pwm questions

[nina]

I have searching on this forum about pwm and I realized it can be used to control DC motors and comunications RF as well.
My questions are:

- How select PWM frequency?
- What is recommended frequency to work with DC motors? Is there any relationship with motor power?
- Is there a way to calculate before use or PWM is gotten trying one by one?
- Is there a frequency range?
- Power that PWM can provide is directly linked to transistor?

Tks

Nina

[Ttelmah]

The PWM frequency is set by Timer2.

Constraints on the frequency
Go too low, and you are likely to get unacceptable audio noise.
Go even lower, and you may get roughness in the movement on a motor.
The higher the PWM frequency, the better design needs to be on the driver feeding the output transistor (normally a FET), and the higher the energy involved in repeatedly charging/discharging the gate.
As you switch faster, switching losses will increase for a given design.
Radio frequency interference problems may increase at higher frequencies (but remember with a brushed DC motor you have a big problem here already....).
At lower frequencies, magnetic components are more likely to saturate, and the energy storage in magnetics needs to be larger (this is why you can use so much smaller transformers for a high frequency power supply). If driving a 'stepper' motor, or a brushless DC motor, this also applies to these, but not to a brushed DC motor.

So, generally, for things like PWM PSU's, it is better to spend extra time, design the switches well, and push frequencies higher. 20KHz, to perhaps 250KHz, are 'typical' areas for these.
However for DC brushed motor control, such frequencies are not needed, and typically, provided the noise is not unacceptable, frequencies of a few kHz, are commonly used.

Remember with _any_ PWM, involving inductive components (a motor contains coils, so is inductive), you _must_ have circuitry to do something with the energy in the coil, when the PWM switches off. Not handling this, is the commonest cause of part destruction in a PWM circuit...

Yes, power is limited by what the transistor(s) can handle. Remember also that the _peak_ involved in a DC motor, may be many times it's normal running current. For low voltages, FETs able to handle many hundreds of amps, are relatively common. In general, unless a FET specifically says that it includes a flyback diode, do _not_ rely on the internal diodes in a FET to handle flyback (there are a few specialist transistors from Hitachi, and a number of other designers, with separate diodes built into the die - the internal 'intrinsic' diode has poor characteristics as a flyback diode - handles less current than the FET itself, is generally quite slow etc....).

Best Wishes

[nina]

thank you Ttelmah for explanation....

so for DC motor what frequencies are commonly used? you said "few KHz. But how much?

About lighting control using PWM what frequency are commonly used?

Could you explain me how resolution can affect those controls?

Tks again

Nina

[PCM programmer]