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'Need another set of eyes for MOSFET capacitive swi'
1999\12\29@181644 by

In the process of building a DC-DC converter for my car MP3 player I've
come across the need for  -5 and -12 volt power. While I am aware of that
there are a number of single chip solutions, especially from Maxim, I'm
attempting to attack a capacitive switcher for reasons of understandability
and parts availablity. In short I have no idea where to get parts for or
how to wind flyback transformers. Also I wanted to practice/learn some
discrete design skills.

I understand capacitive switchers. An example can be found here:

http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#5volts.gif

That uses a 555 to generate the drive the charge pump. Now the problem is
that the current available from the circuit depends on several factors:

1) The current available from the 555.
2) The size of the caps.
3) The discharge rate of the reservoir cap.

So what I'm trying to figure out is how to get 0.5 A at both voltages from
the circuit.

For the sake of argument I imagined the circuit with a fixed load that
drew 0.5A at all times. For -5V that would be a 10 ohm load. The input
voltage is a nominal 12V with a 1.4V drop due to the diodes. Lastly to
ensure regulation we'd like to have the reservior cap never go above
-7 volts. Now the discharge rate depends on the RC constant (specifically
Vt = V0*e(-t/RC). Through experimentation I figured out that when t/RC is
0.4 that the cap discharges to 2/3's of its original value, which is near
the ratio from -10.6 to -7 that I'm looking for. So plugging in:

0.4=t/10C (R=10)
0.4*10C=t
4C=t

So the discharge time and hence the required charge frequency is directly
related to the size of the reservoir cap. Some common values results in
easily achievable frequencies. Even the 100 uC shown in the circuit only
requires a 2500 Hz frequency to maintain its charge.

Now all I have to do is charge it fast enough. I decided to use a pair of logic
level Power MOSFETs to charge the flying cap. The IRFZ44 with its 0.028 ohm
Ron resistance will completely charge a 100 uC flying cap in 14 uS which is
more than fast enough.

The only thing I was trying to figure out is how to put the MOSFETs in a
totem pole configuration. After researching CMOS configurations I finally
think I understand that the purpose of the P-channel MOSFET is so that the
complementary MOSFETS can be driven by the same input. But since the
characteristics of the N-channel is so much better I want to use a pair of
N-channels. So I plan to use this config:

+12
|
D
A-->G
S
|
|-----| | ------
|
D
B-->G
S
|
|
gnd

with A and B having to be complemetary. Whichever point A or B that's 5V
will conduct with the other off. Right?

Now on to current limiting resistors. Since the gate capacitance is 1900 pf
there will need to be current limiting resistors on A and B presuming that
I'm driving them with a PIC. I saw this in the previous MOSFET thread here.
Also will I require current limiting resistors between the MOSFET's and
+12 and GND? Since the Ron is so low, it's possible to draw more than 50A
(rated max current) through the MOSFET. Or do I not need to worry about it
because the regulator at the far end cannot draw that much current?

One last thing. To prevent both MOSFET's from conducting linearly together
I plan to program the pic with brief delays where both MOSFET's are turned off.
So instead of a AB pattern of 01 10 01 10 using 01 00 10 00 01 00 with the 00
periods much shorter than the power on periods. BTW this is where the PIC
comes into play, using it to drive the MOSFETs in the above pattern. That's
why I didn't lable the thread as OT.

One last question. Can anyone explain the difference between using a Zener and
a 7905 regulator?

That's all I have so far. Just wanted to see if I doofed up anything really
badly in the design before I put this together.

Thanks,

BAJ

On Wed, Dec 29, 1999 at 06:15:20PM -0500, Byron A Jeff wrote:
> So I plan to use this config:
>
>     +12
>      |
>      D
> A-->G
>      S
>      |
>      |-----| |------
>      |
>      D
> B-->G
>      S
>      |
>      |
>     gnd

To activate MOSFET A you would need the gate to be 5V higher than the
source.  This is easy with MOSFET B because the source is at ground.
But MOSFET A would only trigger if the voltage at source is at or below
ground potential, right?

