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PWM Frequency Selection Issues:
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For a 0 to
PWM signal, power loss through a resistor, R, is calculated as:
For a constant current signal of value
, power loss through R is:
As DC decreases, more and more power is lost through the
resistance to heat when using a PWM signal compared to a
constant signal of the same average value.
Switching Losses: An ideal switch is either fully on or off which means it never dissipates any power.
Real switches don’t switch instantaneously, they have a transition time during which they dissipate power.
The transition time is usually fixed per edge. For example, using a switch that requires a 1 microsecond
transition time in a 25kHz PWM (40 microsecond period) means that the transition time is 1/40 of the
total. If the PWM frequency were increased to 250kHz (4 microseconds), this ratio would increase to ¼
which would significantly and adversely affect performance.
Vibration and Noise: An electric motor uses wire coils to produce magnetic force. Every length of wire
in the motor undergoes lateral movement proportional to the current being passed through it. When the
current is cycled, i.e. a PWM signal, the movement becomes a regular vibration audible to the human
ear. Since the range of human hearing is generally considered to be 20Hz to 20kHz, PWM frequency is
often selected to be 20kHz or greater to eliminate audible noise being produced.
Resolution: Normally a resolution of 10 bits (1/1024 = .001) is adequate. Based on your CPU speed
and PWM period, confirm your resolution is adequate. If not consider using the “High Resolution Timer”,
adjusting the PWM period, overclocking, or moving to a faster CPU.