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H-Bridge with level shift - MOSFET gate voltage issues?

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Tim Watts
Guest

Sat Jan 05, 2019 8:45 pm   



On 05/01/2019 15:46, John Larkin wrote:
Quote:
On Sat, 5 Jan 2019 10:03:39 +0000, Tim Watts <tw_at_example.com> wrote:

On 04/01/2019 19:34, John Larkin wrote:

I have an Amazon stick-on dimmable LED strip for by workbench. It PWMs
at about 4 KHz.

It was a good point to raise - to not try to get to clever and bang the
PWM through the roof for the sake of flicker control :)

In reality, we wouldn't want to try to get too high with a Pi, but kHz
sounds like a good place to be for LEDs.

If you switch at, say, 1 KHz, the radiation efficiency of a string of
LEDs as an antenna will be zilch. At the AM broadcast band, where the
antanna efficiency works its way up to terrible, you'd be looking at
the 1000th harmonic of the drive waveform. That line will contain
microscopic energy.


I was thinking about this driving home - the wire is also a balance
conductor pair which should reduce radiation too.

We're not talking about a giant single core loop, but a star topology
with 2 pair cables. Not twisted pair, but still in close proximity.

Quote:

So don't worry about it.

But for extra fun, go software spread-spectrum.


:-)


--
Email does not work


Guest

Sun Jan 06, 2019 2:45 am   



On Sunday, January 6, 2019 at 2:46:25 AM UTC+11, John Larkin wrote:
Quote:
On Sat, 5 Jan 2019 10:03:39 +0000, Tim Watts <tw_at_example.com> wrote:

On 04/01/2019 19:34, John Larkin wrote:

I have an Amazon stick-on dimmable LED strip for by workbench. It PWMs
at about 4 KHz.

It was a good point to raise - to not try to get to clever and bang the
PWM through the roof for the sake of flicker control :)

In reality, we wouldn't want to try to get too high with a Pi, but kHz
sounds like a good place to be for LEDs.

If you switch at, say, 1 KHz, the radiation efficiency of a string of
LEDs as an antenna will be zilch. At the AM broadcast band, where the
antanna efficiency works its way up to terrible, you'd be looking at
the 1000th harmonic of the drive waveform. That line will contain
microscopic energy.


Sadly, AM radio amplitude at your receiver is also microscopic, and a string of LEDs can be lot closer to your AM radio (and your neighbour's AM radios) than the AM transmitter. A long string of LEDs can be physically long enough to be a appreciable fraction of an AM wavelength so its radiative efficiency can be above the "zilch" level.

> So don't worry about it.

Not good advice.

> But for extra fun, go software spread-spectrum.

It will certainly make it harder for the interference detector vans to lock in on your location, and make it less likely that a particular neighbour will get irritated by noise interfering with their favourite AM station. Having all of them mad at you might be a different problem.

Filtering out the higher frequency components before the current gets to the LEDs might be a safer solution, if more expensive.

--
Bill Sloman, Sydney


Guest

Sun Jan 06, 2019 11:45 am   



On Saturday, January 5, 2019 at 8:43:38 PM UTC-5, bill....@ieee.org wrote:
Quote:
On Sunday, January 6, 2019 at 2:46:25 AM UTC+11, John Larkin wrote:
On Sat, 5 Jan 2019 10:03:39 +0000, Tim Watts <tw_at_example.com> wrote:

On 04/01/2019 19:34, John Larkin wrote:

I have an Amazon stick-on dimmable LED strip for by workbench. It PWMs
at about 4 KHz.

It was a good point to raise - to not try to get to clever and bang the
PWM through the roof for the sake of flicker control :)

In reality, we wouldn't want to try to get too high with a Pi, but kHz
sounds like a good place to be for LEDs.

If you switch at, say, 1 KHz, the radiation efficiency of a string of
LEDs as an antenna will be zilch. At the AM broadcast band, where the
antanna efficiency works its way up to terrible, you'd be looking at
the 1000th harmonic of the drive waveform. That line will contain
microscopic energy.

Sadly, AM radio amplitude at your receiver is also microscopic, and a string of LEDs can be lot closer to your AM radio (and your neighbour's AM radios) than the AM transmitter. A long string of LEDs can be physically long enough to be a appreciable fraction of an AM wavelength so its radiative efficiency can be above the "zilch" level.

So don't worry about it.

Not good advice.

But for extra fun, go software spread-spectrum.

It will certainly make it harder for the interference detector vans to lock in on your location, and make it less likely that a particular neighbour will get irritated by noise interfering with their favourite AM station. Having all of them mad at you might be a different problem.

