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Notches in ground planes for multi-power multi-channel board

J

Joel Koltner

Jan 1, 1970
0
Joerg said:
I have never seen a LM317 do that. Usually when they approach dropout LDOs
steer their pass device into full conduction and naturally it'll take a
while to swing back when Vin jumps back up. OTOH non-LDOs are different,
they typically run out of base drive and that happens kind of gracefully.
Their pass device doesn't really leave the linear range, at least not by
much.

Ah, gotcha.

Do I get a 'pass' in that the class I took in college was called "CMOS IC
Analog Design" and therefore I don't have to know what a BJT is?

Just kidding. :)
People have done rather strange things with the LM317. One example is in the
above datasheet on page 18 where they built a switcher with it. I've used
them as AM modulators and such, usually producing some forehead wrinkles and
coughing in design reviews.

Nice. Very clever.

---Joel
 
J

Jamie Morken

Jan 1, 1970
0
Joerg said:
Easy fix if you need good rejection: Use a npn-Darlington with enough
heat dissipation capability to drop around 1.5V at your maximum current.
Collector to switcher output, emitter to linear regulator input, 1K
resistor from base to collector, 47uF cap from base to ground. Plus a
couple discharge diodes. That'll behave like having a shoe size 15
electrolytic in there.

Hi,

Is this npn-darlington solution good at rejecting HF better than a
linear regulator like the LM317? Also where do the discharge diodes go
in this circuit?

cheers,
Jamie
 
J

Joerg

Jan 1, 1970
0
Jamie said:
Hi,

Is this npn-darlington solution good at rejecting HF better than a
linear regulator like the LM317? Also where do the discharge diodes go
in this circuit?

Yes, for smaller transistors and small currents. Mostly I don't even use
Darlington, just a transistor with a good guaranteed minimum beta. One
diode goes from emitter to collector (cathode to collector which would
be at the input rail), the other from base to collector (also cathode to
collector). The diodes make sure the capacitors don't zap the
transistor(s) in case someone drops the pliers and abruptly shorts out
the input rail.

Remember that this drops 700-800mV or so, or twice that in case of a
Darlington. And it is not too useful if the output of this stage must be
very stable. Of course, then you could run a secondary loop back to the
switcher but that becomes esoteric and is not for the faint of heart.

[...]
 
J

Jamie Morken

Jan 1, 1970
0
Joerg said:
Yes, for smaller transistors and small currents. Mostly I don't even use
Darlington, just a transistor with a good guaranteed minimum beta. One
diode goes from emitter to collector (cathode to collector which would
be at the input rail), the other from base to collector (also cathode to
collector). The diodes make sure the capacitors don't zap the
transistor(s) in case someone drops the pliers and abruptly shorts out
the input rail.

Remember that this drops 700-800mV or so, or twice that in case of a
Darlington. And it is not too useful if the output of this stage must be
very stable. Of course, then you could run a secondary loop back to the
switcher but that becomes esoteric and is not for the faint of heart.

I am making an isolated bipolar powersupply that needs to be quite
clean, for 12bit ADC/DAC's, here is what I have so far:

"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"

For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V rails.

cheers,
Jamie



 
J

Joerg

Jan 1, 1970
0
Jamie said:
I am making an isolated bipolar powersupply that needs to be quite
clean, for 12bit ADC/DAC's, here is what I have so far:

"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"


For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

It's correct, pnp for the negative rail. But you need to flip both D117
and D118 around.

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V rails.

I don't think you need those high PSRR regulators. The LM317 is actually
pretty good. Don't know what you are using for the negative rail though,
your schematic doesn't say. Also, you need ADJ pin dividers, else you'll
only get 1.2V out of it. Bypass that ADJ pin to GND with a 10uF MLCC cap
for even better noise muffling.

Add a 0.1uF each in parallel to C22 and C29 to make sure RF spikes won't
sail through. When picking a negative regulator watch out. Many of those
are LDOs and LDO usually means trouble. Make sure it's unconditionally
stable.
 
J

John Larkin

Jan 1, 1970
0
Joerg said:
Yes, for smaller transistors and small currents. Mostly I don't even use
Darlington, just a transistor with a good guaranteed minimum beta. One
diode goes from emitter to collector (cathode to collector which would
be at the input rail), the other from base to collector (also cathode to
collector). The diodes make sure the capacitors don't zap the
transistor(s) in case someone drops the pliers and abruptly shorts out
the input rail.

