Flyback regulator / transformer problems.

[[email protected]]

Hi all
I am working on a stepdown flyback regulator and am having a few
problems. It steps 220Vac down to 5Vdc(3A) , 12Vdc(2A) and 20Vdc(0.5A).
I designed a similar cct a little while ago(Had some postings on this
group) , using the uc3843/2 ic. That took 220Vac down to 13.8Vdc. I got
that unit working great , running up to 60W continuously with good
regulation etc.
I have essentially the same cct with a different transformer(turns
ratio , and more outputs) , and feedback components changed for a
5V(regulated) output.The other outputs are not regulated,
I have only wound the 5V winding and populated the pcb with the
required components for 5V out so far.
So far my results have not been great. I can get a stable 5V out , but
there are a few problems. The drain voltage of the FET is going WAY to
high.Up to about 800V spike.
The auxilary winding (that supplies the uc3842 with power after start
up) goes from about 13V with no load on the 5V winding to about 40V
when the 5V winding is loaded with 1,5A.This is causing the 1n5819
diode in this part of the cct to blow.I got around this by putting 2
diodes in series , but this is curing the symptom , not the cause :0(
Why should this windings voltage shoot up like this.The switching
FET is also getting to hot.
The previous cct(220 > 13.8v) used a transformer that was wound by a
transformer company. For this cct I am winding my own transformer , so
I suspect that this is where my problems are comming from. The
switching frequency is about 100KHz.I am using a EC35 core with N27
material with a 0.5mm gap.
I have tried various designs(different Dmax 0.3 - 0.45) for calculating
the windings , all get similar poor results.
My current windings are. Lp 53 turns(600uH) . N (aux) 8 turns and the
5V winding 3 turns.
I have tried changing the order of the windings , with little change.
ie
primary , then aux , then 5v.
aux , primary , 5v.
aux , 5v , primary.
1/2 primary , aux , 1/2primary then 5V

Hope I am making sence.
Is there something I am missing here.
A simulation shows the circuit working ok without the huge voltage
appearing on the primary(coupling set at 0.9).
I thaught that getting 5V would not be any harder than getting 13.8V!!
The power level at this stage(without the other windings) is only about
15W.
Any help would be appreciated.
Cheers
Rob

 

[Graham Holloway]

Hi all
I am working on a stepdown flyback regulator and am having a few
problems. It steps 220Vac down to 5Vdc(3A) , 12Vdc(2A) and 20Vdc(0.5A).
I designed a similar cct a little while ago(Had some postings on this
group) , using the uc3843/2 ic. That took 220Vac down to 13.8Vdc. I got
that unit working great , running up to 60W continuously with good
regulation etc.
I have essentially the same cct with a different transformer(turns
ratio , and more outputs) , and feedback components changed for a
5V(regulated) output.The other outputs are not regulated,
I have only wound the 5V winding and populated the pcb with the
required components for 5V out so far.
So far my results have not been great. I can get a stable 5V out , but
there are a few problems. The drain voltage of the FET is going WAY to
high.Up to about 800V spike.
The auxilary winding (that supplies the uc3842 with power after start
up) goes from about 13V with no load on the 5V winding to about 40V
when the 5V winding is loaded with 1,5A.This is causing the 1n5819
diode in this part of the cct to blow.I got around this by putting 2
diodes in series , but this is curing the symptom , not the cause :0(
Why should this windings voltage shoot up like this.The switching
FET is also getting to hot.
The previous cct(220 > 13.8v) used a transformer that was wound by a
transformer company. For this cct I am winding my own transformer , so
I suspect that this is where my problems are comming from. The
switching frequency is about 100KHz.I am using a EC35 core with N27
material with a 0.5mm gap.
I have tried various designs(different Dmax 0.3 - 0.45) for calculating
the windings , all get similar poor results.
My current windings are. Lp 53 turns(600uH) . N (aux) 8 turns and the
5V winding 3 turns.
I have tried changing the order of the windings , with little change.
ie
primary , then aux , then 5v.
aux , primary , 5v.
aux , 5v , primary.
1/2 primary , aux , 1/2primary then 5V

Hope I am making sence.
Is there something I am missing here.
A simulation shows the circuit working ok without the huge voltage
appearing on the primary(coupling set at 0.9).
I thaught that getting 5V would not be any harder than getting 13.8V!!
The power level at this stage(without the other windings) is only about
15W.
Any help would be appreciated.
Cheers
Rob

Rob

Have you checked the phasing of the other windings WRT the 5V winding and
the primary?


