuit protections like DESAT in IGBT and MOSFET drivers are of critical importance for the safe operation of the device. How this protection works, when it is activated, and how DESAT protection is integrated into applications are addressed in the blog.


2.The Cause of DESAT

In IGBTs, excessive current leads to a "desaturation" situation, transitioning from the saturation region to the active region. This phenomenon typically occurs when, as a result of short-circuit events, the device's maximum limits are rapidly exceeded. When an IGBT is desaturated, it operates at its maximum power limit, leading to overheating and damage. Therefore, controlling the current, designated as Ic, to keep the IGBT's operating point within the saturation region is of utmost importance.


With Power SiC MOSFETs, the situation is slightly different. As seen in Figure 2, excessive current leads to a "desaturation" situation, transitioning from the linear (Ohmic) region to the saturation region. This phenomenon can occur during short-circuit events and also during the device's turn-on and turn-off moments. Given that SiC MOSFETs have a shorter SCWT, it's worth reiterating how crucial timing is when detecting situations like excessive current and short circuits. Therefore, instead of DESAT protection, monitoring current using a shunt resistor method is much more effective for SiC MOSFETs. However, it should be noted that the shunt resistor method has a drawback, such as high power loss.


3.Detection of DESAT Condition

The transition to the active region starts with an area known as the "Current Knee", as illustrated in Figure 1. Along with this region, the rate of increase in the Ic current diminishes, and the rise rate of Vce, the voltage drop of the switching element during conduction, escalates. DESAT protection circuits typically employ the Vce level of the switching element during conduction to detect excessive current conditions, aiming to protect the IGBT.


One of the most commonly used overcurrent and short-circuit protection mechanisms is DESAT protection. This structure monitors the Vce level of the switching element in its conduction state to determine whether Vce reaches a predefined threshold. For instance, as shown in Figure 3, it tracks the Vce level, considering the Vdesat voltage level corresponding to a predefined operating point (Point A) as the threshold. Typically, the VDESAT threshold is chosen from a point in the "Current Knee" region where the Vce's increase accelerates. If the "Current Knee" region lies outside the operating limits, the threshold level is chosen at a reasonable distance from the Vce level at the switching element's operating point. For instance, it's selected to be far enough from the operating point so that other system parameters can endure without showing stress, and close enough not to approach the switching element's limits.


If a scenario emerges where Vdesat is less than Vce,q, the DESAT protection is activated and ensures the safe shutdown of the switching element. Concurrently, it sends an error signal related to this situation to the control mechanism, thereby shielding the device from potential damage. DESAT protection not only ensures system safety but also prolongs the switching element's lifespan by preventing undue stress.


The A threshold point on the output characteristic graph of an IGBT operating at point Q.
Figure 3: The A threshold point on the output characteristic graph of an IGBT operating at point Q.


4.How DESAT Protection Works

Figure 4 shows the structure of a DESAT protection.

Demonstration of the DESAT protection structure at the application level.
Figure 4: Demonstration of the DESAT protection structure at the application level.

When the IGBT is in the off state, and thus there is no overcurrent issue, the operation of the protection circuit will be as shown in Figure 5.


When the IGBT is in the off state, the direction of the charging current.
Figure 5: When the IGBT is in the off state, the direction of the charging current.

When the IGBT is conducting, and there is no overcurrent issue, the protection circuit will operate as shown in Figure 6.

The direction of the charging current when the IGBT is in conduction and there is no overcurrent.
Figure 6: The direction of the charging current when the IGBT is in conduction and there is no overcurrent.

The response of the protection circuit in a situation where the IGBT is in conduction and there's an overcurrent problem will be as shown in Figure 7.

The direction of the charging current when the IGBT is in conduction and there is an overcurrent.
Figure 7: The direction of the charging current when the IGBT is in conduction and there is an overcurrent.

A while after the event in Figure 7 starts to occur (determined by the capacitance value), the voltage between points A and COM rises above the Vds voltage, and this is detected by the DESAT comparator, which then sends this information to the control mechanisms.


4.1.Protection Delay

The charging time of the Cds capacitor determines how long it will take for the DESAT condition to be reported to the control system. The delay time is calculated according to equation 1.

IGBT DESAT Protection Delay  Equation.
IGBT DESAT Protection Delay Equation.

Such a delay acts as a filter for numerous false triggering situations like noise, voltage fluctuations, and transitions. However, this value should not be longer than the SCWT of the switching element.


4.2.DESAT Threshold Voltage

The DESAT threshold voltage (Vth) is the voltage at which diodes cease conduction. Based on whether this threshold is exceeded or not, it is determined if the Vce for the system is high. The DESAT threshold voltage is calculated according to equation 2. [4]

IGBT DESAT Protection Threshold Voltage  Equation.
IGBT DESAT Protection Threshold Voltage Equation.

5.DESAT Protection in Practice

The DESAT protection circuit can be constructed using discrete components. However, many modern IGBT and MOSFET drivers come equipped with a DESAT protection feature. This feature is typically paired with an integrated DESAT condition sensor and a rapid response mechanism. The DESAT sensor monitors the voltage across the component and activates the protection circuit when a certain threshold value is exceeded.


6.Conclusion

Protecting IGBT and MOSFET components ensures the reliability and longevity of power electronic systems. DESAT protection assists in safeguarding these components from potentially harmful conditions, preventing severe malfunctions and component damages. As such, the DESAT protection feature is crucial in high-performance electronic systems.


7.Contact

Lentark Electronics

Website 1: www.lentark.com

Website 2: www.lentarkstore.com

E-mail : info@lentark.com


8.References

  • [1] Product corresponding to the graph: CM600DU-24NF MITSUBISHI IGBT.

  • [2] TI, “IGBT & SiC Gate Driver Fundamentals.”, Q4 2021, p28.

  • [3] Product corresponding to the graph: ROHM, BSM300D12P3E005 SiC Power Module.

  • [4] TI, “How can we adjust the DESAT detection threshold?”

176 views0 comments

Recent Posts

See All

Comments

Rated 0 out of 5 stars.
No ratings yet

Add a rating
bottom of page