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4hv.org :: Forums :: General Science and Electronics
<< Previous thread | Next thread >>   

sg3525 manual duty cycle adjust

Author Post
kell
Tue Feb 03 2009, 08:32PM Print View
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
Someone might find this useful.
I worked out a good way to set the duty cycle on the SG3525 manually.
You certainly won't see this in the datasheet.
It tested well at 50 kHz and at several hundred kHz.


    Vref
     |
     |
    33k
     |      pin1
     |      -in
     \       |
 10k /       |
     \<--1k--+
     /       |
     |       |
     |      10k
    gnd      |
             |
            comp
            pin9

Tie the noninverting input to Vref.
Interestingly, as you adjust the duty cycle up and down, the voltage across the 10k compensation resistor stays at a constant 1 volt.
With these component values, you can adjust down to about 15% duty cycle.
If you require the ability to adjust all the way down to 0% duty cycle, use 6k2 for the compensation resistor.
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Dr. Dark Current
Tue Feb 03 2009, 10:01PM
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3356
Well you can do it with just a single potentiometer. Set the output of the error amp to low (inverting to vref, noninverting to gnd) and then use ~50k pot from pin 9 to vref to change your duty.

How is this possible? Because the error amp only uses single pull-up transistor on its output, and there is a 0.1mA current sink pulldown on pin 9, so by varying the resistance, you bring the voltage up and down

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kell
Wed Feb 04 2009, 03:27PM
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

How did you suss out the fact that the error amp has a pullup resistor on its output? The error amp diagram in the datasheet shows what looks like a current mirror there.
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Dr. Dark Current
Wed Feb 04 2009, 09:53PM
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3356
kell wrote ...


How did you suss out the fact that the error amp has a pullup resistor on its output? The error amp diagram in the datasheet shows what looks like a current mirror there.

The transistor on the output of the error amp has a pull-down 100uA current sink/mirror. That's why you can connect a "pullup" pot to it to vary the voltage. I have done that and it works.

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Thomas F
Fri Feb 06 2009, 07:55PM
Registered Member #503
Joined: Sun Nov 19 2006, 06:37PM
Location:
Posts: 47
Hi guys,

Sorry for piling on this thread,but I did not want to start a new one for a similar topic :).
I have this circuit from Dr.Kilovolt , I think.
Can this be used to generate lower frequencies ( about 25~50Hz ) and duty cycles down to about 1% ? Any modifications would be required ?
I would be running the entire setup from a 10V supply.

Cheers
Thomas


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Dr. Dark Current
Fri Feb 06 2009, 08:47PM
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3356
Probably yes, you would need to increase the tuning resistor value as well as timing capacitor. Duty cycle of 1% should be theoretically possible, but I think you will have a really hard time setting that value precisely. Maybe you could connect a second, much smaller pot in series with the big one for precise control. There's also the problem of pulse width drift with temperature, but I think it shouldn't be that bad.

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Thomas F
Sat Feb 07 2009, 05:54AM
Registered Member #503
Joined: Sun Nov 19 2006, 06:37PM
Location:
Posts: 47
Thanks Dr.KV.
I was actually thinking of going for a multi turn poti,more expensive,but just one control to twiddle.
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stress
Mon Mar 11 2013, 10:26PM
Registered Member #10265
Joined: Mon Feb 25 2013, 08:16PM
Location:
Posts: 8
hello
i need a schematic with sg3525 with 310V DC at output and 18 Hhz, can anyone help me with some schematics?
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klugesmith
Mon Mar 11 2013, 11:41PM
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1165
stress wrote ...
hello
i need a schematic with sg3525 with 310V DC at output and 18 Hhz, can anyone help me with some schematics?

Welcome.
Please fix or clarify your output frequency.
Unit multiplier symbols in SI don't include H.
Hempohertz?

Have you spent more than a minute trying the forum search tools here?

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Ben Solon
Tue Mar 12 2013, 12:22AM
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location: Holliston M.A., United States
Posts: 598
Addressing the original topic, that's what the discharge resistor in the timing portion of the circuit. 0ohms is zero dead time, more resistance adds more dead time, which for all intents and purposes is duty cycle. One pot.
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stress
Tue Mar 12 2013, 12:35AM
Registered Member #10265
Joined: Mon Feb 25 2013, 08:16PM
Location:
Posts: 8
if i have 390v DC with this sg3525 how do i stabilize it to 310v DC ?
that is my problem... any schematics?
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