Regards

Graham Holloway

 

[Robert Baer]

Hi all
I am working on a stepdown flyback regulator and am having a few
problems. It steps 220Vac down to 5Vdc(3A) , 12Vdc(2A) and 20Vdc(0.5A).
I designed a similar cct a little while ago(Had some postings on this
group) , using the uc3843/2 ic. That took 220Vac down to 13.8Vdc. I got
that unit working great , running up to 60W continuously with good
regulation etc.
I have essentially the same cct with a different transformer(turns
ratio , and more outputs) , and feedback components changed for a
5V(regulated) output.The other outputs are not regulated,
I have only wound the 5V winding and populated the pcb with the
required components for 5V out so far.
So far my results have not been great. I can get a stable 5V out , but
there are a few problems. The drain voltage of the FET is going WAY to
high.Up to about 800V spike.
The auxilary winding (that supplies the uc3842 with power after start
up) goes from about 13V with no load on the 5V winding to about 40V
when the 5V winding is loaded with 1,5A.This is causing the 1n5819
diode in this part of the cct to blow.I got around this by putting 2
diodes in series , but this is curing the symptom , not the cause :0(
Why should this windings voltage shoot up like this.The switching
FET is also getting to hot.
The previous cct(220 > 13.8v) used a transformer that was wound by a
transformer company. For this cct I am winding my own transformer , so
I suspect that this is where my problems are comming from. The
switching frequency is about 100KHz.I am using a EC35 core with N27
material with a 0.5mm gap.
I have tried various designs(different Dmax 0.3 - 0.45) for calculating
the windings , all get similar poor results.
My current windings are. Lp 53 turns(600uH) . N (aux) 8 turns and the
5V winding 3 turns.
I have tried changing the order of the windings , with little change.
ie
primary , then aux , then 5v.
aux , primary , 5v.
aux , 5v , primary.
1/2 primary , aux , 1/2primary then 5V

Hope I am making sence.
Is there something I am missing here.
A simulation shows the circuit working ok without the huge voltage
appearing on the primary(coupling set at 0.9).
I thaught that getting 5V would not be any harder than getting 13.8V!!
The power level at this stage(without the other windings) is only about
15W.
Any help would be appreciated.
Cheers
Rob

Start with a new coil former and wind only the primary (what the FET
drives; i take it that is 53 turns for 600uH.
Measure it on a bridge if possible.
Set up a "dumb" circuit using the same FET as in the final circuit.
Source to gnd, drive gate with (adjustable) pulse generator at about
10V, inductor from adjustable supply to drain (use lead long enough for
current probe); also put good bypassing on the supply end (i use short
lead 0.1uF 200V small ceramic caps and then a bit longer lead 1-5uF 600V
film cap as bypass).
If you cannot use a current probe, then use surface mount resistor(s)
in series with the FET source and ground; 100 ohm to 1K surface mount
from source to tie point for scope probe.
Now you can start with a low inductor supply voltage and alter the
frequency and pulse width as you see fit.
Keep the frequency (pulse repetition rate) low so that the power
level is low; makes it easier on supplies and parts.
At some voltage and gate drive width, you will notice the current
ramp swing upwards exponentially = that is where core saturation starts.
Measure that current to get Ampere-Turns for saturation; divide by
effective length from data sheet on the core size/shape.
Refer to the B-H curve of the material, and you will find this point
is close to what the data sheet indicates.
** That is your reference.
Now adjust the pulse width and then the inductor supply voltage,
while looking for saturation problems.

I suspect that you may be saturating the poor core.
In any event, with a Ferroxcube P14/8 core of 3C18 material (or
similar for that matter), i can easily get 1000V for a 56 turn 1000uH
inductor; supply from about 5V to 200V (adjusted pulse width for
constant peak current of 240mA).

On the "input" side (when the FET is on), the equation E=-L*(dI/dT)
is useful, in that measured values fit it fairly well. Errors are due to
leakage inductance (pesky ringing) and core losses at the high
frequencies seen (ignore pulse repetition rate for core loss here).
So, use that equation, where E is the inductor supply voltage, dT is
the gate width, dI is the peak ramp current, and L is the nominal
inductance.

