FET-based DC motor drive control circuit


Long, linear DC motor with its good characteristics, excellent control performance characteristics of a majority of variable speed motion control and closed loop position servo control system the best choice. With the computer in the control area in particular, the high switching frequency, the second generation of full-controlled power semiconductor devices (GTR, GTO, MOSFET, IGBT, etc.) development and pulse width modulation (PWM) DC Motor Control technology, DC been widely used. To meet the needs of small DC motor used, the semiconductor company introduced a DC motor control ASIC, microprocessor-based control constitutes a DC motor servo system. However, application specific integrated circuit composed of DC motor drive output power is limited, not suitable for high power DC motor drive demand. Therefore, enhanced by N-channel FET H bridge construction, to achieve high-power DC motor drive control. The driver circuit can meet the needs of all types of DC motors, and fast, precise and efficient, low power consumption can be directly interfaces with the microprocessor can be applied to achieve DC PWM speed control technology.

2 DC motor drive control circuit overall structure

DC motor drive control circuit is divided into optical isolation circuit, motor drive logic circuit, driving the signal amplification circuit, charge pump circuit, H bridge power driver circuit of four parts, the circuit diagram shown in Figure 1.

FET-based DC motor drive control circuit

Can be seen from the diagram, the external motor drive control circuit simple interface. The main direction control signal with the motor running Dir motor control signal PWM signal and electrical brake signal Brake, Vcc for the driver logic circuit section provides power supply, Vm for the electrical power supply, M +, M-machine interfaces for the DC.

In the high-power drive system, will drive circuit and control circuit electrical isolation, reduce the drive control circuit of the external control circuit interference. After isolation control signal generated by the motor drive logic circuit motor logic control signals, control the H bridge's upper and lower arm. As the H bridge N-channel enhanced by a high-power FET structure, not by a direct drive motor logic control signal must be amplified by the driving signal control circuit and charge pump circuit to amplify the signal, and then drive H bridge power driver circuit to drive DC motor.
Principle 3 H bridge power driver

DC motor drive is the most widely used H-type full bridge circuit, this drive circuit easily achieve a four quadrant DC motor, corresponding to positive change, positive change brake, reverse, reverse brake. H bridge power driver schematic diagram shown in Figure 2.

H-bridge driver circuit all four switch states are working in the chopper. S1, S2 as a group, S3, S4 as a group, these two complementary states, when a group of conduction, the other group must be shut down. When the S1, S2 turns on, S3, S4 off, the motor plus the forward voltage across the motor brake the forward or reverse; when the S3, S4 turns on, S1, S2 off, the electrical ends of the anti- to the voltage, the motor reversal or are transferred braking.

Actual control, the need to keep the motor switch between the four quadrants, that is to switch and reverse switch between, that is, S1, S2 turns on and S3, S4 off to the S1, S2 and S3 off , S4 turns on switching between these two states. This theory demands two sets of control signals completely complementary, but because the actual switching devices are present on-and off-time, absolute complementarity control logic will lead to short circuit through the upper and lower leg. In order to avoid short circuit through the various switch and ensure coordinated action and synchronization, two sets of control signals required each other down with theory, but the actual difference must be dead a long enough time, this correction process not only by hardware, that is, two sets of upper and lower leg increased delay between the control signal can also be realized by software.

Figure 2 4 switch for the freewheeling diode for coil windings to provide continued flow loop. When the motor is running, the drive current flowing through the main switch motor. When the motor is braking, the motor work in the power generation state, the rotor current must flow through the freewheeling diode, otherwise the motor will be fever, severe and even burned.

