Fault-detection mechanisms are a necessity in high-power industrial systems such as motor drives and solar inverters, as well as automotive systems including electric vehicle (EV) chargers, traction inverters, onboard chargers and DC/DC converters. DC bus-based overcurrent protection is widely used in electric motor drives. The traditional implementation of overcurrent (OC) fault detection is discrete with a combination of non-isolated multichannel comparators and either optocouplers or digital isolators. To meet the growing needs of fault detection, TI is introducing a new family of basic and reinforced isolated comparators to the TI isolation portfolio. The primary use case is ultra-fast overcurrent, overvoltage, over temperature detection in high-voltage industrial and automotive HEV/EV systems. Its smaller PCB area is particularly suitable for applications with miniaturization and high-power density needs. Compared with traditional solutions, it has significant advantages in CMTI, response time, threshold accuracy, hysteresis, and latching function.

The power supply on DC+ for overcurrent protection can be generating by either adding a transformer or adding an additional secondary winding to an existing transformer. However, technical challenges such as transformer size limitations or the proximity of these transformers to the actual OC implementation will practically limit such a transformer-based implementation.

Figure 2-1 shows how the OC protection based on the DC+ bus can protect three short-circuit situations: a shoot-through fault of IGBT (blue), DC+ ground fault (red), and phase-to-phase short fault (yellow).

Adding an isolated comparator such as a single threshold comparator AMC23C11 for OC detection ensures appropriate detection and protection of power circuits from these three conditions. The dual-threshold version (AMC23C14) can achieve both short circuit and overload protection.

As there is no need for a power supply on DC+ other than for an isolated comparator, a simple, low-cost, and low BOM implementation is critical.

Figure 3-1 shows the implementation of a DC+ power supply.

Figure 3-1 DC+ Power Supply Solution

The DC+ power supply solution consists of two parts. The first part is a bootstrap circuit consisting of a high-voltage diode D1, a capacitor C7 and current-limiting resistor R6. C7 is charged by the low-side gate driver power supply. The second part is a charge pump consisting of a high-voltage diode D2, a capacitor C2 and current-limiting resistor R4. When the half bridge starts to operate there are three operating states as follows:

• When the low-side IGBT turns on, C7 is charged by the power supply of the low-side gate driver and D2 is turned off.
• When the high-side IGBT turns on, D2 conducts, C7 charges the isolated comparator and capacitor C2.
• When the low-side IGBT turns on again, C7 is charged by the low-side power supply and C2 continues to supply power to the isolated comparator.

In summary, this charge-pump based bootstrap circuit provides power to the isolated comparators with the help of energy stored in capacitors C7 and C2 during the IGBT switch phases.