Determine PWM frequency, how high?

[julianto]

Hi all. I have a plan on trying to use a simple PWM averaging using RC filter. By Googling I have found some information about determing the RC value. It seems that the higher PWM freq we use the better. I't's kinda a rule thumb.

My question is, how high should it goes? Is it merely depends on the uC (generator system) capabilitiy (incl. design requirement) or is there somekind of a golden rule there? If there is not any "golden frequency" then what is the most common frequency used (if there is any of such) and also why (the technical background)?

Thanks in advance :smile:

 

[randell_xtian]

it should be based on what type of application your gonna used it.

 

[FvM]

I guess, you want to use the PWM circuit as a DAC. What are your requirements in DAC terms? Resolution, update rate, settling time? What are the available hardware PWM parameters of your microcontroller, respectively the expected frequency, resolution and accuracy of a software pwm solution? You'll notice, that there several absolute limitations and possible trade-offs. Do some calculations and you'll hopefully understand.

 

[julianto]

Thx for replying randell :smile:
I was thinking about using it as DAC (FvM has pointed that out) feed into voltage regulators feedback pin such as LM317, LM2576 and such. The reason I'm asking is that people, especially engineers, usually picks many rule of thumb along the way :smile:

---------- Post added at 23:20 ---------- Previous post was at 22:51 ----------

Hi FvM, first contact ... thx for reply :smile:
You are absolutely right, I have a thought to use it as a means to control voltage regulator. You have what they use to say, a senior instict, I guess. I am trully sorry that this time I cannot provide you the detail of my design simply because there is simply none, yet. In this phase I'm just looking as many information as I can to re-read and synthesize later on. I have a reasonable basic knowledge of electronics but it cannot be said as a good working skills, so you might get "the picture" now. But here is what I have in mind already ...

I'm planning to use Arduino (Mega then my own built NG board, the older) as the controller. If I succeed I will look forward to implement it on a system with atmega8/atmega8535/atmega32/attiny2313 as the controller, depends on what other system function dictates. I suppose learning pwm using Arduino is very straight forward than directly on the bare AVR. As I remember most Arduino operates with 16MHz crystal. I have no other plan yet to share the uC time budget, sole purpose is as the pwm controller. Other function I will learn to add later, as the basic digital power supply works well.

I hope I can manage to have 1.25V (or better 0V) all the way up to 15V, for the first effort system.

I have found this in Wikipedia:

"The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. Typically switchings have to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies."

~Pulse-width modulation - Wikipedia, the free encyclopedia

I have read elsewhare about similar project, mainly based on the PICs. But I cannot find the common "usual value" as stated in the Wikipedia for other apps. Since you mention it, limitations and trade-offs, I sens that too (the reason I'm asking). But I am crawling in the dark here. If you can elaborate more, I will be gratefull, FvM.

 

[bigdogguru]

Here are a few appnotes concerning DAC, PWM and filtering, while PIC based the general theory can be extended to other MCU families such as Atmel AVR:

AN655 - D/A Conversion Using PWM and R-2R Ladders to Generate Sine and DTMF Waveforms

Source for Above Appnote

AN823 - Analog Design in a Digital World Using Mixed Signal Controllers

AN1086 - Switching Power Supply Design with the PIC16F785

TB053 - Generating High Voltage Using the PIC16C781/782

BigDog

 

[Tahmid]

This should really help you: www.datadog.com/pwm_tutorial.pdf

If you increase the switching frequency you get lower losses in the magnetics, smaller capacitors and magnetics (transformers/inductors), but higher losses in the electronics (FETs and diodes).

Hope this helps.
Tahmid.

 

[julianto]

Thanks BigDog. I will study those files carefully. So temporarily I will follow what was stated in the AN655:

The designer should experiment with this value to find the most optimal number for the application.

I will be back to ask more information about limits and tradeoff I try some experiments myself.

---------- Post added at 05:53 ---------- Previous post was at 05:45 ----------

Hi Tahmid. Thanks for the link. I will study all reading materials that I already have so far. Then after I did some experiments myself I'll be back asking for your experience about those losses trade-offs.

 

[bigdogguru]

I just picked up a couple Arduinos to play around with, mainly to checkout the Arduino programming environment. I have a few links to PWM projects using the Arduino, I'll post them here later today.

 

[FvM]

AN565 linked by bigdogguru and the PWM tutorial linked by Tahmid is referring exactly to your DAC application. Main performance parameters are resolution respectively accuracy and speed/settling time. If speed isn't a primary objective of your design, then selection of PWM frequency and filter dimensioning isn't very critical. For high resolution (12 to 16 Bit), you'll end up with a rather low pwm frequency and slow filter. I've been using active 2nd order low-pass filters in this situation to get low ripple and an acceptable settling time.

PWM for switch mode converters, which has also been addressed in some contributions, is a different topic and imposes different requirements.

 

[shreyas_patel21]

How to decide pwm frequency

hello everyone I am using PWM for fan control in power supply.

I am using pic 18f66k90

I used 50% duty cycle for slow speed and 90% duty cycle for high speed.

but I don't know what should be the frequency for fan control.

thank you.
-shreyas

 

[FvM]

Why don't you just try? Newer BLDC fans have input capacitors and don't work well with simple on-off PWM. Best behaviour can be expected with low PWM frequencies in this case.

 

[Tahmid]

Re: How to decide pwm frequency

hello everyone I am using PWM for fan control in power supply.

I am using pic 18f66k90

I used 50% duty cycle for slow speed and 90% duty cycle for high speed.

but I don't know what should be the frequency for fan control.

thank you.
-shreyas

Start with about 10 kHz and then move the frequency up or down by experimentation if you feel you need to.

 

[shreyas_patel21]

I tried with different frequency.
I observed that at around 60kHz it doesn't make noise,
up to 20-23KHz it makes some noise.

But for speed of the fan, result almost same in all cases.

What should be the frequency for higher efficiency or for longer lifetime of fans?, lower or higher?

what are the advantages and disadvantages of higher and lower frequencies.?

thank you!
Regards,
shreyas.

 

[FvM]

Small motor inverters work well with 16 to 25 kHz. Almost no audible noise, acceptable losses. It's usually not reasonable to go for higher PWM frequencies.