power electronics,make a flyback converter

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I've tried using ic sg 3524 but I get the output signal does not square, what causes it?

please help..
best wishes...:-(
 

Can we see the circuit you are using for?
 

i'll post the circuit soon.


the output sinyal from IC pwm 3524 that i expected to switch the transistor.

i want to make a charger battrey from flyback converter,using ferrit transformator..

please help..
best wishes.

---------- Post added at 11:24 ---------- Previous post was at 10:12 ----------

http://circuits.datasheetdir.com/66/SG3524-circuits.jpg
 

In a fisrt order, the output waveform is a square voltage. Depending on the connected load, it may look different. To drive the high gate capacitance of a MOSFET, you would want e.g. to have a push-pull driver.
 

The output has to be a square wave waveform. It is, as suggested by FvM, it's due to high gate capacitance of MOSFET. You should note that SG3524, unlike the SG3525 and other such drivers, does not have totem-pole drivers at the output. You're attempting to drive a MOSFET at a high frequency, but due to the gate capacitance and the high resistance in the way of charging that capacitance (the 2k pull up resistor), the wave form "changes" or distorts". It probably something that curves upwards and reaches a peak and stays there, ie, it has a high rise time to maximum voltage. Use a totem-pole or push-pull driver at the output that can charge the MOSFET gate capacitance quickly, thus not distorting the output waveform. A simple totem-pole would suffice.

Hope this helps.
Tahmid.
 

I use pwm ic sg 3524, to determine the frequency that we need to use as a pre-determined formula, I've tried it but did not square the output waveform.

maybe you guys know my stok will measure the output waveform of the IC sg3524 using an oscilloscope

please help..
 

Show us your schematics with component values.You cant get exact 50% duty cycle from sg3524. See datasheet.
 

the formula to get the frekuensi, F= 1.3/rt.ct ( from datasheet)


i want get 100 kHz using sg 3524...so i take the r=2,6k adn c=5nf,
but,when i measure with osciloscope i get 75 khz, and the output wave is not square..
why is that??

 

I'm looking at the ST datasheet which says that the formula is F= 1.18/(RtCt)

Using your values, I get 90kHz frequency.

The error is likely due to resistor and capacitor tolerance.

If the capacitor and resistor both have 10% tolerance, Rt may be 2.6*1.1=2.86kohm and Ct may be 5*1.1=5.5nF.

Using the formula F=1.18/(RtCt) and using Rt = 2.86kohm and Ct = 5.5nF gives frequency F = 75.0kHz. That is exactly what you got. Vary the resistance a little to adjust, or use more precise resistors, or use a pot to alter the resistance and "tune" it.

As for the waveform, this has already been mentioned before. But, you should post the waveform pictures. That would help clarify what you are seeing and help us identify the problem.

Hope this helps.
Tahmid.
 


Hi,

I have some doubts. Since the MOSFET have high gate-capacitance, how come by adding a totem pole amplifier can decrease it and let the signal pass through with least distortion?
 

The high resistance pull-up charges the gate capacitance slowly. Think about the wave shape when charging a capacitor with a 2k resistor and when charging when a 10 ohm resistor. The 10 ohm resistor charges quicker, much quicker, and so, the rise time is very small as compared to the 2k resistor which limits the current, thus making the capacitor take longer to charge. The totem-pole driver charges the MOSFET gate almost instantaneously by charging it with greater current and providing minimal resistance.

Hope this helps.
Tahmid.

---------- Post added at 22:28 ---------- Previous post was at 22:25 ----------

Google around for this and you will find many documents and articles that will describe this in more detail.
 

Feeding dc bias at pin#2 will further reduce on time( PWM control through error amplifier).
 

how to determane the transisitor for the ferrit tfaro???
 

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