Re: Bootstrap diode


"legg" <legg@xxxxxxxxxxxxxxx> wrote in message
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On Tue, 29 Sep 2009 21:52:52 -0500, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:


"legg" <legg@xxxxxxxxxxxxxxx> wrote in message
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On Tue, 29 Sep 2009 19:46:02 -0500, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:


"Joerg" <invalid@xxxxxxxxxxxxxxx> wrote in message
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Jon Slaughter wrote:
<snip>


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.

If the way you use it prevents it's normal function, you are
misapplying the part. I think this is one of the reasons why app notes
are published.


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.

In a full bridge there are four switches. The low side switches in
each case are responsible for refreshing the bootstrap cap specific to
the high-side switch on it's own side of the bridge. The load is
applied between the switch pair junctions on each side of the bridge.

Your reference app note describes a half-bridge driver; one side of a
bridge is used.

An asymmetrical bridge replaces two diagonally situated switches of
the full bridge with rectifiers - it is capable of producing load
current in only one direction.

Ok? For bi-directional application you use two asymmetrical half-bridges...
I have done that with the MAX5062 but with a much larger motor it causes
them to burn up. The MAX5062 seems to be designed similar to the app note.
In the data*** for the MAX5062 it says it can be used for Motor control. I
can't think of any other way than to use it asymetrically. It uses a boot
cap and high side sensing and shows circuits similar to other half bridge
drivers. it does not show one for a motor though.

It works fine on a small 12V DC motor(<1A) but the chips burn up very
quickly on a 15A 12V DC motor. I have no clue why this happens. I get bi-dir
and speed control(I use a pot and a pic to determine the speed/dir then
setup the pwm to feed the max's) just fine with small motor. I get dead IC's
when connectin the large one. (even with added diode supression across the
mosfets)


This was about a year ago and I got tired of going wasting so many chips I
just gave up.




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.


A 'leak' resistor between the source and the negative rail could serve
this function, prior to the first high-side gate drive signal. Once
any motor load current is established, its own flyback energy will
ensure that the source is pulled low in the freewheeling interval.


This might cause some problems with the gate drive since the HS source is no
longer floating. One would need to match the resistance to the capacitance
so that it can be charged fast enough yet not pull the source down to ground
to much reducing the HS gate drive.



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