20 mA LED SMPS buggy

[Bernhard Kuemel]

Hi sed!

I want to illuminate our hall with 7 white LEDs so we don't have to turn
on the main light at night when we want to save energy or avoid too much
light.

I started to build a SMPS with a TL494 PWM controller salvaged from an
ATX SMPS [2]. Image/video gallery [0].

Before I hook it up to 230V AC (325V rectified) I'm testing it with 12V
and something is wrong.

When I decrease the voltage at error amplifier 1 (EA1) with the 150K
pot, the duty cycle increases which at first lets the current rise
higher in the flyback primary and produces more LED current. But at some
point the primary current drops sharply as if the transistor were shut
off, but stops at 240 mA and then settles at 300 mA for the rest of the
duty cycle, no matter how long that is [1]. The LED current drops and
remains at about 0.6 mA. I want 20 mA.

At higher duty cycles the transistor gets hot.

Without the flyback connected the base current of the transistor
(BULK128 or BULT118) is very clean [3]. With the flyback there are some
bumps in the base current [4].

What's going on at these higher duty cycles? How can I get the primary
current to keep increasing during the transistor on time?

I abused the TL494 quite a lot but without flyback it seems to work
fine. Do you think it may be damaged?

Any other suggestion for improvement? I want this to be relatively
efficient, so no linear PS.

Thanks, Bernhard

PS: The flyback primary is 139 mH. The secondary is self wound.

[0]
http://www.bksys.at/bernhard/img/LED-SMPS/-gallery.html
(click on medium size pictures for full size)

[1]
http://www.bksys.at/bernhard/img/LED-SMPS/P1080833.JPG
http://www.bksys.at/bernhard/img/LED-SMPS/P1080839.MOV

[2]
breadboard:
http://www.bksys.at/bernhard/img/LED-SMPS/P1080819.JPG
schematic:
http://www.bksys.at/bernhard/img/LED-SMPS/P1080848.JPG

[3]
voltage across base resistor:
http://www.bksys.at/bernhard/img/LED-SMPS/P1080835.JPG
http://www.bksys.at/bernhard/img/LED-SMPS/P1080842.MOV

[4]
voltage across base resistor with flyback connected:
http://www.bksys.at/bernhard/img/LED-SMPS/P1080836.html
http://www.bksys.at/bernhard/img/LED-SMPS/P1080837.html
http://www.bksys.at/bernhard/img/LED-SMPS/P1080841.MOV
base resistor voltage while disconnecting the flyback:
http://www.bksys.at/bernhard/img/LED-SMPS/P1080843.MOV

 

[Bernhard Kuemel]

how about some from here and be done with it:
http://s.dx.com/search/led+driver

I haven't found a suitable driver. 230V AC, 20 mA, 7 LED (7*3.2 V = 22
V). Hmm, maybe I could modify R(sens) of a driver. I'm also interested
in doing/learning this myself.

 

[Bernhard Kuemel]

I abused the TL494 quite a lot but without flyback it seems to work
fine. Do you think it may be damaged?

I just tried with another TL494 which I have not abused except by
desoldering it with a hot air gun. Same problem.

Bernhard

 

[Bernhard Kuemel]

Hi sed!

I want to illuminate our hall with 7 white LEDs so we don't have to turn
on the main light at night when we want to save energy or avoid too much
light.

I started to build a SMPS with a TL494 PWM controller salvaged from an
ATX SMPS [2]. Image/video gallery [0].

Before I hook it up to 230V AC (325V rectified) I'm testing it with 12V
and something is wrong.

When I decrease the voltage at error amplifier 1 (EA1) with the 150K
pot, the duty cycle increases which at first lets the current rise
higher in the flyback primary and produces more LED current. But at some
point the primary current drops sharply as if the transistor were shut
off, but stops at 240 mA and then settles at 300 mA for the rest of the
duty cycle, no matter how long that is [1]. The LED current drops and
remains at about 0.6 mA. I want 20 mA.

