Understand MOSFET Switch Behavior Via An LED Driver Simulation

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2020-07-01 08:11:42

Automotive incandescent bulbs have largely given way to more efficient, reliable, stylish, and even safer light emitting diodes (LEDs). LEDs turn on in a fraction of the time and are especially useful in brake lamps, where fractions of a second matter. The challenge in designing an automotive LED lamp is in satisfying government requirements for light output while also being cost effective. Another hurdle is design complexity where a switch-mode power supply (SMPS) is often required. A SMPS is more complex, expensive, and can be a source of electromagnetic interference (EMI), especially in comparison with the body control module (BCM) high-side-driver (HSD) that is typically used to turn on an incandescent bulb. Generating sufficient light output often requires a series string of LEDs. Consequently, a boost converter is often used to create the LED string compliance voltage from a car battery that can vary from 6V under start-stop and cold-crank condition to 19V.

The boost topology uses a MOSFET switch and diode, as illustrated in Figure 1, and the switch can turn on in nanoseconds. If this switch action resonates with parasitic elements on the PCB, it can cause EMI in the form of radiated and conducted emissions. One technique used to mitigate EMI in switching regulators is to slow down the input supply current and voltage transitions (di/dt and dv/dt) with a MOSFET gate resistor. But what exactly does this do and how should it be selected? There are two methods of analyzing the effect of the gate resistor. The first and more common method is to use a circuit simulator like SPICE or SIMPLIS. Another approach would be to use a 2D or 3D electromagnetic (EM) simulator like ADS to run a co-simulation with SPICE. The latter is much more complicated, but equally more comprehensive. It also requires expensive software and a user who is familiar with these tools. SPICE, on the other hand, is more commonly understood and available as freeware.

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