A PIC based boost converter

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A PIC based boost converter

The old design as discussed here last week had problems starting oscillating
with heavy capacitive loads.
The efficiency was only about 50 % too.
The TI super switcher chip is invisibly small and difficult to obtain without
forking out $$$ for shipping and maybe customs.
That left the junk(well its valuable) box.
I already have 2 PICs 18F14K22 in this design, plenty of those around,
why not give it a try>?
These PICs start working at about 1.8 V,
not quite enough to drive a power MOSFET like the IRLZ34N I have,
but just enough to get output...
So I added a diode circuit that supplies the PIC from the 5 V output once it
starts.
Used the PIC internal 1 V reference as reference for a voltage comparing diff
amp.
The output of that controls the reference level of a cycle by cycle current
limiter.
So true current mode.
The PIC's internal comparator is used to stop a PWM cycle.

There will be a second power MOSFET in series with the batteries, controlled by
a 'power good'
signal from this same PIC, to completely disconnect the batteries below say 1.9 V
(PIC can measure that with ADC or the other comparator.

I forgot to draw the 100k resistor from gate MOSFET to ground,
to prevent any charge on the gate during power up when the PIC output is
tristate,
from shorting the supply.

The trimpot for the output can be a normal voltage divider, but I only had the
100k in the box.
The diagram:
 
http://panteltje.com/pub/PIC_current_mode_boost_converter_diagram_img_3222.jpg

The test setup:
 
http://panteltje.com/pub/PIC_current_mode_boost_converter_test_setup_img_3221.jpg

Putting all those littel circuits together, and it works.
 
http://panteltje.com/pub/ee_spider_was_here_img_3219.jpg

People interested in the PIC software can ask here.


Re: A PIC based boost converter

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PIC_current_mode_boost_converter_diagram_img_3222.jpg
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PIC_current_mode_boost_converter_test_setup_img_3221.jpg
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Looks interesting.  Schematic?

--
www.wescottdesign.com

Re: A PIC based boost converter
On a sunny day (Tue, 22 Nov 2011 12:25:13 -0600) it happened Tim Wescott
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Re: A PIC based boost converter
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Looks nice. Just curious, have you taken into account if the PIC runs
wild, so it cannot check the voltage or current and will blow the FET?

Perhaps add a seperate loop with high priority interrupt routed to a
crude check of the output voltage set 10% higher than the nominal
value?

Cheers

Klaus

Re: A PIC based boost converter
On a sunny day (Tue, 22 Nov 2011 14:56:50 -0800 (PST)) it happened Klaus

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You need to know a bit more of the PIC hardware.
What I use here is a hardware analog comparator.
The PWM unit is a hardware counter too.
The comparator switches of the PWM in hardware each time the current
reaches a preset value.
The external diff amp controls this preset value.
So it is a true current mode regulator.
It would be short circuit proof IF it was not for the
boost configuration where the diode would conduct from input to output.
So for that in this sort of regulator you need a normal fuse.
For the rest of loads it nicely current limits.

The only software so far this uses is the initialisation of the chip sub units.


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No, you are still thinking software.
PICs are great for this sort of thing with all the build in things,
counters, PWM unit, UART, reference voltages, DAC, comparators, ADC,
internal clock generator, what not.

I added a soft start in software that slowly increases the max PWM value allowed
after power up.
This is done from the timer interrupt.
Nothing can go wrong with that, as no timer tick then the PWM stays zero.
There is a watchdog and brownout detection too.

PICs are cool (use little power).


Re: A PIC based boost converter

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If that is the case, what is the added value of the PIC? If you want
to do PWM with a micro at least make the control loop in software so
you can control overshoot, load regulation, fold-back current
limiting, power reduction based on the temperature of the power stage,
etc, etc.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
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Re: A PIC based boost converter
On a sunny day (Wed, 23 Nov 2011 14:15:59 GMT) it happened snipped-for-privacy@puntnl.niks

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allowed
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Well, slow start for a start, remote control of parameters,
remote measuring of now:
battery voltage
output voltage

Less eye strain as not to have to solder a 3 mm^2 chip,
no shipping costs, no 12 Euro 50 custom handling costs,
****no time lost****, I have the PICs,

I know you do not like PICs, you have made that very clear.
But I like them and I have plenty of soft for it.
So development is very fast.
To do the temp thing you mention only takes a few extra lines of code
and a LM335 sensor.
Or use a 8 pin PIC as sensor.
But in this case there is no overheating expected so why bother.
I would like to see you do foldback current limiting in a boost converter of
this type

 in --- L ---- diode --- out
           |
          switch
           |
          ///
Hey?
Foldback diode?
I use a fuse.

PICs are nice, and as main() is free here, it can
do other things I think of.




Re: A PIC based boost converter
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snip

I remember seeing some put a cap in series with the diode to make it
short
circuit proof


-Lasse


Re: A PIC based boost converter
On a sunny day (Wed, 23 Nov 2011 12:18:44 -0800 (PST)) it happened

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mm but caps do not conduct DC?

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Re: A PIC based boost converter
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True. That would make it short-circuit proof. Right?

Re: A PIC based boost converter
On a sunny day (Wed, 23 Nov 2011 15:07:48 -0600) it happened John S

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OK :-)
But what about output?


Re: A PIC based boost converter
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What output?

Re: A PIC based boost converter
On a sunny day (Wed, 23 Nov 2011 15:12:09 -0600) it happened John S

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DC ouput?

But I think I have that solved, see my other reply.

Re: A PIC based boost converter
On a sunny day (Wed, 23 Nov 2011 15:07:48 -0600) it happened John S

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OK :-)
But what about output?

Maybe like this?
  in
  |
  L
  |   C         a   k    
  ---||---------|>|--------- out
  |          |            |
  |MOSFET   ---           |
  /switch   / \          ===
  |         ---          ---
  |          |            |
 ///        ///          ///

2 diode losses..
          

Re: A PIC based boost converter
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Yes. If no added diode, no output. That's why I said "What Output?"

However, you must admit that it will work to limit current.



Re: A PIC based boost converter
On a sunny day (Wed, 23 Nov 2011 15:40:20 -0600) it happened John S

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Yes, sure, in some situations this could be a really good solution.

Re: A PIC based boost converter
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However, I don't like the current spikes it will produce. I would
probably look for a cleaner solution.


Re: A PIC based boost converter
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in normal operation the switch node is not DC

but when there's no dc path from input to output, current flow stops
when you
stop switching


-Lasse

Re: A PIC based boost converter

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Even though its pulsed, it still is DC. Hint: current flows in one
direction.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
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Re: A PIC based boost converter

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Give it up. You'll save more hair :)

Jamie



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