Remember that this drops 700-800mV or so, or twice that in case of a
Darlington. And it is not too useful if the output of this stage must be
very stable. Of course, then you could run a secondary loop back to the
switcher but that becomes esoteric and is not for the faint of heart.

I am making an isolated bipolar powersupply that needs to be quite
clean, for 12bit ADC/DAC's, here is what I have so far:

"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"

For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V rails.

cheers,
Jamie




I don't understand this. It looks like a pwm controlled forward
converter, but the secondary circuit (D110, L5, C29) looks strange.
There's usually another "catch" diode to keep the current circulating
in L5 when the pwm is off. L5 might ring like a banshee when the pwm
snaps off.

As far as 100 KHz noise is concerned, you have more normal-mode ripple
rejection than you need. The bigger problem might be common-mode noise
on the (floating?) common, coupled through the winding capacitances.

John
 
J

Jamie Morken

Jan 1, 1970
0
Joerg said:
It's correct, pnp for the negative rail. But you need to flip both D117
and D118 around.



I don't think you need those high PSRR regulators. The LM317 is actually
pretty good. Don't know what you are using for the negative rail though,
your schematic doesn't say. Also, you need ADJ pin dividers, else you'll
only get 1.2V out of it. Bypass that ADJ pin to GND with a 10uF MLCC cap
for even better noise muffling.

Add a 0.1uF each in parallel to C22 and C29 to make sure RF spikes won't
sail through. When picking a negative regulator watch out. Many of those
are LDOs and LDO usually means trouble. Make sure it's unconditionally
stable.

Thanks, I updated the circuit:

"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply2.jpg"

I used a LM337 negative voltage regulator on the negative rail. I saw
an AC regulator circuit in the LM317 datasheet but am not sure if you
can really use the LM317 for a negative regulator for this?

I left the second voltage regulators in the circuit in case 8V and 5V
are both needed :)

cheers,
Jamie
 
J

Jamie Morken

Jan 1, 1970
0
John said:
Joerg said:
Jamie Morken wrote:
Joerg wrote:
[email protected] wrote:
On Feb 10, 7:43 pm, Joerg <[email protected]>
wrote:
[email protected] wrote:
All,
I am designing amulti-channelboard and I have individual linear
regulators for each channel running off a common switcher.
Now, my concern is that due to the split power planes, which are like
thin fingers running through the board, I will cause potential
differences based on the current in different channels, especially,
because I have a common ground plane. Now, I was thinking my choices
are:
1) Split the ground plane as well, so I would have thin finger like
power and ground plane separated by narrow notch but connected at the
linear regulator end and at the input end. (All the channels are
w.r.t
common ground plane.)
Splitting a ground plane is usually a recipe for disaster. I have
yet to
see a case where that really worked and it's been decades now. In
audio
designs it can work but only until a strong RF field shows up, upon
which all hell breaks loose.

OTOH if nobody split planes anymore I'd have less work so it does have
some upsides ;-)

The trouble is, that I have common (to all channels) control lines
runnning across my planes (though separated by 2 layers) and even
if I
isolate the grounds, some noise is bound to take the signal lines.
That's one of the reasons why it's a recipe for disaster ;-)

2) Keep split power planes, but add a narrow slice ground plane
between the power plane 'fingers' in between them on the same
layer as
the power planes.
Copper pour on the same plane as power? You can do that but via it
through here and there. And no thermal reliefs for those vias unless
something needs to be soldered in there.

3) Keep 1, but put connections between ground planes at regular
intervals.
That's like saying "Let's split it, oh, wait, maybe split them only a
little". I would not split the ground at all.

Anyone have thoughts on this. The voltages are +5 or less and it is
all analog..there are digital lines and a digital section, but they
come into the analog area through an isolator.
Make sure your power is clean and the switcher doesn't chatter
through.
Use shielded versions for the magnetics if it's close by. Design your
stuff with reasonable PSRR or bypass extra good where you can't, like
for transistor stages.

