# Patent

## Title
Circuit and method for de-energizing a field coil

## Abstract

A circuit includes a first half bridge including a first controllable semiconductor switch and a first diode. The first controllable semiconductor switch is coupled between a first constant supply potential and a center tap of the first half bridge. The first diode is coupled between the center tap and a constant reference potential. A second half bridge includes a second diode and a second controllable semiconductor switch. The second diode is coupled between a second constant potential higher than the first potential and a center tap of the second half bridge. The second controllable semiconductor switch is coupled between the center tap and the constant reference potential. Driver circuitry controls the conducting state of the first and the second semiconductor switch thus controlling the current flow through a field connectable between the center taps.

## Background
This is a continuation application of U.S. patent application Ser. No. 12/703,361, entitled “Circuit and Method for De-Energizing a Field Coil,” which was filed on Feb. 10, 2010, and which is hereby incorporated herein by reference in its entirety.


TECHNICAL FIELD
The invention relates to a driver circuit for driving a field coil of a generator, especially an electric generator (alternator) for automotive applications.
BACKGROUND
During the operation of motor vehicle generators sudden surge voltages occur in response to a load dump because the magnetomotive force and the stored magnetic energy of the field coil of the generator can only be degraded at a finite rate. In common generator control circuits the field coil of a generator is de-energized by means of expensive components, such as power zener diodes, in order to prevent the output voltage of the generator to increase to high values which may cause damage to various electric components connected to the generator.
Various circuits for de-energizing field coils in order to reduce the magnetic energy stored therein are known in the art. Although such circuits may alleviate the problem of surge voltages that may occur especially in response to a load dump, there is still the need for improved circuits that allow a quick and reliable de-energization of field coils.
SUMMARY OF THE INVENTION
A circuit arrangement is disclosed. The circuit arrangement includes a first half bridge including a first controllable semiconductor switch and a first diode. The first controllable semiconductor switch is coupled between a first constant supply potential and a center tap of the first half bridge. The first diode is coupled between the center tap and a constant reference potential. A second half bridge includes a second diode and a second controllable semiconductor switch. The second diode is coupled between a second constant potential higher than the first potential and a center tap of the second half bridge. The second controllable semiconductor switch is coupled between the center tap and the constant reference potential. Driver circuitry controls the conducting state of the first and the second semiconductor switch thus controlling the current flow through a field connectable between the center taps.
Further a method for controlling the magnetic energy stored in a field coil of a generator is disclosed. During normal operation, current is supplied to the field coil thus generating a desired magnetic field. The current flows from a circuit node providing a first constant supply potential via the field coil to a terminal providing a constant reference potential. A load dump of a load coupled to the generator is detected. In case a load dump is detected, current supplied to the field coil stopped and a current path is provided from the terminal providing the constant reference potential via the field coil to a node providing a second constant potential higher than the first supply potential.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, instead emphasis being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts. In the drawings:
FIG. 1 is a circuit diagram illustrating a known H-bridge for controlling the magnetic energy stored in a field coil;
FIG. 2 is a circuit diagram illustrating a first example of the present invention wherein for de-energizing the coil a counter voltage is applied thereto which is higher in magnitude of a system voltage;
FIG. 3 is a circuit diagram illustrating the example of FIG. 2 in more detail; and
FIG. 4 is a circuit diagram illustrating an alternative to the circuit of FIG. 3.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
As mentioned above, sudden surge voltages may occur during the operation of generators in response to a load dump because the magnetomotive force (i.e., because of the stored magnetic energy of the field coil of the generator). For this reason, common generator control units include circuits for de-energizing the field coil thus considerably reducing the amplitude and the duration of the surge voltages.
FIG. 1 illustrates a so-called H-bridge circuit for reducing the energy of a field coil. The circuit has two power switches M1, M2 which may be implemented, for example, as MOSFETs or other type of transistors. During normal operation of the circuit, both power switches M1, M2 are closed thus providing current from the system power supply (voltage VB) to the field coil LF of the generator thus generating an exciting field. Thereby a certain amount of magnetic energy is stored in the coil LF. The MOSFETs M1, M2 are usually driven by a pulse-shaped control signal such as, for example, a pulse-width-modulated (PWM) signal, for controlling the average current through the field coil. As long as the transistors M1, M2 are in a conducting state, current flows from the power system supply (voltage VB) via power switch M1, field coil LF, and power switch M2 to ground potential GND.
During a non-conduction state of transistor M1, when transistor M1 is driven in an off-state while transistor M2 is still on, the current path VB-M1-LF-M2-GND is interrupted and the coil discharges via free-wheeling diode D1. The effective counter voltage present at the field coil LF during such a free-wheeling phase is the forward voltage of the free-wheeling diode D1, e.g., approximately 0.7 V.
