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switching regulator questions

I came across the LM2578, while looking really closely at a picture of
eBay item 350090122136.

The LM2578 datasheet, page 25 gives an interesting flyback regulator
circuit providing +/- outputs.
http://www.national.com/mpf/LM/LM2578A.html

I'm tempted to build one, with a supersized flyback transformer and
pass transistor.

My question is this: if the load on the transformer increases, such
that the pulses become very wide, then will the + and - outputs still
be symmetric? In other words, is there any possibility that the |+V|
output could be greater in magnitude than the |-V| output, because the
square waves would no longer be symmetric?

Michael
 
B

Bob Eld

Jan 1, 1970
0
I came across the LM2578, while looking really closely at a picture of
eBay item 350090122136.

The LM2578 datasheet, page 25 gives an interesting flyback regulator
circuit providing +/- outputs.
http://www.national.com/mpf/LM/LM2578A.html

I'm tempted to build one, with a supersized flyback transformer and
pass transistor.

My question is this: if the load on the transformer increases, such
that the pulses become very wide, then will the + and - outputs still
be symmetric? In other words, is there any possibility that the |+V|
output could be greater in magnitude than the |-V| output, because the
square waves would no longer be symmetric?

Michael

The data sheet has 17 pages, where is page 25?
 
B

Bob Eld

Jan 1, 1970
0
I came across the LM2578, while looking really closely at a picture of
eBay item 350090122136.

The LM2578 datasheet, page 25 gives an interesting flyback regulator
circuit providing +/- outputs.
http://www.national.com/mpf/LM/LM2578A.html

I'm tempted to build one, with a supersized flyback transformer and
pass transistor.

My question is this: if the load on the transformer increases, such
that the pulses become very wide, then will the + and - outputs still
be symmetric? In other words, is there any possibility that the |+V|
output could be greater in magnitude than the |-V| output, because the
square waves would no longer be symmetric?

Michael

This is a flyback configuration meaning that the secondary delivers power
when the primary is off. Energy is stored in the inductor/transformer during
the on part of the cycle then transferred to the secondary side during the
off time.

Regulation is determined by feedback from the + side of the output, back to
the chip. The negative side goes for a ride without direct feedback.

Regulation will depend on how closely coupled the + and - windings are. It
would be best to bifilar wind them, both wires together to reduce leakage
inductance. Leakage inductance will store energy that is not necessarily
coupled to the secondary or may couple unevenly. The minus probably will NOT
track the plus very well especially if they have differing currents.

Depending on the application, it would be best to follow the +and - outputs
with linear regulators like LM7815 and LM7915 to insure reasonably matched
and quiet + and -15 volts. Of course the outputs would have to be a few
volts higher.

You should avoid very wide on time voltages because the secondaries have to
ring off and supply power with the same (volt )(time)/(turns) product as the
primary. Usually the on time should not exceed the off time, depending on
turns ratio.
 
M

MooseFET

Jan 1, 1970
0
This is a flyback configuration meaning that the secondary delivers power
when the primary is off. Energy is stored in the inductor/transformer during
the on part of the cycle then transferred to the secondary side during the
off time.

Regulation is determined by feedback from the + side of the output, back to
the chip. The negative side goes for a ride without direct feedback.

Regulation will depend on how closely coupled the + and - windings are. It
would be best to bifilar wind them, both wires together to reduce leakage
inductance. Leakage inductance will store energy that is not necessarily
coupled to the secondary or may couple unevenly. The minus probably will NOT
track the plus very well especially if they have differing currents.

You can make it better if you do this:

Imagine two isolated outputs that both make +15V. The diode is in the
positive side. of both windings.

Now hook those two supplies in series.

The plus end of the two windings still have the same AC waveform on
them but are at different DC points.

Now put a capacitor between the two positive ends of the windings.

They are now forced to have nearly exactly the same waveform even with
the leakage inductance ringing.

The result is much better regulation on the non-servoed output
 
You can make it better if you do this:

Imagine two isolated outputs that both make +15V. The diode is in the
positive side. of both windings.

Now hook those two supplies in series.

The plus end of the two windings still have the same AC waveform on
them but are at different DC points.

Now put a capacitor between the two positive ends of the windings.

They are now forced to have nearly exactly the same waveform even with
the leakage inductance ringing.

The result is much better regulation on the non-servoed output


Ah. Thanks. Requires two coils, sets of diodes, ... but more
stable...

Thanks to all who replied

Michael
 
You can make it better if you do this:

Imagine two isolated outputs that both make +15V. The diode is in the
positive side. of both windings.

Now hook those two supplies in series.

The plus end of the two windings still have the same AC waveform on
them but are at different DC points.

Now put a capacitor between the two positive ends of the windings.

They are now forced to have nearly exactly the same waveform even with
the leakage inductance ringing.

The result is much better regulation on the non-servoed output


Say... the key word here is isolated.

Am I correct in assuming that using two circuits with simple
inductors, connected in series, would be a bad idea?

So, I should use two 1:1 isolation flyback transformers in series,
correct?

Michael
 
M

MooseFET

Jan 1, 1970
0
Ah. Thanks. Requires two coils, sets of diodes, ... but more
stable...

There are not more coils or diodes just one more capacitor.
 
M

MooseFET

Jan 1, 1970
0
Say... the key word here is isolated.

Am I correct in assuming that using two circuits with simple
inductors, connected in series, would be a bad idea?

So, I should use two 1:1 isolation flyback transformers in series,
correct?

I think you have it completely wrong. There is:

One core.

On this core there are 3 windings

One winding is the primary and is driven by the MOSFET

The two others are identical windings that are the secondaries.
 
I think you have it completely wrong. There is:

One core.

On this core there are 3 windings

One winding is the primary and is driven by the MOSFET

The two others are identical windings that are the secondaries.


Gotcha!

I'm missing the forest for the trees... thanks.

Michael
 
S

Simon S Aysdie

Jan 1, 1970
0
You can make it better if you do this:

Imagine two isolated outputs that both make +15V.  The diode is in the
positive side. of both windings.

Now hook those two supplies in series.

The plus end of the two windings still have the same AC waveform on
them but are at different DC points.

Now put a capacitor between the two positive ends of the windings.

They are now forced to have nearly exactly the same waveform even with
the leakage inductance ringing.

The result is much better regulation on the non-servoed output





You can make it better if you do this:

Imagine two isolated outputs that both make +15V. The diode is in the
positive side. of both windings.

Now hook those two supplies in series.

The plus end of the two windings still have the same AC waveform on
them but are at different DC points.

Now put a capacitor between the two positive ends of the windings.

They are now forced to have nearly exactly the same waveform even with
the leakage inductance ringing.

The result is much better regulation on the non-servoed output

http://www.linear.com/pc/downloadDocument.do?navId=H0,C3,P1544,D4299
 
E

Eeyore

Jan 1, 1970
0
I came across the LM2578, while looking really closely at a picture of
eBay item 350090122136.

The LM2578 datasheet, page 25 gives an interesting flyback regulator
circuit providing +/- outputs.
http://www.national.com/mpf/LM/LM2578A.html

I'm tempted to build one, with a supersized flyback transformer and
pass transistor.

My question is this: if the load on the transformer increases, such
that the pulses become very wide, then will the + and - outputs still
be symmetric? In other words, is there any possibility that the |+V|
output could be greater in magnitude than the |-V| output, because the
square waves would no longer be symmetric?

With flyback ? Or any other topology that instantly comes to mind. How do
you think that would happen ? It wouldn't matter much anyway.

Audio is assymetric itself.

Graham
 
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