--
Regards, Joerg

http://www.analogconsultants.com/- Hide quoted text -

- Show quoted text -
Well, actually, I am not splitting the ground planes, in the sense
that they are connected at the same potential at all the linear
regulators and at the input. But with my control lines, crossing
across my parts, my split is not going to have much effect. I could
put de-coupling caps across the notch to allow high frequencies to
pass, but again they might go through my control lines and not bother
with the caps...basically it is big ?..so yeah, I think I am not going
to split the ground...best to do it when I have free time as a test
project.

For my thin ground slice in between the power planes (on the same
layer0, I put vias every 50 mils or so and no themal reliefs and it is
a relatively unpopulated location, so that will help. My switcher and
linear regulator are LT parts 3430/1763, but I have been told by just
about every amplifier part vendor (and LT themselves!) that linear
regulators don't attenuate frequencies above 100 kHz very well, so I
might keep a 100 kHz switching frequency and maybe put ferrites at the
linear regulator input.

Easy fix if you need good rejection: Use a npn-Darlington with enough
heat dissipation capability to drop around 1.5V at your maximum
current. Collector to switcher output, emitter to linear regulator
input, 1K resistor from base to collector, 47uF cap from base to
ground. Plus a couple discharge diodes. That'll behave like having a
shoe size 15 electrolytic in there.
Hi,

Is this npn-darlington solution good at rejecting HF better than a
linear regulator like the LM317? Also where do the discharge diodes go
in this circuit?

Yes, for smaller transistors and small currents. Mostly I don't even use
Darlington, just a transistor with a good guaranteed minimum beta. One
diode goes from emitter to collector (cathode to collector which would
be at the input rail), the other from base to collector (also cathode to
collector). The diodes make sure the capacitors don't zap the
transistor(s) in case someone drops the pliers and abruptly shorts out
the input rail.

Remember that this drops 700-800mV or so, or twice that in case of a
Darlington. And it is not too useful if the output of this stage must be
very stable. Of course, then you could run a secondary loop back to the
switcher but that becomes esoteric and is not for the faint of heart.
I am making an isolated bipolar powersupply that needs to be quite
clean, for 12bit ADC/DAC's, here is what I have so far:

"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"

For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V rails.

cheers,
Jamie




I don't understand this. It looks like a pwm controlled forward
converter, but the secondary circuit (D110, L5, C29) looks strange.
There's usually another "catch" diode to keep the current circulating
in L5 when the pwm is off. L5 might ring like a banshee when the pwm
snaps off.

Thanks, I added those diodes.
As far as 100 KHz noise is concerned, you have more normal-mode ripple
rejection than you need. The bigger problem might be common-mode noise
on the (floating?) common, coupled through the winding capacitances.

Would a common mode filter be the solution for this I guess? I've seen
some capacitors that can be used for this I think, they have 4
terminals. Also would a common mode choke from each rail to the
floating ground be enough to get rid of the common mode noise or are
there other techniques to use?

cheers,
Jamie
 
J

John Larkin

Jan 1, 1970
0
Thanks, I added those diodes.


Would a common mode filter be the solution for this I guess? I've seen
some capacitors that can be used for this I think, they have 4
terminals. Also would a common mode choke from each rail to the
floating ground be enough to get rid of the common mode noise or are
there other techniques to use?

cheers,
Jamie

Hi, Jamie,

I couldn't make suggestions without understanding the application. Is
the circuit that this thing powers floating? What does it do?

We recently did a 16-channel thermocouple input board. Each channel
has a dc/dc converter based on an ISDN line transformer, driven by
some small mosfets at 60 KHz. The tc input channel is fully floating,
with optoisolators for data i/o. The biggest noise problem is
common-mode 60 KHz coupled into the floating circuits through
transformer capacitance. It's only a problem in the sense that we
don't want to poke a lot of 60 KHz into the customer's thermocouple
leads. We added a 1 (or maybe 2?) nF cap from the floating ground to
real ground as a compromise... too big a cap has problems of its own.

John
 
J

Joerg

Jan 1, 1970
0
Jamie said:
Thanks, I updated the circuit:

"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply2.jpg"


I used a LM337 negative voltage regulator on the negative rail. I saw
an AC regulator circuit in the LM317 datasheet but am not sure if you
can really use the LM317 for a negative regulator for this?

