GB2117143A - Apparatus for supplying a regulated d.c. voltage - Google Patents

Abstract

A permanent magnet alternator 10 supplies a d.c. output voltage to terminals 18, 20 by way of a rectifier 14. A semiconductor switch 21 is connected across the rectifier output terminals and a capacitor 26 is connected across the terminals 18, 20. A diode 19 prevents the capacitor 26 from discharging through the switch 21. Inductances 17 in the output circuit of the alternator 10 limit the current which can flow through the switch 21 and this current limit is in excess of the anticipated current between the terminals 18, 20, the excess current being available to charge the capacitor 26 when the switch 21 is open. The open/shut times of the switch 21 are controlled by a circuit 23 in accordance with any difference between an internal reference voltage and the voltage across the terminals 18, 20. <IMAGE>

Description

SPECIFICATION Apparatus for supplying a voltage-regulated direct current This invention relates to apparatus for supplying a voltage-regulated direct current, and in particular to such apparatus for use in an aircraft, where the maximum loads to be supplied may be determined in advance for each such apparatus.

Direct current supplies for aircraft are commonly derived from an alternator which is driven from an engine of the aircraft. In addition to the electrical load varying up to the aforesaid maximum value, the driving aircraft engine may operate over a range of speeds in the ratio 1:11. It is required that the d.c. supply voltage shall be maintained substantially constant over the anticipated load range and over the engine speed range.

Though it may be possible to regulate the alternator voltage by varying excitation of the rotor, such excitation usually requires the excitation current to be supplied to the rotor through slip rings, and at high altitudes the operation of slip rings becomes unreliable. It has been proposed to use permanent magnet alternators and to provide a drive device for coupling the alternator to the engine, so that the alternator output voltage is substantially constant over the engine speeds range and in varying conditions of load. A drive device of this type is disclosed in the UK Patent 1 586963 and its corresponding US Patent 4278928, Such devices are, however, heavy and complex.

It is an object of the invention to provide a d.c.

supply apparatus in which the foregoing problems are substantially overcome.

In aircraft it is known to provide separate electrical power supplies for specific electrical loads, or classes of load, for example de-icing or electronic controls. Power supplies allocated are commonly referred to as dedicated supplies, and the maximum electrical loads thereon may be estimated accurately in advance. The present invention makes use of this feature of dedicated power supplies to provide for voltage regulators.

According to the invention there is provided an apparatus for supplying a regulated d.c. voltage, comprising a permanent magnet alternator, a pair of d.c. output terminals for connection to an external load, a full wave rectifier responsive to an output from said alternator for providing a d.c.

supply to said output terminals. A semi-conductor switch across the output connections of said rectifier, a capacitance connected across said d.c.

output terminals, a diode device connected to prevent said capacitance from discharging through said switch when the latter is switched on, and means responsive to the voltage level across said d.c. output terminals for controlling operation of said switch, each current path between said alternator and said rectifier including an inductance whose value is such that when said switch is closed the current flow therethrough is greater than an anticipated maximum value of current flowing through said output terminals to an external load, whereby when said switch is open currents can flow to said external load and can charge said capacitance.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagram of the apparatus as a whole.

Figure 2 indicates voltage variations at the output terminals of the apparatus of Figure 1, and Figure 3 shows details of a control circuit forming part of the apparatus of Figure 1.

As shown in Figure 1 a three-phase permanent magnet alternator, indicated at 10 is driven by a shaft 11 of an aircraft engine. The alternator 10 supplies a bridge rectifier 14 which provides a d.c.

output on lines 1 5, 1 6. Three inductances 1 7 are connected in series with the alternator 10 and rectifier 1 4. In the drawing the inductances 1 7 are shown separately from the alternator 10, but it will be understood that the inductances 1 7 could, in fact, form part of the stator windings of the alternator 17, provided that the total inductance in each line to the rectifier 14 has a value which is determined as indicated hereafter. The line 1 5 is connected to a d.c. output terminal 1 8 through a diode 19, and the line 1 6 is connected to the other d.c. output terminal 20.

Connected across the terminals 1 8, 20 is an inverter circuit 1 2 which is responsive to a 28v d.c. output between the terminals 1 8, 20 to provide a 1 0v d.c. supply on a line 13.

Connected across the lines 1 5, 16 is a field effect transistor 21 whose gate is connected by a line 22 to a control circuit 23. Two voltages dropping resistors 24 are connected across the output terminals 1 8, 20 to supply on a line 25 to the circuit 23, a voltage which is a predetermined fraction of the output voltage Vo on the terminals 1 8, 20. In the present example the nominal value of the output voltage Vois 28 volts and the voltage on line 25 is 5 volts. The control circuit 23 generates a series of switching pulses on the line 22 for the transistor 21 , the frequency of these pulses being 100 kHz.The control circuit 23 generates an internal 5 volt reference which is compared with the signal on line 25, and the ratio of on/off times of the transistor 21 is dependent on a difference between the aforesaid reference voltage and the voltage on line 25.

A capacitor 26 is connected across the terminals 18, 20, and zener diode 27 is connected in parallel with the capacitor 26. The zener diode 27 has a breakdown voltage which is lower than the maximum acceptable voltage of loads connected to the terminals 1 8, 20.

