Re: Bootstrap diode

Jon Slaughter wrote:
"Joerg" <invalid@xxxxxxxxxxxxxxx> wrote in message news:7ihse6F31jnevU1@xxxxxxxxxxxxxxxxxxxxx
Jon Slaughter wrote:
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Jon Slaughter wrote:
"legg" <legg@xxxxxxxxxxxxxxx> wrote in message news:hve5c5lmac70tcq24okmaa7nlvi3l5at61@xxxxxxxxxx
On Tue, 29 Sep 2009 19:46:02 -0500, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:

"Joerg" <invalid@xxxxxxxxxxxxxxx> wrote in message
news:7ifimaF2vl82lU2@xxxxxxxxxxxxxxxxxxxxx
Jon Slaughter wrote:
"Jon Slaughter" <Jon_Slaughter@xxxxxxxxxxx> wrote in message
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Can someone explain how it works and/or give a schematic/circuit? I just
know they are used in some high-side mosfet drivers for creating the
high-side gate voltage but can't seem to find out any details.

http://www.national.com/an/AN/AN-1317.pdf

Seems to imply that such a configuration cannot work unless the low side
is used often? Probably just as often as the high side?
Pretty much, no exercise -> no voltage. How often depends on the size of
the cap and the hunger of your high side circuitry. Bootstraps are meant
for complementary circuits where the whole chebang runs at some reasonable
frequency and both high- and low-side are switched.

I guess then I can't use this for a full bridge since the low side drain and
high side source must connected directly. I guess with a motor inbetween the
cap will not be charged properly?

The bootstrap cap is normally referenced to the high-side source. As
long as the source is pulled low, either by the low side fet or the
load, the bootstrap cap will charge.

But with a motor the cap will not be pulled to ground or even close.

In ~self-oscillating full-bridge situations, where the drive signals
depend on the output state, care must be taken to ensure that the
bottom fet does in fact turn on at least once before drive to the
upper fet is required.

This may complicate start-up and limiting situations.

My confusion is actually how it accomplishes. In the pdf link it has a diagram of a basic driver. It shows a level shifter and some logic then an omp amp.

My guess it that it has some logic to level shift the signal to whatever floating hi and low it uses. The op amp is for drive capabilities. The op amp is floating as the cap supplies power to it. Essentially as Rich as mentioned it is similar to a voltage doubler. The cap is charged on the low side then "lifted" to the high side giving effectively 2Vcc from ground but puts Vcc across the op amp to power it and drive the high side gate.

If a load is between the high side source and low side drain then the cap will not be charged up properly.

It can be, depends on the load. If the low side FET conducts then its drain pulls towards zero. Then the load tugs on the source of the high side FET and pulls that to ground.

Of course if the load is sluggish this falls apart.

if you use a resistive load I don't see how this can happen except at the moment of switching. If the low side switch is on and the high side is off then the cap will charge up through the resistor which means it is not grounded... but if the cap is small enough it might approach ground. The higher the load the better as it will approach ground more. But then it also depends on the switching times and requires a dead time.

I could use a small low side mosfet or maybe even a bjt that alternates with the high side. This allows the cap to charge every time the high side is used. Then just use the drivers for high side and use something else for a low side. This might cause some problems with inductive loads. Also if the low-side small mosfet is ever turned on without the motor's low side then it will take all the current and burn up rather quickly. (this shouldn't happen if everything is consistent though)

Well, of course you can't leave both switches on for too long. Then you are better off with a real motor driver.


I imagine I could create some additional circuitry to charge the cap up on the low side. Basically disconnect the cap and connect it to ground during the low side. This is so that the cap can be charged during the load side but is separate from the load. Of course this creates more problems than it's worth.

In that case I'd use a real motor driver or something wit a separate charge pump.
They seem to be hard to find compared to these "half-bridges".

Not really:

http://www.freescale.com/files/analog/doc/data_sheet/MC33486.pdf

But I don't know what it exactly is that you want to do with it. This is an example for high power, there's cheaper ones for less amps.

Thanks though, I've seen some of hte "pre-drivers" which is what I'm basically looking for. Again I've done this with the MAX5062's and can't see why such a device would burn up just because I'm using a larger motor. The external mosfets are the only ones that should care about the side of the motor and they were just fine(even after the drivers burned up).

I'm going to try some of those full-bridge pre-drivers though and see what I can get.


I never use Maxim so I can't say. One scenario for a burn-up is stray spikes finding their way into the chip. Traditionally such spikes will have 10 times the gusto when you go from a 1A motor to a 10A motor :-)

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
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Relevant Pages

  • Re: Bootstrap diode
    ... the cap and the hunger of your high side circuitry. ... If a load is between the high side source and low side drain then the ... I could use a small low side mosfet or maybe even a bjt that alternates ... Then just use the drivers for high side and use something ...
    (sci.electronics.design)
  • Re: Bootstrap diode
    ... the cap and the hunger of your high side circuitry. ... In the pdf link it has a diagram of a basic driver. ... If a load is between the high side source and low side drain then the cap will not be charged up properly. ... If the low side switch is on and the high side is off then the cap will charge up through the resistor which means it is not grounded... ...
    (sci.electronics.design)
  • Re: Bootstrap diode
    ... the cap and the hunger of your high side circuitry. ... the bootstrap cap will charge. ... If a load is between the high side source and low side drain then the ... Then just use the drivers for high side and use something ...
    (sci.electronics.design)
  • Re: Bootstrap diode
    ... the cap and the hunger of your high side circuitry. ... long as the source is pulled low, either by the low side fet or the ... If a load is between the high side source and low side drain then the cap will not be charged up properly. ... In that case I'd use a real motor driver or something wit a separate charge pump. ...
    (sci.electronics.design)
  • Re: Bootstrap diode
    ... know they are used in some high-side mosfet drivers for creating the ... the cap and the hunger of your high side circuitry. ... the bootstrap cap will charge. ... If a load is between the high side source and low side drain then the cap ...
    (sci.electronics.design)