Apparatus for discharging a high-voltage bus

1. A discharge circuit for a high-voltage bus that is electrically connected to a high-voltage DC power source, wherein the high-voltage bus includes positive and negative conductors, comprising:

a discharge switch electrically connected in series with a discharge resistor between the positive and negative conductors of the high-voltage bus, wherein the discharge switch includes a gate;

a bi-stable switch including a control gate, an input line that is electrically connected to the high-voltage bus and an output line that is electrically connected to the gate of the discharge switch; and

a trigger device in communication with the control gate of the bi-stable switch;

wherein the input line of the bi-stable switch is electrically connected to the high-voltage bus; and

wherein the discharge switch is controllable to a closed state to provide an electric current flow path through the discharge resistor between the positive and negative conductors of the high-voltage bus in response to an activation signal communicated from the trigger device to the control gate of the bi-stable switch.

2. The discharge circuit of claim 1 , wherein the trigger device further comprises a composite trigger gate having a plurality of input lines and an output line in communication with the control gate of the discharge switch.

3. The discharge circuit of claim 2 , wherein the plurality of input lines includes a first trigger signal that is output from a first trigger circuit that is electrically connected to a signal line electrically connected to a first contactor that is electrically connected between the high-voltage DC power source and one of the positive and negative conductors of the high-voltage bus.

4. The discharge circuit of claim 3 , wherein the first trigger circuit is in communication with an ignition switch.

5. The discharge circuit of claim 1 , wherein the input line of the bi-stable switch electrically connected to the high-voltage bus comprises a capacitive divider electrically connected between the positive and negative conductors of the high-voltage bus, wherein the capacitive divider includes a first capacitor electrically connected in series with a second capacitor at a first node, and wherein the input line of the bi-stable switch is electrically connected to the first node.

6. The discharge circuit of claim 5 , wherein the first capacitor and the second capacitor are sized to achieve a magnitude of voltage at the first node that activates the gate of the discharge switch when the bi-stable switch is activated.

7. The discharge circuit of claim 5 , further comprising a pull-down resistor arranged in parallel with a third capacitor between the gate of the discharge switch and the negative conductor of the high-voltage bus.

8. The discharge circuit of claim 1 , further comprising a bulk capacitor electrically connected between positive and negative conductors of the high-voltage bus, wherein the discharge switch is controllable to a closed state to provide an electric current flow path through the discharge resistor between the positive and negative conductors of the high-voltage bus to discharge the bulk capacitor.

9. The discharge circuit of claim 1 , wherein the high-voltage bus electrically connects the high-voltage DC power source to a power inverter.

10. The discharge circuit of claim 1 , wherein the discharge switch is a MOSFET device.

11. The discharge circuit of claim 1 , wherein the bi-stable switch comprises a thyristor.

12. A power inverter disposed to control an electric machine, the power inverter being electrically connected to a high-voltage DC power source via a high-voltage bus, the power inverter comprising:

a multi-phase inverter circuit electrically connected to positive and negative conductors of the high-voltage bus;

a bulk capacitor electrically connected between the positive and negative conductors of the high-voltage bus; and

a discharge circuit for the high-voltage bus, including:

first and second contactors being controllable to electrically connect a respective one of the positive and negative conductors of the high-voltage bus to the high-voltage DC power source,

first and second trigger circuits in communication with a respective one of the first and second contactors,

a capacitive divider electrically connected between the positive and negative conductors, and including a first capacitor electrically connected in series with a second capacitor at a first node,

a discharge resistor electrically connected in series with a discharge switch between the positive and negative conductors,

a bi-stable switch electrically connected between the first node and a gate of the discharge switch, and

the first and second trigger circuits being in communication with a gate of the bi-stable switch,

wherein the first and second trigger circuits communicate with the gate of the bi-stable switch to control the discharge switch to an open state when the ignition switch is in a key-ON state, and

wherein the bi-stable switch controls the discharge switch to a closed state to provide a low-impedance electric current flow path through the discharge resistor between the positive and negative conductors of the high-voltage bus in response to an activation signal that is communicated to the bi-stable switch.

13. The power inverter of claim 12 , wherein the discharge switch is controllable to the closed state to provide a low-impedance electric current flow path through the discharge resistor between the positive and negative conductors of the high-voltage bus in response to the ignition switch being in a key-OFF state.

14. The power inverter of claim 12 , wherein the discharge switch is controllable to an open state to provide a high-impedance resistance to current flow through the discharge resistor between the positive and negative conductors of the high-voltage bus when the ignition switch is in a key-ON state and the input lines to the composite trigger gate are in a LOW state.

15. The power inverter of claim 12 , wherein the power inverter electrically connects to an electric machine.

16. The power inverter of claim 12 , wherein the discharge resistor comprises a resistor having an impedance that is selected to minimize time to discharge the bulk capacitor.

17. The power inverter of claim 12 , wherein the bulk capacitor is electrically connected between the positive and negative conductors of the high-voltage bus in parallel with the discharge switch that is electrically connected in series with the discharge resistor.

18. A method for discharging electric power stored on a high-voltage bus, the method comprising:

activating, by a trigger device, a bi-stable switch in response to a trigger signal,

wherein the bi-stable switch includes an input line that is electrically connected to the high-voltage bus, wherein the input line is connected to a first node that connects a first capacitor electrically connected in series with a second capacitor between positive and negative conductors of the high-voltage bus;

wherein the bi-stable switch includes an output line that is electrically connected to a gate of a discharge switch; and

wherein the discharge switch is electrically connected in series with a discharge resistor between the positive and negative conductors of the high-voltage bus.