AC VOLTAGE REGULATOR FOR PERMANENT MAGNET GENERATORS

DESCRIPTION Technical Field

The present invention relates to an alternate voltage regulator for generators and more particularly for permanent magnet generators.

Prior Art

Voltage regulation systems working with generators connected with internal combustion engines, motorbike engines for instance, are well known. The generators are generally permanent magnet type. These generators give voltage signal affected by large variations in frequency and amplitude due to the wide range of rotational velocity of the engine. In the case of motorbikes, the voltage signal can have an amplitude spanning in the range from few volts up to 300 V and as frequency variation from nearly zero to 1000 Hz. Some voltage regulator are mainly composed by an electronic switch connected in parallel to the power source which is the generator.

During the operating phase, the electronic switch is closed at given instants in order to regulate the average output voltage. This closure corresponds to a short circuit shunt of the generator and consequently causes a high energy dissipation on the internal resistances of both the switch and the generator. This happens just because these generators have a low internal inductance (few mH) which implies fast increases of the short circuit currents. Furthermore, the engine load increases while the generator is shunted. Therefore the increased power dissipation requires heat dissipating assembling structures which are oversized with respect to the real power given to electrical load. Disclosure of the Invention

The aim of the present invention is to provide a voltage regulating circuit for an AC generator, especially of permanent magnet type, allowing to extract from the generator the electrical power strictly necessary for a correct biasing of the electrical load. A further aim of the present invention is to provide a voltage regulating circuit for an AC permanent magnet generator that prevents the damages of the load and also

improves the lifetime of the biased devices.

A further aim of the present invention is to provide voltage regulating circuit for an AC generator which is able to decrease the breaking effects on the engine and therefore to reduce the energy wasting and hence the carburant consumption. These and others aims are achieved, according to the present invention, by an AC voltage regulator device, including a bidirectional switch, in conformity of the appended claims.

The advantages and technical characteristics of the invention will become more apparent from the following detailed description of a non-limiting example embodiment of it. Brief Description of the Drawings

In the drawings:

- Figure 1 shows an example of known electronic voltage regulator having a switch in parallel with the generator;

- Figure 2 shows an example of electronic voltage regulator according to the present invention, having a bidirectional electronic switch;

- Figure 3 shows a block diagram of a control circuit for a voltage regulator according to the present invention;

- Figure 4 shows a simplified diagram of the main waveforms within the said circuit;

- Figure 5 shows a simplified electrical diagram of a preferred embodiment of the bidirectional switch within the voltage regulator.

Preferred Embodiment of the Invention

With reference to Figure 1, a known voltage regulation circuit is shown which is associated to a generator 1 which is driven by an engine (not showed), for example of a motorbike. The generator 1, being connected to the motor shaft, produces a voltage signal with a wide range of amplitude and frequency in the ranges 1-300 V and 0-1000 Hz respectively.

This circuit includes an electronic switch 6 in parallel to the power source, which is the generator 1. The internal inductance 2 and resistance 3 are referred to the generator 1 and therefore are characteristic of the generator itself. During the operating phase, the switch 6 is closed in a programmed way in order to

regulate the average voltage at the output user 5. When the switch 6 is closed, the generator 1 is shunted causing an high energy dissipation since both the generator and the switch have an internal resistance over which the energy is dissipated into heat . Furthermore the engine feels an increasing load in every time cycle in which the generator is shunted.

Figure 2 shows an example of an AC voltage regulating circuit according to the present invention. This circuit have a couple of terminals 55 and 56 which can be respectively connected to an AC voltage generator - represented by an ideal voltage generator 30, by a series inductance Lm 31 and the series resistance Rm 32 which represents the internal impedance of the real generator - and to a load 39 to be biased with a given rms voltage. The circuit, which in figure 2 is shown in a basic schematic form, comprises a bidirectional electronic switch 33 and control means - not shown in the figure and better described below - causing the switch ON and OFF during a positive half wave of the voltage produced by the generator 30 in a given direction and the switch ON and OFF during the negative half wave of the voltage produced by the generator 30 in the opposite direction.

In Figure 2 the bidirectional switch is shown as two series one directional switches 34 and 35, each of them has in parallel one of two opposite diodes 8 and 9. Figure 3 shows the block diagram of an example of control electronic circuit for a voltage regulation device according to the invention in which the block 33 represents a bidirectional switch having the above cited characteristics.

In particular the bidirectional switch 33, in the following also addressed as Power AC Switch, is a power bidirectional switch whose aim is to close or open the branch connecting the terminals 55 and 56 allowing or not the current flow in the load 39. The resistance Ri 48 placed in series to the load resistance Rc 39 is a resistor with a value which is known and low enough to avoid any variation in the circuit behaviour. From the voltage drop Vri across Ri 48 it is possible to calculate the current value Iload across the load 39 by means of the well known relationship Iload=Vri/Ri. The voltage Vrc across the load 39 and the voltage Vri across the resistance Ri 48 are the inputs of two analog electronic circuits 36 and 38 whose aim is to output two DC

voltages which are directly proportional to the effective values of the two input voltages. These DC voltages are the inputs of a processing block 42, which can be realized using analog or digital technology, by means of which the effective values for the voltage and current on the output load 39 can be calculated and consequently the electrical power and the resistance Rc of the load.

In fact, the value of the load resistance Rc is not known 'a priori' but it changes according to the various operating conditions of the motorbike, f.i. whether lights are on or off, etc.. The knowledge of the dissipating power and the load resistance Rc allows to perform diagnostic and control actions, such as protecting against the short circuits, limiting the maximum absorbed current and so on.