Not that way. I prefer the LM317 over the LM337 because the 337 has the
architecture of an LDO and is not as stable. But it would require
another transformer winding. Or maybe you could used the one at pins 9
and 10 with a bridge rectifier. Then you could build a separate +8V
supply and just tie the positive side to GND. But you should be able to
get the LM337 to work as well, it's a simpler circuit.

I left the second voltage regulators in the circuit in case 8V and 5V
are both needed :)

Ok. 3V ain't a lot of head room but for 100mA it's fine.
 
J

Joerg

Jan 1, 1970
0
John said:
Joerg said:
Jamie Morken wrote:
Joerg wrote:
[email protected] wrote:
On Feb 10, 7:43 pm, Joerg <[email protected]>
wrote:
[email protected] wrote:
All,
I am designing amulti-channelboard and I have individual linear
regulators for each channel running off a common switcher.
Now, my concern is that due to the split power planes, which are like
thin fingers running through the board, I will cause potential
differences based on the current in different channels, especially,
because I have a common ground plane. Now, I was thinking my choices
are:
1) Split the ground plane as well, so I would have thin finger like
power and ground plane separated by narrow notch but connected at the
linear regulator end and at the input end. (All the channels are
w.r.t
common ground plane.)
Splitting a ground plane is usually a recipe for disaster. I have
yet to
see a case where that really worked and it's been decades now. In
audio
designs it can work but only until a strong RF field shows up, upon
which all hell breaks loose.

OTOH if nobody split planes anymore I'd have less work so it does have
some upsides ;-)

The trouble is, that I have common (to all channels) control lines
runnning across my planes (though separated by 2 layers) and even
if I
isolate the grounds, some noise is bound to take the signal lines.
That's one of the reasons why it's a recipe for disaster ;-)

2) Keep split power planes, but add a narrow slice ground plane
between the power plane 'fingers' in between them on the same
layer as
the power planes.
Copper pour on the same plane as power? You can do that but via it
through here and there. And no thermal reliefs for those vias unless
something needs to be soldered in there.

3) Keep 1, but put connections between ground planes at regular
intervals.
That's like saying "Let's split it, oh, wait, maybe split them only a
little". I would not split the ground at all.

Anyone have thoughts on this. The voltages are +5 or less and it is
all analog..there are digital lines and a digital section, but they
come into the analog area through an isolator.
Make sure your power is clean and the switcher doesn't chatter
through.
Use shielded versions for the magnetics if it's close by. Design your
stuff with reasonable PSRR or bypass extra good where you can't, like
for transistor stages.

--
Regards, Joerg

http://www.analogconsultants.com/- Hide quoted text -

- Show quoted text -
Well, actually, I am not splitting the ground planes, in the sense
that they are connected at the same potential at all the linear
regulators and at the input. But with my control lines, crossing
across my parts, my split is not going to have much effect. I could
put de-coupling caps across the notch to allow high frequencies to
pass, but again they might go through my control lines and not bother
with the caps...basically it is big ?..so yeah, I think I am not going
to split the ground...best to do it when I have free time as a test
project.

For my thin ground slice in between the power planes (on the same
layer0, I put vias every 50 mils or so and no themal reliefs and it is
a relatively unpopulated location, so that will help. My switcher and
linear regulator are LT parts 3430/1763, but I have been told by just
about every amplifier part vendor (and LT themselves!) that linear
regulators don't attenuate frequencies above 100 kHz very well, so I
might keep a 100 kHz switching frequency and maybe put ferrites at the
linear regulator input.

Easy fix if you need good rejection: Use a npn-Darlington with enough
heat dissipation capability to drop around 1.5V at your maximum
current. Collector to switcher output, emitter to linear regulator
input, 1K resistor from base to collector, 47uF cap from base to
ground. Plus a couple discharge diodes. That'll behave like having a
shoe size 15 electrolytic in there.
Hi,

Is this npn-darlington solution good at rejecting HF better than a
linear regulator like the LM317? Also where do the discharge diodes go
in this circuit?

Yes, for smaller transistors and small currents. Mostly I don't even use
Darlington, just a transistor with a good guaranteed minimum beta. One
diode goes from emitter to collector (cathode to collector which would
be at the input rail), the other from base to collector (also cathode to
collector). The diodes make sure the capacitors don't zap the
transistor(s) in case someone drops the pliers and abruptly shorts out
the input rail.

