sg3525 manual duty cycle adjust

kell, Tue Feb 03 2009, 08:32PM

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.
Re: sg3525 manual duty cycle adjust
Dr. Dark Current, Tue Feb 03 2009, 10:01PM

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 smile

Re: sg3525 manual duty cycle adjust
kell, Wed Feb 04 2009, 03:27PM


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.
Re: sg3525 manual duty cycle adjust
Dr. Dark Current, Wed Feb 04 2009, 09:53PM

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.

Re: sg3525 manual duty cycle adjust
Thomas F, Fri Feb 06 2009, 07:55PM

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

1233950104 503 FT63013 Flydriver1
Re: sg3525 manual duty cycle adjust
Dr. Dark Current, Fri Feb 06 2009, 08:47PM

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.

Re: sg3525 manual duty cycle adjust
Thomas F, Sat Feb 07 2009, 05:54AM

Thanks Dr.KV.
I was actually thinking of going for a multi turn poti,more expensive,but just one control to twiddle.
Re: sg3525 manual duty cycle adjust
stress, Mon Mar 11 2013, 10:26PM

hello
i need a schematic with sg3525 with 310V DC at output and 18 Hhz, can anyone help me with some schematics?
Re: sg3525 manual duty cycle adjust
klugesmith, Mon Mar 11 2013, 11:41PM

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?

Re: sg3525 manual duty cycle adjust
Ben Solon, Tue Mar 12 2013, 12:22AM

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.
Re: sg3525 manual duty cycle adjust
stress, Tue Mar 12 2013, 12:35AM

if i have 390v DC with this sg3525 how do i stabilize it to 310v DC ?
that is my problem... any schematics?