need pwm controller

[Yzordderrex]

I'm looking for a very simple pwm controller. I would prefer just 8
pins and an absolute minimum of functions. Maybe soft start and cycle
by cycle Ilimit. I don't even need an error amp. I actually thought
about doing discreet, and may end up that way.

Application is for a forward off line controller to be used as a
modulator for a rf deck. 375w carrier 1500wpep. Voltage mode control
0-100vdc output.

I looked at the TI UCC35701 part, but looks like I will have some
difficulty dealing with their fb scheme.

regards,
Bob

 

[Pooh Bear]

I'm looking for a very simple pwm controller. I would prefer just 8
pins and an absolute minimum of functions. Maybe soft start and cycle
by cycle Ilimit. I don't even need an error amp. I actually thought
about doing discreet, and may end up that way.

Application is for a forward off line controller to be used as a
modulator for a rf deck. 375w carrier 1500wpep. Voltage mode control
0-100vdc output.

I looked at the TI UCC35701 part, but looks like I will have some
difficulty dealing with their fb scheme.

How do you plan to implement *voltage mode control* with no error amp ?


Graham

 

[Winfield Hill]

Pooh Bear (Graham) wrote...

How do you plan to implement *voltage mode control* with no error amp ?

The ucc35701 is a nice chip. I imagine he's surprised by its
"voltage feedforward" capability and uncertain what to do with
it. He could ignore it by setting it to a fixed value, but it's
too valuable a capability to ignore. The Vff voltage sets Irt,
and is used to instantaneously set the Ct capacitor charge rate,
thereby proportionally setting the pulse width without using the
feedback loop, which is useful for offline applications (in the
longer term it has no effect, since Vrt/Irt cancel each other).
A little power-line sag or dip comes along, no problem. Note,
older-style non-PFC smps are especially susceptible to the line-
dip issue, because they work off the very peak-ety-peak voltage,
and respond to any lack of pure power-line sine-wave integrity.

While Yzordderrex could use the rf output as a voltage-feedback
term, he may be wise to add a driver output voltage-limit path.
If he wants more applications info, he can check out Unitrode's
lower-frequency ucc3570 predecessor.

 

[Yzordderrex]

Thanks for replies.

I would use an outboard error amp. Hell, I would settle for a nice
triangle generator in 8 pins.

As far as the feedforward pin goes I know what to do with it, and give
it the respect it deserves. There is a group of... well, here, A
thread I am involved in on another board. I am N9NEO. Guys there
build these modulators that are totally open loop and require massive
capacitor banks in order to get the 120Hz hum out of carrier. They DO
NOT use the feedforward pin. I have already made the suggestion they
use that pin.

http://classe.monkeypuppet.com/viewtopic.php?t=443


Ok, so maybe I will take another look at the 35701 or 3570. I have
already built a full bridge modulator that does full legal power and
fits into a small cigar box. This forward switching converter is going
to feed into an analog output section. Just keep a few volts headroom
above the bipolar to run it. I plan to run the bipolar output section
with enough bandwith to get rid of the 140kHz switching noise from the
forward converter.

regards,
Bob

 

[Genome]

I'm looking for a very simple pwm controller. I would prefer just 8
pins and an absolute minimum of functions. Maybe soft start and cycle
by cycle Ilimit. I don't even need an error amp. I actually thought
about doing discreet, and may end up that way.

Application is for a forward off line controller to be used as a
modulator for a rf deck. 375w carrier 1500wpep. Voltage mode control
0-100vdc output.

I looked at the TI UCC35701 part, but looks like I will have some
difficulty dealing with their fb scheme.

regards,
Bob

Hmmmm I once had a troll for 8 pin voltage mode off-line controllers. They
are few and far between..... like I found two. One was the TI part you
mention.... but it needs an external driver.

I was going to suggest the Fairchild FAN7556 but when I went to their
website I couldn't find it no more. Bastards.

DNA

 

[John Woodgate]

I read in sci.electronics.design that Yzordderrex

I am N9NEO.

Good name for a non-humanoid autonomous robot.

Guys there build these modulators that are totally open loop and
require massive capacitor banks in order to get the 120Hz hum out of
carrier.

How does this hum problem arise? Keeping hum out of audio amplifiers
(which I suppose is what the modulator is, in a sense) isn't rocket
science, at any power level.

 

[Yzordderrex]

John,

No Hum, but they pay big money to eliminate it. The hum would be
noticable if there were 120Hz on the bus caps.

They run a big 60Hz transformer into an even bigger bank of caps. Some
nice optos to drive the transistors fast in order to try and keep it
all linear. Big butterworth filters on the output to get rid of the
85kHz switching garbage. Nice big signals with Hi-Fi audio. Can be
found pretty much coast to coast near 3880-3990kHz

I suspect they just like to build these big transmitters reminescent
of the old boatanchor days. They are a very nice bunch, but a little
set in their ways. I am going to try and give them a modulator which
isn't so far removed from the present technology.

regards,
Bob

 

[John Woodgate]

I read in sci.electronics.design that Yzordderrex

No Hum, but they pay big money to eliminate it.

Have they ever heard of a smoothing choke? Or maybe even better, a
choke-input filter?

The hum would be noticable if there were 120Hz on the bus caps.

Low PSRR in the output stage? that can be fixed.

They run a big 60Hz transformer into an even bigger bank of caps. Some
nice optos to drive the transistors fast in order to try and keep it
all linear. Big butterworth filters on the output to get rid of the
85kHz switching garbage. Nice big signals with Hi-Fi audio. Can be
found pretty much coast to coast near 3880-3990kHz

I'm in UK and I don't have an HF receiver any more.