At higher duty cycles the transistor gets hot.

Without the flyback connected the base current of the transistor
(BULK128 or BULT118) is very clean [3]. With the flyback there are some
bumps in the base current [4].

What's going on at these higher duty cycles? How can I get the primary
current to keep increasing during the transistor on time?

I abused the TL494 quite a lot but without flyback it seems to work
fine. Do you think it may be damaged?

Any other suggestion for improvement? I want this to be relatively
efficient, so no linear PS.

Thanks, Bernhard

What is the switching frequency and required output current?

From http://www.bksys.at/bernhard/img/LED-SMPS/P1080835.JPG I get 1493 Hz.

I don't
see any feedback or any other means of regulating the output current
through the LEDs.

I'm currently using a pot to control the duty cycle. Since the load is
intended to be constant, I think I could do without feedback. Dumping
the same amount of energy through the flyback each cycle should produce
a constant current.

Driving a bipolar switching transistor through a 1k
base resistor from the emitter of the TL494's internal transistor is
also unlikely to work very well.

I thought I could drive the transistor with a 1K to Vcc and pull it down
with C2 (collector of the TL494 output transistor). But that would eat
12 mA continuously instead of only during turn on.

I don't understand why my approach is bad. The AND gate driving the
internal base (B1) [1] seems to produce enough voltage to drive two
base/emitter diodes.

Additionally, a flyback converter
designed to work properly with a 12 volt input certainly isn't going to
be very happy with 325 volts applied.

I'm going to increase the frequency for 325 V to something like 100-200
kHz. That should limit the primary current to about 8 mA. I derived some
equations and made a little program that inputs the duty cycle and
prints operating frequency and peak primary current.

Switching converter design is hard to get right by just putting parts
together without having a lot of experience or doing some calculations.

In one of the SMPS tutorials I read someone wrote that one shouldn't
make SMPS unless one really had to do so. Yet, I can't let go of them.

It would probably be best to look at some established designs for
similar circuits and go from there, when experimenting.

That's probably good advice. Maybe I'll look again for a suitable
circuit that I can understand.

[1]
http://www.bksys.at/bernhard/img/LED-SMPS/TL494.pdf

 

[Bernhard Kuemel]

On 6/11/2012 5:27 PM, Bernhard Kuemel wrote:

duty cycle, no matter how long that is [1]. The LED current drops and
remains at about 0.6 mA. I want 20 mA.

[...]

What is the switching frequency and required output current?

From http://www.bksys.at/bernhard/img/LED-SMPS/P1080835.JPG I get 1493 Hz.

Required output current is 20 mA.

 

[mike]

Hi sed!

I want to illuminate our hall with 7 white LEDs so we don't have to turn
on the main light at night when we want to save energy or avoid too much
light.

I have no idea what goes on where you live, but here in 120VAC land,
there are a lot of options.
I've given this issue a lot of thought.

A string of led xmas lights is cheap, especially if you buy 'em on
Dec. 26. I would have just done that except that there ain't no outlets
in my hallway to plug it in.

The dollar store has a large variety of LED night lights.

Light switches with built-in motion detection didn't work
for the geometry of my hallway. Even the stick-on X10
wireless motion sensors didn't work from all the directions
I needed. I guess there really is a reason this stuff shows
up in a garage sale free box.

I light my house with three 1.5W LED nightlights from
lights of America from Costco or Walmart. ~$5 each.
The newest versions are almost too bright.
You can do a lot with aluminum foil reflector/baffle.
OR if you get the older version with the individual packaged
LED's,
you can pull some out and string 'em down the hall.
I rely on reflections and that works well enough.

Other house lights are on a tiny fraction of the time.

I did the math. Turning on a 13W light for a minute when
I get up to pee uses a LOT less energy than leaving the 4.5W on
24/7. But, I haven't tripped over anything since I implemented
that plan. Saving 8 cents a month has not reached actionable
level on the todo list.

