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Jon Slaughter · Guest

http://i36.tinypic.com/2vblekk.jpg

Can anyone see any issues with that circuit?

There seems to be some problem I>m having but not sure. I>m assuming that
the mosfet drivers(MAX5062A) are somewhat ideal(very little input current
needed to get good transitions).

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))

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.

(the circuit seems to work fine for driving small resistive loads but
unfortunately I burned up my last 2 IC>s so I can>t do any more analysis on
it until I get some more)

Thanks,
Jon

Joel Koltner · Guest

Hi Jon,

I>m not at all an expert on this sort of thing, but I have a few suggestions:

[quote]http://i36.tinypic.com/2vblekk.jpg
Can anyone see any issues with that circuit?
[/quote]
Here>s what I>d do:

1) Add a capacitor across your output pins (P3.1/P3.2). The idea is that the
catch diodes (those intrinsic to U1-U4 or separate ones you>ve added) take a
finite amount of time to turn on (usually in the ns range... they act like
little capacitors), and during that time you can get little spikes applied do
your upper FET sources and lower FET drains. This can exceed Vgs of the upper
FETs (likely) or Vds of the lower FETs (less likely) and cause failures. If
you want to get fancy, you can ues an R-C network instead (a proper
"snubber") -- this can waste less power than a capacitor alone does, but you
probably don>t care for a low-power circuit.
2) Add a ~10V Zener diode to each upper FET between gate and source (cathode
at gate, anode at source). The idea here is that rapidly rising/falling
drain-source voltages will capacitively couple back to your gate, potentially
exceeding Vgs and breaking it down. See section 3 here:
http://www.irf.com/technical-info/appnotes/an-936.pdf
3) Add some small (~22 ohm) resistors in series between your driver outputs
and the FETs' gates. Although this will slightly slow down your switching,
it also damps any oscillations on the gate of the FET and (somewhat) protects
the Max5062>s outputs. (Such oscillations are generally bad news to the
health of your circuit...) Note that you might not *have* any such
oscillations (sometimes they>re high enough in frequency that they>re
difficult to see, though), but the idea is to add the resistors, make sure
everything works, then remove them, and make sure everything still works
rather than risking something blowing up while you>re troubleshooting other
problems.

[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))
[/quote]
At a few Hz, the high side drive voltage storage capacitors (C2 & C1) might be
losing their charge -- have you looked at the gate drive voltage on your upper
FETs while driving the LED?

[quote]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
;/
[/quote]
Do the FETs fry as well? I>m guessing that the most likely cause is one of
your upper FETs getting punched through by inductive spikes from the load, and
those high-voltage spikes next see the gate drivers and kill them as well.

---Joel

James Arthur · Guest

Joel Koltner wrote:
[quote]Hi Jon,

I>m not at all an expert on this sort of thing, but I have a few suggestions:

http://i36.tinypic.com/2vblekk.jpg
Can anyone see any issues with that circuit?

Here>s what I>d do:

1) Add a capacitor across your output pins (P3.1/P3.2). The idea is that the
catch diodes (those intrinsic to U1-U4 or separate ones you>ve added) take a
finite amount of time to turn on (usually in the ns range... they act like
little capacitors), and during that time you can get little spikes applied do
your upper FET sources and lower FET drains. This can exceed Vgs of the upper
FETs (likely) or Vds of the lower FETs (less likely) and cause failures. If
you want to get fancy, you can ues an R-C network instead (a proper
"snubber") -- this can waste less power than a capacitor alone does, but you
probably don>t care for a low-power circuit.
2) Add a ~10V Zener diode to each upper FET between gate and source (cathode
at gate, anode at source). The idea here is that rapidly rising/falling
drain-source voltages will capacitively couple back to your gate, potentially
exceeding Vgs and breaking it down. See section 3 here:
http://www.irf.com/technical-info/appnotes/an-936.pdf
3) Add some small (~22 ohm) resistors in series between your driver outputs
and the FETs' gates. Although this will slightly slow down your switching,
it also damps any oscillations on the gate of the FET and (somewhat) protects
the Max5062>s outputs. (Such oscillations are generally bad news to the
health of your circuit...) Note that you might not *have* any such
oscillations (sometimes they>re high enough in frequency that they>re
difficult to see, though), but the idea is to add the resistors, make sure
everything works, then remove them, and make sure everything still works
rather than risking something blowing up while you>re troubleshooting other
problems.