> One last question. Can anyone explain the difference between using a Zener and
> a 7905 regulator?

As sole regulation?  Zener would try to sink any excess current.  7905
would instead partially close output in order to avoid exceeding volts.

I still can't figure out why a 7805 can't be used upside down.

--
James Cameron   quozlus.netrek.org   http://quozl.us.netrek.org/

{Quote hidden}

Normally true except I'm using IRFZ44 logic level MOSFETs. They have
internal circuitry so that they are fully on at 5V.

>
> > One last question. Can anyone explain the difference between using a Zener and
> > a 7905 regulator?
>
> As sole regulation?  Zener would try to sink any excess current.  7905
> would instead partially close output in order to avoid exceeding volts.

So I guess I'll use a 7905.

BAJ

On Wed, Dec 29, 1999 at 08:49:26PM -0500, Byron A Jeff wrote:
> Normally true except I'm using IRFZ44 logic level MOSFETs. They have
> internal circuitry so that they are fully on at 5V.

I know that.  I think you may have missed my point.  Will the gate of
the upper MOSFET be fully on despite the source not being at ground
potential due to the capacitor?  What potential will it have?

--
James Cameron   quozlus.netrek.org   http://quozl.us.netrek.org/

Byron, my man.  you are doing it the hard way.

The charge pump chips I have used are suited for low currents.  I know you are
going to reinvent for more current, but why?  6+2.5 watts is not a place to start
breaking ground on leading edge inductorless concepts.

National simple switcher (and linear tech and others, for that matter) show an easy
to build Buck-Boost that goes from +12 to -12 with a relatively easy cookbook
circuit.  And even though a flyback would be a fabulous solution, yes, you need to
have them wound for you (\$\$\$).  The apps on the simple switchers show easy to get
toroid inductors.  They list Schott, Pulse, and Renco.  I quickly crossed a 68uH 2A
inductor to a Miller 5707 in Digikey.  It is oversize, but what the hell, you are
only building one.  I see this app under LM2576, near the end.  It shows a 700mA
output, +12 to -12 converter.  Build two of those, and you are in business!

Remember, when you build it and it has a massive amount of ripple, to go back to
the app note and study their recommended current path connection diagram.
Following layout practices like they say will take a horendously noisy switcher and
make it very manageable.  The dead giveaway is when you ground the scope, then put
the probe tip on the same ground point, and you get lots of nasty noise.  If so, it
is time to take an asprin and warm up your iron.  Make the layout paths correct,
and recheck.

And with the home made cap converter, watch the RMS current through the reservoir
capacitors.  You must study the ESR and max ripple ratings. If you go over them,
the cap heats up inside and becomes damaged over time, like about when you have
your babe in the car flying down the highway on a sunny spring day and the stereo
system goes (BBBBZZZZEEEEAAAAACCCCKKKKKK   POP).

Byron A Jeff wrote:

> In the process of building a DC-DC converter for my car MP3 player I've
> come across the need for  -5 and -12 volt power. While I am aware of that
> there are a number of single chip solutions, especially from Maxim, I'm
> attempting to attack a capacitive switcher for reasons of understandability
> and parts availablity. In short I have no idea where to get parts for or
> how to wind flyback transformers. Also I wanted to practice/learn some
> discrete design skills.

PS real engineers don't use Maxim.

1999\12\30@010209 by
>
> On Wed, Dec 29, 1999 at 08:49:26PM -0500, Byron A Jeff wrote:
> > Normally true except I'm using IRFZ44 logic level MOSFETs. They have
> > internal circuitry so that they are fully on at 5V.
>
> I know that.  I think you may have missed my point.  Will the gate of
> the upper MOSFET be fully on despite the source not being at ground
> potential due to the capacitor?  What potential will it have?

The opposite end of the cap is connected to ground via a diode. The source
will start at 0V and rise to just below the drain voltage as the cap
charges. My understanding is that the IRFZ44 will raise the gate voltage
to above the drain voltage. So it shouldn't be an issue.

