4.3.6.2. Fixed stop time (Frequency Modulation switching control)

A key difference between this type of control and PWM control is the absence of the oscillator. In the downtime control fixed, as its name suggests, is set downtime or time of recirculation. However, the driving time is left, according to all parameters of control, and engine power. What produces practically a frequency modulation according to the reference voltage.

Figure 4.42. Chopper downtime bipolar fixed (Frequency Modulation switching control).

The implementation of power drivers is done naturally, just connect the circuit. The current flowing through the motor winding resistance on the sensor Rs. When the voltage across it reaches to the reference Vref, the comparator acts on the monostable, activating it. The cycle of monostable putting into operation its output to low, disabling the driver and making the circuit between state recirculation. This time will come set by the monostable time, usually between 25 and 35 microseconds. Once occurs after activation of a new driver into a new driving cycle (Figure 4.43)..

The period of the oscillator is determined and consists of the sum of downtime, which in this case is always fixed and the monostable time plus driving time, which will depend on all parameters involved in the circuit , the supply voltage, equivalent inductance and resistance of the winding, reference voltage, time delay ...

4.3.6.3. Comparison of the two types of control

The technique of frequency modulation control or control of a fixed stopping time, presents a number of advantages that make it suitable for current or microstep control. This technique minimizes the power dissipation in power transistors, and has a ripple in the current very low. While the pulse width modulation (PWM) shows some instability problems when the duty-cycle exceeds 50%.