Design of DC motor drive control circuit based on field effect tube

1 Introduction

For a long time, DC motors have become the best choice for most variable speed motion control and closed-loop position servo control systems due to their good linear characteristics and excellent control performance.

Especially with the development of computers in the control field, high switching frequency, fully controlled second-generation power semiconductor devices (GTR, GTO, MOSFET, IGBT, etc.), and the application of pulse width modulation (PWM) DC speed control technology, DC motors have been widely used. In order to meet the use needs of small DC motors, various semiconductor manufacturers have launched DC motor control dedicated integrated circuits to form a DC motor servo system based on microprocessor control. However, the output power of the DC motor driver composed of dedicated integrated circuits is limited and is not suitable for high-power DC motor drive needs. Therefore, an N-channel enhancement field effect transistor is used to construct an H bridge to achieve high-power DC motor drive control. This drive circuit can meet the needs of various types of DC motors, and has the characteristics of fast, accurate, efficient, and low power consumption. It can be directly interfaced with a microprocessor and can use PWM technology to achieve DC motor speed control.

2 Overall structure of DC motor drive control circuit

The DC motor drive control circuit is divided into four parts: optoelectronic isolation circuit, motor drive logic circuit, drive signal amplifier circuit, charge pump circuit, and H-bridge power drive circuit. The circuit block diagram is shown in Figure 1.

As can be seen from the figure, the peripheral interface of the motor drive control circuit is simple. Its main control signals include the motor running direction signal Dir, the motor speed control signal PWM and the motor brake signal Brake. Vcc provides power for the drive logic circuit, Vm is the motor power supply voltage, and M+ and M- are DC motor interfaces.

In a high-power drive system, the drive circuit is electrically isolated from the control circuit to reduce the interference of the drive control circuit on the external control circuit. The isolated control signal is passed through the motor drive logic circuit to generate a motor logic control signal to control the upper and lower arms of the H-bridge respectively. Since the H-bridge is composed of a high-power N-channel enhancement field effect transistor, it cannot be directly driven by the motor logic control signal. The control signal must be amplified by the drive signal amplifier circuit and the charge pump circuit, and then drive the H-bridge power drive circuit to drive the DC motor.

3 H-bridge power drive principle

The most widely used DC motor drive is the H-type full-bridge circuit. This drive circuit conveniently realizes the four-quadrant operation of the DC motor, corresponding to forward rotation, forward braking, reverse rotation, and reverse braking. The H-bridge power drive schematic is shown in Figure 2.

The four switch tubes of the H-type full-bridge drive circuit all work in the chopping state. S1 and S2 form a group, and S3 and S4 form a group. The two groups are complementary. When one group is turned on, the other group must be turned off. When S1 and S2 are turned on, S3 and S4 are turned off, and a positive voltage is applied to both ends of the motor to achieve forward or reverse braking of the motor; when S3 and S4 are turned on, S1 and S2 are turned off, and a reverse voltage is applied to both ends of the motor, and the motor is reversed or forward braked.

In actual control, it is necessary to continuously switch the motor between the four quadrants, that is, switch between forward and reverse rotation, that is, switch between the two states of S1, S2 on and S3, S4 off to S1, S2 off and S3, S4 on. In theory, this situation requires that the two sets of control signals are completely complementary, but since the actual switching devices have on and off times, the absolute complementary control logic will cause the upper and lower bridge arms to short-circuit directly. In order to avoid direct short-circuit and ensure the coordination and synchronization of the actions of each switch tube, the two sets of control signals are theoretically required to be inverted, but in practice they must differ by a sufficiently long dead time. This correction process can be implemented either through hardware, that is, adding a delay between the two sets of control signals of the upper and lower bridge arms, or through software.

The four switch tubes in Figure 2 are freewheeling diodes, which can provide a freewheeling circuit for the coil winding. When the motor is operating normally, the drive current flows through the motor through the main switch tube. When the motor is in a braking state, the motor works in a generating state, and the rotor current must flow through the freewheeling diode, otherwise the motor will heat up, and even burn out in serious cases.

4 DC motor drive control circuit design

It can be seen from the block diagram of the DC motor drive control circuit that the drive control circuit has a simple structure and is mainly composed of four circuit parts. Among them, the optoelectronic isolation circuit is relatively simple and will not be introduced here. The other parts of the DC motor drive control circuit are introduced in detail below.