BAJ

>
> Byron, my man.  you are doing it the hard way.

I know. I know. But every time I start to look at inductors I get the willies.
Every time I read a switcher datasheet, it seems to come up with an inductor
component that's completely unavailable. The cross referencing you list
below isn't a cut and dried activity.

Also I'm trying to limit myself to stuff I can get in town here in Atlanta.
Fortunately we have one of the few RadioShack.com stores which carries
everything in their commercial catalog, including inductors. So I'm listening.

{Quote hidden}

I see. I read that data sheet. Every other example show's an input voltage
that's higher than the regulated voltage.

>
> Remember, when you build it and it has a massive amount of ripple, to go back to
> the app note and study their recommended current path connection diagram.
> Following layout practices like they say will take a horendously noisy switcher and
> make it very manageable.  The dead giveaway is when you ground the scope, then put
> the probe tip on the same ground point, and you get lots of nasty noise.  If so, it
> is time to take an asprin and warm up your iron.  Make the layout paths correct,
> and recheck.

I see that I can get the parts intown. Will take a crack at it.

BAJ

As someone else noted - a Natsemi "simple switcher" or similar is very very
easy to get going and probably better suited to this application.

However - the FET drive totem-pole you have shown is OK BUT the gate of the
top FET MUST be driven to eg 5 volts above its Source when it is turned on.
As the source will be at +12 volts at this point then the gate must be
driven to 17 volts (or more). For this you need a "high side" voltage
source. A capacitor pump of sorts is one way of getting this :-)
This is one reason to use the P channel upper FET - in that case you pull
the gate down towards ground to turn the FET on so no high side drive
voltage is needed. There are many app notes around showing high side
supplies.
One basic form works like this.
- Connect  capacitor Cpump side A to junction of 2 FETS.
- Connect a diode, anode to +12V and cathode to Cpump side B.
- Connect second diode, anode to Cpump side B, cathode to Vhigherstill.
- Connect a reservoir capacitor from Vhigherstill to ground.

When the FET pair output is low Cpump will charge to +12v less a diode drop.
When the top FET turns on it will pull the FET junction towards +12V so the
top of Cpump will be pulled to ABOVE +12v.
Second diode will conduct raising Vhigherstill to about 2 x +12v.
This is a low power diode pump being used to provide drive voltage for a
high power diode pump :-).
The first time its a bit flaky as the drive voltage is being made as the
upper FET is turned on (bootstrapping) but once the top rail is at about 2 x
+12v it's away.

BUT - use a simple switcher :-)

PS -Re someone's comment on Maxim.  Real engineers will often find a cheaper
solution than a Maxim part but anyone who will NEVER use Maxim is not a real
engineer :-). Maxim make some very useful products. In small quantity use
they are often very hard to beat. In volume use they compete with everyone
else in the market and are sometimes the right solution, sometimes not.

Russell McMahon
_____________________________

>From other worlds - http://www.easttimor.com
http://www.sudan.com

What can one man* do?
Help the hungry at no cost to yourself!
at  http://www.thehungersite.com/

(* - or woman, child or internet enabled intelligent entity :-))

{Original Message removed}
Hi Byron,
you have not mentioned how you are getting your 12v and 5v?  I used
inductive switching for the + rails and overwound an extra coil to get my
-ve by stealth :)  In general I have found that inductance is not very
critical (at least when using lm2576's :)

have a look at http://members.xoom.com/andrewmuck/PSU.htm

cya,    Andrew...

> {Original Message removed}
> I know. I know. But every time I start to look at inductors I get the
willies.
Me too,
Who is going to develop a PIC based measuring instrument to give me the
Henry's (milli, micro or whatever) of all the coils, and other wound or
round pieces of wire (sri, pieces of wire that have been wound on something
or nothing)  I have in my junk box.....
Henk - VK2GWK
http://www.users.bigpond.com/tobbe/

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