However, in case of a load dump the magnetic energy stored in the coil has to be reduced more quickly than is achievable in the above-mentioned free-wheeling phase during normal operation. For a rapid de-energization of the magnetic energy of field coil LF, both power switches M1 and M2 are driven into an off state (non-conduction state). In this case, the current flows via diode D1, field coil LF, and diode D2. Thereby, current flows through the diodes D1 and D2 in a forward direction. Hence the current flows contrary to the direction of the voltage VB (for example, 14 V) of the system power supply augmented by the sum of the forward voltages of the diodes D1 and D2, i.e., a total of about 15.4 V. The counter-voltage required for this (approximately 15.4 V in the present example) is built up by the self-inductance of the field coil.
FIG. 2 is a block diagram illustrating an improved circuit for controlling the magnetic energy stored in a field coil LF. Similar to the circuit of FIG. 1, the field coil LF (e.g., for generating an exciting field of a generator) is coupled between the center taps of two half bridges thus forming an H-bridge.
A first half bridge is connected between a terminal providing a constant first supply potential VB (e.g., from an automotive battery) and a terminal providing a constant reference potential GND. The first half bridge includes a series circuit of a first controllable semiconductor switch M1 and a diode D1. Thereby the first semiconductor switch M1 is connected between the terminal providing the constant first supply potential VB and the center tap, whereas the first diode D1 is connected between the center tap and the terminal providing the constant reference potential GND.
Similarly the second half bridge is connected between a terminal providing a constant second potential VD higher than the first potential VB and the terminal providing the constant reference potential GND. The second half bridge includes a series circuit of a second diode D2 and a second controllable semiconductor switch M2. Thereby the second Diode D2 is connected between the terminal providing the constant second potential VD and the center tap whereas the second semiconductor switch M2 is connected between the center tap and the terminal providing the constant reference potential GND.
One of ordinary skill will understand that the diodes D1, D2 might be replaced by other appropriate switching elements which provide the same function. Further, the controlled semiconductor switches may be implemented as, for example, MOSFETs, BJTs, IGBTs or other types of transistors.
In the example of FIG. 2, the switching state (on or off) of the first semiconductor switch M1 is controlled by a first driver circuitry 10 that controls the current flow through the first semiconductor switch M1 and thus the magnetic field provided by the field coil LF. During normal operation the strength of the excitation field is controlled by modulating the driver signal supplied to a control terminal of the first semiconductor switch M1. In many applications this modulation is a pulse width modulation. However, other types of modulation (pulse frequency modulation, pulse density modulation, etc.) may be appropriate. During such normal operation the second semiconductor switch M2 is driven into an on state by a second driver circuitry 20 that is coupled to a control terminal of the second semiconductor switch M2 and thus controls the current flow through it. Consequently, during normal operation the current through the field coil LF takes the current path VB-M1-LF-M2-GND or GND-D1-LF-M2-GND dependent on the conduction state of the first semiconductor switch M1.
In case of a load dump, i.e., in case an electric load suddenly disconnects from the generator that is excited by the field coil LF, both semiconductor switches M1 and M2 are driven into an off state. Therefore the second control circuitry 20 is configured to detect such load dumps, for example, by monitoring the total load current provided by the generator. During this de-energizing mode of operation of the circuit of FIG. 2 the current through the field coil LF takes the current path GND-D1-LF-D2-VD, where VD is a constant potential higher than the system supply potential VB. Thus the resulting counter voltage present at the field coil is VD+2.VF wherein VF is the forward voltage of the diodes D1 and D2. As in the present example the field coil LF de-energizes at a higher counter voltage (since constant potential VD>system supply potential VB) compared to the circuit of FIG. 1 the speed of de-energizing is improved.
The example illustrated in the circuit diagram of FIG. 3 is similar to the example of FIG. 2. However, the example of FIG. 3 includes one possibility of how to provide the high potential VD. In the present example a DC/DC converter 30 is connected between the terminal providing the constant system supply potential VB and the terminal providing the constant potential VD. As the potential VD is higher than supply potential VB the DC/DC converter may include a boost converter.
An alternative to the example of FIG. 3 is illustrated in FIG. 4 by means of a further circuit diagram. In this example the boost converter 30 is replaced by a zener diode DZ, which is connected between the terminal providing the constant system supply potential VB and the terminal providing the constant potential VD. Thus, in the de-energizing mode of operation (see description above with reference to FIGS. 1 and 2) the constant potential VD equals the sum of supply potential VB and zener voltage VZ. Thus, during this de-energizing mode of operation of the circuit of FIG. 4 the current through the field coil LF takes the current path GND-D1-LF-D2-DZ-VB, wherein the potential present at the common node between diode D2 and zener diode DZ is VD=VB+VZ.
Although the present invention has been described in accordance to the embodiments shown in the figures, one of ordinary skill in the art will recognize there could be variations to these embodiments and those variations should be within the spirit and scope of the present invention. Accordingly, modifications may be made by one ordinarily skilled in the art without departing from the spirit and scope of the appended claims.