FET-based DC motor drive control circuit

4 DC Motor Control Circuit

By a DC motor drive control circuit diagram can be seen driving control circuit is simple, mainly by the four-part circuits, including optical isolation circuit is relatively simple, not going to introduce, following the DC motor drive control circuit of the other parts in detail.
4.1 H bridge driver circuit design

Commonly used in DC motor control circuit as drive H bridge power drive circuit. As the power MOSFET is a voltage-controlled device, with a large input impedance, high switching speed, no secondary breakdown characteristics of the phenomenon, to meet the demand for high-speed switching, so commonly used form power MOSFET H bridge circuit leg. H bridge circuit of four N-channel power MOSFET were used to type and P-channel type, while the P-channel power MOSFET is generally not used in the bridge arm drive motor, so there are two possible options: one is the upper and lower leg respectively P-channel power MOSFET 2 and two N-channel power MOSFET; the other is to use upper and lower leg are N-channel power MOSFET.

In contrast, use of two N-channel power MOSFET and two P-channel power MOSFET drive motor program, the control circuit is simple and low cost. However, the reasons for processing, P-channel power MOSFET performance than the N-channel power MOSFET of the poor, and the drive current, much less for the power driver circuit. The N-channel power MOSFET, while the migration rate of carrier high frequency response is better, higher transconductance; on the other hand can increase the conduction current, reduce resistance, reduce costs, reduce the area . Considering the system power, reliability requirements, and the N-channel power MOSFET advantages, the design uses four N-channel power MOSFET the same H-bridge circuit, with good performance and high reliability, and has more large drive current. The circuit shown in Figure 3. The figure for the motor supply voltage Vm, four diodes as freewheeling diodes, a small output capacitor in parallel C6, generated by the motor for reducing the emotional component of the peak voltage.

FET-based DC motor drive control circuit

4.2 Charge Pump Circuit Design

The basic principles of charge pump capacitor of the charge by the cumulative effects caused by high pressure, so that current flows from low potential of high potential. The first charge pump model is the ideal J. Dickson made in 1976, when this circuit is erasable EPROM to provide the required voltage. Later, J. Witters, Toru Tranzawa J. Dickson and others on the charge pump model is improved, made more precise theoretical model, to be confirmed by experiments presented the theoretical formula. With the continuous development of integrated circuits, based on low power, low cost considerations, the charge pump circuit design more and more widely.

Simple charge pump circuit shown in Figure 4 Principle. Capacitor C1 through diode D1 A-side access Vcc, capacitor C1, the amplitude of Vin B termination of the square wave. When the B point potential was 0, D1 turns on, Vcc capacitor C1 to begin charging, until node A's potential to Vcc; when the B point rose to high potential Vin, the voltage across the capacitor can not because of mutation, at this time point A potential increased to Vcc + Vin. Therefore, A point is a square wave voltage, the maximum is Vcc + Vin, the minimum value is Vcc (assuming an ideal diode diode). A point of a square wave through a simple rectifier, which provides a voltage higher than Vcc.

FET-based DC motor drive control circuit

In the drive control circuit, H bridge by the four N-channel power MOSFET formed. To control the various MOSFET, the MOSFET gate voltage must be sufficiently higher than the gate voltage. Usually make the MOSFET turn completely reliable, its gate voltage is 10 V or more generally, that VCS> 10 V. The H bridge leg, directly above the voltage of 10 V applied can make it turn; while for the bridge arm of the two MOSFET, make VGS> 10 V, it must meet the VG> Vm +10 V, the drive circuit must be able to provide a voltage higher than the supply voltage, which requires an additional step-up circuit driver circuit to provide higher than 10 V gate voltage. VGS has required taking into account the upper limit, the general MOSFET turns on VGS is 10 V ~ 15 V, which controls the gate voltage with the gate voltage change, namely the floating gate drive. Therefore, the drive control circuit in charge pump circuit design, to provide higher voltage Vm Vh, drive power tube conduction. The circuit shown in Figure 5.

