Gate drivers with DESAT protection have a built-in threshold which is effective for many IGBTs, but could be slightly too high or too low for other IGBTs or other types of device. This DESAT detection threshold can be tailored to suit different devices simply by changing discrete components in the DESAT circuit.

This Document is available as in PDF format, link below

Adjusting the DESAT Threshold.pdf

ToC

1. Intro
2. What is Desaturation in an IGBT?
3. How does the DESAT detection circuit work?
4. Why adjust the DESAT threshold?
5. How do we adjust the DESAT threshold?
6. References & additional resources
   

Intro

DESAT is a short-circuit/overcurrent detection system integrated in UCC5870, UCC217xx, and ISO5x5x. As its name suggest, DESAT detects "desaturation" of an IGBT and is triggered when VCE rises above a certain threshold set by the design. VCE rising corresponds with the IGBT leaving saturation region and approaching the active region. After DESAT detection, gate drivers (including ISO5x5x and UCC217xx families) will immediately begin pulling the gate low to turn off the IGBT and end the short circuit condition.

Gate drivers with DESAT protection have a built-in threshold which is effective for many IGBTs, but could be slightly too high or too low for other IGBTs or other types of device. This DESAT detection threshold can be tailored to suit different devices simply by changing discrete components in the DESAT circuit.

What is Desaturation in an IGBT?

The optimal operating point of IGBTs in power applications is in the saturation region, where V_CE and thus dissipated power are controlled. In the event of a short circuit condition (for example if both high and low-side IGBTs get turned ON simulateously), VCEwill rise quickly while collector current saturates. This puts the IGBT in the active region of operation. A general IC V_CE curve is shown in Figure 1.

Active region is very undesirable for IGBTs in power applications because this causes unsustainably high power dissipation in the bipolar junction and a swift failure of the IGBT itself. This "desaturation" concept is in contrast to FETs, where rising  V_GS leads to the device approaching saturation region (rather than leaving it), and where devices are operated in the linear region for power applications. Please see here for additional information on Desaturation.

In FETs, DESAT detection would therefore be utilized for detecting "saturation" rather than "desaturation," though for SiC FETs in particular direct Over-current can be preferable due to SiC's drain characteristics. The difference in DESAT efficacy between IGBT and SiC is explained further in this eBook.

Figure 1: IGBT Collector Characteristics

How does the DESAT detection circuit work?

DESAT system detects when the collector voltage V_CE is above a threshold, V_DESAT, though not directly.

Figure 2 shows a standard DESAT implementation.

Figure 2: Typical DESAT circuit

During normal operation, the IGBT is in saturation, V_CE is low and and the diodes D_HV1,2 are forward biased. The I_CHG current can then flow through the diodes to the collector, preventing the blanking capacitor C_BLK from charging, as shown in Figure 3.

Figure 3: DESAT circuit during normal IGBT operation

In short-circuit, V_CE rises and reverse-biases D_HV1,2, so the I_CHG current now charges the blanking capacitor C_BLK , as shown in Figure 4.

Figure 4: DESAT during Short-circuit

After a time (set by the capacitance value), the voltage on the cap exceeds the internal threshold  V_DESAT and the comparator indicates that DESAT is detected. The collector voltage where I_CHG begins charging the blanking capacitor is V_CE  is considered the system DESAT detection threshold.

 Due to the presence of the D_HV1,2, the voltage at the DESAT pin when the current source begins charging the blanking capacitor will be higher than V_CE  by 1 or 2 forward voltage drops , or V_CE +2⋅V_F . We must also consider the drop across the series resistor R_LIM due to the internal current source. Therefore, we find the effective DESAT threshold, measured at the collector as V_CE(DESAT),=V_DESAT −2⋅V_F−I_CHG⋅R_LIM.

In short, the DESAT threshold is the voltage when the diodes stop conducting, which tells the system VCE is too high.

Why adjust the DESAT threshold?

Not all IGBTs are identical, so their VCE / IC transfer characteristics are also different.

The DESAT detection threshold is usually chosen at the "knee voltage" (often 7-10V), depicted in Figure 5, which is the point at which the collector current saturates and is akin to VDSAT for MOSFETs. Designers may wish to choose the effective DESAT detection threshold to suit an IGBT with a different knee voltage, or possibly choose a threshold below the knee voltage to have earlier DESAT detection.

Figure 5: IGBT Current Knee

How do we adjust the DESAT threshold?

The methods to adjust the DESAT detection threshold involve making changes to the components in series between the DESAT pin and the IGBT collector. These methods are summarized in Table 1

Changing the number of series HV blocking diodes allows us to decrease the threshold in steps of the diode V_F. Similarly, we can increase the detection threshold in larger increments by adding a zener diode, as shown in Figure 6. With this, we find that

V_CE(DESAT),=V_DESAT −V_Z−n⋅V_F−I_CHG⋅R_LIM, where n is the number of HV blocking diodes, minimum 1, though often 2 are necessary in order to stand off working voltages of 1kV and up.

To a lesser extent, small adjustments to the threshold can also be made by changing the value of R_LIM, though this should not be used for adjustments larger than a few hundred mV. A zener diode may be used to make larger ( for example, 3V) changes to the DESAT detection threshold and are available in several knee voltages. When selecting a zener, the voltage drop will depend on the current bias–in this case,  I_CHG.

Figure 6

Change	Effect on DESAT Threshold
Add (or remove) HV Blocking diodes	Decreases threshold by total diode VF
Add a zener diode	Decreases threshold  by zener VZ
Change limiting resistor value	Decreases threshold by ICHG⋅RLIM

Table 1: DESAT Threshold Adjustments Summary

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References & additional resources

• IGBT & SiC Gate Driver Fundamentals
• High Voltage Interactive - How to protect SiCMOSFETs... the best way!
• Microsemi - IGBT Tutorial
• [E2E FAQ] Increasing DESAT Charging Current