Technology: PWM and inductive load

Internal PWM of the terminal

Integrated for each channel in the EL2212 two-channel digital output terminal is a compact PWM output stage with an ultra-small design that drives one or two actuators in 3 phases. In each case the supply voltage is switched through to the actuator. In order to counteract possible impermissibly high load currents, depending on to the resistance of the actuator, the voltage is only switched through to the actuator in pulses in the various phases by means of pulse width modulation (PWM). On average over time, this results overall in a lower current through the actuator, which is smoothed almost to a DC voltage due to the inductive components of the actuator. This procedure is used in the EL2212 terminal and always takes place by presetting the holding current and supply voltage: the preset current is regulated as a setpoint value even if the supply voltage together with the interior resistance of the actuator enables a much larger load current.

Pulse width modulation for current control

By means of the PWM output stage the pulse width modulation (PWM) of the supply voltage is used to regulate the output current of a connected ohmic/inductive load. The full supply voltage, pulsating with a certain frequency, is thereby fed to the output. A load current only develops at the inductance at the high level. The load current is not changed by changing the voltage level, but by the duration of the switch-off (pulse width) in relation to the period duration. This results in a duty factor corresponding to the pulse width divided by the period duration of between 0 and 100% and is proportional to the load current.

The figure "Operation at load with inadequate inductance" illustrates operation with an inadequate inductance. Continuous current flow is not reached. The current has "gaps". This mode of operation is not permitted.

Pulse width current terminals require inductive loads The load inductance should have a minimum inductance of 1 mH. Operation of the pulse width current terminals at loads with an inductance of less than 1 mH is not recommended, since the intermittent current flow prevents reference between the set value and the arithmetic mean of the current.