4.1 H-bridge drive circuit design

In DC motor control, H-bridge circuits are often used as the power drive circuit of the driver. Since power MOSFET is a voltage-controlled element with the characteristics of large input impedance, fast switching speed, and no secondary breakdown phenomenon, it meets the requirements of high-speed switching action, so power MOSFET is often used to form the bridge arm of the H-bridge circuit. The four power MOSFETs in the H-bridge circuit are N-channel and P-channel respectively, and P-channel power MOSFET is generally not used for the lower bridge arm to drive the motor. In this way, there are two feasible solutions: one is to use 2 P-channel power MOSFETs and 2 N-channel power MOSFETs for the upper and lower bridge arms respectively; the other is to use N-channel power MOSFETs for both the upper and lower bridge arms.

Relatively speaking, the scheme of using two N-channel power MOSFETs and two P-channel power MOSFETs to drive the motor has a simple control circuit and low cost. However, due to the processing technology, the performance of P-channel power MOSFET is worse than that of N-channel power MOSFET, and the driving current is small, so it is mostly used in driving circuits with lower power. On the other hand, N-channel power MOSFET has higher carrier mobility, better frequency response, and larger transconductance; on the other hand, it can increase the on-current, reduce the on-resistance, reduce the cost, and reduce the area. Considering the system power and reliability requirements, as well as the advantages of N-channel power MOSFET, this design uses an H-bridge circuit of four identical N-channel power MOSFETs, which has better performance and higher reliability, and has a larger driving current. Its circuit diagram is shown in Figure 3. In the figure, Vm is the motor power supply voltage, the four diodes are freewheeling diodes, and a small capacitor C6 is connected in parallel at the output end to reduce the peak voltage generated by the inductive element motor.

4.2 Charge Pump Circuit Design

The basic principle of the charge pump is to generate high voltage through the accumulation effect of capacitors on charges, so that the current flows from low potential to high potential. The earliest ideal charge pump model was proposed by J.Dickson in 1976. At that time, this circuit was used to provide the required voltage for erasable EPROM. Later, J.Witters, Toru Tranzawa and others improved J.Dickson's charge pump model, proposed a more accurate theoretical model, and verified it through experiments and proposed relevant theoretical formulas. With the continuous development of integrated circuits, based on low power consumption and low cost considerations, charge pumps are increasingly used in circuit design.

The principle circuit diagram of a simple charge pump is shown in Figure 4. The A terminal of capacitor C1 is connected to Vcc through diode D1, and the B terminal of capacitor C1 is connected to a square wave with amplitude Vin. When the potential at point B is 0, D1 is turned on, and Vcc starts to charge capacitor C1 until the potential at node A reaches Vcc; when the potential at point B rises to the high level Vin, because the voltage across the capacitor cannot change suddenly, the potential at point A rises to Vcc+Vin. Therefore, the voltage at point A is a square wave.

The maximum value is Vcc+Vin, and the minimum value is Vcc (assuming the diode is an ideal diode). The square wave at point A can provide a voltage higher than Vcc after simple rectification and filtering.

In the drive control circuit, the H-bridge is composed of 4 N-channel power MOSFETs. To control each MOSFET, the gate voltage of each MOSFET must be sufficiently higher than the gate voltage. Usually, to make the MOSFET fully and reliably turned on, its gate voltage is generally above 10 V, that is, VCS>10 V. For the lower arm of the H-bridge, a voltage of more than 10 V can be directly applied to make it turned on; and for the two MOSFETs in the upper arm, to make VGS>10 V, it must satisfy VG>Vm+10 V, that is, the drive circuit must be able to provide a voltage higher than the power supply voltage, which requires a boost circuit to be added to the drive circuit to provide a voltage higher than 10 V of the gate. Considering that VGS has an upper limit requirement, VGS is generally 10 V to 15 V when the MOSFET is turned on, that is, the control gate voltage changes with the change of the gate voltage, that is, floating gate drive. Therefore, a charge pump circuit is designed in the drive control circuit to provide a voltage Vh higher than Vm to drive the power tube to turn on. Its circuit schematic is shown in Figure 5.

