Pwm Low Pass Filtering

Pulse width modulation (PWM) amplifiers require low pass filtering of the output to demodulate the PWM Carrier Some applications also utilize the filter as a way to achieve an imped- ance transformation which draws less power supply current than is delivered to the load. These filters CAN be as simple as a single inductor, to multiple LC nodes depending on the application. In some applications the load will have enough inductance to act as its own filter. Deciding on the type and size of a filter CAN be time consuming since the calculations CAN be tedious and often give component values that are not easily obtainable. This application note is an effort to reduce filter calculation time. Using the Apex Power Design tool, available at will further reduce time. Power Design.exe is a self-extracting Excel97 spreadsheet and will be used extensively in this application note. 2.0 Filter TYPES PWM filters are normally a low pass configuration. These exhibit low attenuation to the frequency spectrum from 0 Hertz to the frequency of cutoff (FC). This low attenuation region is called the pass band. Beyond the FC, attenuation increases at a rate determined by the filter type and the number of poles (order). Figure 1 indicates the general response of the low pass filter.
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