Power semiconductors like MOSFETs and IGBTs ideally operate in a fully saturated state: ohmic region for MOSFETs and saturation region for IGBTs. The voltage across, V_DS for MOSFETs or V_CE for IGBTs, remains low to minimize power dissipation. However, when these devices enter an oversaturated state as shown in Figure 1-1, small increases in current cause large voltage increases, leading to excessive power dissipation and potential damage.

Figure 1-1 Example IGBT I-V Curve

Some isolated switch drivers include a desaturation (DESAT) protection feature that monitors V_DS or V_CE and quickly shuts down the semiconductor during an overcurrent event. The TPSI31xx is a family of fully integrated isolated switch drivers, which when combined with an external power switch, forms a complete isolated solid-state relay solution.

The TPSI3133, as shown in Figure 1-2, is a variant specifically intended for DESAT due to its internal pulldown MOSFET on the fault comparator input. This internal MOSFET prevents false positives while the IGBT/SiC MOSFET is off by pulling down the fault comparator input and keeping it pulled down for an additional 100ns after EN goes high, allowing the IGBT/SiC MOSFET to fully turn on.

Figure 1-2 DESAT With TPSI3133

DESAT protection circuits are typically configured with IGBTs because IGBTs show distinct voltages (V_CE) across current (I_C) in the saturation region which makes detection easier, have sharper transitions into the active region, and generally able to handle more power due to larger die size (many have a short circuit withstand rating).

DESAT can also work with SiC MOSFETs. The problem with low voltage MOSFETs is since the overcurrent threshold is typically set well above normal operation to avoid false positives, a low voltage MOSFET may be damaged by overheating well before the circuit even detects an overcurrent event.

By implementing DESAT protection, engineers can ensure power semiconductors stay within safe operating area.

1. Blocking diode: Protects the circuit from high voltage while the IGBT/SiC MOSFET is off.
2. Current limiting resistor (R_LIM) to limit diode forward current.
3. DESAT threshold detection (R_DIV1, R_DIV2): Defines the voltage threshold for detecting oversaturation.
4. Blanking capacitance (C_BLK): Filters noise and prevents false triggering. While blanking capacitance helps avoid false positives, it may also extend stress time on the IGBT/SiC MOSFET during overcurrent events.

At power-up while EN is low, TPSI3133 receives power and begins to transfer power to its secondary rails (VDDM and VDDH). The internal MOSFET pulls down the comparator input to avoid a false trigger. Once EN goes high, the two current paths (paths 1 and 2) compete to set the comparator input voltage as shown in Figure 2-1. Since Path 1 normally would be faster than path 2 due to IGBT turn-on delay, once EN goes high, the TPSI3133 keeps the comparator input pulled down for an additional 100ns, allowing the IGBT to fully turn on before fault detection. Adding blanking capacitance (C_BLK) can provide additional delay but must be carefully selected in order to minimize stress time. During an overload condition, the IGBT V_CE rises, which brings up the voltage at the HV diode anode, trips the fault comparator threshold. Once the TPSI3133 detects the fault, it shuts off the driver which turns off the IGBT, protecting the system.

Figure 2-1 DESAT KVL Paths

Table 3-1 lists the requirements for an example DESAT design.

Selecting appropriate resistance values ensures that the blocking diode becomes forward biased while the IGBT is on, allowing proper voltage sensing.

1. Condition for sensing voltage, allows the blocking diode to become forward biased when IGBT is on:
   Equation 1. $\left(V_{CE\left(DESAT\right)}+V_{F\left(DIODE\right)}\right)<\frac{{R_{DIV1}+R}_{DIV2}}{R_{DIV2}+R_{DIV1}+R_{LIM}}\times VDDH$
2. Overcurrent threshold equation, select the desired voltage threshold (V_{CE (DESAT)}) to trip the comparator (V_REF+):

Using the example IGBT I-V curve from earlier, it looks like I_C = 10 A at V_CE = 7.5V. With this overcurrent threshold value, solve for the resistances. Please keep in mind that I-V curves are typical and devices may individually have variation.

Figure 3-1 DESAT Selection V_CE Threshold

Select R_LIM to limit diode forward current. Given the example HV diode I-V curve in Figure 3-2, selecting 54.9kΩ would limit V_F to 0.7V. Please keep in mind that the TPSI3100 family has 25mW available for powering auxiliary circuitry, R_LIM = 54.9kΩ consumes up to 5.26mW.

Figure 3-2 Example HV Diode I-V Curve

1. $(7.5V+0.7V)\times {1.2}_{\left(20\%margin\right)}<\frac{{R_{DIV1}+R}_{DIV2}}{R_{DIV2}+R_{DIV1}+54.9e3}\times 17V$
2. $\frac{R_{DIV2}}{R_{DIV2}+R_{DIV1}}\times \left(7.5V+0.7V\right)>1.23V*{1.2}_{\left(20\%margin\right)}$

1. $R_{DIV1}>-\frac{41}{50}$
2. $R_{DIV2}>\frac{1}{73}\left(5325300-73\times R_{DIV1}\right)$

Select a C_BLK value based on the 10µs maximum delay requirement, starting with the general RC equation. If we use R_RESP < 10kΩ, propagation delay time can be as long as 460ns and needs to be accounted for. Use Thevenin/Norton equivalent to determine the equivalent resistance seen by C_BLK.