In many architectures, when initially connecting or biasing a high-voltage network to a high-voltage potential, a pre-charge system is utilized. This pre-charge approach is especially prevalent when a high-voltage contactor is used to connect a previously-disconnected high-voltage subdomain, such as a traction inverter, to another already-biased high-voltage domain, such as the high-voltage bus in an HEV. If a pre-charge network is not used, when the contactor or relay is thrown to bridge the domains, a surge of current passes through the contactor to charge the deliberate and parasitic capacitances in the subdomain. If the magnitude of the surge current exceeds the contactor rating, the resulting heat can weld the contactor shut, shorting the domains together and introducing a safety risk.

The pre-charge network acts before the main contactor is thrown, providing a path to slowly ramp up the voltage of the subdomain in a controlled and current-limited manner. Once the subdomain has pre-charged to within a volt or several volts of the main voltage domain, the main contactor is thrown to fully bridge the domains. Since the two domains are at approximately the same voltage due to the pre-charging, the inrush current through the contactor is significantly reduced. For more detailed information, refer to the Why Pre-Charge Circuits are Necessary in High-Voltage Systems application note.

Because the RES60A-Q1 is commonly used to perform voltage measurements on either side of the contactor, the device can be charged by the pre-charge network before the main contactor is thrown. In the event of a fault with the pre-charge network, however, or in architectures without a dedicated pre-charge pathway, the device can be exposed to a voltage spike that is not current-controlled. Other failure conditions such as a load dump can result in similar stresses at the device. While current-limiting resistors can provide additional protection, using unmatched external resistors in series with the RES60A-Q1 introduces new mismatch errors that compromise the voltage divider accuracy. Therefore, the RES60A-Q1 must withstand a large voltage pulse with a fast edge rate, without external current limiting.

Development of a hot swap board facilitated evaluation of the RES60A-Q1 response to this scenario, incorporating a high-voltage relay and a large bank of high-voltage capacitors. The capacitors are charged to a high potential, in this case 1000Vdc. When the relay is thrown, the high potential is applied to the HVIN pin. The device transient response to this stress test, as recorded for several ratios of RES60A-Q1, is shown below.

While there is an initial voltage spike at the MID pin, the spike settles quickly, and the devices are not damaged. Incorporating a filtering capacitance, as discussed in Section 7.4.1, reduced the magnitude of the voltage spike to protect downstream circuitry. Though circuit designers must still use caution and consider the possibility of implementation-specific inductive effects, the data suggest the RES60A-Q1 is resilient to this fault condition. For additional information on the hot swap board architecture and experimental results, refer to the RES60A-Q1 Extended Reliability Testing application note.