The energy put into the core during that time becomes available
during the "output" side, mostly during the collapse of the magnetic field.
You should see a fairly narrow high voltage pulse that is roughly
flat on the top. There will be a bit of a peak after the rise, the top
is usually sloped down some before the fall. Rise and fall times are
approximately the same.
Monitored current during that time will be roughly zero; actually
starting from small positive, sloping thru zero near middle of voltage
pulse, to small negative at pulse end.
Then the current will be like a backwards ramp; jumps large negative
and slowly falls back to zero, continuing positive, etc (ringing); seen
at low pulse rates (say 10Khz).

You are obviously putting to much energy into the core for the load
it is seeing; the open-circuit flyback peak is zapping your poor FET (i
use the IRFBG20, a 1000V FET for my tests), and the voltage you see is
limited by the FET avalanching. It is geting hot due to that, and the
high pulse rate ("frequency").

At least you have the right idea in interleaving the feedback winding
with the drive winding, and the mixing of the secondaries.
All of that bother tends to reduce leakage inductance - at the
expence of added capacitance and need for insulation against high voltages.

Hope this is of help.

 

[[email protected]]

Hi to all.
Thanks the help.
The phasing is correct as far as I know. I changed the aux winding
around , but things got worse , so I assume I had it correct.The 5V
output seems good , so I assume that it is correct.Tried changing it
around and the supply weny haywire!!
As to the core saturating. Surely if the core saturates the output
voltage would fall.
The output seems nice and steady under load.
I'll try to go through all the tests you mentioned , sounds like fun!!
I don't have a current probe , but I'll use a resistor to measure the
current.100 ohm to 1k seems like a large value though. I'd have thaught
a few ohms would be more appropriate , or am I miss-understanding you?
Once I have the amp/turns figure ,
I'm not sure where to find the "effective length" from the data sheet.
As you may have guessed ,I'm fairly inexperienced with this sort of
stuff , so please excuse my ignorance!!


I'm not sure what you mean when you say
"
In any event, with a Ferroxcube P14/8 core of 3C18 material (or
similar for that matter), i can easily get 1000V for a 56 turn 1000uH
inductor; supply from about 5V to 200V (adjusted pulse width for
constant peak current of 240mA).

"

cheers
Rob

 

[legg]

Hi all
I am working on a stepdown flyback regulator and am having a few
problems. It steps 220Vac down to 5Vdc(3A) , 12Vdc(2A) and 20Vdc(0.5A).
I designed a similar cct a little while ago(Had some postings on this
group) , using the uc3843/2 ic. That took 220Vac down to 13.8Vdc. I got
that unit working great , running up to 60W continuously with good
regulation etc.
I have essentially the same cct with a different transformer(turns
ratio , and more outputs) , and feedback components changed for a
5V(regulated) output.The other outputs are not regulated,
I have only wound the 5V winding and populated the pcb with the
required components for 5V out so far.
So far my results have not been great. I can get a stable 5V out , but
there are a few problems. The drain voltage of the FET is going WAY to
high.Up to about 800V spike.

Even using the same basic transformer structure, the change in turns
ratio (12V to 5V) would increase leakage inductance by a factor of 4.
An RCD voltage clamp is one common method of limiting voltage peaks
and absorbing this energy.

The auxilary winding (that supplies the uc3842 with power after start
up) goes from about 13V with no load on the 5V winding to about 40V
when the 5V winding is loaded with 1,5A.This is causing the 1n5819
diode in this part of the cct to blow.I got around this by putting 2
diodes in series , but this is curing the symptom , not the cause :0(
Why should this windings voltage shoot up like this.The switching
FET is also getting to hot.

Voltage stress on the auxiliary winding, when configured as a flyback,
occurs during switch 'on' time. Using your turns ratio, this stress
will always exceed 50V + Vaux at nominal line input, so schottkys are
not the best solution for this job. Use UF4004 or other jellybean 1A
ultrafast rectifiers.

The auxiliary tolerance can be made more immune to primary spikes, by
placing a resistor in series with it's rectifier. It's tolerance will
be best when closely coupled to the winding supplying the regulated
output.