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))

At a few Hz, the high side drive voltage storage capacitors (C2 & C1) might be
losing their charge -- have you looked at the gate drive voltage on your upper
FETs while driving the LED?
[/quote]
a) Use smaller bootstrap caps. 10-20x gate charge is plenty.
Bigger caps are harder to charge quickly, and force high
currents through the drivers' bootstrap diodes.

b) You can>t leave the outputs high--you have to switch them.
Otherwise the gate drive bleeds down, the top FETs go
linear then fry.

[quote]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
;/

Do the FETs fry as well? I>m guessing that the most likely cause is one of
your upper FETs getting punched through by inductive spikes from the load, and
those high-voltage spikes next see the gate drivers and kill them as well.
[/quote]
Could be.

HTH,
James Arthur

Jamie · Guest

Jon Slaughter wrote:

[quote]http://i36.tinypic.com/2vblekk.jpg

Can anyone see any issues with that circuit?

There seems to be some problem I>m having but not sure. I>m assuming that
the mosfet drivers(MAX5062A) are somewhat ideal(very little input current
needed to get good transitions).

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))

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.

(the circuit seems to work fine for driving small resistive loads but
unfortunately I burned up my last 2 IC>s so I can>t do any more analysis on
it until I get some more)

Thanks,
Jon
[/quote]
Are you pulling the low side Fets (both) low in the off duty cycle with the
PIC ?
I can see right off what would happen if you only trigger lets say U1
for the HIgh side and U4 for the low side.
In this config, the driver on that side is discharging the Boost cap
for U3 how ever, you>re not using U3 your using U1 which needs it>s
boost cap properly discharged so that it can create a boost for the next
on duty cycle. This simply gives you no boost for U1.

The same would apply if you were to reverse your direction on the
other side.

Now what would work maybe depending on what you>re driving here is to
have both low side FETS come on during the off duty cycle. Other wise,
all I see here is a set of high side fets (N channels) getting no boost
due to the cap not getting discharged in the correct order.

If you looked at the data sheet for these drivers, they used PWM into
a half bridge using both N-channels. Those examples force the cap to
be pulled (discharged) on the off cycle of the PWM so the next ON cycle
for the high side will have a boost.

Maybe you are already pulling both low sides down at the same time
now during the off cycle ? I don>t remember you saying that you were? If
could cause an issue depending on what you have for a load on the
circuit for example a DC motor? the Regen could get loaded in the low
side of the bridge when doing this since both leads would then be
shorted at that time.

Some food for thought.

http://webpages.charter.net/jamie_5"

legg · Guest

On Wed, 30 Jul 2008 12:25:00 -0500, "Jon Slaughter"
<Jon_Slaughter@Hotmail.com> wrote:

[quote]http://i36.tinypic.com/2vblekk.jpg

Can anyone see any issues with that circuit?

There seems to be some problem I>m having but not sure. I>m assuming that
the mosfet drivers(MAX5062A) are somewhat ideal(very little input current
needed to get good transitions).

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))
[/quote]
The boost capacitor is required to supply ~9uA of quiescent current to
the driver circuit. With a 0.1uF boost capacitor, you>ll lose 5V from
the voltage headroom on this capacitor in 55mSec. (~20Hz)

So if your pwm control frequency is low enough to produce visible
blinking in the led load, you>ll have to use a larger boost capacitor.

A static 'ON' state is not possible in either direction - ensure that
the switching function is faithfully maintained at either control
extreme.

Rapid charging of the severely depleted boost capacitor may produce
unpredictable supply voltage and ground trace perturbations.

RL

Jon Slaughter · Guest

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.

Guest · Posted: Thu Jul 31, 2008 7:51 pm Post subject: Re: Problem with Hbridge circuit?

