Question: How does constant current operation via PWM drive work?
Answer: As an example, a small resistor can be inserted at the groundside side of the output transistor to detect current flowing to the motor. The voltage across this resistor is compared with the reference voltage, and if greater the ON output of the output transistor high side is turned OFF. From this point the motor current continues to flow, but will gradually decrease. After a certain period of time this high side output will turn ON and the motor current will increase until the resistance voltage reaches the reference voltage, turning OFF the high side once again. This operation is repeated, with a triangular current wave flowing up to a peak current value obtained by dividing the reference voltage with the current detection resistor. Setting the high side OFF time sufficiently small can enable virtually constant current operation.
Figure 1. Example of constant current operation with a PWM drive
Figure 1 shows a circuit diagram of constant current operation utilizing a PWM drive.
A current detection resistor Rs is connected to the groundside (RNF) of the power transistor. A comparator compares this RNF voltage to the reference voltage pin (Vref) and outputs Low if the RNF voltage exceeds Vref, turning OFF the power transistor at the power supply side responsible for supplying current. (It is also ok to turn OFF the power transistor at the power supply side and turn ON the power transistor at the groundside.)
Next, in the above example, counting the oscillator (OSC) frequency allows arbitrary OFF time (toff) setting. During this OFF time regenerative current flows. Once the OFF time ends, the power transistor at the power supply side turns ON again, supplying current. Since the OFF time is sufficiently small with respect to the electrical time constant, the motor can be operated through constant current control with a peak current Ipeak of Vref/Rs. Constant current drive makes it possible to turn the motor at a constant torque.
Figure 2. Constant current waveforms with a PWM drive
Figure 2 shows the constant current waveforms with PWM drive.
While regenerative current flows, since current does not flow through Rs, once current supply resumes the current change at Rs will be large.
Large voltage noise may be generated due to the parasitic inductor component, allowing current to flow to charge the parasitic capacitance of the power transistor and possibly causing Vref to be exceeded.
To prevent an OFF operation due to this voltage noise, a time where no reaction occurs (tblnk) is required in order to ignore the peak current for a short period of time. This is necessary to ensure no malfunctions occur due to voltage noise even when the motor rotation switches.
In addition to this time setting there is another way to remove noise using a filter.
Figure 3. Current pathways during current supply and current regeneration with a PWM drive
Figure 3 illustrates the current pathway during motor current supply and current regeneration with the PWM drive.
In Figure 3 above, when current is supplied (a), Q1 and Q4 are turned ON, connecting the motor to the power supply. During Current Regeneration 1 (b), Q1 is turned OFF, Q2 is turned ON, and Q4 remains ON (similar to shorting the motor terminals), while in Current Regeneration 2 (c), Q1 and Q2 are turned OFF, Q4 stays ON, and the motor current cycles through the parasitic diode of Q2.