Remember that this drops 700-800mV or so, or twice that in case of a
Darlington. And it is not too useful if the output of this stage must be
very stable. Of course, then you could run a secondary loop back to the
switcher but that becomes esoteric and is not for the faint of heart.
I am making an isolated bipolar powersupply that needs to be quite
clean, for 12bit ADC/DAC's, here is what I have so far:

"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"

For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V rails.

cheers,
Jamie




I don't understand this. It looks like a pwm controlled forward
converter, but the secondary circuit (D110, L5, C29) looks strange.
There's usually another "catch" diode to keep the current circulating
in L5 when the pwm is off. L5 might ring like a banshee when the pwm
snaps off.

Good point, I had only looked at the DC side. Also, the primary driver
side does not look right.

Jamie, the best way to prevent DC runaway of the core is to AC couple.
Then second best is to use bridge rectifiers (and then you could
actually use a LM317 for the neg rail as well).

Or at least connect the anode of D119 to xfmr pin 7, flip D120 around
and connect its cathode to xfmr pin 4.
 
J

John Larkin

Jan 1, 1970
0
"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"

For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V rails.

cheers,
Jamie




[...]

I don't understand this. It looks like a pwm controlled forward
converter, but the secondary circuit (D110, L5, C29) looks strange.
There's usually another "catch" diode to keep the current circulating
in L5 when the pwm is off. L5 might ring like a banshee when the pwm
snaps off.

Good point, I had only looked at the DC side. Also, the primary driver
side does not look right.

Jamie, the best way to prevent DC runaway of the core is to AC couple.
Then second best is to use bridge rectifiers (and then you could
actually use a LM317 for the neg rail as well).


That's actually a pretty slick configuration. When the fets turn off,
the flyback energy is dumped back into the power supply.

Looks like Q92 drives the gate of p-fet Q91, also slick. Interesting
things happen over the pwm range from 0 to 50%, but I think it's OK.

Why didn't I think of that?

John
 
J

Jamie Morken

Jan 1, 1970
0
John said:
Hi, Jamie,

I couldn't make suggestions without understanding the application. Is
the circuit that this thing powers floating? What does it do?

We recently did a 16-channel thermocouple input board. Each channel
has a dc/dc converter based on an ISDN line transformer, driven by
some small mosfets at 60 KHz. The tc input channel is fully floating,
with optoisolators for data i/o. The biggest noise problem is
common-mode 60 KHz coupled into the floating circuits through
transformer capacitance. It's only a problem in the sense that we
don't want to poke a lot of 60 KHz into the customer's thermocouple
leads. We added a 1 (or maybe 2?) nF cap from the floating ground to
real ground as a compromise... too big a cap has problems of its own.

Hi John,

Yes its a floating supply for powering ADC/DAC and opamps that are
interfaced to by opto from the microcontroller side.

The primary side is the isolated microcontroller supply, and the
secondary side is 120VAC generated by a truesine inverter.

Thanks I'll add the 2nF cap across grounds. Would a common mode
choke, maybe 3 coils in this bipolar supply case be effective as well?

What about these 4 terminal caps, they seem to be more effective than
a larger common mode choke.

http://www.johansondielectrics.com/x2y/

cheers,
Jamie
 
J

Jamie Morken

Jan 1, 1970
0
John said:
"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"

For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V rails.

cheers,
Jamie




[...]

I don't understand this. It looks like a pwm controlled forward
converter, but the secondary circuit (D110, L5, C29) looks strange.
There's usually another "catch" diode to keep the current circulating
in L5 when the pwm is off. L5 might ring like a banshee when the pwm
snaps off.
Good point, I had only looked at the DC side. Also, the primary driver
side does not look right.

Jamie, the best way to prevent DC runaway of the core is to AC couple.
Then second best is to use bridge rectifiers (and then you could
actually use a LM317 for the neg rail as well).


That's actually a pretty slick configuration. When the fets turn off,
the flyback energy is dumped back into the power supply.

Looks like Q92 drives the gate of p-fet Q91, also slick. Interesting
things happen over the pwm range from 0 to 50%, but I think it's OK.

