FAQ
Q.
Please explain the effects of motor drive applications on switching power supplies.
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A. Problems such as the generation of counter electromotive force due to inductance of the coil and lowered output of the power supply can occur if a standard switching power supply were to be used for motor drive applications.
These phenomena are found in mainly output capacitance types of below 300W that do not have independent auxiliary power supplies in their power supplies.

Figure 1 Block Diagram (Example)

About DC370V of boosted voltage is applied on both ends of the input capacitor (C1) of the smoothing circuit from the rectifier circuit through the harmonic current suppression circuit when the power supply is turned on. Furthermore, voltage is then supplied to the VCC of the PWM control IC (hereunder “IC”) through the starting resistance R1. The MOS FET (Q1) starts switching when this VCC reaches or exceeds the activation start voltage of the IC and the IC starts to move. The driving voltage of the IC receives a supply from the auxiliary winding tap (winding 3) from the main transformer after that. In other words, the voltage that passes through the R1 is only used only for starting up the IC during the startup of the system (steady operation).
Here is an explanation of the mechanism where the output voltage is decreased when the motor is not running with the waveform of the parts in the block diagram of Figure 1.
 
Figure 1 Waveforms of parts when the motor is not running

Figure 2 Waveforms of parts when the motor is not running
A counter electromotive force is generated if the motor stops during the steady operation of the power supply. The counter electromotive current flows into the secondary side of the power supply, the output voltage increases, and the IC tries to make the pulse narrow in order to decrease the output voltage after detecting its increase. However, the pulse cannot be made narrow instantly due to lag in the control system and a phenomenon where the output voltage exceeds the rated voltage occurs (1).
The IC is in then a state where the pulse is closed for a while without the output voltage instantly returning to the rated voltage because there is an output capacitor (C2) on the secondary side of the power supply.
On the other hand, when the IC pulse closes, Q1 stops operating and the voltage supply from the auxiliary winding tap of the main transformer is cut off. Therefore, VCC gradually decreases (②). Furthermore, when VCC drops to IC stop voltage (③), the IC enters restart mode, and voltage is supplied to the IC’s VCC again via the startup resistor R1, requiring several +ms time to rise to IC startup voltage (④). Therefore, the output voltage will drop during that time (⑤). For these reasons, when connecting a motor load to a switching power supply, the voltage at the output end of the power supply must be prevented from rising due to reverse EMF. It is recommended that a diode for reverse current protection be connected outside the power supply.