The previous cct(220 > 13.8v) used a transformer that was wound by a
transformer company. For this cct I am winding my own transformer , so
I suspect that this is where my problems are comming from. The
switching frequency is about 100KHz.I am using a EC35 core with N27
material with a 0.5mm gap.
I have tried various designs(different Dmax 0.3 - 0.45) for calculating
the windings , all get similar poor results.
My current windings are. Lp 53 turns(600uH) . N (aux) 8 turns and the
5V winding 3 turns.
I have tried changing the order of the windings , with little change.
ie
primary , then aux , then 5v.
aux , primary , 5v.
aux , 5v , primary.
1/2 primary , aux , 1/2primary then 5V

1/2 primary, iso, 5V, iso, aux, 1/2 primary.


RL

 

[Robert Baer]

Hi to all.
Thanks the help.
The phasing is correct as far as I know. I changed the aux winding
around , but things got worse , so I assume I had it correct.The 5V
output seems good , so I assume that it is correct.Tried changing it
around and the supply weny haywire!!
As to the core saturating. Surely if the core saturates the output
voltage would fall.
The output seems nice and steady under load.
I'll try to go through all the tests you mentioned , sounds like fun!!
I don't have a current probe , but I'll use a resistor to measure the
current.100 ohm to 1k seems like a large value though. I'd have thaught
a few ohms would be more appropriate , or am I miss-understanding you?
Once I have the amp/turns figure ,
I'm not sure where to find the "effective length" from the data sheet.
As you may have guessed ,I'm fairly inexperienced with this sort of
stuff , so please excuse my ignorance!!


I'm not sure what you mean when you say
"
In any event, with a Ferroxcube P14/8 core of 3C18 material (or
similar for that matter), i can easily get 1000V for a 56 turn 1000uH
inductor; supply from about 5V to 200V (adjusted pulse width for
constant peak current of 240mA).

"

cheers
Rob

One ohm from FET source to ground; for current sensing.
SMT for low inductance; if cannot do, use two 1/4 watt 2 ohm carbon
comps, at 30-45 degree angle ontop of the ground plane - shortest leads
possible.
Then a 1K from the source to a test point; short lead at the soruce end.
This isolates capacitive loading and other effects of probe loading
on the circuit.
**
Every core has a magnetic path with an effective length associated
with its size and shape.
Perhaps you can access a PDF of the core i mentioned via:
http://www.ferroxcube.com/prod/assets/p148.pdf
Look at the second page, top left for effective length (for that core).

**
I gave an example of what i had done that was chosen to be close to
what you had mentioned.
That way, you have a rough idea as what to expect.
Sorry cannot give examples of power inverters, but i thought
something was better than nothing.

 

[[email protected]]

One ohm from FET source to ground; for current sensing.
SMT for low inductance; if cannot do, use two 1/4 watt 2 ohm carbon
comps, at 30-45 degree angle ontop of the ground plane - shortest leads
possible.
Then a 1K from the source to a test point; short lead at the soruce end.
This isolates capacitive loading and other effects of probe loading
on the circuit.
**
Every core has a magnetic path with an effective length associated
with its size and shape.
Perhaps you can access a PDF of the core i mentioned via:
http://www.ferroxcube.com/prod/assets/p148.pdf
Look at the second page, top left for effective length (for that core).

**
I gave an example of what i had done that was chosen to be close to
what you had mentioned.
That way, you have a rough idea as what to expect.
Sorry cannot give examples of power inverters, but i thought
something was better than nothing.

Hi there.
Thanks for the help.I see you are refering to magnetic path length , so
I should be able to make some progress from here, I hope.
I did a measurement of the current on my board as it's running , and
the core may very well be saturating. The FET source is connected to
GND via a 1ohm
sence resistor which is connected to the uc3842 via a low pass filter
consisting of 1K to the current monitor pin and 470pF to gnd.
The voltage across the 1ohm resistor rises linearly(about) to 400mV ,
and then sharply to about 3V. This indicates to me that the current is
rising as it should to about 400mA and then the core is saturating and
the current ramps up to 3A os so. The only thing stopping the smoke
comming out is the uc3842s current sensing switching the output off.If
the core is indeed
saturating , how can I prevent this.I have used 2 app notes from
different manufacturers(unitrode and infineon) and come out with
similar parameters(turns , inductances etc)for the transformers.
If the transformer is saturating , I must be doing somethig
fundementally wrong:0(
Any ideas.
Cheers
Rob

