Re: Problem with Hbridge circuit?


"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@xxxxxxxxxxx> wrote in message
news:dZMkk.5563$LF2.1623@xxxxxxxxxxxxxxx
Jon Slaughter wrote:
Lack of understanding can cost you a lot in R&D
http://webpages.charter.net/jamie_5";



Well, I'm glad your such a god in everything.... Just wish I was as
perfect as you... maybe I should go should myself for being such an
idiot?
No, I'm not a god and I'm not perfect, You just were not listening and
I can understand why. You were more interested in hearing answers to
resolve your problem instead of listening to why it wasn't working.


No, I wanted to know why it wasn't working. That is the reason I asked. It
does me no good to magically fix my problem without understanding it...
except that sometimes it does end up becoming a waste of trime trying to
figure out why it isn' t work.

The problem is that a lot of things you say seem to contradict my
understanding of how fundamentally the thing works... such as your next
statement


And as I said, depending on what type of load you have on that circuit
you can fix it.

Why does the load matter? Obviously it works with inductors because they
show all there schematics with them... and since I'm mainly using it with
inductors it should be ok... but a purely resistive load should work fine
just as long as its within spec. (in fact an inductive load is much worse
because of the inductive kickback... of course it's current is frequency
dependent but that is also not an issue)

Yes, those drivers are made for a half bridge circuit for which they
work just fine using all N channels fets.


Yes, which is why I thought you were nuts when you kept harping on P
channels. In fact it would be much easier to use P-ch and also cheaper...
But when I was researching different types for purchasing I could not find
any good p-ch and they tended to be more expensive(I was trying to find
larger current and low Rds_on and Nch were the best). I then read on several
sites that N-ch's were more popular because they were cheaper and more
efficient at the same task. (of course you then need a driver).


Since you need to drive both low and high sides with enough current I need a
driver anyways... so getting a driver that drives Nch high side isn't really
any worse than getting one that drives a p-ch high side.

change the code in the pic so that both low side fets come on during
the off cycle or, if you want to keep the driving low side on just turn
on the low side on the non driving side in the off cycle of the PWM.


I was thinking about that but when I read about it I remember someone saying
that its not efficient. (essientially you end up running the motor as a
generator on the "off" cycle and it then brakes itself instead of
freewheeling)

This seems like it might help with the problem of the boost caps though...
but there might be some cross-conduction involved causing the fets to heat
up more(probably not really but possible).

This in effect will also cause braking if you were to
have something like a DC motor on the output otherwise, it'll just
discharge what ever else is there during the off cycle. If you see this as
a problem, then increase the carrier freq and use series inductors/chokes
in the load circuit to help with this.


Yeah, that was the problem I was talking about. It seems much worse than
leaving the fet off... of course this was before I found out that that the
chip didn't work as I thought ;/


You may find it to work fine simply by turning on the low side fet that
isn't supplying the low side drive to the load just to keep the
boost cap in check!


yes, probably so but also might cause other problems.

I'm wondering if there is simply a way to trick the boost part into
charging/discharging but I cant' see it without making the circuit more
complicated.

I think its probably best to just try and find a better driver that doesn't
use that trick or to use some discrete fet's (dual n and p ch in cmos
config) and p ch high side.

I think the cost will be about the same and it will be easier to
troubleshoot and route.. I was just trying to minimize the heat dissipation
of the fets as much as possible. (IIRC the p-ch had almost 2x the on
resistance of equivalent n-ch and about 1.3x the cost if not more)


.

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