Why didn't I think of that?


Terry Given suggested that one in a thread earlier :)

cheers,
Jamie
 
J

Joerg

Jan 1, 1970
0
Jamie said:
John said:
"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"


For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V
rails.

cheers,
Jamie




[...]

I don't understand this. It looks like a pwm controlled forward
converter, but the secondary circuit (D110, L5, C29) looks strange.
There's usually another "catch" diode to keep the current circulating
in L5 when the pwm is off. L5 might ring like a banshee when the pwm
snaps off.

Good point, I had only looked at the DC side. Also, the primary
driver side does not look right.

Jamie, the best way to prevent DC runaway of the core is to AC
couple. Then second best is to use bridge rectifiers (and then you
could actually use a LM317 for the neg rail as well).


That's actually a pretty slick configuration. When the fets turn off,
the flyback energy is dumped back into the power supply.

Looks like Q92 drives the gate of p-fet Q91, also slick. Interesting
things happen over the pwm range from 0 to 50%, but I think it's OK.

Why didn't I think of that?


Terry Given suggested that one in a thread earlier :)

If Terry gave it to you then it should work. But make sure to obtain
enough information regarding the core. A DC run-off situation or core
saturation is no fun.
 
T

Terry Given

Jan 1, 1970
0
Joerg said:
Jamie said:
John said:
On Wed, 20 Feb 2008 17:25:50 GMT, Joerg



"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"


For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V
rails.

cheers,
Jamie




[...]

I don't understand this. It looks like a pwm controlled forward
converter, but the secondary circuit (D110, L5, C29) looks strange.
There's usually another "catch" diode to keep the current circulating
in L5 when the pwm is off. L5 might ring like a banshee when the pwm
snaps off.

Good point, I had only looked at the DC side. Also, the primary
driver side does not look right.

Jamie, the best way to prevent DC runaway of the core is to AC
couple. Then second best is to use bridge rectifiers (and then you
could actually use a LM317 for the neg rail as well).



That's actually a pretty slick configuration. When the fets turn off,
the flyback energy is dumped back into the power supply.

Looks like Q92 drives the gate of p-fet Q91, also slick. Interesting
things happen over the pwm range from 0 to 50%, but I think it's OK.

Why didn't I think of that?

high praise indeed.
If Terry gave it to you then it should work. But make sure to obtain
enough information regarding the core. A DC run-off situation or core
saturation is no fun.

Thanks Joerg!

I came up with that circuit in 1994 - I did a 2MHz modulated
smps/gatedriver circuit. initially I had a clamp zener, but I used the
same zener for all 6 gatedriver/smps, and although the leakage energy
was OK from one, 6 was too much. so I thunked a bit, and figured out the
self-driven PNP diagonal half-bridge. the switching speed of the
PNP/PFET is pretty much irrelevant too - as long as one device switches
fast its ok - at least until you muck about for so long that the next
switching cycle has been and gone.....

we built about 300,000 of these, with no problems. running from +24V,
using FMMT491A and FMMT591A SOT23 BJTs. We could probably have patented
it - though we did patent the toroid winding technique I came up with.

as far as the core is concerned, in theory all is well even at 50% duty
cycle, as Vclamp = Vcc + 2*Vd, but Von = Vcc - 2*Vce. ware remanent flux
though - that can bite quite a chunk out of the available flux
excursion, rendering the typical "hey, Bsat < 300mT, no worries"
approach somewhat fraught with peril

its really good for open-loop flybacks too, if you choose Vout =
Vin*Ns/Np, as load reductions just mean more energy spins round in
circles on the primary.


Cheers
Terry
 
J

John Larkin

Jan 1, 1970
0
Joerg said:
Jamie said:
John Larkin wrote:

On Wed, 20 Feb 2008 17:25:50 GMT, Joerg



"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"


For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V
rails.

cheers,
Jamie




[...]

I don't understand this. It looks like a pwm controlled forward
converter, but the secondary circuit (D110, L5, C29) looks strange.
There's usually another "catch" diode to keep the current circulating
in L5 when the pwm is off. L5 might ring like a banshee when the pwm
snaps off.

Good point, I had only looked at the DC side. Also, the primary
driver side does not look right.

