High side driving N-channel Mosfet with PWM

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So I would have thought that this would be a simple problem but the more
information I get the more I get confused......


I want to PWM a whole lot of LEDs. The current consumption of the bank of
LEDs is in the order of 15 Amps, at 12V.

For reasons I won't go into, my only option is to use a high side mosfet.

I have chosen an N-channel MOSFET (IRL3803) mainly because of its huge
current capability and very low on resistance of 7 milli ohms.

I am hoping to use PWM at a reasonably high frequency, say 20khz or higher,
and I want to use an efficient MOSFET driver IC that is designed for the
job.

I understand that to switch an N channel mosfet on the high side of the
supply line, I have to drive the gate with 10V **more** than the supply
voltage. This is correct yes?

This is where the question comes in. I am looking at the option of using the
IRS2001 to switch the high side fet. But looking at the datasheet, I am
confused. How can the  IRS2001 generate a voltage greater than VCC without
actually being connected to the high side voltage????? There is no talk of a
charge pump in the description and in any case it does not know what
potential the fet is connected to..

Maybe it is not suitable for my application as the IRS2001 seems to be used
for switch mode power supplies? I can't make sense of the first diagram on
the datasheet.

here is a link to the datasheet:

http://www.irf.com/product-info/datasheets/data/irs2001spbf.pdf



OK so if the above IC is not suitable, I have been looking at the MAX621.

http://datasheets.maxim-ic.com/en/ds/MAX620-MAX621.pdf

This one looks to be more straight forward. But I'm not sure how this IC
will go with 20Khz switching. There are many **low side** mosfet drivers
that are in the 2 to 7 amp range to ensure that the mosfets are hard on or
hard off within a few nS. This one doesn't look like it will do anywhere
near that switching current.


Any other ideas for a high side N channel driver IC that packs a punch?













Re: High side driving N-channel Mosfet with PWM

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For a gate capacitance that large ou might need more than the 20mA or so of
peak gate charge current avaialble from the IRS part.

The IR2110 is a very commonly used part that is similar to the irs2001 in
functionality.  The high side gate bias voltage is essentially free due to
the flying bootstrap capacitor supply.

The way it works is that when the low side fet is turned on the vs node is
at ground potential (or 1 diode drop lower for a buck) and the VB node
(bootstrap) is held to 1 diode drop below the 12V rail. This changes up the
capacitor between Vb and Vs.  When the high side is turned on, the high side
circuit uses the capacitor as a floating power supply to supply the gate
drive current.

Pretty easy to use and works great.  You will need to generate your own PWM
signals to feed this guy as it is only a driver.

Read up on the ap notes in the IRF website for more operational use and
proper layout considerations for that chip.  wirewrap is NOT recomended.









Re: High side driving N-channel Mosfet with PWM
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Why so high? If it's just for brightness control, you don't need that
surely?

Re: High side driving N-channel Mosfet with PWM
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if you strobe LEDs too slow, then it causes a strobe effect if there
is any  movement... if you scan your eye past some LED rear car lights
you can get the strobe effect and it is very annoying... even if the
LED is not moving, your eye is always moving

I don't know how fast you need to pulse to avoid this but a few
hundred Hz is too slow.   5 kHz might be OK  I don't know...

At 5 kHz you might get audio interference so that is a good reason to
go to 20 kHz.

Mark

Re: High side driving N-channel Mosfet with PWM

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32 frames/Sec I believe is the minimum needed to prevent strobe flash in
the human eye.. or some figure like that.



http://webpages.charter.net/jamie_5 "


Re: High side driving N-channel Mosfet with PWM

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Fine for a stationary source you are staring at.  But all
you have to do is blink and you will see a trail of dots
(especially if the LED is very much brighter than its
surroundings) at 32 flashes per second, and a lot faster,
too.  If you haven't seen the effect, I suggest you try it.

--
Regards,

John Popelish

Re: High side driving N-channel Mosfet with PWM
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It shows up on scrolling marquees very dramatically - when it's
scrolling, the letters look tilted, but I believe that's an artifact
of the strobing. If you move your eyes to follow the letters, they
square up.