The voltage on the node 55 is sent to the input of an analog circuit 41 able to detect the zero crossings. Its output is also sent to an input of the processing block 42 so that this last one is able to calculate the AC voltage frequency and its phase and consequently the rotational speed of the engine. Starting from the knowledge of these parameters the processing block 42 generates, with the help of two saw generators 46 and 47, a couple of voltage waveforms S+ and S- which are synchronous with voltage output of the generator and with a constant and suitable maximum amplitude.

The effective value of the load 39 voltage Vrms is compared with a reference value Vrif, which corresponds to the effective voltage value we want to apply to the load. Vrif is placed at one input of a subtractor 37. The output of the subtractor 37 is processed by a control block 43, which is of PID type (proportional-integrative-derivative) and whose aim is to speed up the system response and to eliminate the regulation error in the stationary regime. The output voltage Vpid of the controller 43 is compared with the signals S+ and S-, (produced by the two saw generators 46 and 47) by means of two comparators 44,45; the signals at the output of the comparators drive a pulse former 40 whose signals are the four control voltages 57, 58, 59 and 60 for the bidirectional Power Ac Switch 33 The plots of Figure 4 show a schematic view of the main waveforms in the circuit. In particular, the waveform 63 (63', 63", 63'") is the voltage of the ideal generator 30. During the positive half wave 63' at the instant "t on up" (70) S+ (65) becomes greater

than Vpid (66) and the "on up" voltage (58) is turned to high level which implies the closure of the switch 33 and starts the current flow across the load. When S+ (65) becomes again lower than Vpid (66), the voltage "on up" is turned to low level and a pulse is generated on "off up" (57) opening the switch 33 so that the current flow across the load is blocked at the "t off up" (69) instant. During the negative half wave the behaviour of the circuit is exactly dual because the Vpid is compared with the signal S- (64). The time intervals in which the bidirectional switch is closed have been graphically marked with bold dashed lines along the plot of the voltage 63. If the effective voltage Vrms across the load increases with respect to the reference value Vrif because of, for instance, a variation of the load resistance Rc 39, the voltage Vpid increases. This fact changes the crossing points between the S+ (65) and S- (64) signals and the Vpid, retarding the closure of the switch at the instants "t on up"(70) and "t on down" (71), and therefore reducing the period during which the current flows across the load. As a consequence the Vrms value decreases until the matching with Vrif.

The circuit behaves in similar but opposite way in the case of a decreasing of Vrms with respect to the reference value Vrif. It is now evident that the circuit is able to implement a negative feedback by means of which the effective voltage value can be stabilized around a given reference value 37. The feedback operates also in presence of variation of the engine rotational speed. In fact, if this parameter increases, the amplitude of the generated AC voltage increases as well and therefore an increase of the effective voltage across the load occurs. If this happens the circuit operates by changing the switching instants of the device 33 in order to re-establish the desired voltage value across the load. It is important to notice that the particular shape of the S+ (65) and S- (64) signals causes the switch opening exactly when the voltage 63 crosses the zero level. This solution was chosen in order to avoid as much as possible the generation of voltage spikes causing electromagnetic interferences. Spikes are generated only at the switch opening if the current flowing is different than zero; spikes are not produced at the closing of the switch thanks to the filtering

properties of the circuit. It is possible to set the switch opening also at different instants just by programming the processing block 42.

A preferred embodiment of the power bidirectional switch 33 is schematically shown in Figure 5 The two nMOS power transistors 75 and 76 duly driven by the remaining part of the circuit, have the aim to allow or not the current flow across the load 39. The configuration chosen to drive them has the advantage to use, for both the positive (63') and the negative (63") voltage half wave (see figure 4), two n-channel enhancement MOS having a channel resistance which has lower value than the corresponding p- channel devices.

The use of BJT or IGBT transistors in place of the two MOS 75 and 76 is also possible without significant changes in the circuit. The use of SCR devices is also possible but these last ones cannot be automatically turned off until the current flow crosses the zero value. Instead the nMOS transistors 75 and 76 are able to cut off the current flow at any time.

The following description of the circuit behaviour, with reference to Figures 4 and 5, is limited to the case of the positive half wave of the generator voltage signal, since in the opposite case of the negative half wave the circuit behaviour is exactly dual and the lower processing block 52 operates instead of the upper processing block 51 . In the circuit, Sl, S2, S3 and S4 represent four electronic switches which are closed if the respective control voltages 58, 57, 59, and 60 are at high level, while they are open in the opposite case. Let consider the following situation: the voltage generator is in the positive half wave, the transistor 75 is off and the voltages on ON UP (58) and OFF UP (57) are at low level (switches Sl and S2 open). Because of the positive voltage on the node 55, the bootstrap capacitor Cl (80) is positively charged, according to the polarity shown in Figure 5, through the resistance 81 and the load resistance Rc (39) thanks to the anti-parallel diode (9) of the transistor 76. The Cl capacitor holds the charge also during the negative half wave thanks to the presence of the diode D2 that prevents the discharge. If during the next positive half wave (see figure 4) the voltage on ON UP (58) is turned

at high level, the Sl switch closes and transfers the voltage stored on Cl (80) between the gate and source of the transistor MOS 75 which immediately starts to drive current. To protect the Ml MOS (75) against over voltages, a zener diode (not shown in the figure) can be placed in a suitable position. If during the conduction of the transistor 75 the voltage on ON UP (58) is turned to low level and the voltage on OFF UP (57) is turned to high level, the Sl switch is open and the S2 switch closes, therefore a short circuit occurs between the gate and the source of transistor 75 causing its switching off and blocking the current flow in the load 39. The foregoing description of a specific embodiment will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such an embodiment without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiment. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.