How to design the output freewheeling circuit of the I/O module?

Designing a proper freewheeling circuit for the I/O module output is crucial for protecting switching devices and suppressing interference. When driving inductive loads, the freewheeling diode must be connected in parallel with the load, with the diode's cathode connected to the positive terminal of the power supply and the anode connected to the junction of the load's negative terminal and the switching transistor, forming the freewheeling path. Fast recovery or ultra-fast recovery diodes, such as the FR or ES series, should be selected due to their short reverse recovery time, reducing switching losses and noise. The physical area of the freewheeling circuit on the PCB must be minimized to reduce parasitic inductance and radiation. For AC loads or applications requiring rapid turn-off, an RC snubber circuit, varistor, or bidirectional TVS diode can be used instead of a single diode. For MOSFET drivers, sometimes an integrated diode is used, but its performance may be insufficient, requiring an external fast recovery diode. The peak current and reverse voltage that the freewheeling diode needs to withstand must be calculated during the design, with margins allowed. Measuring the voltage waveform across the switching transistor using an oscilloscope can verify whether the freewheeling circuit effectively suppresses voltage spikes.