Circuit and method for coil current control
Circuits and methods to control a current in a coil are disclosed. The circuit and methods provide over-dwell protection and soft shut-down functionality to safely discharge the coil. The safe discharge of the coil is facilitated by a soft-start ramp signal that reduces the coil current gradually by controlling a switching device according. A profile of the soft-start ramp signal over time determines the gradual reduction. The profile of the soft-start ramp signal can be adjusted to set (i) an over-dwell period of the coil current, after which the coil current is shut down, and (ii) a soft shut-down period, over which the coil current is gradually reduced.
1. A circuit for controlling a current in a coil, the circuit comprising:
a capacitor connected between a voltage source (V S ) and an output node;
a first switching device connected between the voltage source and the output node such that when the first switching device is in an ON state the output node is coupled to the voltage source, and when the first switching device is in an OFF state the output node is coupled to the voltage source through the capacitor; and
a voltage-controlled current-source (VCCS) connected between the output node and a ground, the VCCS outputting a current that charges the capacitor such that when the first switching device is moved from the ON state to the OFF state, a voltage is generated at the output node that decreases over a period from V S to a ground voltage according to a soft-start-profile.
2. The circuit for controlling a current in a coil according to claim 1 , further comprising an insulated gate bipolar transistor (IGBT) having a collector coupled to the coil, an emitter coupled to ground, and a gate coupled to the output node such that the voltage generated at the output node reduces a gate-emitter voltage (V GE ) of the IGBT to reduce a current through the IGBT and the coil.
3. The circuit for controlling a current in a coil according to claim 2 , further comprising a non-inverting amplifier coupled between the gate coupled and the output node.
4. The circuit for controlling a current in a coil according to claim 1 , wherein the current output by the VCCS between a minimum current (I START ) and a maximum current (I CHG ) corresponds to a voltage (V CAP ) across the capacitor as the capacitor is charged.
5. The circuit for controlling a current in a coil according to claim 4 , wherein for the current output by the VCCS equals I START when V CAP equals zero volts.
6. The circuit for controlling a current in a coil according to claim 4 , wherein the current output by the VCCS between a minimum current (I START ) and a maximum current (I CHG ) corresponds to a voltage (V CAP ) is linearly related to V CAP by a slope (dI/dV).
7. The circuit for controlling a current in a coil according to claim 1 , wherein the voltage generated at the output node drives a gate of a second switching device to control the current in the coil.
8. The circuit for controlling a current in a coil according to claim 7 , wherein a soft-start profile of the voltage generated at the output node driving the gate of the second switching device configures the second switching device to reduce the current in the coil to zero over a soft shut-down period that begins after an over-dwell period.
9. The circuit for controlling a current in a coil according to claim 8 , wherein the soft shut-down period and the over-dwell period are each controlled by the soft-start profile.
10. The circuit for controlling a current in a coil according to claim 8 , wherein the soft-start profile is determined by characteristics of the voltage-controlled current-source.
11. The circuit for controlling a current in a coil according to claim 10 , wherein the characteristics include a minimum current (I START ), a maximum current (I CHG ), and a slope (dI/dV) of an output current versus an input voltage.
12. A method for controlling a current in a coil, the method comprising:
receiving a charge coil trigger signal
generating a soft-start ramp signal that has a voltage profile that decreases with time; and
applying the soft-start ramp signal to a gate of a switching device to gradually shut down the current in the coil over a soft shut-down period after an over-dwell period, wherein the soft shut-down period and the over-dwell period correspond to the voltage profile of the soft-start ramp signal.
13. The method according to claim 12 , wherein the generating a soft-start ramp signal includes:
coupling a first side of a capacitor to a source voltage;
controlling a current charging the capacitor using a voltage controlled current source (VCCS) that is connected between a second side of the capacitor and a ground voltage; and
outputting the soft-start ramp signal as a voltage at the second side of the capacitor.
14. The method according to claim 13 , wherein a voltage across the capacitor is applied to an input of the VCCS to control current charging the capacitor.
15. The method according to claim 13 , wherein the voltage profile that decreases with time is controlled by operating characteristics of the VCCS.
16. The method according to claim 15 , wherein the operating characteristics of the VCCS include a minimum current, a maximum current, and a slope (dI/dV).
17. The method according to claim 15 , further comprising:
adjusting the operating characteristics of the VCCS to provide a particular over-dwell period or a particular soft shut-down period.
18. The method according to claim 12 , wherein the switching device is an insulated gate bipolar transistor (IGBT).
19. A soft-start ramp generator, comprising:
a voltage source terminal;
a ground terminal;
an output node;
a capacitor coupled between the voltage source terminal and the output node; and
a voltage controlled current source (VCCS) coupled between the output node and the ground terminal, the VCCS configured to control a current through the capacitor based on a voltage across the capacitor so that a voltage at the output node decreases from a voltage at the voltage source terminal to a voltage at the ground terminal according to a soft-start profile.
20. The soft-start ramp generator according to claim 19 , wherein voltage at the output node of the soft-start ramp generator is coupled to a switching device to control a current in a coil to gradually decrease over a soft shut-down period after an over-dwell period.