Bridge Circuit for Driving a Brushless DC Motor

1. Field of the Invention

The invention pertains to a bridge circuit comprising a plurality of field-effect power transistors for driving a brushless DC motor and in particular a drive circuit for driving said power transistors as a bridge circuit for driving a brushless DC motor having a plurality of n-channel field-effect transistors (FETs) which are fed by one single DC voltage source.

2. Description of the Prior Art

It is desirable to provide a simple drive circuit which facilitates a safe driving of power transistors connected to the positive line of a supply voltage without requiring the utilization of an additional voltage source in form of a DC-DC transformer or p-channel transistors.

It is also very desirable for cost reasons to construct a bridge circuit consisting of field-effect transistors exclusively with n-channel transistors since p-channel transistors with identical capacity are in accordance with the current state of the art substantially larger and more expensive than n-channel transistors. However, there exists the specific problem that the transistors connected to the positive line of the supply voltage require a gate potential in order to be switched on, with said gate potential lying substantially above the potential of the positive line of the supply voltage and thus not easily available. The obvious solution of generating this auxiliary potential by means of a DC-DC voltage transformer or a battery is not very advantageous for cost reasons.

It is an object of the invention to solve various problems in the prior art.

Another object of the invention is attained by the provision of an auxiliary potential generated by at least one potential sliding capacitor to drive transistors which are connected to the positive line of a DC voltage source whereby the auxiliary potential is at a greater potential than the positive line of the DC voltage source.

A further object of the invention is to turn off the transistors connected to the positive line of the DC power supply voltage before the start of the motor during which at least one potential sliding capacitor is charged.

Briefly summarized, the present invention relates to an n-channel field effect transistor bridge circuit connectable across a positive output and a negative output of a single DC voltage source for driving a brushless DC motor. An auxiliary potential generated by at least one potential sliding capacitor is provided for driving the transistors connected to the positive power supply output voltage. The auxiliary potential is provided through a charged capacitance prior to the starting of the motor by switching the sliding potential capacitor across the voltage source.

FIG. 1 is a schematic diagram depicting an embodiment of a bridge circuit for driving a brushless DC motor in accordance with the present invention.

According to the invention, the auxiliary potential is instead generated by means of at least one potential sliding capacitor (13, 13') which during operation of the bridge circuit is at times switched with transistor (12, 12') causing the negative lead (19, 19') of the capacitor (13, 13') to switch the -V.sub.cc (18), a relatively lower potential in order to be able to charge said capacitor with the supply voltage. The potential sliding capacitor is subsequently raised to a relatively higher potential (10, 10'), whereby the positively charged pole of said capacitor reaches a potential which lies above the positive potential of the positive line of the supply voltage and may thus serve to drive the gates of the aforementioned transistors.

Such an arrangement of a (generally known) potential sliding capacitor operates perfectly once the capacitor has been charged. However, this required charge does not exist when first starting the motor, namely because at least one of the transistors connected to the positive line of the supply voltage has to be switched on without providing the potential sliding capacitor driving said transistor with a possibility of previously charging itself.

An additional variation of the invention thus suggests that all transistors (11, 11') connected to the positive line (1) of the supply voltage remain turned off at first during a preset period of time after switching on the driving voltage before the start of the motor or another starting command for the motor, so that the potential sliding capacitors (13, 13') driving said transistors have an opportunity to charge themselves during the preset time period to an auxiliary voltage (17) of, for example, 12 V via the diodes (16, 16'), whereby the negative poles (19, 19') of the aforementioned capacitors (13, 13') are connected to the relatively negative potential of the negative line -V.sub.cc (18) of the supply voltage by means of the already switched on transistors (12, 12') connected to the negative line -V.sub.cc (18) of the supply voltage. The turning off of the aforementioned transistors (11, 11') is obtained by the fact that all outputs of the optocouplers (15, 15') are switched into a conducting mode. The charging process for these capacitors (13, 13') practically requires only a very short period of time, for example, less than one msec. This duration is in accordance with the invention determined by a suitable time control (4), for example, a microprocessor. The turning off of the transistors (11, 11') connected to the positive line of the supply voltage is eliminated after this charging process, and the commutation device (2) which controls the power transistors as a function of the rotor position (for example, by means of at least one Hall generator (5)) takes over the driving of all power transistors (11, 11', 12, 12') by the optocouplers (15, 15') of the aforementioned bridge circuit. An additional characteristic of the invention suggests that the transistors (12, 12') connected to the negative line of the supply voltage may be additionally controlled by means of a pulse width modulator (3).

In the described bridge circuit for driving a brushless DC motor, the transistors which are connected to the positive line of the DC supply and are driven by an auxiliary potential source which provides a greater positive potential than the positive line of the DC supply voltage and is generated by at least one potential sliding capacitor configuration. After switching on the drive voltage, the capacitors (13, 13') are charged by the microprocessor 4 which turns off the upper FET (11, 11') with the optocoupler (15, 15') by causing the optotransistor of the optocoupler to be turned on. Then the FET (12, 12') is turned on with the microprocessor 4 allowing the sliding potential capacitor (13, 13') to be charged. When the FET (12, 12') is turned on, the negative lead (19, 19') of the capacitor (13, 13') is electrically connected with the negative lead 18 of the power supply and the capacitors (13, 13') are charged via the 12 volts supplied to the diode (16, 16'). After the capacitors (13, 13') are charged, the FET (11, 11') can be turned on via the resistor (14, 14') as soon as the optocoupler (15, 15') is turned off. The switching of the FETs (11, 11'), (12, 12') proceeds as follows. When FET (11, 11') is turned off, the capacitor (13, 13') is very slowly discharged and the capacitor (11, 11') remains at a high enough potential to keep FET (11, 11') in a turned on state. When the FET (12, 12') is turned on, the capacitor (11, 11') is then rapidly charged again.

The drive currents flowing through the gates of the field-effect transistors are negligibly small, so that a capacity of, for example, 2.2 .mu.F suffices for the aforementioned potential sliding capacitors (13, 13') in order to maintain the aforementioned transistors (11, 11') switched on for an extended period of time, for example over several minutes. The charge of these capacitors would, however, be slowly depleted for example by self-discharge, if the motor remained hindered or stopped for an extended period of time. The auxiliary potential to drive the transistors (11) or (11') would slowly decrease to a level which no longer suffices for a maximum conductivity of these transistors, so that the power loss due to these transistors would slowly increase to impermissible values. In order to eliminate this hazard, an additional variation of the invention suggests that the aforementioned bridge circuit (11, 11', 12, 12') is switched off in case of a stopped rotor of the motor after a predetermined period of time of, for example, a few seconds by the fact that the transistors (11, 11') connected to the positive line of the supply voltage remain turned off, so that the potential sliding capacitors (13, 13') are charged anew. This turning off of the transistors (11, 11') can be eliminated for a short duration may after an additional preset time period in order to initiate the restarting of the motor. If this attempt to start the motor is not successful, a renewed turning off of the transistors (11) or (11') is performed, and the described process is repeated. The turning off and switch-on times are preferably controlled by a corresponding time control circuit (4), for example by means of a suitably programmed microprocessor.

Although the previous description pertains to a single-phase motor, this fact by no means indicates that the invention is limited to a motor of this type. The previously described invention may also be successfully applied for a two-phase or a three-phase motor. In the instance of a three-phase motor, the number of end stage transistors would be increased to six, whereby three transistors are connected to the positive line of the supply voltage, and three transistors are connected to the negative line of the supply voltage.