AN-1160 Power Switching for Inductive Loads in Surge and Noise Sensitive Applications

Electronic designs often involve switching power to a load. It is important to control the rate of switching in order to control the rate of rise of voltage and current (dV/dt and dI/dt) at the switching node. Excessive dV/dt and dI/dt can cause a power supply to droop, couple noise to nearby circuits, or cause transients which can be potentially harmful.

Motors can be especially problematic to switch. Motors can be roughly modeled as the inductance of the stator. If a motor's source power is switched on too quickly, an inductor resists a rapid change in current by producing back-EMF, following the inductor's constitutive equation, V = L * dI/dt. This results in a potentially harmful voltage transient at the switching node.

Furthermore, the inrush current will be large because as the motor starts from a standstill, the rotor is not moving, thereby not creating a back-EMF to resist current flow. As such, the inrush current will be large and similar to the stall current.

In this app note, we will evaluate two high-side switching methods: a relay for “instantaneous” switching, and the Silego SLG59H1012V HFET which has a programmable output dV/dt.


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