Filtering out the higher frequency components before the current gets to the LEDs might be a safer solution, if more expensive.

--
Bill Sloman, Sydney


JL is overlooking edge rate of the current pulses. One can expect unattenuated spectral components in the GHz range. The LED string is a very nonlinear load which greatly exacerbates the interference problem in this respect, in the way it will snap into high conduction after just a few 10s millivolt output excursion past LED cut-in.

Tim Watts
Guest

Sun Jan 06, 2019 3:45 pm   



On 06/01/2019 01:43, bill.sloman_at_ieee.org wrote:

Quote:
Filtering out the higher frequency components before the current gets to the LEDs might be a safer solution, if more expensive.


Appropriately sized choke?


--
Email does not work

Piotr Wyderski
Guest

Sun Jan 06, 2019 4:45 pm   



Lasse Langwadt Christensen wrote:

> for something simple, old, and cheap how about L298

Is there a MOS version of this antediluvian part? I could still
find several applications of it.

Best regards, Piotr

Lasse Langwadt Christense
Guest

Sun Jan 06, 2019 4:45 pm   



s√łndag den 6. januar 2019 kl. 16.05.03 UTC+1 skrev Piotr Wyderski:
Quote:
Lasse Langwadt Christensen wrote:

for something simple, old, and cheap how about L298

Is there a MOS version of this antediluvian part? I could still
find several applications of it.


there are lots of more modern DC-motor and stepper-motor drivers that
are similar, all depends on what you need

John Larkin
Guest

Sun Jan 06, 2019 6:45 pm   



On Sun, 6 Jan 2019 02:22:26 -0800 (PST),
bloggs.fredbloggs.fred_at_gmail.com wrote:

Quote:
On Saturday, January 5, 2019 at 8:43:38 PM UTC-5, bill....@ieee.org wrote:
On Sunday, January 6, 2019 at 2:46:25 AM UTC+11, John Larkin wrote:
On Sat, 5 Jan 2019 10:03:39 +0000, Tim Watts <tw_at_example.com> wrote:

On 04/01/2019 19:34, John Larkin wrote:

I have an Amazon stick-on dimmable LED strip for by workbench. It PWMs
at about 4 KHz.

It was a good point to raise - to not try to get to clever and bang the
PWM through the roof for the sake of flicker control :)

In reality, we wouldn't want to try to get too high with a Pi, but kHz
sounds like a good place to be for LEDs.

If you switch at, say, 1 KHz, the radiation efficiency of a string of
LEDs as an antenna will be zilch. At the AM broadcast band, where the
antanna efficiency works its way up to terrible, you'd be looking at
the 1000th harmonic of the drive waveform. That line will contain
microscopic energy.

Sadly, AM radio amplitude at your receiver is also microscopic, and a string of LEDs can be lot closer to your AM radio (and your neighbour's AM radios) than the AM transmitter. A long string of LEDs can be physically long enough to be a appreciable fraction of an AM wavelength so its radiative efficiency can be above the "zilch" level.

So don't worry about it.

Not good advice.

But for extra fun, go software spread-spectrum.

It will certainly make it harder for the interference detector vans to lock in on your location, and make it less likely that a particular neighbour will get irritated by noise interfering with their favourite AM station. Having all of them mad at you might be a different problem.

Filtering out the higher frequency components before the current gets to the LEDs might be a safer solution, if more expensive.

--
Bill Sloman, Sydney

JL is overlooking edge rate of the current pulses.


No, but I am considering that the 1000th harmonic of a square wave has
pretty low amplitude. The math is well documented.


> One can expect unattenuated spectral components in the GHz range.

At power levels that are undetectable in the thermal/atmospheric
noise. Do the math, for Pete's sake.



> The LED string is a very nonlinear load which greatly exacerbates the interference problem in this respect, in the way it will snap into high conduction after just a few 10s millivolt output excursion past LED cut-in.

A forward-biased LED string will "snap into high conduction" in 10s of
mV?

That sort of reasoning prevents anything from getting done. But you
don't design electronics.

--

John Larkin Highland Technology, Inc

lunatic fringe electronics


Guest

Mon Jan 07, 2019 5:45 am   



On Monday, January 7, 2019 at 4:01:50 AM UTC+11, John Larkin wrote:
Quote:
On Sun, 6 Jan 2019 02:22:26 -0800 (PST),
bloggs.fredbloggs.fred_at_gmail.com wrote:

On Saturday, January 5, 2019 at 8:43:38 PM UTC-5, bill....@ieee.org wrote:
On Sunday, January 6, 2019 at 2:46:25 AM UTC+11, John Larkin wrote:
On Sat, 5 Jan 2019 10:03:39 +0000, Tim Watts <tw_at_example.com> wrote:

On 04/01/2019 19:34, John Larkin wrote:


<snip>

Quote:
JL is overlooking edge rate of the current pulses.