 

[Paul Mathews]

to prevent saturation: shorter pulse duration, more turns, longer path
length, large cross section, ...
Paul Mathews

 

[legg]

Hi there.
Thanks for the help.I see you are refering to magnetic path length , so
I should be able to make some progress from here, I hope.
I did a measurement of the current on my board as it's running , and
the core may very well be saturating. The FET source is connected to
GND via a 1ohm
sence resistor which is connected to the uc3842 via a low pass filter
consisting of 1K to the current monitor pin and 470pF to gnd.
The voltage across the 1ohm resistor rises linearly(about) to 400mV ,
and then sharply to about 3V. This indicates to me that the current is
rising as it should to about 400mA and then the core is saturating and
the current ramps up to 3A os so. The only thing stopping the smoke
comming out is the uc3842s current sensing switching the output off.If
the core is indeed
saturating , how can I prevent this.I have used 2 app notes from
different manufacturers(unitrode and infineon) and come out with
similar parameters(turns , inductances etc)for the transformers.
If the transformer is saturating , I must be doing somethig
fundementally wrong:0(

At 100KHz, this is a core saturation limited design, not a core loss
limited one.

The core you are using shouldn't saturate under regulated conditions,
using the turns, gap, voltages and power levels indicated (1A5 peak
primary current @60W). Recheck your gap size.

The normally functioning circuit will saturate if the output voltage
or power is doubled at high line, or if transient duty cycle exceeds
twice that normally required, at normal line input voltage.

You should be limiting your duty cycle at start up, with a slow-start
circuit, and reduce your control loop transient response to avoid
this. Alternately, using a chip with a 50% duty cycle limit might also
avoid the situation.

Your circuit is limiting reliably at a 3A level, in reaction to some
other fault that is preventing the formation of flyback voltseconds.
Output shorts, component breakdown, miswiring or phasing errors are
candidates, though your normal initial ramp suggests that these are
not present.

Recheck your frequency and voltage readings on the scope - are you
reading the right scale?

RL

 

[Robert Baer]

to prevent saturation: shorter pulse duration, more turns, longer path
length, large cross section, ...
Paul Mathews

--------- SNIPped for brevity -------
No, *LESS* turns if all else same.
It is the Ampere-Turns that produces a given magnetizing field; more
turns makes it larger, and more saturation at a given current (core the
same).

 

[analog]

No, *LESS* turns if all else same.
It is the Ampere-Turns that produces a given magnetizing field;
more turns makes it larger, and more saturation at a given current
(core the same).

True enough as far as it goes, but probably not very helpful to the
original poster. To get more flyback transformer power throughput
one usually combines an increase in turns with an increase in gap
length (of course the transformer will operate at a lower efficiency).
Very little energy is stored in the core - most all is stored in the
gap. For a given maximum flux density, increasing the gap volume
increases the maximum energy stored.

PS: Were you able to understand and correct the errors you were
making that lead you to incorrectly claim to have found "Another
discrepancy (LTspice/switchcad3)" over in sci.electronics.cad, or
do you require more help with the basics of circuit simulation?

 

[Robert Baer]

True enough as far as it goes, but probably not very helpful to the
original poster. To get more flyback transformer power throughput
one usually combines an increase in turns with an increase in gap
length (of course the transformer will operate at a lower efficiency).
Very little energy is stored in the core - most all is stored in the
gap. For a given maximum flux density, increasing the gap volume
increases the maximum energy stored.

PS: Were you able to understand and correct the errors you were
making that lead you to incorrectly claim to have found "Another
discrepancy (LTspice/switchcad3)" over in sci.electronics.cad, or
do you require more help with the basics of circuit simulation?

I found that if i picked a FET from the available list, or faked a
FET (IRFBG20 in particular - but not very well due to limited data sheet
info), then the simple model of FET switching inductor "worked" - the
waveforms matched the bench reasonably well.
So now i have the agony of trying to model a pot core with autoformer
winding for my flyback.

 

[Tony Williams]

So now i have the agony of trying to model a pot core with
autoformer winding for my flyback.

