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DC/DC converter with uC interface

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Simon:

--- Quote from: free_electron on May 05, 2019, 07:21:42 pm ---you can't speed control a bldc properly by changing its voltage.

get a 4 wire fan and use a pwm on the control line to regulate speed. the way that works is that they are chopping the power rail to the bridge. the rail there has a capacitor to smoothen the voltage. the control ic reads the pwm signal and changes the drive pattern of the mosfets to adjust the speed.

the voltage on a bldc only controls torque. ( indirectly through current)
the rotational speed is purely controlled by speed of commutiation on the phases.

--- End quote ---

Correct. many fons will state a working voltage range and go a bit under the minimum before they cut out but they tend to slow down at the bottom end as they use that back emf on the unswitched phase to detect the position so work out for themselves how fast they can go.

aix:

--- Quote from: free_electron on May 05, 2019, 07:21:42 pm ---
get a 4 wire fan and use a pwm on the control line to regulate speed. the way that works is that they are chopping the power rail to the bridge. the rail there has a capacitor to smoothen the voltage. the control ic reads the pwm signal and changes the drive pattern of the mosfets to adjust the speed.

--- End quote ---

Thanks for this insight.  I've looked into getting a PWM-controlled BLDC fan, but couldn't find one in the form factor I need at a suitable cost.


--- Quote from: free_electron on May 05, 2019, 07:21:42 pm ---you can't speed control a bldc properly by changing its voltage.

the voltage on a bldc only controls torque. ( indirectly through current)
the rotational speed is purely controlled by speed of commutiation on the phases.

--- End quote ---

I've measured the rotational speed of the fan I have and it does change with voltage (see the attachment).  I am not sure how to reconcile this with your suggestion that it shouldn't.

aix:

--- Quote from: mariush on May 05, 2019, 07:23:12 pm ---The MC34063 is a shitty cheap ancient switching regulator IC with poor efficiency from the start, and then it actually See my post above where I showed you some more modern regulators like that MP1540, which claim to be as efficient as over 90% [snip]
 
Another good example would be  LT1961  : https://www.analog.com/en/products/lt1961.html#product-overview
Datasheet : https://www.analog.com/media/en/technical-documentation/data-sheets/1961fa.pdf


--- End quote ---

Just wanted to say thanks and mention that I'm not ignoring your suggestions.  The MC34063A is all I had in my parts box on a Sunday, and so that's what I ended up playing with.  I knew next to nothing about DC/DC converters this morning.  I still do, but at least I have a working circuit.  This feels like progress.  :)

Once I get to grips with the current design, I might venture into playing with more advanced ICs.

Simon:

--- Quote from: aix on May 05, 2019, 08:13:22 pm ---
I've measured the rotational speed of the fan I have and it does change with voltage (see the attachment).  I am not sure how to reconcile this with your suggestion that it shouldn't.

--- End quote ---

I explained this earlier. Ideally brushless fan's don't want their supply voltage altering as there is control circuitry that needs a minimum to work on. Usually the fans can work down to about half the maximum at lower speeds and may be able to go lower in voltage but it will be attempting to run full speed at all times. It will conceed to go as fast as it can at the lower voltage but it will be speed limited by the throttling of power. while the fan will work at a lower than specified voltage it is of course not guaranteed to work.

i have some 16-32V 65W fans at work that i can run down to 5V but I only do that when I use them on my bench to cool me down in the summer. i would never set them up to do this in a working design.

aix:

--- Quote from: Simon on May 05, 2019, 09:17:43 pm ---I explained this earlier. Ideally brushless fan's don't want their supply voltage altering as there is control circuitry that needs a minimum to work on. Usually the fans can work down to about half the maximum at lower speeds and may be able to go lower in voltage but it will be attempting to run full speed at all times. It will conceed to go as fast as it can at the lower voltage but it will be speed limited by the throttling of power. while the fan will work at a lower than specified voltage it is of course not guaranteed to work.

--- End quote ---

This agrees with what I'm seeing with my fan.  Its datasheet specifies 7-13.8 VDC working voltage, but it spins (even starts) all the way down to at least 3V.  With the current design I plan to push it to 4.5V.  Given the nature of my project it's OK to drive the fan out of spec.  The lower voltage threshold will be configurable in software in case I find the fan unreliable at lower voltages or have to replace it with a different model.

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