 

[colin]

I'm looking for a very simple pwm controller. I would prefer just 8
pins and an absolute minimum of functions. Maybe soft start and cycle
by cycle Ilimit. I don't even need an error amp. I actually thought
about doing discreet, and may end up that way.

Application is for a forward off line controller to be used as a
modulator for a rf deck. 375w carrier 1500wpep. Voltage mode control
0-100vdc output.

I looked at the TI UCC35701 part, but looks like I will have some
difficulty dealing with their fb scheme.

regards,
Bob

the uc3843 is 8 pin curent mode controler but its easy to run it in voltage
mode, you just feed the triangle waveform from the oscillator into the
curent sense pin, its shown how in the datasheet, although if u want curent
limit too you would have to combine the two signals.

Colin =^.^=

 

[colin]

I'm looking for a very simple pwm controller. I would prefer just 8
pins and an absolute minimum of functions. Maybe soft start and cycle
by cycle Ilimit. I don't even need an error amp. I actually thought
about doing discreet, and may end up that way.

Application is for a forward off line controller to be used as a
modulator for a rf deck. 375w carrier 1500wpep. Voltage mode control
0-100vdc output.

I looked at the TI UCC35701 part, but looks like I will have some
difficulty dealing with their fb scheme.

regards,
Bob

just wondered why you would be using forward conversion rather than flyback,
is it posible the power output of the transmiter can be controled with
current
as efectivly as with voltage ?

current mode control would provide a hum free output as it is not influenced
by line voltage.

obviously you would need to control the maximum output voltage, but this
would not be dificult.

Colin =^.^=

 

[Yzordderrex]

Colin,

Going to use forward mode.

Flyback mode trying to get to 1500w pep would be unmanagable. Would
need to store too much energy. Forward mode is probably a leap as
well. I've already done it the right way (bridge) now I feel this
perverse need to do it again. (It's actually the audience that I'm
playing to.)

This is not just a power supply. This converter has to control the
voltage at an audio rate. Need gain well beyond 5kHz - It's a
modulator.

regards,
Bob

 

[colin]

Colin,

Going to use forward mode.

Flyback mode trying to get to 1500w pep would be unmanagable. Would
need to store too much energy. Forward mode is probably a leap as
well. I've already done it the right way (bridge) now I feel this
perverse need to do it again. (It's actually the audience that I'm
playing to.)

This is not just a power supply. This converter has to control the
voltage at an audio rate. Need gain well beyond 5kHz - It's a
modulator.

yes i apreciated its a modulator, assuming the transmiter apears resistive
modulating the current would efectivly modulate the voltage, so you could
feed the input directly into the curent comparator. (with some limit on the
peak voltage)

i agree 1500w probably is rather a lot for flyback, would mean a large
transformer, although im not sure how feasable it would be to have a
seperate inductor for energy storage in paralell with an ordinary
transformer.

you can use a bridge with flyback too, just the use diodes in place of
diagonaly oposite switches.

It was just an alternative idea, just for thought, as I read your link out
of interest and noticed the need for very large capacitors to get rid of
hum, but output power with curent mode control is unafected by line voltage
so does away with the need for large caps.

Colin =^.^=

 

[Ken Smith]

I'm looking for a very simple pwm controller. I would prefer just 8
pins and an absolute minimum of functions. Maybe soft start and cycle
by cycle Ilimit. I don't even need an error amp. I actually thought
about doing discreet, and may end up that way.

Application is for a forward off line controller to be used as a
modulator for a rf deck. 375w carrier 1500wpep. Voltage mode control
0-100vdc output.

The LT1246 can be made to do a forward converter. The datasheet examples
are done as a flyback but the basic PWM action is not dependant on the
type of converter you are doing.

The "slope compensation" circuit can be copied to make a circuit with
normal (non-current mode) operation if you want.

Using an extra part or two you can make it into a "constant off time" or
"constant dead time" controller if you want.

It works ok up to about 750KHz and can drive the gate of a largish MOSFET.


Doing 1500W in any single ended topology converter is likely to be
trouble. You may want to consider a multiphase design to reduce the size
of the inductive elements.

 

[Ken Smith]

yes i apreciated its a modulator, assuming the transmiter apears resistive
modulating the current would efectivly modulate the voltage, so you could
feed the input directly into the curent comparator. (with some limit on the
peak voltage)

Yes, in designs like this, the supply input impedance of the output
section is nearly resistive. Its value determined mostly by the load
impedance connected to the output stage.

For this reason the OP will want to feedback both the voltage and current
to the modulator section. This way the system can be made well behaved
for modest mismatches and self defending for larger ones.

i agree 1500w probably is rather a lot for flyback, would mean a large
transformer, although im not sure how feasable it would be to have a
seperate inductor for energy storage in paralell with an ordinary
transformer.

Breaking the inductor and transformer action apart makes for more troubles
not less. Making 1500W out of, lets say 5 300W switchers would be what I
would suggest if I have to do this with flyback designs

 

[Yzordderrex]

Thanks for the input Ken (and others)

Yes, the LT part could probably do a good job. I like the idea of
constant off-time. I did some thumbnail simulations and could probably
get the thing to soft switch over most of the modulation envelope.
Looks like I would have problems at the bottom of the envelope at the
low currents. Probably not a big deal down there anyway.

Multiphase might be trouble coordinating without current mode control,
and probably harder to keep resonant. I might like the challenge of
the 1500w peak with the one converter. No fun doing things the easy
way. Heheh.

thanks
Bob