GB2481211A - Alternator start-up circuit - Google Patents

Abstract

A start-up system for a motor vehicle comprises an alternator 10 having a field coil 12 and a switching circuit 18. The switching circuit provides a pulsed output current I to the field coil of the alternator. The output current may be controlled by a pulse width modulated signal to the switching circuit provided by a microcontroller. The start-up circuit also includes means for inhibiting the current provided to the alternator either when the charge on the vehicle battery is too high or the engine speed is too low. The switching circuit may include inputs signals determined by the vehicle battery voltage and the current engine speed and the output of a PWM signal is inhibited when the input signals reach specified threshold values.

Description

Alternator Start-up Circuit

BACKGROUND

a. Field of the Invention

This invention relates to alternators for motor vehicles, and in particular to start-up circuits for alternators.

b. Related Art Alternators convert mechanical energy into electrical energy and work by rotating an electromagnet, in the form of the rotor or field coil, past stationary, or stator, coils. As the magnetic field around the stator coil changes a current is induced in the coil, therefore as the rotor rotates, a voltage is generated in the stator coil.

The alternator only begins to generate voltage when the alternator is rotating at sufficient speed. Although the iron core of the rotor will possess some residual magnetism, this will, in general, not be enough to guarantee that the alternator will start to generate when rotated. Therefore, a start-up current, typically about mA, has to be applied to the field coil. This start-up current produces a voltage in the stator coils which is then fed back to the field coil. As a result of this feedback, the current in the field coil increases, which in turn produces an increased stator voltage. This feedback loop causes a rapid escalation in the stator voltage, so that within a few tenths of a second the stator voltage is typically greater than 15 V and the current in the field coil may be as high as 4 A, depending on the load on the alternator. In this way, the initial start-up current of mA is completely redundant within a short time, and the start-up circuitry is designed to switch off this current when it is no longer required.

The traditional start-up circuit includes a low power incandescent bulb in the instrument cluster, also used as the generator warning light, which is wired between the ignition voltage and the high-side of the field coil. This lamp gives sufficient rotor current to set up a magnetic field strong enough to begin generation. Once the alternator is generating normally, the voltage at the high-side of the field coil becomes equal to the ignition voltage and the warning light goes out. Recently, the warning light on the vehicle instrument cluster has been replaced by a light emitting diode (LED), and a power resistor, typically 100 C) and 6 W, has been added to provide the start-up current. The large power rating, of around 6W is needed to handle the worst case of when the ignition is on but the engine is stopped. In these conditions, the resistor may be dissipating 2.5 W in an ambient temperature of 85 °C.

In one design of instrument cluster the power resistor is provided by three 300 C) and 2 W surface mount resistors wired in parallel. However, these resistors are relatively expensive, costing about $0.29 each.

It is an object of the present invention to reduce the cost of providing a start-up circuit for a vehicle alternator.

SUMMARY OF THE INVENTION

According to the invention, there is provided a start-up system for a motor vehicle comprising an alternator having a field coil and a switching circuit, wherein the switching circuit provides an output current to the field coil of the alternator, and wherein the output current is pulsed.

Providing a pulsed current to an alternator field coil decreases the power dissipation of the start-up circuit permitting the use of lower power components in the circuit, thereby decreasing the cost and size of the circuitry required.

Preferably the pulsed output current comprises a first period of time during which the current is 0 A and a second period of time during which the current is greater than 0 A, and wherein the second period of time is shorter than the first period of time.

Preferably the first period of time is between 100 and 200 ms and the second period of time is between 20 and 40 ms. More preferably the first period of time is between 108 and 192 ms and the second period of time is between 24 and 36 ms.

More preferably the first period of time is between 129 and 171 ms and the second period of time is between 27 and 33 ms. Ideally the first period of time is 150 ms and the second period of time is 30 ms.

The switching circuit includes a supply voltage from the vehicle battery and preferably the output current from the switching circuit is controlled by an input provided by a microcontroller. The microcontroller provides a pulse width modulated signal to the switching circuit.

It is also desirable if the switching circuit includes an input corresponding to an engine speed of the motor vehicle, and the pulsed output current is only provided to the alternator when the engine speed is greater than a threshold value.

Preferably the engine speed threshold value is 500 revolutions per minute because, typically, an alternator cannot generate power when the engine is rotating at speeds less than this.

Additionally, it is advantageous if the switching circuit includes an input corresponding to a voltage of a battery of the motor vehicle, and the pulsed output current is only provided when the battery voltage is less than a threshold value.

