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Jon Slaughter
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 Posted: Fri Aug 01, 2008 7:05 am Post subject: Re: Problem with Hbridge circuit? |
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<langwadt@fonz.dk> wrote in message
news:ba14422e-9a32-443a-bf0b-0762c4bfa2ee@r66g2000hsg.googlegroups.com...
[quote]On 31 Jul., 20:50, "Jon Slaughter" <Jon_Slaugh...@Hotmail.com> wrote:
"James Arthur" <bogusabd...@verizon.net> wrote in message
news:8alkk.628$JH5.170@trnddc06...
Jon Slaughter wrote:
Thanks guys... I>ll try some of the suggestions but remember that the
circuit was taken directly from the datasheet applications section...
of
course I doubled it so I could have a full H-Bridge but that shouldn>t
make any difference.
Joel, RL, and I all agree: you can>t drive this thing at
a few Hz. It won>t work.
I saw the datasheet recommended the 100nF boost caps, which
is probably excessive (you didn>t specify which FETs you>re
driving), but that>s not the problem now.
You have to drive it faster.
Can you explain to me for my own benefit why it matters how fast I drive
it?
If I pulse it once a minute or 10000 times a second it shouldn>t matter?
(assuming transitions are fast enough and its not too fast)
Basically your telling me if I turned on one of the inputs then it would
blow up cause its at 0hz. This makes no sense to me. Maybe it has to do
with that boost cap thing though? (I>m not sure how it works but maybe
that
cap is to smooth out the pulses to read the average value so it knows how
much to boost or something? too slow and it gets the wrong reading?)
the boost cap is the power supply for the upper fet gate drive
inside the driver chip is a diode from Vcc to the boost cap, so
everytime the lower fet
is on the boost cap is charged to Vcc.
[/quote]
strange ;/ why the hell would they generate the voltage like this? ;) Seems
like there are other ways that are just as good? Essentially it is a charge
pump that depends on the lower?
(so essentially these chips are not really made for H-bridge driving?
I wonder if there is a way to modify it, say by takign the boost cap off the
hbridge and somehow tricking the chip? (Just wondering... not like I will do
it)
[quote]when the lower fet is off, the driver will slowly discharge the boost
cap and eventually the
voltage is too low to fully turn on the fet.
_unless_ you at regular intervals turn off the upper fet and turn on
the lower fet
to recharge the boost cap
[/quote]
Yes, this isn>t occuring. I>m leaving one fet on and one fet off for large
periods of time while the other side will have one off and the other
toggling. Hence if your explination is correct about the boost cap(which I>m
not saying it isn>t) then this means I am not operating the chips properly.
Although they did work when driving an LED so I guess because it was such a
low current that it was working? But eventually it should stop working
because the cap would be drained? |
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Jon Slaughter
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| Posted: Fri Aug 01, 2008 7:05 am Post subject: Re: Problem with Hbridge circuit? |
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[quote]There could be other problems--you still haven>t told us
which parts fried, or did I miss that?
[/quote]
One time a mosfet did go bad(didn>t fry or anything... not sure what
happened) but its mainly been the IC>s.
[quote]
"I took my pwm circuit and ran it one of the mosfets and it took
relatively significant amount of current to get good transitions.
(I was driving and LED and it was acting funky with low current..
I assume essentially the mosfet was probably always conducting
because it didn>t blink like it was suppose to(I>m only driving
it at a few hz))"
Ahh, here it is:
"But I seem to be having issues when my load is large and
inductive (a large motor). The driver chips burn up for
some reason that I can>t figure out why;/ I>ve added diodes
across the mosfets which are not shown in the circuit
but still same problem."
Those better be fast recovery diodes you>ve added across
the FETs...are they?
[/quote]
Yes but the fets themselfs have built in diodes and I was using those at
first with same problem.
[quote]
And are the FETs still good after the driver pops?
[/quote]
I think so(at least in one occasion I replaced the chip and it worked).
[quote]What does the driver do after it>s fried? Stuck
high? or low? or open?
You could be blowing the driver>s boost diodes.
[/quote]
One time it was stuck low. I don>t remember the last time and I already
removed the ICs.
[quote]We need those clues too.
Joel mentioned inductive loads can backdrive the FET
gates negative, exceeding their Vgs rating.
Maybe, depending on the FET, but more likely they>ll
zap the driver first--the voltage undershoot goes
with the ratio of Crss / (Crss + Ciss), typically
about 1:10. With a 125volt supply, you>d not usually
get more than -12v at the gate, not enough to kill it.
[/quote]
The voltage is only 12V for the drain on the high side and 12V input to the
drivers.
[quote]But the driver might need protection against such
undershoot.
These drivers switch a 5nF gate in 33nS--a loose layout,
inductive sense resistor, oscillating FET, or stray
drain inductance can screw the thing easily. Adding
series gate resistors will ease the situation.
