Simple Split-Rail Brushless Motor Controller

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Title: Simple Split-Rail Brushless Motor Controller

Filed: 14/10/2011

Number: GB1117751.6

Figure 1

This invention relates to brushless DC motor controller and AC synchronous motor controller and “Brushless Motor with Resonant Tuning” (GB0904679.8)


It is well known to have brushless motors called “AC synchronous motor”, “brushless DC motor”, “induction motor” or “reluctance motor”

It is well known to control a brushless motor using a conversion and control circuit known as a brushless motor controller.

It is also well known to employ MOSFET (metal oxide semiconductor field effect transistor) or IGBT (insulated gate bipolar transistor) devices as the output stages of the controller circuit.

These transistors are controlled by a low power input switching voltage which is called the “gate” voltage.

A MOSFET or IGBT is a transistor which has 3 terminals: a “gate” which is the control input and 2 output terminals to control the power output.

Both devices are in 2 types: N-channel and P-channel which mainly indicates the polarity of the gate input.

These MOSFET/IGBT devices are required to allow the control of industrial motors using low power digital circuits.

P-channel IGBTs are in fact quite rare and are never used for high power applications.

In N-channel MOSFETs the outputs are called “drain” (positive) and “source” (negative).

In high power IGBTs the outputs are called “collector” (positive) and “emitter” (negative).

These transistors provide control of the motor by switching the high voltage and current in sympathy with the gate voltage.

There exists a problem when trying to switch on some of the transistors since it is normal operation to have their emitter/source (the negative terminal) voltage at or close to the positive of the power supply.

The problem is you need a significant voltage difference (usually at least 3v) between the gate and the emitter/source for the transistor to switch on.

This requires a special arrangement of driver circuit to allow the gate voltage at times to be set above the high voltage supply rail from the power supply to the circuit.

It is known that this is provided by using separate power supplies for the gate driver circuits.

This is usually 4 or 6 isolated gate driver power supplies as well as the motor high voltage power supply.

There are also known isolation devices called “opto-isolators” comprising a light emitting diode (LED) and photo-transistor in the same package.

The LED and photo-transistor parts of the opto-isolators are connected to different circuits or different parts of the same circuit.

The Invention

The invention seeks to provide a simple method for controlling the high power transistors described above.

According to the present invention there is provided a simple split-rail brushless motor controller comprising:
a) A low power supply for control electronics,
b) A high power supply for motor power,
c) The positive connection of the high power supply is shared with the ground connection of the low power supply,
d) Isolation devices between the high power circuit and the low power circuit.

Preferably the controller is used in an electric vehicle.

Preferably the high power supply is provided by a high voltage battery in the vehicle.

Preferably the low power supply is shared with the other control electric/electronics of the vehicle.

Preferably the low power supply is 12v DC and the high power supply is approximately 300v DC.

Preferably this would mean the high power supply would actually be described as -300v DC.

Preferably the isolation devices are opto-isolators.

Preferably the high power transistors are IGBTs, but MOSFETs can be used in this circuit without significant alteration.

An embodiment of the invention will now be described with reference to figure 1.

Referring to Figure 1, there are 2 power supplies A-B, and B-C.

There is a low power supply A-B which combines with the high power supply B-C to form a single split-rail supply A-B-C.

In the split-rail supply A-B-C, A is the positive (+12v), B is the ground (0v) and C is the negative (-300v).

Referring to the top section of figure 1 above the ground B, Controller G is a representation of a micro-controller or digital controller circuit providing the logic and sequencing to control the motor.

Controller G has power supply connections to the split-rail positive supply A, the split-rail ground B and 6 outputs connected to the LED inputs of opto-isolators D via limiting resistors H.

The LED inputs of the opto-isolators D are also connected to the split-rail power supply ground B.

Referring to the bottom section of figure 1 below the ground B, there are 6 transistors E and K, switch off resistors I and J, and a brushless motor F.

Of the 6 transistors E and K, transistors K are connected to the ground B and transistors E are connected to the negative C.

Transistors K are the high-side transistors.

Transistors E are the low-side transistors.

The transistors E and K are connected in pairs to form the 3 phases of the brushless motor F.

The circuit continues from the photo-transistors of the opto-isolators D (above the ground B) to the gates of the 6 transistors E and K.

Switch off resistors I are connected between the gates of the high-side transistors K and their individual emitters.

Switch off resistors J are connected between the gates of the low-side transistors E and the negative C which is common with all the emitters of low-side transistors E and K.

The invention provides that the voltage at the gates of the transistors E and K naturally can rise above the emitter voltages of all the transistors E and K and thus switch them on reliably.


1. A simple split-rail brushless motor controller comprising:
a) a single low power supply,
b) a high power supply,
c) a common connection between the low power supply negative and the high power supply positive outputs.

2. A method as in claim 1 of controlling a brushless motor using a single simple power supply only.

3. A method of controlling a brushless motor significantly similar to claims 1 and 2


The invention relates to a brushless motor controller.

Normally a brushless motor controller requires the use of a complex system of multiple, isolated power supplies to operate the high power output transistors which drive the motor.

This invention simplifies this using a single split-rail supply A-B-C which powers the controllers and brushless motor F without requiring isolation.

This invention provides a method of powering the brushless motor and controller from the same power supply thus decreasing complexity and ultimately cost.