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Filtering PWM to smooth DC

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IanB:

--- Quote from: Hero999 on September 22, 2018, 09:15:45 pm ---Attached is a simulation which takes things to the extreme R = 1R, C = 1000µF and a load resistance of 80R to be close to the 170mA. The PWM frequency is a bit higher at 2kHz and a duty cycle of 20%. The voltage across the load settles at 13V, with 830mA current spikes being drawn each pulse.
--- End quote ---

If you were to replace the resistor with an inductor, what size inductor would produce reasonable performance?

Zero999:

--- Quote from: IanB on September 22, 2018, 09:57:31 pm ---
--- Quote from: Hero999 on September 22, 2018, 09:15:45 pm ---Attached is a simulation which takes things to the extreme R = 1R, C = 1000µF and a load resistance of 80R to be close to the 170mA. The PWM frequency is a bit higher at 2kHz and a duty cycle of 20%. The voltage across the load settles at 13V, with 830mA current spikes being drawn each pulse.
--- End quote ---

If you were to replace the resistor with an inductor, what size inductor would produce reasonable performance?

--- End quote ---
Big, especially if the frequency is 500Hz as per the original poster's circuit, not the 2kHz I used in my simulation. Without calculating anything properly, just plugging figures into the simulation, I get around 47mH, which would need to be rated to 200mA.

Of course a resistor is much more economical, but it depends on what voltage drop and performance at low duty cycles is acceptable.

There is another possibility: filter the PWM to pure DC using a rectifier and capacitor and use an emitter follower with a higher value RC circuit to drive the load. I'll post a schematic tomorrow.

Zero999:
And here it is. It will drop nearly 2V at full output but that can be minimised by replacing D1 with a MOSFET rectifier and R3 with a current source. Another possibliity is to change it to low drop-out configuration, with a P device for Q1, but it will be difficult to stabilise.

Rapsey:
Wow, thank you so much Hero999, you are really going above and beyond!

I've spent a few hours playing around with different RC vaues and studying the results. Getting a good feel for how it all fits together now. It seems that for the specific scenario of feeding PWM through an RC LPF you could say that:

* Capacitor value primarily affects ripple (smaller cap will drop more in voltage during off-portion of PWM cycles)
* Resistor value primarily affects control range in two ways:

* Limits the top end of your PWM control (smaller resistor will drop less voltage and thus get closer to the 12V max)
* Shifts your control curve (smaller resistor will charge the capacitor faster, e.g. 10% pulses could be enough to fully charge the capacitor and if it is big enough to carry through the remaining 90% this would shift your effective control curve to ~0-10% PWM)Therefore in practice it seems best to do a balancing act choosing your resistor (as low as possible for the upper limit but not so low that it destroys your control curve) and then choose an appropriate capacitor based on how much ripple you can tolerate. Of course all of the above depends on how great the load is. Greater loads will require smaller resistors and larger capacitors to maintain similar results.

A few questions about your RC circuit:

* Is that LTSpice?
* How do you arrive at a load resistance of 80R?
Do you simply use Ohm's law and divide the voltage by the rated current? If so, then judging by my volt/amp measurements this remains fairly constant at lower speeds.
Can you measure the ESR of a component like this by simply measuring resistance with a multimeter?
* You mention 830mA current spikes but the graph shows the spike going as high as 14A and eventually settling at 830mA?And the LC circuit:

* Would, say, a B82791H2251N020 be a good inductor here? (47mH, max 250mA / 2.4R, 30% tolerance)
* How would the characteristics differ from an RC circuit? Why is this better?
If these big inductors have resistance values in the same ballpark as the resistors we used for RC, is there still a point in using them?As for the rectifier circuit... I think I have some more studying to do. :)

wraper:

--- Quote from: Rapsey on September 24, 2018, 02:36:48 pm ---And the LC circuit:

* Would, say, a B82791H2251N020 be a good inductor here? (47mH, max 250mA / 2.4R, 30% tolerance)
* How would the characteristics differ from an RC circuit? Why is this better?
If these big inductors have resistance values in the same ballpark as the resistors we used for RC, is there still a point in using them?As for the rectifier circuit... I think I have some more studying to do. :)

--- End quote ---
No, it's stupid. Also that inductor is large and made for mains filters, has two 47mH coils, that thing likely would experience tremendous losses. Viable would be like 30kHz PWM and inductor in hundreds of uH range. If done decently there will be barely any energy losses and 100-220uF capacitor would be more than enough.

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