In the description of the PWM drive principle of the brush DC motor, the current regeneration method based on the short circuit of the motor is introduced, and there are other current regeneration methods in the PWM drive of the motor, which actually brush DC. Each method in the PWM driver of the motor has a matter that should be considered.
Use the PWM output to drive the brush DC motor: current regeneration method
Here is a schematic diagram for explaining the principle of the PWM drive. Among them, there is no need to listed transistors. (A) is a current regeneration when the voltage is applied, and (b) is a current regeneration of the short-circuit method of both ends of the motor.
Since the transistor (in this example is MOSFET) to switch the H bridge, as a reality problem, the on-resistance of the transistor is required as the loss in each path. This idea is equally applicable to subsequent current regeneration methods.
In addition to (b), there are three current regeneration methods. (C) a method of only disconnecting Q1 (Q2 and Q3 holding off, while Q4 holding turning on) in a state where the voltage is applied. In this case, the regenerative current is the same as the parasitic (body) diode flow of Q2 in turn, is the same as in (b). At this time, the path includes not only the on-resistance, but also the forward voltage Vf of the parasitic diode. Therefore, the current will be rapidly attenuated. Further, the equivalent average voltage applied to the motor will lose the amount of Vf, so it will be less than the corresponding voltage corresponding to the duty cycle.
(D) is a method of closing all transistors. In this case, the regenerative current flows via the shutdown Q2 and Q4 parasitic diode. In this path, the VF of the two parasitic diodes of Q2 and Q3 will become loss.
Further, since the power supply EA enters the current path and flows in the opposite direction, the decay of the regenerative current is very fast, and the equivalent average voltage applied to the motor relative to the ON duty cycle is very low. When driving at a 50% duty cycle, the equivalent average voltage applied to the motor will be close to zero. At this time, due to the presence of a diode, the regeneration will be stopped when the current becomes zero, and no current flows in the opposite direction.
(E) a method of turning on the transistor to the applying voltage, that is, from the Q1 and Q4, the voltage applied in the Q2 and Q3 turn off the Q1 and Q4 turn-on, Q2 and Q3 are turned on. , That is, the bias state.
In this case, the power source EA enters the current path and flows in reverse flow, so the attenuation of the regeneration current is very fast, and when the regeneration current via the diode, when the current is changed to zero, the regeneration is stopped, but since the transistor is in turn State, the current will flow in the opposite direction.
Therefore, if the drive is driven according to the same 50% duty cycle with (d), the equivalent average voltage applied to the motor will become zero. When the duty cycle is 100%, the equivalent average voltage applied to the motor is the largest. The duty cycle is between 100% and 50%, and the applied voltage from the maximum to zero can be proportional to the duty cycle. In addition, when the duty cycle is from 50% to 0%, the current flows in reverse flow from 100% to 50%. The PWM voltage and current waveform under this condition are as follows.
The problem mentioned in the instructions mentioned in the PWM drive of the brush DC motor will be introduced separately in the next article.
Key points:
PWM drive with brush DC motor will repeatedly apply voltage and current regeneration.
PWM drives with brush DC motor use the H-bridge current regeneration method, and the regenerative current path different loss is also different.
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