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O/T - switching regulators

 
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Darren Logan
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O/T - switching regulators
PostPosted: Wed Mar 26, 2003 3:31 pm     Reply with quote

Hi all,

As always apologies for O/T post however I thought some of you might find my problem an interesting one.

I have a PIC based circuit that requires 5V to run, however I need to drive 3 x 12V DC (0.8A) solenoids. For this, I am using the famous ULN2003 7 x darlington driver IC.

The power supply coming in is 12V DC to 24V DC, but of course I need to regulate to 12V.

The solenoids have a +/- 5\% voltage tolerance so minimum power input is dictated by this.
Also, Vce sat. of the ULN2003 is at worste case 1.6V - yes ok I could use FETS with near zero rdsON but I'm not OK!.

My only solution is to use a switching reg. which can both step up and down (i.e. a buck/boost or SEPIC as it's now know as) circuit.

My question to you gurus is:

a) How would you achieve this?
b) Do you know of any cheap switching reg. circuits which could accept say 5 to 26V in and give 12V at 0.8A out?

Hint: LM3478 from National Semi. ! - one heck of a switching reg. for little cost.

Cheers,
Darren
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This message was ported from CCS's old forum
Original Post ID: 13108
Sherpa Doug
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Re: O/T - switching regulators
PostPosted: Wed Mar 26, 2003 3:57 pm     Reply with quote

:=Hi all,
:=
:=As always apologies for O/T post however I thought some of you might find my problem an interesting one.
:=
:=I have a PIC based circuit that requires 5V to run, however I need to drive 3 x 12V DC (0.8A) solenoids. For this, I am using the famous ULN2003 7 x darlington driver IC.
:=
:=The power supply coming in is 12V DC to 24V DC, but of course I need to regulate to 12V.
:=
:=The solenoids have a +/- 5\% voltage tolerance so minimum power input is dictated by this.
:=Also, Vce sat. of the ULN2003 is at worste case 1.6V - yes ok I could use FETS with near zero rdsON but I'm not OK!.
:=
:=My only solution is to use a switching reg. which can both step up and down (i.e. a buck/boost or SEPIC as it's now know as) circuit.
:=
:=My question to you gurus is:
:=
:=a) How would you achieve this?
:=b) Do you know of any cheap switching reg. circuits which could accept say 5 to 26V in and give 12V at 0.8A out?
:=
:=Hint: LM3478 from National Semi. ! - one heck of a switching reg. for little cost.
:=
:=Cheers,
:=Darren

Are you REALLY sure you can't run your 12V solenoids off of the 12V input minus the darlington loss? What is the source of your 12 to 24V? Is the 24V just a momentary fault condition? Could your 12V solenoids withstand brief 24V faults?

One oddball solution might be to regulate a negative 5V from your source. Run the PIC from 0V and -5V. Regulate 13.6V for the solenoids from (12 to 24) + 5V = 17 to 29V.

Tell us more about the application and we might give you better advice.

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This message was ported from CCS's old forum
Original Post ID: 13109
Darren Logan
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Re: O/T - switching regulators
PostPosted: Wed Mar 26, 2003 4:18 pm     Reply with quote

>Are you REALLY sure you can't run your 12V solenoids off of >the 12V input minus the darlington loss?

Actually, funny you should mention this because they seem to run fine right down to 7V on the bench but "apparently" if you apply full flow pressure to them then they need full power.
I cannot quantify or verify this (sorry to be so vague!).

>What is the source of your 12 to 24V? Is the 24V just a >momentary fault condition? Could your 12V solenoids withstand >brief 24V faults?

The source is a DC power supply which could be anything between 12 and 24V. No the 24V could be continuous and no the solenoids do not like 24V!

>One oddball solution might be to regulate a negative 5V from >your source. Run the PIC from 0V and -5V. Regulate 13.6V for >the solenoids from (12 to 24) + 5V = 17 to 29V.

Yes that would work!, however the -5V being the new GROUND would need to sink up to 0.8A of solenoid current. (Only one solenoid is on at a time). Can you suggest such a -5V device?.
Also I would still need a 12V regulator which would be dissipating high power when the input is 24V.

Regards,
Darren
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Original Post ID: 13111
John P
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Re: O/T - switching regulators
PostPosted: Thu Mar 27, 2003 12:39 am     Reply with quote

I've been there and done that (power supply was 12-28V), but since the valves were custom made, I got them to wind the coils for an 8V supply. Then we used a switching setup designed to deliver a constant current--close to a 100\% duty cycle for the minimum voltage, low duty cycle if the input voltage was at the max. It worked very nicely and I won kudos and plaudits.

I can't imagine how a complicated voltage converter would ever be better than a few MOSFETs. Remember, you don't need to feed the coils a constant voltage, all they care about is the current. And the coils are highly inductive, so they'll smooth out the ripple if you pulse-width modulate the drivers.

If you're really forced to use a voltage converter, maybe you could use one that always steps up (to 28 or 30, say) and then use a PWM drive to operate the coils with the proper current. This would at least be simpler than a step-up-or-down converter.

Another appoach would be to measure the power supply voltage and set the duty cycle via the PIC, and assume that this will make the average current come out right. Less hardware, but I'd guess not as accurate. Maybe worth thinking about.