Jamie, the best way to prevent DC runaway of the core is to AC
couple. Then second best is to use bridge rectifiers (and then you
could actually use a LM317 for the neg rail as well).



That's actually a pretty slick configuration. When the fets turn off,
the flyback energy is dumped back into the power supply.

Looks like Q92 drives the gate of p-fet Q91, also slick. Interesting
things happen over the pwm range from 0 to 50%, but I think it's OK.

Why didn't I think of that?

high praise indeed.
If Terry gave it to you then it should work. But make sure to obtain
enough information regarding the core. A DC run-off situation or core
saturation is no fun.

Thanks Joerg!

I came up with that circuit in 1994 - I did a 2MHz modulated
smps/gatedriver circuit. initially I had a clamp zener, but I used the
same zener for all 6 gatedriver/smps, and although the leakage energy
was OK from one, 6 was too much. so I thunked a bit, and figured out the
self-driven PNP diagonal half-bridge. the switching speed of the
PNP/PFET is pretty much irrelevant too - as long as one device switches
fast its ok - at least until you muck about for so long that the next
switching cycle has been and gone.....

we built about 300,000 of these, with no problems. running from +24V,
using FMMT491A and FMMT591A SOT23 BJTs. We could probably have patented
it - though we did patent the toroid winding technique I came up with.


BJT's? With base resistors? It looks much nicer with fets.

as far as the core is concerned, in theory all is well even at 50% duty
cycle, as Vclamp = Vcc + 2*Vd, but Von = Vcc - 2*Vce. ware remanent flux
though - that can bite quite a chunk out of the available flux
excursion, rendering the typical "hey, Bsat < 300mT, no worries"
approach somewhat fraught with peril

its really good for open-loop flybacks too, if you choose Vout =
Vin*Ns/Np, as load reductions just mean more energy spins round in
circles on the primary.


Cheers
Terry

Nice circuit, a few parts doing a lot of stuff. The only warning I'd
give Jamie is that he may not want fast edges getting into his
isolated analog stuff, and it would be tricky to slow this one down.
Our thermocouple supply was a classic open-loop forward converter,
dual nfets driving a center-tapped transformer, and we shaped the gate
drives to soften things up. Efficiency didn't matter much here. And we
got a nice clean square wave out of the secondaries, which also
clocked the delta-sigma ADC!

John
 
J

Joerg

Jan 1, 1970
0
John said:
Joerg said:
Jamie Morken wrote:

John Larkin wrote:

On Wed, 20 Feb 2008 17:25:50 GMT, Joerg



"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"


For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V
rails.

cheers,
Jamie




[...]

I don't understand this. It looks like a pwm controlled forward
converter, but the secondary circuit (D110, L5, C29) looks strange.
There's usually another "catch" diode to keep the current circulating
in L5 when the pwm is off. L5 might ring like a banshee when the pwm
snaps off.

Good point, I had only looked at the DC side. Also, the primary
driver side does not look right.

Jamie, the best way to prevent DC runaway of the core is to AC
couple. Then second best is to use bridge rectifiers (and then you
could actually use a LM317 for the neg rail as well).


That's actually a pretty slick configuration. When the fets turn off,
the flyback energy is dumped back into the power supply.

Looks like Q92 drives the gate of p-fet Q91, also slick. Interesting
things happen over the pwm range from 0 to 50%, but I think it's OK.

Why didn't I think of that?
high praise indeed.
Terry Given suggested that one in a thread earlier :)

If Terry gave it to you then it should work. But make sure to obtain
enough information regarding the core. A DC run-off situation or core
saturation is no fun.
Thanks Joerg!

I came up with that circuit in 1994 - I did a 2MHz modulated
smps/gatedriver circuit. initially I had a clamp zener, but I used the
same zener for all 6 gatedriver/smps, and although the leakage energy
was OK from one, 6 was too much. so I thunked a bit, and figured out the
self-driven PNP diagonal half-bridge. the switching speed of the
PNP/PFET is pretty much irrelevant too - as long as one device switches
fast its ok - at least until you muck about for so long that the next
switching cycle has been and gone.....

we built about 300,000 of these, with no problems. running from +24V,
using FMMT491A and FMMT591A SOT23 BJTs. We could probably have patented
it - though we did patent the toroid winding technique I came up with.


