# Pulse Wave Modulation



## Mr. Sharkey (Jul 26, 2007)

Pulse _Width_ Modulation is how just about all power is controlled these days. It has been around for a long time, although the devices that actually control the power have changed over the years. PWM describes the method of switching a high power controller using variable width, low level frequencies. We went from transistors doing the switching (high resistance, much loss) to SCR's (slightly better, but frequency limited), to MOSFET devices (low resistance, good efficiency) to the current level of technology, IGBT's (Insulated Gate Bipolar Transistors), which have very good power handling capabilities and have the added benefit of allowing a high degree of isolation between the circuit being controlled and the input control signal, making them useful for high voltage control circuits.

In the olden days, PWM frequencies were in the audio range (~1 - 10 KHz), so controllers using this system "sing", that is, you can hear the switching pulses, and the vehicle motor makes a distinct whining noise that rises in pitch as the power is increased. Modern circuit design and faster switching components have allowed the frequencies to be increased to the supersonic range, 15-25 KHz and above (100 KHz, depending on the application), so dogs may be able to hear the circuits operating, but humans have it nice and quiet.

PWM has matured since the days of early controllers. Back then, discrete components generated saw-tooth wave signals, and then modulated them with the desired accelerator position to integrate a signal that would control the motor. There are now one-chip solutions to PWM control which require a minimum of external parts and have built-in current limiting, soft-start, etc. If you have electronics experience, building your own basic controller is not difficult. For more sophisticated controllers, PIC's, or microcontroller PC devices can be programmed to do a bang-up job running a PWM control circuit.

In all cases, it's necessary to differentiate between the PWM signal that is generated, and the components that use that signal to actually control the power. An EV controller may have both inside the case, but PWM is the control method, while MOSFET or IGBT devices do the actual power switching control.


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