On 31 Jul., 20:50, "Jon Slaughter" <Jon_Slaugh...@Hotmail.com> wrote:
[quote]"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?)
[/quote]
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.

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

-Lasse

legg · Guest

On Thu, 31 Jul 2008 08:44:22 -0500, "Jon Slaughter"
<Jon_Slaughter@Hotmail.com> wrote:

[quote]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.

[/quote]
The application note unlikely included the two types of IC>s, as you
originally hooked them together, nor did it suggest adopting audible
or sub-audible operating frequencies.

Even a well-proven schematic requires considerable discipline in it>s
physical rendering.

Drawings from app notes should be considered as suggestions or
starting points to a practical end-use application. There is no
reasurance that they accurately reflect a working circuit, typos
aside.

We have only your assertion that the schematic presented for
examination reflects the breadboard that you have assembled, or your
description accurately relates real physical circumstances, as they
actually occured.

You really should use a scope to troubleshoot a circuit like this.

RL

James Arthur · Guest

Jon Slaughter wrote:
[quote]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.
[/quote]
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.

Cheers,
James Arthur

Joel Koltner · Guest

"legg" <legg@nospam.magma.ca> wrote in message
news:jlk3945gcf2h852rgbo90n5asmcc7i3abs@4ax.com...
[quote]Drawings from app notes should be considered as suggestions or
starting points to a practical end-use application.
[/quote]
<<smirk>> I have a friend who>s an FAE, and he tells me that a huge percentage
of the circuits he sees at customers are directly copied from the data sheet
example

Jon Slaughter · Guest

"James Arthur" <bogusabdsqy@verizon.net> wrote in message
news:8alkk.628$JH5.170@trnddc06...
[quote]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.

[/quote]
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?)

legg · Guest

On Thu, 31 Jul 2008 09:54:52 -0700, "Joel Koltner"
<zapwireDASHgroups@yahoo.com> wrote:

[quote]"legg" <legg@nospam.magma.ca> wrote in message
news:jlk3945gcf2h852rgbo90n5asmcc7i3abs@4ax.com...
Drawings from app notes should be considered as suggestions or
starting points to a practical end-use application.

smirk>> I have a friend who>s an FAE, and he tells me that a huge percentage
of the circuits he sees at customers are directly copied from the data sheet
example

You don>t always get what you think you>re paying for.[/quote]

Power supplies have always been considered as simple or an
after-thought. It breaks my heart, sometimes.

RL

Joel Koltner · Guest

"Jon Slaughter" <Jon_Slaughter@Hotmail.com> wrote in message
news:oQnkk.13874$LG4.2800@nlpi065.nbdc.sbc.com...
[quote]Can you explain to me for my own benefit why it matters how fast I drive it?
[/quote]
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.)

But it does appear to be the reason that driving your LEDs at a couple Hz
wasn>t working.

---Joel

James Arthur · Guest

langwadt@fonz.dk wrote:
[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.

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

-Lasse
[/quote]
Exactly right. You have to switch the driver to keep its boost
cap charged.

There could be other problems--you still haven>t told us
which parts fried, or did I miss that?

"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?

And are the FETs still good after the driver pops?

What does the driver do after it>s fried? Stuck
high? or low? or open?
You could be blowing the driver>s boost diodes.

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.

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.

HTH,

James Arthur

Jamie · Guest

langwadt@fonz.dk wrote:
[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.

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

-Lasse

Finally some one understands the real problem![/quote]

And getting back to the author of this fine design ;)

It aint going to work unless you turn on the lower FET that is
part of the half bridge of that side!. of course, that>s not the side
that you are turning on in the low cycle!. so there you go.

Oh well, such as life.

Kind of reminds me of the mini DC drive boards inside of our
electric reel movers at work.. It uses a double Half bridge using
N channels and when the battery gets low, the high side don>t fully
switch on thus, taking out a couple of fets if they hammer it.
When I say take out, I mean split the cases open!

Lack of understanding can cost you a lot in R&D
http://webpages.charter.net/jamie_5"


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