No, but I am considering that the 1000th harmonic of a square wave has
pretty low amplitude. The math is well documented.


But neglecting the fact that AM radio signal with which that harmonic would interfere also has a pretty low amplitude.

Quote:
One can expect unattenuated spectral components in the GHz range.

At power levels that are undetectable in the thermal/atmospheric
noise. Do the math, for Pete's sake.


Not that John Larkin seems to have a clue about what mathematics he might be suggesting that Fred Bloggs ought to do. John Larkin isn't exactly famous for posting his mathematical work, and some of us suspect that he doesn't know any more about what the word actually means than he does about word "design".

Quote:
The LED string is a very nonlinear load which greatly exacerbates the interference problem in this respect, in the way it will snap into high conduction after just a few 10s millivolt output excursion past LED cut-in.

A forward-biased LED string will "snap into high conduction" in 10s of
mV?


LED's are diodes, and the current through them is an exponential function of voltage. I've never heard of LEDs acting as "snap-off" diodes.

They are varicaps, so I suppose that it is theoretically possible.

http://www.hanssummers.com/varicap/varicapled.html

Quote:
That sort of reasoning prevents anything from getting done. But you
don't design electronics.


No. That sort of reasoning leads people to test what is actually happening.

The idea of one LED in long string of LED acting as a snap-off diode is arresting, but it clearly didn't register with John Larkin (who does know about snap-off diodes, but only when they are diodes that he wants to use as step-recovery diodes).

--
Bill Sloman, Sydney


Guest

Mon Jan 07, 2019 7:45 pm   



Quote:

No, but I am considering that the 1000th harmonic of a square wave has
pretty low amplitude. The math is well documented.

But neglecting the fact that AM radio signal with which that harmonic would interfere also has a pretty low amplitude.


the 60 Hz harmonics of a standard lamp dimmer certainly can and DO interfere with AM radio.

m

Lasse Langwadt Christense
Guest

Mon Jan 07, 2019 11:45 pm   



mandag den 7. januar 2019 kl. 23.16.49 UTC+1 skrev Piotr Wyderski:
Quote:
Lasse Langwadt Christensen wrote:

there are lots of more modern DC-motor and stepper-motor drivers that
are similar, all depends on what you need

Basically I need a dual low-side MOSFET driver, but their
static load properties are rarely specified.


if you just need low side there are millions of FETs that will work or if you want short circuit and temperature protected the numerous low side switches

Piotr Wyderski
Guest

Mon Jan 07, 2019 11:45 pm   



Lasse Langwadt Christensen wrote:

> but then you have to find a real transformer instead of a cheap switching supply

O tempora, o mores!

Best regards, Piotr

Piotr Wyderski
Guest

Mon Jan 07, 2019 11:45 pm   



Lasse Langwadt Christensen wrote:

Quote:
there are lots of more modern DC-motor and stepper-motor drivers that
are similar, all depends on what you need


Basically I need a dual low-side MOSFET driver, but their
static load properties are rarely specified.

Best regards, Piotr

Piotr Wyderski
Guest

Tue Jan 08, 2019 12:45 am   



Lasse Langwadt Christensen wrote:

> https://www.allegromicro.com/~/media/Files/Datasheets/A4950-Datasheet.ashx

Now this is something very close to my needs, thanks!

Best regards, Piotr

Lasse Langwadt Christense
Guest

Tue Jan 08, 2019 12:45 am   



mandag den 7. januar 2019 kl. 23.48.16 UTC+1 skrev Piotr Wyderski:
Quote:
Lasse Langwadt Christensen wrote:

if you just need low side there are millions of FETs that will work

Low side *driver*, not a FET. I need two full half-bridge pairs
and their static current and transient voltage specs. Vendors
don't care much about that, given the usual application.


https://www.allegromicro.com/~/media/Files/Datasheets/A4950-Datasheet.ashx

?

Piotr Wyderski
Guest

Tue Jan 08, 2019 12:45 am   



Lasse Langwadt Christensen wrote:

> if you just need low side there are millions of FETs that will work

Low side *driver*, not a FET. I need two full half-bridge pairs
and their static current and transient voltage specs. Vendors
don't care much about that, given the usual application.

Best regards, Piotr

Goto page Previous  1, 2, 3, 4, 5  Next

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