Preferably, the battery voltage threshold value is 16 V, corresponding to the battery being overcharged. If the alternator is permitted to start to generate further power to charge the battery under these conditions, damage to the battery and other electrical systems may occur.

Also according to the invention, there is provided a method of providing a start-up current to an alternator, the method comprising providing a pulsed output current including a first period of time during which the current is 0 A and a second period of time during which the current is greater than 0 A. Also according to the invention, there is provided a motor vehicle comprising an alternator having a field coil and a switching circuit, wherein the switching circuit provides an output current to the field coil of the alternator, and wherein the output current is pulsed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is a block diagram illustrating a preferred embodiment of a start-up circuit of the present invention; and Figure 2 is a circuit diagram showing the switching circuit of Figure 1.

DETAILED DESCRIPTION

Figure 1 is a schematic block diagram illustrating a start-up circuit 1 of the present invention. The circuit 1 comprises a switching circuit 18, an output 16 of which is connected to one side of a field coil of an alternator 10. The input for the switching circuit 18 is generated by a microcontroller 2 which provides a pulse width modulated signal 3 to the switching circuit 18. The switching circuit 18 is arranged to use the logic signal output of the microcontroller 2 to control a flow of current (I) from the vehicle battery 4 to the field coil of the alternator 10.

A further output of the switching circuit 18 is arranged to provide a voltage (Vo) 7 for a warning light circuit 6, as will be described in more detail below. A monitoring circuit 8 is also provided to monitor both the engine speed of the vehicle and a supply voltage (Vs) 15 of the vehicle battery 4. The monitoring circuit 8 provides an output signal 9 to the microcontroller 2 that prevents the microcontroller 2 providing an output signal to the switching circuit 18 if the engine speed is too low or the battery voltage is too high.

Referring now to Figure 2, the alternator 10 of a motor vehicle comprises a rotor or field coil 12 and stator coils or stator windings 14. The field coil 12 is arranged to rotate within the stator windings 14, and the magnetic field around the field coil 12 induces an alternating voltage in the statorwindings. Typically, the alternator 10 will include three sets of stator windings 14 so that the alternator 10 produces a three phase current.

Before the alternator 10 can start to generate power, the field coil 12 must be provided with an initial current. This start-up current is only provided for a short period of time and is then switched off.

The output 16 of the switching circuit 18 provides a voltage (Vi) 17 at a high-side 11 of the field coil 12. A low-side 13 of the field coil 12 is connected, via a transistor 24 to ground (GND) 19. When the voltage (Vi) 17 is applied to the high-

side, a current is produced in the field coil 12.

The switching circuit 18 comprises a printed circuit board 20 having a number of surface mount components. The supply voltage (Vs) 15 to this circuit is typically 12 V and is supplied by a standard vehicle battery 4 (open circuit voltage at full charge 12.6V).

An input voltage (V2) 22 is connected to a base 21 of a first transistor (Qi) 23.

Preferably the transistor 23 is a digital transistor or bias resistor transistor, and more preferably the transistor 23 is a BCR1 16 series NPN silicon digital transistor in a 3-pin SOT-23 package. The transistor 23 is connected in series with two resistors (Ri) 25 and (R2) 27 such that an emitter 29 of the transistor 23 is connected to ground 19 and a collector 31 of the transistor 23 is connected via these two resistors to the supply voltage (Vs) 15.

A base 33 of a second transistor (Q2) 35 is connected to the junction 37 between the two resistors (Ri) 25 and (R2) 27. The transistor (Q2) 35 is a PNP transistor, and is preferably a MMBTA56 PNP general purpose amplifier. An emitter 39 of this transistor 35 is connected to the supply voltage (Vs) 15 and a collector 41 is connected via a third resistor (R3) 43, diode (Dl) 45 and capacitor (Cl) 47 to ground 19.

A fourth resistor (R4) 49 is connected between the supply voltage (Vs) 15 and a junction 51 between the series resistor (R3) 43 and diode (Dl) 45, such that the fourth resistor (R4) 49 lies in parallel to the second transistor (Q2) 35 and the third resistor (R3) 43.

Base input voltage (V2) 22 is provided by the signal 13 from the microcontroller 2.

The microcontroller 2 includes suitable pulse width modulated timer hardware and appropriate software, all of which is known in the prior art. In a preferred embodiment, the microcontroller 2 provides a pulse width modulation signal so that the base voltage (V2) 22 has a square waveform. Base voltage (V2) 22 is a logic signal and therefore has a maximum value of 5 V. The microcontroller 2 controls the duty cycle of the input voltage (J2) 22 and in a preferred embodiment the duty cycle is less than 50%. Preferably, the period of the square wave is between 108 and 192 ms and the input voltage is high for between 24 and 36 ms.