I found a "clapper" for a quarter at a garage sale.
I plugged one of the 1.5W lights into it. They pass
current to sense whether something is plugged in, so
the light is dim. A couple of claps later, it's up to
the full 1.5W. Bad news is that it comes on if you cough
or close the dresser drawer or whistle a happy tune....maybe
it's just glad to see me.
But when you want it on, it's rather discriminating
about the timing of your claps...go figger..

The switcher is the easy part. The hard part is packaging,
making it safe so you don't electrocute a visitor or burn
your house down.

Bottom line...

Building your own switcher is a good idea only if you can't
think of anything better to do with your time. If that's
the case, I've got some yard work that needs doing. ;-)

 

[Martin Riddle]

Hi sed!

I want to illuminate our hall with 7 white LEDs so we don't have to
turn
on the main light at night when we want to save energy or avoid too
much
light.

I started to build a SMPS with a TL494 PWM controller salvaged from an
ATX SMPS [2]. Image/video gallery [0].

Before I hook it up to 230V AC (325V rectified) I'm testing it with
12V
and something is wrong.

When I decrease the voltage at error amplifier 1 (EA1) with the 150K
pot, the duty cycle increases which at first lets the current rise
higher in the flyback primary and produces more LED current. But at
some
point the primary current drops sharply as if the transistor were shut
off, but stops at 240 mA and then settles at 300 mA for the rest of
the
duty cycle, no matter how long that is [1]. The LED current drops and
remains at about 0.6 mA. I want 20 mA.

At higher duty cycles the transistor gets hot.

Without the flyback connected the base current of the transistor
(BULK128 or BULT118) is very clean [3]. With the flyback there are
some
bumps in the base current [4].

What's going on at these higher duty cycles? How can I get the primary
current to keep increasing during the transistor on time?

I abused the TL494 quite a lot but without flyback it seems to work
fine. Do you think it may be damaged?

Any other suggestion for improvement? I want this to be relatively
efficient, so no linear PS.

Thanks, Bernhard

PS: The flyback primary is 139 mH. The secondary is self wound.

[0]
http://www.bksys.at/bernhard/img/LED-SMPS/-gallery.html
(click on medium size pictures for full size)

[1]
http://www.bksys.at/bernhard/img/LED-SMPS/P1080833.JPG
http://www.bksys.at/bernhard/img/LED-SMPS/P1080839.MOV

[2]
breadboard:
http://www.bksys.at/bernhard/img/LED-SMPS/P1080819.JPG
schematic:
http://www.bksys.at/bernhard/img/LED-SMPS/P1080848.JPG

[3]
voltage across base resistor:
http://www.bksys.at/bernhard/img/LED-SMPS/P1080835.JPG
http://www.bksys.at/bernhard/img/LED-SMPS/P1080842.MOV

[4]
voltage across base resistor with flyback connected:
http://www.bksys.at/bernhard/img/LED-SMPS/P1080836.html
http://www.bksys.at/bernhard/img/LED-SMPS/P1080837.html
http://www.bksys.at/bernhard/img/LED-SMPS/P1080841.MOV
base resistor voltage while disconnecting the flyback:
http://www.bksys.at/bernhard/img/LED-SMPS/P1080843.MOV

As Bitrex pointed out, Driving with the Emitter may be the problem.
I used the 494 for an isolated supply and it works fine for a push pull
configuration.
I would lookinto that, and connect C1 and E1 as well to get full
brightness.

but I think these work better.....
<http://www.amazon.com/Energizer-ENLPLFPA2-Design-Automatic-Nightlight/dp/B0055OMAWC>

Cheers

 

[hamilton]

I don't understand why my approach is bad.

I think I see the problem.


h

 

[Glenn]

Hi Bernhard

Please do not play with 230V - it is lethal. Also for experienced designers!

-

Apart from that I will congratulate you with your curiousity with smps.

It is fun - but you will probably blow many circuits/components.

I have inserted my responses.