## Claims

1. A circuit comprising:
a first half bridge including a first controllable semiconductor switch and a first switching element, wherein the first controllable semiconductor switch is coupled between a first substantially constant supply potential and a center tap of the first half bridge, and the first switching element is coupled between the center tap and a constant reference potential;
a second half bridge including a second switching element and a second controllable semiconductor switch, wherein the second switching element is coupled between a second substantially constant potential that is higher than the first substantially constant supply potential and a center tap of the second half bridge, and the second controllable semiconductor switch is coupled between the center tap and the second substantially constant potential; and
driver circuitry for controlling a conducting state of the first semiconductor switch and the second semiconductor switch thus controlling current flow through an inductance connectable between the center taps.

2. The circuit of claim 1, wherein:
the first switching element comprises a first diode; and
the second switching element comprises a second diode.

3. The circuit of claim 1 further comprising:
a field coil of a generator coupled between the center tap of the first half bridge and the center tap of the second half bridge;
wherein the driver circuitry is configured to control an exciting field for the generator by providing a modulated driver signal to the first semiconductor switch during normal operation, and
wherein the driver circuitry is further configured to detect a load dump and to drive the first and second semiconductor switches to an off state in case a load dump has been detected.

4. The circuit of claim 1, wherein:
the center tap of the first half bridge is directly connected to a first terminal of the first controllable semiconductor switch and to a first terminal of the first switching element; and
the center tap of the first half bridge is directly connected to a first terminal of the second controllable semiconductor switch and to a first terminal of the second switching element.

5. The circuit of claim 1, further comprising a DC/DC-converter coupled to the second half bridge and providing the second constant potential.

6. The circuit of claim 5, wherein the DC/DC-converter is coupled between the second constant potential and the first constant supply potential.

7. The circuit of claim 1, further comprising a zener diode coupled between a terminal providing the first constant supply potential and a terminal providing the second constant potential.

8. The circuit of claim 7, wherein the second constant potential is the sum of the first constant supply potential and a zener voltage of the zener diode.

9. A circuit comprising:
a first switch coupled between a first supply voltage node and a first terminal of an inductance;
a first diode coupled between the first terminal of the inductance and a reference voltage node;
a second diode coupled between a second supply voltage node and a second terminal of the inductance, wherein the second supply voltage node is configured to carry a higher voltage than the first supply voltage node; and
a second switch coupled between the second terminal of the inductance and the reference voltage node.

10. The circuit of claim 9, further comprising a DC/DC converter coupled between the first voltage node and the second voltage node.

11. The circuit of claim 9, further comprising a Zener diode coupled between the first voltage node and the second voltage node.

12. The circuit of claim 9, wherein the first supply voltage node comprises a terminal of an automotive battery.

13. The circuit of claim 9, further comprising drive circuitry coupled to a control terminal of the first switch and a control terminal of the second switch.

14. The circuit of claim 13, wherein the drive circuitry is configured to provide a modulated driver signal to the first switch during normal operation.

15. The circuit of claim 14, wherein the drive circuitry is further configured to turn off the first switch and the second switch when a load dump has been detected.

16. The circuit of claim 9, further comprising a third diode coupled in parallel with the first switch and a fourth diode coupled in parallel with the second switch.

17. The circuit of claim 9, wherein the first supply voltage node is coupled to a 14 volt supply and wherein the reference voltage node is a ground node.

18. A circuit comprising:
a load dump detector coupled to a load of a generator; and
a first circuit configured to:
supply current to a field coil of a generator from a circuit node providing a first substantially constant supply potential via the field coil to a terminal providing a substantially constant reference potential during a normal operation, and
stop supplying current to the field coil and providing a current path from the terminal providing the constant reference potential via the field coil to a node providing a second constant potential that is higher than the first constant supply potential when a load dump is detected by the load dump detector.

19. The circuit of claim 18, further comprising a DC/DC converter configured to provide the second constant potential.

20. The circuit of claim 18, wherein the second constant potential is provided by a zener diode coupled between a node providing the first constant supply potential and a node providing the second constant potential, the second constant potential being the sum of the first constant supply potential and a zener voltage of the zener diode.

21. A method of operating a circuit having a first switch coupled between a first supply voltage node and a first terminal of an inductance, a first diode coupled between the first terminal of the inductance and a reference voltage node, a second diode coupled between a second supply voltage node and a second terminal of the inductance, and a second switch coupled between the second terminal of the inductance and the reference voltage node wherein the second supply voltage node is configured to carry a higher voltage than the first supply voltage node; and
providing a first supply voltage to the first supply voltage node; and
providing a second supply voltage to second supply voltage node, wherein the second supply voltage is higher than the first supply voltage.

22. The method of claim 21, further comprising:
providing a modulated drive signal to a first switch during normal operation;
detecting a load dump condition; and
turning off the first switch and the second switch when the load dump condition has been detected.

23. The method of claim 21, wherein providing the second supply voltage comprises coupling a DC/DC converter between the first supply voltage node and the second supply voltage node.

24. The method of claim 21, wherein providing the second supply voltage comprises coupling a Zener diode between the first supply voltage node and the second supply voltage node.

25. The method of claim 21, wherein providing the first supply voltage comprises coupling an automotive battery to the first supply voltage node.