BJT's? With base resistors? It looks much nicer with fets.

as far as the core is concerned, in theory all is well even at 50% duty
cycle, as Vclamp = Vcc + 2*Vd, but Von = Vcc - 2*Vce. ware remanent flux
though - that can bite quite a chunk out of the available flux
excursion, rendering the typical "hey, Bsat < 300mT, no worries"
approach somewhat fraught with peril

its really good for open-loop flybacks too, if you choose Vout =
Vin*Ns/Np, as load reductions just mean more energy spins round in
circles on the primary.


Cheers
Terry

Nice circuit, a few parts doing a lot of stuff. The only warning I'd
give Jamie is that he may not want fast edges getting into his
isolated analog stuff, and it would be tricky to slow this one down.
Our thermocouple supply was a classic open-loop forward converter,
dual nfets driving a center-tapped transformer, and we shaped the gate
drives to soften things up. Efficiency didn't matter much here. And we
got a nice clean square wave out of the secondaries, which also
clocked the delta-sigma ADC!

Hey, you re-invented the modern electro-cardiograph :)

Except that we (usually) feed back the analog signals and do the AD
conversion on the system side.
 
T

Terry Given

Jan 1, 1970
0
John said:
Joerg said:
Jamie Morken wrote:


John Larkin wrote:


On Wed, 20 Feb 2008 17:25:50 GMT, Joerg




"http://rocketresearch.nekrom.com/new/isolated bipolar powersupply/isolated bipolar powersupply.jpg"


For the npn solution I put in a pnp on the negative rail, not sure if
this is correct, or if it should still be an npn?

Thanks for any comments on the circuit. I would like to get as clean
a supply as possible, maybe 0.5mV noise would be nice on the +-5V
rails.

cheers,
Jamie





[...]


I don't understand this. It looks like a pwm controlled forward
converter, but the secondary circuit (D110, L5, C29) looks strange.
There's usually another "catch" diode to keep the current circulating
in L5 when the pwm is off. L5 might ring like a banshee when the pwm
snaps off.


Good point, I had only looked at the DC side. Also, the primary
driver side does not look right.

Jamie, the best way to prevent DC runaway of the core is to AC
couple. Then second best is to use bridge rectifiers (and then you
could actually use a LM317 for the neg rail as well).



That's actually a pretty slick configuration. When the fets turn off,
the flyback energy is dumped back into the power supply.

Looks like Q92 drives the gate of p-fet Q91, also slick. Interesting
things happen over the pwm range from 0 to 50%, but I think it's OK.

Why didn't I think of that?
high praise indeed.

Terry Given suggested that one in a thread earlier :)


If Terry gave it to you then it should work. But make sure to obtain
enough information regarding the core. A DC run-off situation or core
saturation is no fun.

Thanks Joerg!

I came up with that circuit in 1994 - I did a 2MHz modulated
smps/gatedriver circuit. initially I had a clamp zener, but I used the
same zener for all 6 gatedriver/smps, and although the leakage energy
was OK from one, 6 was too much. so I thunked a bit, and figured out the
self-driven PNP diagonal half-bridge. the switching speed of the
PNP/PFET is pretty much irrelevant too - as long as one device switches
fast its ok - at least until you muck about for so long that the next
switching cycle has been and gone.....

we built about 300,000 of these, with no problems. running from +24V,
using FMMT491A and FMMT591A SOT23 BJTs. We could probably have patented
it - though we did patent the toroid winding technique I came up with.



BJT's? With base resistors? It looks much nicer with fets.

they were cheaper. and the 6 npn base drives had to go a long way on a
single layer (no 0V plane) to get from the CPLD to the gatedrivers, so
Vbe being < Vt was a good thing.
Nice circuit, a few parts doing a lot of stuff. The only warning I'd
give Jamie is that he may not want fast edges getting into his
isolated analog stuff, and it would be tricky to slow this one down.
Our thermocouple supply was a classic open-loop forward converter,
dual nfets driving a center-tapped transformer, and we shaped the gate
drives to soften things up. Efficiency didn't matter much here. And we
got a nice clean square wave out of the secondaries, which also
clocked the delta-sigma ADC!

John

Lovely!


Cheers
Terry
 
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