Hello Robert. I don't know if it helps, but
this is a quick fudge for an autotransformer
from an old textbook. It assumes that the
auto is wound such that the pri and sec each
occupy equal areas of the available space.

2
Calculate the factor (1-r), where r is the
*fractional* voltage ratio, Vp/Vs or Vs/Vp.

Model a two-winding transformer of equal size
and flux density. Calc the numbers for copper
loss, resistance refered to the primary, and
leakage inductance.

To model the auto, use that two-winding transformer,
but multiply each of those numbers by the fudge factor.

 

[[email protected]]

Hello Robert. I don't know if it helps, but
this is a quick fudge for an autotransformer
from an old textbook. It assumes that the
auto is wound such that the pri and sec each
occupy equal areas of the available space.

2
Calculate the factor (1-r), where r is the
*fractional* voltage ratio, Vp/Vs or Vs/Vp.

Model a two-winding transformer of equal size
and flux density. Calc the numbers for copper
loss, resistance refered to the primary, and
leakage inductance.

To model the auto, use that two-winding transformer,
but multiply each of those numbers by the fudge factor.

Hi to all.
I am finally getting somewhere!!! Thanks to all he help I've received
here.
I have changed my winding scheme to match that mentioned by RL. Seems
to help a lot.I have increaced my source resistor to 3 ohms. This has
stopped the core from saturating. I think that this was my main
problem. I think I misunderstood the functioning of the chip in this
regard. I assumed this was to limit short cct currents. It seems that
this is not the case.It is there to terminate the "switch" conduction
when a certain current is reached.I was therefor terminating the switch
after the core had already saturated.I still have a bit to do , but the
cct is currently running quite well(next to me now). The drain voltage
of the fet is still going quite high , about 600V , but it is much
better that it was , and also seems to have the correct "shape" as it
where.A peak of 600V , rings for 3 cycles and then flat at 400V Just
before the voltage falls to 0 (on time) it falls at an angle to about
300V .I have fairly minimal "snubbing" as it where. A bigger snubbing
network will help kill some of the ringing. I think a bit more
attention to my windings will help with the peak voltage , which is
obviously caused by leakage inductance.(primary and reflected from
output).
Its currently running 2.5A on the sec(5V) . The FET is still nice and
cool. The output diode is pretty hot(77 dec C) , but I expect this is
about right.I am using a byv32 - 200V(To220 package) diode .It's Vf is
about 0.8V. I assume its dissipating about 1W without a heatsink. I am
currently looking a diodes with a better Vf at the moment.Lots more
experimenting required!!
Thanks again for all the help guys.
I let you know how things work out.
Cheers
Rob

 

[Rich Grise]

Hi all
I am working on a stepdown flyback regulator and am having a few
problems. It steps 220Vac down to 5Vdc(3A) , 12Vdc(2A) and 20Vdc(0.5A).

....

Then, after much discussion, On Fri, 03 Jun 2005 02:59:50 -0700, seegoon99
wrote:

Thanks again for all the help guys.
I let you know how things work out.

I'm still wondering what ever possessed you to use a flyback in
a stepdown converter????

I'd have thought a buck, or forward for multiple tracking voltages,
would have been more "appropriate". Or is this some sort of educational
exercise?

Thanks,
Rich

 

[Robert Baer]

Hello Robert. I don't know if it helps, but
this is a quick fudge for an autotransformer
from an old textbook. It assumes that the
auto is wound such that the pri and sec each
occupy equal areas of the available space.

2
Calculate the factor (1-r), where r is the
*fractional* voltage ratio, Vp/Vs or Vs/Vp.

Model a two-winding transformer of equal size
and flux density. Calc the numbers for copper
loss, resistance refered to the primary, and
leakage inductance.

To model the auto, use that two-winding transformer,
but multiply each of those numbers by the fudge factor.

Thanks; will try that.