Hi sed!

I want to illuminate our hall with 7 white LEDs so we don't have to turn
on the main light at night when we want to save energy or avoid too much
light.

I started to build a SMPS with a TL494 PWM controller salvaged from an
ATX SMPS [2]. Image/video gallery [0].

Before I hook it up to 230V AC (325V rectified) I'm testing it with 12V
and something is wrong.

When I decrease the voltage at error amplifier 1 (EA1) with the 150K
pot, the duty cycle increases which at first lets the current rise
higher in the flyback primary and produces more LED current. But at some
point the primary current drops sharply as if the transistor were shut
off, but stops at 240 mA and then settles at 300 mA for the rest of the
duty cycle, no matter how long that is [1]. The LED current drops and
remains at about 0.6 mA. I want 20 mA.

At higher duty cycles the transistor gets hot.

Without the flyback connected the base current of the transistor
(BULK128 or BULT118) is very clean [3]. With the flyback there are some
bumps in the base current [4].

What's going on at these higher duty cycles? How can I get the primary
current to keep increasing during the transistor on time?

Your topology seems to be:
http://en.wikipedia.org/wiki/Flyback_converter

Look in the datasheet - bipolar transistors are to some extent awful
compared to Power MOSFETs.

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00003551.pdf

You feed the BULT118 with approximatly 12mA during on-time.

The beta is max. 50. Typically 30 at 100mA (see page 4)

50*12mA=600mA (@Vce=1V)

30*12mA=360mA (@Vce=1V)

At 1A, beta is 10...15 (@Vce=1V) !

-

But there is a much worse problem - your on->off-transition time. More
about that later.

-

You want 20mA and approx. 7*3.6V ca.= 25V

0.02*25 = 0.5W

Your average Ic should be 0.5/12 ca.= 42mA (approx. 100% efficiency...)

If you have 50% duty-cycle the on-time average should be
(Ton+Toff)/Ton*42mA = 84mA

If the Ic=Itransformer ramp up linearly you should have a fine
"triangle"/sawtooth current, and the start current of 0mA and peak
current of 2*84mA=168mA.

If your smps part (minus TL494) has e.g. 50% efficiency your peak
current should be 1/0.5*168mA=336mA.

You get 0.6mA and maybe 25V P=0.015W

How can that be?

Answer:

-

More on on->off-transition time:

You do not remove the base-charge (capacitor like) at on->off quickly
enough.

You have this capacitor that you need to remove charge from: Cbe, when
you have peak Ic current and mayby Vce=1V (or more).

In the datasheet you can see that the on page 3 that the storage time is
3uS at Ib=|-Ib|=0.2A

But you supply only 12mA during on-time - and no -Ib! So your on->off
transition-time will be pretty slow.

This means that your Ic will "slowly" decrease - and your Vce will
slowly increase during on->off transition.

I think that your "missing" LED-power is burned of here.

-

But you might encounter other problems, when you do a fast on->off
transition. You might get high Vce voltage oscillations.

That is why you need a snubber circuit:
http://en.wikipedia.org/wiki/Snubber

I you do not, many times 12V (230V) might arise. But your transistor
ought to handle it at 12V.

-

Solution to slow on->off transition:

Use 1kohm between 12V and base - always 12mA.

Use TL494 C1, E1 to discharge Vbe. E1 to ground - and C1 to base. It
would be better to have -5V to switch off the transistor - but I think
using E1 and C1 to discharge should way better than not discharging.

I abused the TL494 quite a lot but without flyback it seems to work
fine. Do you think it may be damaged?

No - not necessarily.

Any other suggestion for improvement? I want this to be relatively
efficient, so no linear PS.

Yes - many!

Will try to send in another post.

Thanks, Bernhard

br,

Glenn

 

[Glenn]

On 11/06/12 23.27, Bernhard Kuemel wrote:
....

PS: The flyback primary is 139 mH. The secondary is self wound.

....