Part A of the circuit is a square wave generator circuit, which is composed of RC and an inverting Schmitt trigger, and generates a square wave with an amplitude of Vin = 5 V. Part B is a charge pump circuit, which is composed of a three-stage charge pump. When point a is at a low level, diode D1 is turned on and capacitor C1 is charged, so that the voltage at point b is Vb=Vm-Vtn; when point a is at a high level, since the voltage of capacitor C1 cannot change suddenly, the voltage at point b is Vb=Vm+Vin-Vtn. At this time, diode D2 is turned on and capacitor C3 is charged, so that the voltage at point c is Vx=Vm+Vin-2Vtn; when point a is at a low level again, diodes D1 and D3 are turned on and capacitors C1 and C2 are charged respectively, so that the voltage at point d is Vd=Vm+Vin-3Vtn; when point a is at a high level again, since the voltage of capacitor C2 cannot change suddenly, the voltage at point d becomes Vd=Vm+2Vin-3Vtn. At this time, diodes D2 and D4 are turned on and capacitors C3 and C4 are charged respectively, so that the voltage at point e is Ve=Vm+2Vin-4Vtn. In this way, a voltage higher than Vm is obtained at point g: Vh = Vm + 3Vin - 6tn = Vm + 11.4 V. Vm is the diode voltage drop, which is generally 0.6 V. This ensures that the upper arm of the H bridge is fully turned on.

4.3 Motor drive logic and amplifier circuit design

The motor drive logic and amplifier circuit in the DC motor drive circuit mainly realizes the conversion and amplification of the external control signal to the drive H-bridge control signal. After the control signals Dir, PWM, and Brake pass through the photoelectric isolation circuit, they are decoded by the gate circuit to generate 4 control signals M1', M2', M3', and M4', which are then amplified by the transistor to generate 4 signals M1, M2, M3, and M4 for controlling the H-bridge. The circuit schematic is shown in Figure 6. Among them, Vh is the voltage of Vm boosted by the charge pump, and Vm is the motor power supply voltage.

When the motor is working, the upper arm of the H-bridge is in a normally open or normally closed state, controlled by Dir, and the lower arm is controlled by the PWM logic level to generate a continuously adjustable control voltage. In this scheme, the upper arm MOSFET is switched only when the motor is commutating, and the commutation frequency of the motor is extremely low. The low end is directly controlled by the logic circuit, and the signal level of the logic circuit switches quickly to meet different frequency requirements. This circuit also has an advantage. Since the upper arm opens slowly and the lower arm closes quickly, the upper and lower arms will not be turned on at the same time during commutation in actual control, which reduces the current impact during commutation and increases the life of the MOSFET.

5 DC motor PWM speed control

DC motor speed n=(U-IR)/Kφ

Where U is the armature terminal voltage, I is the armature current, R is the total resistance of the armature circuit, φ is the magnetic flux per pole, and K is the motor structural parameter.

DC motor speed control can be divided into excitation control method and armature voltage control method. Excitation control method is to control magnetic flux, and its control power is small.

At low speed, it is limited by magnetic saturation, at high speed, it is limited by commutation spark and commutator structure strength, and because the excitation coil inductance is large, the dynamic response is poor, so this control method is rarely used. Most applications use the armature voltage control method. With the advancement of power electronics technology, changing the armature voltage can be achieved through a variety of ways, among which PWM (pulse width modulation) is a commonly used speed control method for changing the armature voltage.

The basic principle of PWM speed control is to connect and disconnect the power supply at a fixed frequency, and change the "duty ratio" of the voltage on the DC motor armature by changing the time ratio (duty ratio) of connection and disconnection within a cycle as needed, thereby changing the average voltage and controlling the motor speed. In the pulse width speed control system, when the motor is powered on, its speed increases, and when the motor is powered off, its speed decreases. As long as the power-on and power-off time is changed according to a certain rule, the motor speed can be controlled. Moreover, the stepless speed control system composed of PWM technology has no impact on the DC system when starting and stopping, and has the characteristics of low starting power consumption and stable operation.

Assuming that the motor is always powered on, the maximum motor speed is Vmax, and the duty cycle is D=t/T, then the average speed Vd of the motor is:

Vd=VmaxD

From the formula, we can know that when the duty cycle D=t/T is changed, different average motor speeds Vd can be obtained, thus achieving the purpose of speed regulation. Strictly speaking, the average speed and duty cycle D are not strictly linear. In general applications, they can be approximately regarded as a linear relationship. In the DC motor drive control circuit, the PWM signal is provided by the external control circuit, and after passing through the high-speed optoelectronic isolation circuit, motor drive logic and amplifier circuit, it drives the switch of the H-bridge lower arm MOSFET to change the average voltage on the DC motor armature, thereby controlling the motor speed and realizing DC motor PWM speed regulation.

6 Conclusion

The drive control circuit based on H-bridge PWM control, with N-channel enhancement field effect transistor as the core, has good working performance for the forward and reverse control and speed regulation of DC motors. The experimental results show that the DC motor drive control circuit operates stably and reliably, and the motor speed regulation responds quickly. It can meet the requirements of practical engineering applications and has good application prospects.