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Original Post ID: 13117
mcafzap



Joined: 07 Sep 2003
Posts: 46
Location: Manchester, UK

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Re: O/T - switching regulators
PostPosted: Thu Mar 27, 2003 4:06 am     Reply with quote

:=Another appoach would be to measure the power supply voltage and set the duty cycle via the PIC, and assume that this will make the average current come out right. Less hardware, but I'd guess not as accurate. Maybe worth thinking about.
:=

This is a great idea - I may use it myself, thanks!
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Original Post ID: 13128
TSchultz



Joined: 08 Sep 2003
Posts: 66
Location: Toronto, Canada

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RE: O/T - switching regulators
PostPosted: Thu Mar 27, 2003 5:45 am     Reply with quote

:=>Are you REALLY sure you can't run your 12V solenoids off of >the 12V input minus the darlington loss?
:=
:=Actually, funny you should mention this because they seem to run fine right down to 7V on the bench but "apparently" if you apply full flow pressure to them then they need full power.
:=I cannot quantify or verify this (sorry to be so vague!).
:=
:=>What is the source of your 12 to 24V? Is the 24V just a >momentary fault condition? Could your 12V solenoids withstand >brief 24V faults?
:=
:=The source is a DC power supply which could be anything between 12 and 24V. No the 24V could be continuous and no the solenoids do not like 24V!
:=
:=>One oddball solution might be to regulate a negative 5V from >your source. Run the PIC from 0V and -5V. Regulate 13.6V for >the solenoids from (12 to 24) + 5V = 17 to 29V.
:=
:=Yes that would work!, however the -5V being the new GROUND would need to sink up to 0.8A of solenoid current. (Only one solenoid is on at a time). Can you suggest such a -5V device?.
:=Also I would still need a 12V regulator which would be dissipating high power when the input is 24V.
:=
:=Regards,
:=Darren

I think you need to consider John's recommendation that you power the solenoids with PWM modulated constant current derrived from your input. Since the solenoids are very inductive you actually wind up with an RL filter which works very well.

For more information on how this is done check out many of the stepper motor drivers, most of them use this method for current regulation. Often the coils on the stepper are rated for 5VDC but they are actually driven from a 24 to 55V source using PWM. The modulation used is normally a variable ON time with a fixed off time, this seems to work best for highly inductive loads.

I have done quite a few stepper and DC motor projects and this method of regulation works very well. It also have the advantage of having quite a variety of control chips already available and they are usually at a very reasonably cost. There is also lots of information on this out there.

If you take care of the solenoids this way then the 5V you need for the rest of the circuit, which is low current, can easily be handled with a small regulator. Linear now have some nice samll switcher regs that handle a very wide input voltage.

-Troy
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This message was ported from CCS's old forum
Original Post ID: 13131
Darren Logan
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Re: O/T - switching regulators
PostPosted: Thu Mar 27, 2003 4:27 pm     Reply with quote

Hi all,

Thanks for the ideas!. It's a question of choice now.

Criteria as always = lowest component count and most importantly cost.

I do not have any I/O pins left on the PIC to control a PWM signal, therefore does anyone know of a PWM ic which controls itself according to PSU voltage?

Or maybe this could be done with a simple op-amp oscillator, running at say 20kHz where the mark space varies according to PSU voltage??????

Regards,
Darren
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This message was ported from CCS's old forum
Original Post ID: 13160
TSchultz



Joined: 08 Sep 2003
Posts: 66
Location: Toronto, Canada

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RE: O/T - switching regulators
PostPosted: Fri Mar 28, 2003 7:21 am     Reply with quote

:=Hi all,
:=
:=Thanks for the ideas!. It's a question of choice now.
:=
:=Criteria as always = lowest component count and most importantly cost.
:=
:=I do not have any I/O pins left on the PIC to control a PWM signal, therefore does anyone know of a PWM ic which controls itself according to PSU voltage?
:=
:=Or maybe this could be done with a simple op-amp oscillator, running at say 20kHz where the mark space varies according to PSU voltage??????
:=
:=Regards,
:=Darren

For some more ideas maybe check out these;

<a href="http://www-s.ti.com/sc/ds/drv101.pdf" TARGET="_blank">http://www-s.ti.com/sc/ds/drv101.pdf</a>
<a href="http://www.ichauscorp.com/pdf/JED0S.PDF" TARGET="_blank">http://www.ichauscorp.com/pdf/JED0S.PDF</a>
<a href="http://www.supertex.com/pdf/datasheets/HV9901.pdf" TARGET="_blank">http://www.supertex.com/pdf/datasheets/HV9901.pdf</a>
<a href="http://www.onsemi.com/pub/Collateral/CS1107-D.PDF" TARGET="_blank">http://www.onsemi.com/pub/Collateral/CS1107-D.PDF</a>
<a href="http://www.chipcenter.com/circuitcellar/december99/c129r20.htm" TARGET="_blank">http://www.chipcenter.com/circuitcellar/december99/c129r20.htm</a>;$sessionid$JGDZ4AIAAOAGSQSNDISSFEQ?PRINT=true
<a href="http://www.powerdesigners.com/InfoWeb/design_center/Appnotes_Archive/1656.pdf" TARGET="_blank">http://www.powerdesigners.com/InfoWeb/design_center/Appnotes_Archive/1656.pdf</a>

-Troy
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This message was ported from CCS's old forum
Original Post ID: 13192
TSchultz



Joined: 08 Sep 2003
Posts: 66
Location: Toronto, Canada

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RE: O/T - switching regulators
PostPosted: Fri Mar 28, 2003 7:29 am     Reply with quote

Here is probably a good place to start. Two of these and you are all set for your 4 solenoids.

<a href="http://us.st.com/stonline/books/pdf/docs/1332.pdf" TARGET="_blank">http://us.st.com/stonline/books/pdf/docs/1332.pdf</a>
<a href="http://us.st.com/stonline/books/pdf/docs/4032.pdf" TARGET="_blank">http://us.st.com/stonline/books/pdf/docs/4032.pdf</a>

-Troy
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Original Post ID: 13193
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