Ideally the on-time will be:

Ipeak=VL*t/L, VL=constant, Istart=0mA
<=>
t=Ipeak*L/VL

t=0.336A*0.139H/11V= 4.2uS

Compare that with the storage time!

Actually you need to start discharging after e.g. 4.2-3.2=1uS because
the base charge takes 3.2 uS to remove!

-

Switching frequency at 50% duty-cycle:

1/(2*4.2uS)= 120kHz

(but only 1uS on-time from TL494 ! theoretically)

br,

Glenn

 

[Glenn]

On 12/06/12 19.36, Glenn wrote:
....

Solution to slow on->off transition:

Use 1kohm between 12V and base - always 12mA.

Use TL494 C1, E1 to discharge Vbe. E1 to ground - and C1 to base. It
would be better to have -5V to switch off the transistor - but I think
using E1 and C1 to discharge should way better than not discharging. ....
Glenn

The above wont work.

The E1, C1 is not the negated of E2, C2.

Instead change 1kohm to 470 ohm and place it between C2 and 12V. E2 goes
to base.

Place 100ohm between base and ground.

Glenn

 

[Glenn]

Hi Bernhard

The conclusion from my other posts, is that BULT118 is to slow - with
the selected primary inductor inductance.

-

IRFD120, 100V, 1.3A 1.3W, Ron=0.27ohm:
http://www.digikey.com/product-detail/en/IRFD120PBF/IRFD120PBF-ND/812474
Datasheet:
http://www.vishay.com/doc?91128
For Rg=18ohm:
* td(off->on) 6.8nS
* td(on->off) 18nS

IRF510, 100V, 5.6A, 43W, Ron=0.54ohm (can handle abuse better on a heat
sink):
http://www.digikey.com/product-detail/en/IRF510PBF/IRF510PBF-ND/811710
Datasheet:
http://www.vishay.com/docs/91015/sihf510.pdf
For Rg=24ohm:
* td(off->on) 6.9nS
* td(on->off) 15nS

TL494 is way slower than the above MOSFETs (TL494 was made in the
previous millenium ;-) ):
* rise time: 200nS
* fall time: 100nS

-

Suggestion:

Use IRF510.

You have to invert/negate the output-signal.

Use E2, C2 as emitter-follower.

-

Crude calculation of pullup and pulldown resistors:

Needed pullup resistor at:

t=r*c (70% charge time 8Vgs)
<=>
r=t/c

c is Cgs (actually Cgd should also be included)
r is resistor.
t is time

t=100nS
c=180pF

r ca.= 560 ohm

-

Rpull-down (between gate and ground) e.g. 3*r 1500 ohm (discharge min.
300nS).

-

Maximum voltage by voltage divider 560 and 1500 ohm (ideal E2,C2):

1500/(560+1500)*12V= 8.7V

min. 5.5Vgs (<70%*8.7V) required, so should be ok.

-

br,

Glenn

 

[Glenn]

Instead of TL494 you could try UCC3806, but it is more expensive:

http://www.ti.com/product/ucc3806

UCC3806N DIL:
http://www.digikey.com/product-detail/en/UCC3806N/296-11475-5-ND/381939

UCC3806 has CMOS output buffers that can sink and pullup to 0.5A to the
external power MOSFET som very fast on->off and off->on transition times
can be realized.

Glenn

 

[mike]

On 06/12/2012 12:24 AM, bitrex wrote:
On 6/11/2012 5:27 PM, Bernhard Kuemel wrote:

duty cycle, no matter how long that is [1]. The LED current drops and
remains at about 0.6 mA. I want 20 mA. [...]

What is the switching frequency and required output current?

From http://www.bksys.at/bernhard/img/LED-SMPS/P1080835.JPG I get 1493
Hz.

Required output current is 20 mA.

There are certain "tricks of the trade" - one can save you the bother of
setting up a switcher altogether.

