Problem with Hbridge circuit?

[Jon Slaughter]

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

 

[James Arthur]

Hi Jon,

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


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.


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?

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.

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.

Could be.

HTH,
James Arthur

 

[Jamie]

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

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]

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))

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]

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.

 

[legg]

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.

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]

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.

Cheers,
James Arthur

 

[Jon Slaughter]

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?)

 

[legg]

<<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.

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

RL

 

[[email protected]]

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

 

[James Arthur]

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

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]

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!

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"

 

[Jon Slaughter]

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.

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)

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

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?

 

[Jon Slaughter]

There could be other problems--you still haven't told us
which parts fried, or did I miss that?

One time a mosfet did go bad(didn't fry or anything... not sure what
happened) but its mainly been the IC's.

"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?

Yes but the fets themselfs have built in diodes and I was using those at
first with same problem.

And are the FETs still good after the driver pops?

I think so(at least in one occasion I replaced the chip and it worked).

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

One time it was stuck low. I don't remember the last time and I already
removed the ICs.

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.

The voltage is only 12V for the drain on the high side and 12V input to the
drivers.

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.

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?)

 

[Jon Slaughter]

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?

 

[Jon Slaughter]

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)?

 

[James Arthur]

The fets never got hot. The drivers did though... sometimes for no apparent
reason.


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)?

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

 

[legg]

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)

Nfets are cheaper on a straight $perVA handling capability, due to fab
constraints at the wafer level.

Bootstrap gate drive is more versatile, simpler and cheaper than many
other options to drive high-side mosfets.

Try setting the pwm frequency in the pic controller to >1KHz, then
experiment with variations on the controller's duty cycle, based on
the known requirements of the drive method.

It's the PIC that is the most versatile element in your breadboard,
and possibly the area of the design that you're most comfortable with.

Beg, borrow or buy some kind of scope, for troubleshooting AC
circuits.

RL

 

[[email protected]]

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

one of the them current running in reverse recovery for the body
diodes
I know there was added discrete diodes but unless they can be
garanteed
to have lower Vf than the fet body diodes I'm not sure they do much
good.

Since the supply 12V a nice gate drive voltage, it might be an idea
to
change the upper fets to P-channel and change or modify the driver a
to drive them instead.

I'm guessing that changing upper fets to P-channel, connecting BST to
Vcc, Cap from BST to ground, HS connected to ground, and inverting the
drive signal for upper fets might work.

-Lasse

 

[James Arthur]

On 1 Aug., 06:57, James Arthur wrote:


One of the them current running in reverse recovery for the
body diodes.
I know there was added discrete diodes but unless they can
be guaranteed to have lower Vf than the fet body diodes I'm
not sure they do much good.

Yep. That's why I asked if the added diodes were fast-recovery
types.

But driving a 30nS edge directly into a motor is just generally
a bad idea.


Cheers,
James Arthur