More preferably the period is between 129 and 171 ms and the input voltage is high for between 27 and 33 ms.

When the base voltage (V2) 22 is 0 V, the first transistor (Qi) 23 is off, and no current flows between the collector 31 and emitter 29. Consequently, no current flows through the first and second resistors (Ri) 25 or (R2) 27, and the base 33 of the second transistor (Q2) 35 is held at a high voltage. Because the second transistor 35 is a PNP transistor, when the base 33 is held high relative to the emitter 39, the transistor is off and no current flows through the transistor.

Therefore, with both transistors 23, 35 in an off state, current flows through the fourth resistor (R4) 49 and diode (Dl) 45, output voltage (Vi) 17 is low and minimal current flows through the field coil 12 of the alternator 10.

When base voltage (V2) 22 is high, the first transistor (Qi) 29 is switched on and current flows between the collector 31 and emitter 29. Because the first transistor 29, in this state, has very little voltage drop across it, and because the second resistor (R2) 27 has a significantly smaller resistance than the first resistor (Ri) 25, the voltage at the junction 37 between these two resistors drops significantly.

The voltage applied to the base 33 of the second transistor (02) 35, therefore, is pulled low compared to the emitter 39 and so this transistor switches on. The low resistance of the second transistor 35 and the small value of the third resistor (R3) 43 compared to the resistance of the fourth resistor (R4) 49, means that more current will flow through the second transistor 35 and the third resistor 43 than through the fourth resistor 49. The output voltage (Vi) 17, therefore, rises to a value of several hundred millivolts. In this state, the increased voltage supplied to the high side 11 of the field coil 12 causes sufficient current to flow through the alternator field coil to begin the generating process.

The capacitor (Cl) 47 is provided to block the flow of the direct current and protect the switching circuitry from high voltage spikes that are sometimes generated by the 12 V system of a vehicle, as is well known in the art. Preferably the capacitor (Cl) 47 is a 100 nF capacitor with polymer terminations to reduce mechanical cracking failures.

In a preferred embodiment, the start-up circuit also includes means for inhibiting the current provided to the alternator either when the charge on the vehicle battery is too high or the engine speed is too low. A first input signal determined by the vehicle battery voltage and a second input signal determined by the current engine speed may be supplied by monitoring circuitry to the microcontroller. Software within the microcontroller inhibits the output of a pulse width modulation signal when the input signals reach specified threshold values.

Preferably, the software inhibits an output from the microcontroller if the engine speed of the vehicle is detected to be below about 500 revolutions per minute (rpm). This is equivalent to the alternator rotating at about 1500 rpm. Typically an alternator cannot generate power when rotating at speeds less than this.

Therefore, it is preferred if the switching circuit only generates a current through the field coil to start the alternator generating when the engine speed is above this threshold value.

Additionally, the software is arranged to further inhibit an output from the microcontroller if the voltage on the vehicle battery is greater than 1 6 V. If the battery voltage is greater than 1 6 V then the battery is overcharged, which may indicate a problem with the alternator or the voltage regulator. If the alternator is permitted to start to generate further power to charge the battery under these conditions, damage to the battery and other electrical systems may occur.

An output voltage (Vo) 7 from the switching circuit provides a signal for a charge warning light circuit and resistor R4 acts as a pull-up resistor for this output voltage. The charge warning light circuitry is standard circuitry, well known in the art, and includes a light emitting diode (LED) used as a warning light on the instrument panel of a vehicle. The circuitry is arranged such that when the voltage Vo is above approximately 6 V, the alternator is assumed to be operating and the LED warning light is not lit. When the voltage V0 drops below 6 V, the LED is illuminated indicating that the alternator is not working.

The fourth resistor (R4) 49 also allows the switching circuit to provide a continuous start-up current of 5 mA to the alternator even if one of the transistors fails open circuit. If required, additional resistors may be placed in parallel with the resistor (R4) 49 to increase the current supplied to the alternators.

The advantage of using a switching circuit as described above is that the power dissipated by the circuit is significantly reduced compared to a traditional start-up circuit. The transistors dissipate minimal power when they are in an on or off state, and although considerable power is dissipated during the transitions between these states, this change of state is rapid, so that the average power dissipation remains low. The decreased power dissipation means that the power resistors used in the start-up circuit can be reduced in value. In a preferred arrangement, the pulse width modulation is set to have a duty cycle of one sixth, so that a high input voltage is supplied for 30 ms in every 180 ms. The power dissipated by the power resistor is therefore reduced by five sixths, and the worst case power dissipation (ignition on, engine stopped) is about 0.42 W rather than 2.5 W, as described above. The expensive 1 00 0, 6 W power resistor may, therefore, be replaced by an inexpensive 100 0, 1 W resistor.