[/quote]
Well, it seems its the way I>m using htem as they don>t seem to be made for
hbridges. That is, if the both high and low side fets on one side need to be
toggled to actually charge up the boost cap. In that case it never happens
in my application. (But it worked with an LED so I>m not sure.. maybe the
small amount of current was the reason?) |
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Jon Slaughter
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| Posted: Fri Aug 01, 2008 7:05 am Post subject: Re: Problem with Hbridge circuit? |
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[quote]Lack of understanding can cost you a lot in R&D
http://webpages.charter.net/jamie_5"
[/quote]
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? |
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Jon Slaughter
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| Posted: Fri Aug 01, 2008 7:05 am Post subject: Re: Problem with Hbridge circuit? |
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"Joel Koltner" <zapwireDASHgroups@yahoo.com> wrote in message
news:l4pkk.375878$ng7.278495@en-nntp-05.dc1.easynews.com...
[quote]"Jon Slaughter" <Jon_Slaughter@Hotmail.com> wrote in message
news:oQnkk.13874$LG4.2800@nlpi065.nbdc.sbc.com...
Can you explain to me for my own benefit why it matters how fast I drive
it?
As Lasse mentioned, the problem is that the "flying" capacitor that drives
the gate of the upper FET will slowly discharge, so if you switch too
slowly that capacitor is no longer a decent ("solid") voltage source and
on every (assumed-to-be-too-slow) switching cycle, your upper FETs first
turn on solidly, and then (over time, as the capacitor charge decreases so
Vgs drops) go through their linear regions (where they look like
resistors) before turning off. If your currents are high enough, this can
case a lot of power dissipation in those FETs... *possibly* enough to kill
them, although this probably isn>t the cause of *your* FET failures, since
the time constants involved are typically going to be on the order of many
seconds and there>s a good chance you>ll notice the FETs getting quite hot
before they die. (And they might not die at all, depending on the
particular circumstances.)
[/quote]
The fets never got hot. The drivers did though... sometimes for no apparent
reason.
[quote]But it does appear to be the reason that driving your LEDs at a couple Hz
wasn>t working.
[/quote]
It was working. Thats why I thought the circuit was working fine.
It seems that I just wasn>t using the chip as it was designed. I didn>t
realize that the high side voltage was generated in such a way. I figured
they generated it independently of the driven circuit and the boost cap was
sort of a sensor to determine how much to drive the high side gate(or
something like that).
I guess that method they use is a little trick to easily generate the high
side voltage in that application(when top and bottom are toggled quickly)? |
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James Arthur
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| Posted: Fri Aug 01, 2008 7:05 am Post subject: Re: Problem with Hbridge circuit? |
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Jon Slaughter wrote:
[quote]"Joel Koltner" <zapwireDASHgroups@yahoo.com> wrote in message
news:l4pkk.375878$ng7.278495@en-nntp-05.dc1.easynews.com...
"Jon Slaughter" <Jon_Slaughter@Hotmail.com> wrote in message
news:oQnkk.13874$LG4.2800@nlpi065.nbdc.sbc.com...
Can you explain to me for my own benefit why it matters how fast I drive
it?
As Lasse mentioned, the problem is that the "flying" capacitor that drives
the gate of the upper FET will slowly discharge, so if you switch too
slowly that capacitor is no longer a decent ("solid") voltage source and
on every (assumed-to-be-too-slow) switching cycle, your upper FETs first
turn on solidly, and then (over time, as the capacitor charge decreases so
Vgs drops) go through their linear regions (where they look like
resistors) before turning off. If your currents are high enough, this can
case a lot of power dissipation in those FETs... *possibly* enough to kill
them, although this probably isn>t the cause of *your* FET failures, since
the time constants involved are typically going to be on the order of many
seconds and there>s a good chance you>ll notice the FETs getting quite hot
before they die. (And they might not die at all, depending on the
particular circumstances.)
The fets never got hot. The drivers did though... sometimes for no apparent
reason.
But it does appear to be the reason that driving your LEDs at a couple Hz
wasn>t working.
It was working. Thats why I thought the circuit was working fine.
It seems that I just wasn>t using the chip as it was designed. I didn>t
realize that the high side voltage was generated in such a way. I figured
they generated it independently of the driven circuit and the boost cap was
sort of a sensor to determine how much to drive the high side gate(or
something like that).
I guess that method they use is a little trick to easily generate the high
side voltage in that application(when top and bottom are toggled quickly)?
[/quote]
Right. The chip *is* made to drive an H-bridge, but you>ve got to
switch it at some minimum rate to keep the boost cap filled.
How often? Worst case it>ll draw 40uA (Ibst) from the boost
cap, which will discharge a 100nF boost cap by 1 volt in 2.5
milliseconds. It looks like the undervoltage lockout kicks
in around 7v, so with the boost cap starting at 11v you>d
have to switch the driver at about 100Hz, minimum (worst
case), or about 25Hz (typical).
If you want the top FET to stay ON all the time, well, the
FET driver won>t do that. You usually just bang the H-output
low for a blip occasionally and settle for the top FET
being ON 95% of the time. The motor won>t notice.
You really should put gate resistors in series with the
driver--motors have large interwinding capacitances; you
don>t want to blast them with 12v in 30nS. 1-5uS switching
time would be fine, and avoid many problems.
HTH,
James Arthur |
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