In the late 80's - early 90's Thorn Consumer Electronics in their hybrid
mono TVs, replaced the heater chain dropper resistor with a "wattless
dropper" a capacitor used in series with 240VRMS 50Hz, having a high Xc in
relation to Rl gives the illusion of constant current - for that application
they used 4.3uf to feed a 300mA heater chain, you can scale this for your
LED current, but you must include a sufficient series resistance to absorb
surges.

I can understand how this works for tubes. I've been unsuccessful with
LED's. Surges can be huge and LED's have a very short time constant to
destruction.
By the time I got the resistor big enough to protect 'em, might as well have
left out the cap.

You can use 2 chains of LEDs to conduct both half-cycles, or use a small
bridge rectifier.

One of these is required to make the cap work.

 

[Bernhard Kuemel]

Hi Bernhard

Please do not play with 230V - it is lethal. Also for experienced
designers!

I always try to avoid those nasty shocks, but occasionally I get one.
Probably because I'm not as careful as when trying to make a jacobs
ladder with my MOT .

When I decrease the voltage at error amplifier 1 (EA1) with the 150K
pot, the duty cycle increases which at first lets the current rise
higher in the flyback primary and produces more LED current. But at some
point the primary current drops sharply as if the transistor were shut
off, but stops at 240 mA and then settles at 300 mA for the rest of the
duty cycle, no matter how long that is [1]. The LED current drops and
remains at about 0.6 mA. I want 20 mA.

Look in the datasheet - bipolar transistors are to some extent awful
compared to Power MOSFETs.

I consider using a MOSFET but atm I use bipolars because IMHO they're
more simple to drive from the TL494 output.

If you have 50% duty-cycle the on-time average should be
(Ton+Toff)/Ton*42mA = 84mA

If the Ic=Itransformer ramp up linearly you should have a fine
"triangle"/sawtooth current, and the start current of 0mA and peak
current of 2*84mA=168mA.

I get that value, too, with my equations. However, I only have a
sawtooth up to a certain point. Further increases in duty cycle ... see
my OP and

http://www.bksys.at/bernhard/img/LED-SMPS/P1080833.JPG
http://www.bksys.at/bernhard/img/LED-SMPS/P1080839.MOV

That's the voltage across a 1 ohm sense resistor in series with the
flyback primary.

That's not a matter of base charge, IMO.

But maybe this problem is current related and won't show up once I
switch to 325V, 100-200 kHz and 8 mA peak current. Maybe I should just
try. Then I will have to deal with turn on/off time.

But you supply only 12mA during on-time - and no -Ib! So your on->off
transition-time will be pretty slow.

This means that your Ic will "slowly" decrease - and your Vce will
slowly increase during on->off transition.

I get a relatively sharp decrease which levels at about 300 mA during
the *on time*.

I tried:

12V - 470 ohm - C2 - E2 - base - 100 ohm - ground

same problem.

With ...

12V - 1K + base - emitter - ground
|
C2 - E2 - ground

.... the transistor is off when Q2 is on, so the duty cycle is reversed.
With the pot all the way down I get from the minimum dead time a minimum
transistor on time. The LEDs get about 1 mA (estimate from the
brightness) and slowly fade out completely - without modifying the duty
cycle. Meanwhile the average primary current rises to 130 mA when the
LEDs are out and keeps rising until I disconnect power when the
transistor get's hot to the touch. I haven't been to the lab to watch
this on the scope.

Bernhard

 

[[email protected]]

I can understand how this works for tubes. I've been unsuccessful with
LED's. Surges can be huge and LED's have a very short time constant to
destruction.
By the time I got the resistor big enough to protect 'em, might as well have
left out the cap.

Have you tried a capacitive voltage divider and connect the bridge,
surge protection resistor and LED chain to the midpoint of the
divider.

Of course, there should be quite a few LEDs in the chain (10-20) to
keep the parallel capacitor at a reasonable size.

I chain of big LEDs (0.5-3 A) could be driven through an choke with
reasonable inductances, e.g. through fluorescent ballasts (at least in
the 230 V world).