Cheers!
Rich



Re: High side driving N-channel Mosfet with PWM
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Depends on the nature of the "flash".  Film looks okay because it moves
quickly and smoothly between frames (at 24FPS).  A light scene moving to a
light scene without dark inbetween is a lot more pleasant than a really
bright scene flashed only every so often.  PWM LEDs will do that, because
they're very firmly either ON or OFF, within microseconds I mean.  You want
to minimize the apparent movement of a PWM cycle with respect to the eye's
movement, which might be...

Pffft... well lemme see here...  I can glance 60 degrees in 0.1 second
pretty easily, which is easily 10' travel at a distance of 8'.  An average
LED is 1/4" across, and you want to keep the "distance" between cycles
below a diameter or so.  The surface rate is around 100 FPS, or 1200 IPS,
or 4800 LEDs/s.  So the minimum duration is around 1/4800 LEDs/s = 1/4800Hz
frequency.  Which hey, that's pretty damn close to the earlier estimate in
this thread of about 5kHz minimum.  There ya go!  Of course, this frequency
can be lower for large LEDs, diffuse arrays, etc., or should be higher for
really close, fast moving stuff (uh, passing pedestrians a few feet away at
60MPH?!?).  And if nothing else, you can add a choke filter and recoil
diode (aka buck converter) to smooth out the pulses and get perfectly level
light output.  Hey, you could use an iron-core choke to justify the
switching loss savings of 400 Hz operation.  :^)

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms



Re: High side driving N-channel Mosfet with PWM
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net says...
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Not at all.  Movies use 24fps, but open the shutter two or three
times per frame to eliminate the flashing.  TV interlaces 30Hz
frames with 60Hz fields for similar reasons.  
  
--
Keith

Re: High side driving N-channel Mosfet with PWM
On Mon, 5 May 2008 21:24:43 +1000, "Phil in Melbourne"

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For your application, VCC will connect to "up to 200V" in the
application schematic, on page #1. The drive supply VB is
conventionally bootstrapped using the diode in the same drawing.

RL

Re: High side driving N-channel Mosfet with PWM

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You're going about this all wrong, no one makes a dimmable LED supply
like this, the main reason being your excessive ripple current
resulting in unnecessary heating and exponential reduction in LED
lifetime. Another good reason for not doing it this way is to avoid the
fairly huge current spikes resulting from what will be a hefty
equivalent load capacitance producing enough EMI to be detected in
another galaxy. And you want to go to all this trouble to produce a crap
design because it's the only thing you understand. Do some background
reading: http://powerelectronics.com/mag/710PET24.pdf and put more
effort into your planning.



Re: High side driving N-channel Mosfet with PWM
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Somebody stuffed up Fig. 2, didn't they.

Re: High side driving N-channel Mosfet with PWM

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Can you explain why you can not just break this down into multiple units
all centrally driven? say 15 units drawing 1amp each, or 30 units drawing
500mA?

Re: High side driving N-channel Mosfet with PWM

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It would be less convenient, but that is possible, and may end up being the
way I go.
There are motor controllers on the market that PWM high current motors. I
thought it would be reasonably trivial to do seeing as MOSFETS come in high
current versions, and seeing that LEDs don't have as much inductance as
motors do.

I will go for the MAX621 and suck it and see. If the fet overheats, I will
decrease the PWM frequency.

If it still overheats, I will break up the supply into groups of 3A loads.

No big deal either way.




Re: High side driving N-channel Mosfet with PWM

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I was thinking that would avoid the all eggs in one basket. if one unit
fails, it only takes out that fraction.

The other point is the frequency and duty cycle might be such that instead
of having to deal with an intermittent 15Amps, you could be dealing with a
consistent 5amps say or less.


Re: High side driving N-channel Mosfet with PWM
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You can get P-channel MOSFETs in similar ratings.  Maybe not quite so small
Rds(on), but certainly in the amperage you need at that voltage.  There are
plenty of lower voltage P-channels (whereas the supply kind of dries up
over 200V, sadly).  Stick a zener on the gate to protect it from nasty
automotive supply voltages and you can practically drive it from a naked
2N3904 collector sinking to ground.

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms



Re: High side driving N-channel Mosfet with PWM

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Aw, c'mon! Why not? This could turn out more interesting than the rest
of the thread! :-)

Thanks,
Rich


Re: High side driving N-channel Mosfet with PWM

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Probably because it is some stupid contraption to go on a car with only the
chassis available as a -ve earth. Probably blow up on the first load dump
it sees anyway :).

--

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