It has been found that replacing a prior art start-up circuit with the switching circuit of the present invention can decrease component costs by more than 90%. In addition, the PCB area taken up by the components of the switching circuit is much reduced compared to prior art start-up circuits that include large power resistors. In one example, the PCB area was decreased by up to 1000 mm2 and this decrease was primarily due to the decrease in thermal pad area that is required due to the lower power dissipation.

In order to decrease the power dissipation as much as possible, it is preferable to set the wave pulse modulation duty cycle to as low a value as possible. However, the time for which the voltage Vi is applied to the field coil must be long enough to produce sufficient start-up current in the field coil.

The inductance of the field coil of the alternator may be as high as 1 H and, typically, an alternator field coil, in association with the above mentioned 100 C) 1 W resistor, will have an L/R time constant of about 10 ms. Therefore, a duty cycle of 30 ms every 180 ms is long enough for the start-up current in the field coil to reach about 95% of the maximum current. Additionally, 30 ms is short enough to avoid any power pulse limitations of the resistor used in the switching circuit.

A pulse of 30 ms has been found to be sufficient to begin the rapid escalation in the stator voltage even when the alternator is rotating at the minimum speed for generation. When the alternator is rotating at 1500 rpm, the field coil completes three quarters of a revolution in 30 ms. In this time, therefore, the field coil will rotate past nine stator poles resulting in nine pulses of voltage being fed back to the field coil, which increases the current flowing in the field coil.

The start-up circuit of the present invention therefore provides a circuit having significantly reduced power dissipation compared to conventional start-up circuits.

This in turn leads to a cost saving as much lower power resistors are required.

Additionally, the reduced power dissipation through these resistors means that the component size is reduced, together with the required thermal pad area, leading to a reduction in the size of the circuit.

The start-up circuit of the present invention also includes means for preventing a start-up current being supplied to the alternator field coil in non-optimal conditions, for example when the engine speed is too low or the battery voltage is too high.

Claims (15)

1. CLAIMS1. A start-up system for a motor vehicle comprising an alternator having a field coil and a switching circuit, wherein the switching circuit provides an output current to the field coil of the alternator, and wherein the output current is pulsed.
2. 2. A start-up system as claimed in Claim 1, wherein the pulsed output current comprises a first period of time during which the current is 0 A and a second period of time during which the current is greater than 0 A, and wherein the second period of time is shorter than the first period of time.
3. 3. A start-up system as claimed in Claim 2, wherein the first period of time is between 100 and 200 ms and the second period of time is between 20 and 40 ms.
4. 4. A start-up system as claimed in Claim 3, wherein the first period of time is between 108 and 192 ms and the second period of time is between 24 and 36 ms.
5. 5. A start-up system as claimed in Claim 4, wherein the first period of time is between 129 and 171 ms and the second period of time is between 27 and 33 ms.
6. 6. A start-up system as claimed in Claim 5, wherein the first period of time is ms and the second period of time is 30 ms.
7. 7. A start-up system as claimed in any preceding claim, wherein the output current is controlled by an input to the switching circuit provided by a microcontroller, and wherein the microcontroller provides a pulse width modulated signal to the switching circuit.
8. 8. A start-up system as claimed in any preceding claim, wherein the switching circuit includes an input corresponding to an engine speed of the motor vehicle, and wherein the pulsed output current is only provided when the engine speed is greater than a threshold value.
9. 9. A start-up system as claimed in Claim 8, wherein the engine speed threshold value is 500 revolutions per minute.
10. 10. A start-up system as claimed in any preceding claim, wherein the switching circuit includes an input corresponding to a voltage of a battery of said motor vehicle, and wherein the pulsed output current is only provided when the battery voltage is less than a threshold value.
11. 11. A start-up system as claimed in Claim 10, wherein the battery voltage threshold value is 16 V.
12. 12. A method of providing a start-up current to an alternator, the method comprising providing a pulsed output current including a first period of time during which the current is 0 A and a second period of time during which the current is greaterthanOA.
13. 13. A motor vehicle comprising an alternator having a field coil and a switching circuit, wherein the switching circuit provides an output current to the field coil of the alternator, and wherein the output current is pulsed.
14. 14. A start-up system for a motor vehicle substantially as herein described with reference to or as shown in the accompanying drawings.
15. 15. A method of providing a start-up current to an alternator substantially as herein described with reference to the accompanying drawings.