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2022.08.24 motor

Single-Switch Circuit Driving and Half-Bridge Circuit Driving

Brushed DC Motor

In this article, as the last methods for driving a brushed motor, driving using a single-switch circuit and using a half-bridge circuit are explained. Both can be used for either DC driving or for PWM driving.

Driving Using a Single-Switch Circuit

This circuit creates two states; in one state, the single switch is used to connect the DC power supply (+) and (-) terminals to the motor, and in the other state, the motor is disconnected from either the (+) or the (-) terminal of the DC power supply. This can be performed by a single switch, and so it is called a single-switch circuit.

When the switch is turned on, the motor rotates in one direction, according to the polarity; when the switch is turned off, a voltage is no longer applied, and so the motor idles and thereafter stops. The switch may be located either on the (-) side, as in ①, or on the (+) side, as in ③. The switch can also be replaced with a semiconductor power transistor (in the diagrams, a MOSFET), making the circuit an electronic circuit. An N- channel MOSFET is used when the device is on the (-) side, as in ②; a P-channel MOSFET is used when on the (+) side, as in ④.

However, a word of caution when using a power transistor. In this circuit, immediately after turn-off the motor inductance causes the current to continue flowing, and so in ① the (-) side of the motor can swing to higher than the power supply voltage, and in ③ the (+) side can swing to below GND potential. This voltage can be at least several times the power supply voltage. Hence when using a power transistor, as already indicated in the circuit diagrams for ② and ④, a power diode must be connected in parallel with the motor, to protect the transistor by suppressing (clamping) the voltage that occurs using the forward voltage of the diode.

When this clamping diode is present, the current due to power generation by the motor flows through the diode while the generated voltage is higher than the diode forward voltage, so that a torque in the opposite direction of the rotation acts, and the short (short-circuit) braking action causes the motor to stop quickly.

PWM driving is also possible using this circuit, and current driving is also possible by detecting the motor current and applying negative feedback.

Driving Using a Half-Bridge Circuit

A circuit in which two switches are connected in series with a power supply, as in the diagrams, is called a half-bridge circuit, because it is one-half of an H-bridge (full bridge) using four switches. Because one H-bridge is counted as one channel, half of the H-bridge is sometimes called a half channel.

When SW2 in ① or SW1 in ③ is turned on, the motor rotates in the direction determined by the polarity. In ①, when with the motor in the rotating state SW2 is turned off and SW1 is turned on simultaneously, short braking occurs, and the motor rapidly stops. And ③ is the opposite: short braking occurs when SW1 is turned off and SW2 is turned on at the same time. In both ① and ③, when while the motor is rotating SW2 and SW1 are both turned off, no voltage is applied, and so the motor idles and then stops.

Similarly to the above single-switch circuits, the switch parts in ① and ③ can be replaced with semiconductor power transistors. In the circuit examples, an N-channel MOSFET is used on the (-) side of the DC power supply, and a P-channel MOSFET is used on the (+) side. Because MOSFETs have parasitic diodes, even when Q1 and Q2 are turned off while the motor is rotating, the parasitic diode in the MOSFET that is parallel-connected to the motor regenerates a current, so that the same action as short braking occurs. This short braking state persists while the voltage generated by the motor does not fall below the forward voltage of the parasitic diode.

Similarly to an H-bridge circuit, in a half-bridge circuit also two switches (or transistors) are series-connected between the power supply and GND, so that the predrive circuit must have a simultaneous-on prevention function to ensure that both switches are not turned on at the same time when switching is performed. If simultaneous-on occurs, a large current called a shoot-through current or the like occurs, and switch (transistor) destruction is possible.

Moreover, in order to create the three output states resulting in rotation, braking, and idling, it is necessary provide either two two-valued inputs or one three-valued input. With a single two-valued input, two out of the three output states would have to be chosen.

These circuits can also be used for PWM driving, and by detecting the motor current and providing negative feedback, current driving is also possible.

Key Points:

・In single-switch circuit driving, a single switch controls switching between the two states of rotation and idling.

・When a power transistor is used as the switch in single-switch circuit driving, there is the possibility that the transistor may be damaged due to a back emf, and so a clamping diode must be used.

・A half bridge can control switching between three states: rotation, idling, and braking.

・When using MOSFETs as the switches in a half bridge, braking operation is possible because of the parasitic diodes in the MOSFETs. Both single-switch and half-bridge circuits are capable of PWM driving and current driving.

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