 

[Spehro Pefhany]

My $7 dealextreme Li-ion 18650 2-cell charger is not potted.
it's also very lightweight,so there's no XFMR in it(I haven't gone into
it...) and the chip and circuit board can probably be accessed and modded.
(unless it's chip-on-board!)

it runs off 100-240VAC.

There is almost without a doubt a HF transformer in there, just not a
mains-frequency transformer.

If it's this one:
http://www.dealextreme.com/p/18650-digital-battery-charger-3499?item=28

... it's indicated to be Class II (double-insulated).

 

[Glenn]

Hi Bernhard

Please do not play with 230V - it is lethal. Also for experienced
designers!

I always try to avoid those nasty shocks, but occasionally I get one.
Probably because I'm not as careful as when trying to make a jacobs
ladder with my MOT .

When I decrease the voltage at error amplifier 1 (EA1) with the 150K
pot, the duty cycle increases which at first lets the current rise
higher in the flyback primary and produces more LED current. But at some
point the primary current drops sharply as if the transistor were shut
off, but stops at 240 mA and then settles at 300 mA for the rest of the
duty cycle, no matter how long that is [1]. The LED current drops and
remains at about 0.6 mA. I want 20 mA.

Look in the datasheet - bipolar transistors are to some extent awful
compared to Power MOSFETs.

I consider using a MOSFET but atm I use bipolars because IMHO they're
more simple to drive from the TL494 output.

If you have 50% duty-cycle the on-time average should be
(Ton+Toff)/Ton*42mA = 84mA

If the Ic=Itransformer ramp up linearly you should have a fine
"triangle"/sawtooth current, and the start current of 0mA and peak
current of 2*84mA=168mA.

I get that value, too, with my equations. However, I only have a
sawtooth up to a certain point. Further increases in duty cycle ... see
my OP and

Is it 10uS/division? If it is, it is slow:

http://www.bksys.at/bernhard/img/LED-SMPS/P1080833.JPG
http://www.bksys.at/bernhard/img/LED-SMPS/P1080839.MOV

That's the voltage across a 1 ohm sense resistor in series with the
flyback primary.

That's not a matter of base charge, IMO.

But maybe this problem is current related and won't show up once I
switch to 325V, 100-200 kHz and 8 mA peak current. Maybe I should just
try. Then I will have to deal with turn on/off time.


I get a relatively sharp decrease which levels at about 300 mA during
the *on time*.

That wont work because C2, E2 is not a negation of C1, E1 - sorry.

I tried:

12V - 470 ohm - C2 - E2 - base - 100 ohm - ground

same problem.

With ...

12V - 1K + base - emitter - ground
|
C2 - E2 - ground

.... the transistor is off when Q2 is on, so the duty cycle is reversed.
With the pot all the way down I get from the minimum dead time a minimum
transistor on time. The LEDs get about 1 mA (estimate from the
brightness) and slowly fade out completely - without modifying the duty
cycle. Meanwhile the average primary current rises to 130 mA when the
LEDs are out and keeps rising until I disconnect power when the
transistor get's hot to the touch. I haven't been to the lab to watch
this on the scope.

Bernhard

Glenn

 

[Glenn]

Glenn

The turn off seems to be 2uS?

If it is 10uS/div, you turn the transistor on for 60uS?

Are you sure that you have phased the transformer for flyback?:
http://en.wikipedia.org/wiki/Flyback_converter

Glenn

 

[Jasen Betts]

I can understand how this works for tubes. I've been unsuccessful with
LED's. Surges can be huge and LED's have a very short time constant to
destruction.
By the time I got the resistor big enough to protect 'em, might as well have
left out the cap.

put a large low ESR capacitor in parallell with the LEDS

it's effectivly a capacitive divider, so if you're dropping 240V
to 7V make the parallel cap 240/7 times the size of the series cap.
then the series resistor only needs to be sized to protect the
rectifier