 

[Pooh Bear]

Hi all
I am working on a stepdown flyback regulator and am having a few
problems. It steps 220Vac down to 5Vdc(3A) , 12Vdc(2A) and 20Vdc(0.5A).
I designed a similar cct a little while ago(Had some postings on this
group) , using the uc3843/2 ic. That took 220Vac down to 13.8Vdc. I got
that unit working great , running up to 60W continuously with good
regulation etc.
I have essentially the same cct with a different transformer(turns
ratio , and more outputs) , and feedback components changed for a
5V(regulated) output.The other outputs are not regulated,
I have only wound the 5V winding and populated the pcb with the
required components for 5V out so far.
So far my results have not been great. I can get a stable 5V out , but
there are a few problems. The drain voltage of the FET is going WAY to
high.Up to about 800V spike.

Reflected voltage too high ? A transient suppressor fixes that btw.

The auxilary winding (that supplies the uc3842 with power after start
up) goes from about 13V with no load on the 5V winding to about 40V
when the 5V winding is loaded with 1,5A.

Sounds a bit like poor coupling

This is causing the 1n5819
diode in this part of the cct to blow.I got around this by putting 2
diodes in series , but this is curing the symptom , not the cause :0(
Why should this windings voltage shoot up like this.The switching
FET is also getting to hot.
The previous cct(220 > 13.8v) used a transformer that was wound by a
transformer company. For this cct I am winding my own transformer , so
I suspect that this is where my problems are comming from. The
switching frequency is about 100KHz.I am using a EC35 core with N27
material with a 0.5mm gap.

N27 !!!!!!

I barely thought you could even still get that !

You should be on at least N87 by now. That's perfect for 100kHz.

I have tried various designs(different Dmax 0.3 - 0.45) for calculating
the windings , all get similar poor results.
My current windings are. Lp 53 turns(600uH) . N (aux) 8 turns and the
5V winding 3 turns.
I have tried changing the order of the windings , with little change.
ie
primary , then aux , then 5v.
aux , primary , 5v.
aux , 5v , primary.
1/2 primary , aux , 1/2primary then 5V

Hope I am making sence.
Is there something I am missing here.
A simulation shows the circuit working ok without the huge voltage
appearing on the primary(coupling set at 0.9).

What are you using to simulate ?

I thaught that getting 5V would not be any harder than getting 13.8V!!
The power level at this stage(without the other windings) is only about
15W.

Your hand wound construction technique may be a problem. Sadly you don't
mention that. Have you measured leakage inductance ?

I have had no trouble with hand winding but I've done a few !

There's quite an art to winding transformers.

I found bags of useful info at http://www.powerint.com. Get their app notes
( esp on transformer construction ). They also have a great little design
tool that works out the magnetics for you and can be 'tweaked' too. It
assumes use of their parts but works for any controller.

Btw - I had a 'sample' transformer made by a 'winding house' for a design I
was working on with a Power Integrations part and it was useless. I did far
better myself. Best to read up on this.

Were you using safety margins btw ?

Graham

 

[Pooh Bear]

...

Then, after much discussion, On Fri, 03 Jun 2005 02:59:50 -0700, seegoon99
wrote:


I'm still wondering what ever possessed you to use a flyback in
a stepdown converter????

Perfectly standard practice. Note 220V *AC* line in - 5V DC out.

I'd have thought a buck, or forward for multiple tracking voltages,
would have been more "appropriate".

Forward for 50W ! You're kidding right ?

Or is this some sort of educational exercise?

Doesn't sound like it.

Graham

 

[Rich Grise]

Perfectly standard practice. Note 220V *AC* line in - 5V DC out.


Forward for 50W ! You're kidding right ?

No, as a matter of fact, I'm not. I know I know dooledy squat about
Shit My Pants mode power Supplies (that's a joke - SMPS - get it?)
but I had always had the impression, from my days as a geek larva
reading TeeVee set schematics, that the thing about a flyback (and
car ignition coils, come to think of it) is that the voltage FLIES
BACK, overshooting the supply so much that you need a catch diode
to catch all the volts. But if you've already got 220 of them, and
you only need five, wouldn't you store the excess volts in a buck
inductor/cap? And I thought "forward" just because it seems so
stodgy and straightforward, excuse the pun. Ya put so many watts
in the primary, go down the turns ratio, and get .9x watts out
the secondary.

When you're going from 220 to 5 volts, what "flies back?"

Doesn't sound like it.

Trust me - it's definitely an educational experience for me!

Thanks!
Rich

 

[Tony Williams]

Thanks; will try that.

Just to correct a possible ambiguity in the typing.

The multiplying factor is the square of (1-r).