At full speed the alternator 10 provides an open circuit output of 272 volts RMS at a frequency of 2.4 kHz. The open circuit output voltage of the alternator 10 increases substantially proportionally with its frequency. However, the total inductive reactance in each alternator output line, that is the combined reactrances of the inductances 1 7 and the stator windings of the alternator 10, are very high in relation to the resistance of the apparatus as a whole, and this resistance may be regarded as negligible. The effect is that an increase in voltage with frequency of the alternator 10 is accompanied by a corresponding increase in the impedance of the circuit, whereby the maximum current which can flow remains substantially constant over the range of alternator speeds.In the present example this current is designed to be 1 amp higher than the anticipated load current. Moreover, the high internal impedance of the alternator 10 and its associated reactances 1 7 has the effect of dropping the voltage supplied to the external load to approximately 28 volts.

With the transistor 21 open circuit current flows through the terminals 18, 20 to the external loads, and the excess current is available to charge the capacitor 26. The transistor 21 is open circuit for the time t1 indicated in Figure 2, this time being appoximately 8.6 microseconds, in which time the voltage across the capacitance 26, and hence across the terminals 1 8, 20, reaches 28.7 volts. The transistor 21 is then rendered conductive by the signal on line 22, for a time t2 of approximately 1.4 microseconds, during which the capacitor 26 discharges down to 28 volts. The diode 19 prevents the capacitor 26 from discharging through the transistor 21. The time t1 + t2 is equal to the 10 microsecond cycle time of the 100 kHz signal on line 22.

If the voltage across the terminals 1 8, 20 falls, the resulting reduction of voltage on line 25 increases the time t1 to increase the charging time of the capacitor 26 and to reduce its discharge time, thereby returning the output voltage Vo to its required value.

The circuit 23 is shown in more detail in Figure 3 and includes a switchmode pulse width modulation control integrated circuit 30 type TL494MJ manufactured by Motorola Inc. The circuit 30 is responsive to the voltage signal on line 25 and includes an oscillator whose frequency is controlled by externally wired components to be 100 kHz. The circuit 30 includes means for generating a 5 volt reference signal for comparison with the signal on line 25, and the pulse duration t1 of a signal on a line 31 is dependent on the aforesaid voltage comparison. A 10 volt power input to the circuit 30 is provided on a line 32. In normal operation the power supply on line 32 is derived from the voltage on line 13 from the circuit 12.During start-up of the generator 10 the power supply on line 32 is derived from a transistor 33 whose base is connected through a resistor 34 and a line 35 to the terminal 18, whose collector is connected through a resistor 36 to the line 35 and whose emitter is connected to the line 32. The resistor 34 is connected through a zener diode 37 and a line 38 to the common, or earth, line 16. The diode 37 maintains a suitable forward bias on the base of transistor 33 and the 28 volts at the terminal.16 is dropped through the resistor 36 to provide the required 10 volts on line 32. When the 10 volts supply is available on line 1 3 the transistor 33 is switched off.

The line 31 is connected to the base of a transistor 39 whose collector is connected to the line 32 and whose emitter is connected to the line 22, thus providing an emitter follower arrangement by means of which a substantial current can be applied to the gate of the transistor 21 to improve the response time of the latter.

A positive voltage can be applied to the base of the transistor 39, and also the line 31, through a voltage dropping resistor 40. When the difference between the voltage on line 25 and the internal 5 volt reference signal is less than a predetermined amount, the circuit 30 connects the line 31 to the line 38, and the gate of the transistor 21 discharges through a diode 41. When the aforesaid difference exceeds the predetermined amount, the connection between lines 31,38 ceases and the positive voltage on line 31 is applied to the base of the transistor 39.

It is envisaged that the regulated voltage at terminals 18, 20 will principaliy be applied to electromagnetic devices, such as solenoid actuators. Such devices will be unaffected by the voltage ripple shown in Figure 2. If required a part of the output current from the terminals 28, 20 may be further smoothed for subsequent application to specific items of equipment.

Claims (6)

1.. An apparatus for supplying a regulated d.c.

voltage, comprising a permanent magnet alternator, a pair of d.c. output terminals for connection to an external load, a full wave rectifier responsive to an output from said alternator for providing a d.c. supply to said output terminals, a semi-conductor switch across the output connections of said rectifier, a capacitance connected across said d.c. output terminals, a diode device connected ta prevent said capacitance from discharging through said switch when the latter is switched on, and means responsive to the voltage level across said d.c.

output terminals for controlling operation of said switch, each current path between said alternator and said rectifier including an inductance whose value is such that when said switch is closed the current flow therethrough is greater than an anticipated maximum value of current flowing through said output terminals to an external load, whereby when said switch is open currents can flow to said external load and can charge said capacitance.

2. An apparatus as claimed in claim 1 in which the value of each said inductance is such that its reactance is very large in comparison with the sum to the total impedance of the apparatus and of an anticipated external load, whereby the load current is substantially constant over a wide range of alternator frequencies.

3. An apparatus as claimed in claim 1 or claim 2 in whichvsaid alternator. is a three phases machine, said inductances being provided in each phase.

4. An apparatus as claimed in any preceding claim in which said means for controlling operation of said switch includes means for generating a series of pulses of a constant frequency and for varying the durations of said pulses in dependence on the voltage across said output terminals.

5. An apparatus as claimed in claim 4 in which said pulse generating means comprises a device for selectively applying either of two voltage levels to a control connection of said semi-conductor switch, one of said levels providing the pulses of said series.

6. An apparatus for supplying a regulated d.c.

voltage, substantially as hereinbefore described with reference to the accompanying drawings.