A part of the circuit is a square wave circuit, the RC and RP Schmitt trigger, resulting in the amplitude of Vin = 5 V square wave. Part B is the charge pump circuit, constituted by the third-order charge pump. When a point is low, the diode D1 conduction charging capacitor C1, so that b point voltage Vb = Vm-Vtn; when a point is high, due to capacitor C1 voltage is not mutation, so b point voltage Vb = Vm + Vin -Vtn, then diode D2 turns on, charging capacitor C3, so that c point voltage Vx = Vm + Vin-2Vtn; when a point again is low, the diode D1, D3 conduction, respectively, the capacitor C1, C2 charge makes the d point voltage Vd = Vm + Vin-3Vtn; when a point again is high, due to capacitor C2 voltage is not mutation, so d point voltage becomes Vd = Vm +2 Vin-3Vtn, then diodes D2, D4 conduction , respectively, the capacitor C3, c4 charge, so that e point voltage Ve = Vm +2 Vin-4Vtn. So this cycle will be at the g points higher than the Vm voltage Vh = Vm +3 Vin-6tn = Vm +11.4 V. Where Vm is diode voltage drop, usually take 0.6 V. H bridge the upper arm to ensure full conduction. 4.3 Logic and zoom motor drive circuit design

DC motor drive motor drive motor drive circuit logic and the main amplifier to achieve an external control signal to drive H-bridge control signal conversion and amplification. Control signal Dir, PWM, Brake by optical isolation circuit, the gate circuit decoding, resulting in four control signals M1 ', M2', M3 ', M4', and then amplified by the transistor to generate control signals H 4 bridge M1, M2, M3, M4. The circuit diagram shown in Figure 6. Where Vh is the voltage Vm by the charge pump upgrade, Vm for the motor supply voltage.

FET-based DC motor drive control circuit

FET-based DC motor drive control circuit

Electrical work, H bridge arm is normally open or normally closed state, controlled by the Dir, the lower arm by the PWM logic level control, continuously adjustable control voltage generated. The program, the upper arm only in the motor commutation MOSFET only when switching, and motor commutation frequency is very low, low-end direct control by the logic circuit, signal level logic circuit switch quickly to meet requirements of different frequencies. The circuit is also an advantage, because the upper arm to open slowly, and the lower arm off quickly, so the actual control of the upper and lower arm does not appear for the moment while conduction phenomenon, reduces the change when the current impact to improve the MOSFET life.

PWM DC Motor Speed Control 5

DC Motor Speed n = (U-IR) / Kφ

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

DC motor speed control can be divided into excitation control law and the armature voltage control method. Excitation control method is to control the flux, the control power of small, low speed limits by the magnetic saturation, high speed, by changing spark and the commutator to the structural strength of the restrictions, and because the dynamic response of large excitation coil inductance less, so this control with very little. Most applications are using armature voltage control method. With the advances in power electronic technology to change the armature voltage can be achieved through a variety of ways, including PWM (pulse width modulation) is commonly used to change the armature voltage as a speed control method.

PWM speed control principle is based on a fixed frequency to connect and disconnect the power, and needed to change a cycle turned on and off time ratio (duty cycle) to change the voltage DC Motor, "accounted for air than "to change the average voltage, control the motor speed. In PWM System, when the motor power when its speed increases, the electrical power failure to reduce its speed. Change according to certain rules as long as the pass, power-off time, you can control the motor speed. And the use of variable speed system composed of PWM technology. Start and stop when no impact on the DC system and has started a small power consumption, stable operation characteristics.

Motor is always connected to the power set, the motor maximum speed Vmax, and set a duty cycle D = t / T, the average speed of the motor Vd as:

Vd = VmaxD

By the formula indicates that, when changing the duty cycle D = t / T when you can get different motor average speed Vd, so as to achieve speed control purposes. Strictly speaking, the average speed and duty cycle D is not strictly linear relationship, in general applications, it can be approximated as linear. In the DC motor control circuit, PWM signal provided by an external control circuit, and by the high-speed optical isolation circuit, motor drive logic and amplifier circuit, the drive H bridge MOSFET switch arm to change the average voltage on the DC Motor to control motor speed, to achieve DC motor PWM speed control.

6 Conclusion

To N-channel enhancement-type FET as the core, based on the H bridge PWM control the drive control circuit for DC motor control and speed adjustment Reversible good performance. Experimental results show that the DC motor drive control circuit is stable and reliable, fast response and motor speed regulation. To meet the requirements of practical application, there are good prospects.

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