Lets slowly....So, I made this circuit:






It is working but not has I want.


Now I can thrigger the FET with 5V, but let me show something:
This is the input of 5V and the output with 1Khz:






Hum.. It is fine, hun? Ok, let me show now with a 10Khz 5V input:






Still working...


But now with 50Khz, something strange starting to occur:






Here you can see better:






And finally with 400 or 300Khz:





The problem is that the mosfet is still conducting even when I put 0V on the gate. This is taking some time to "discharge" the energy on the gate.


I ploted some cicles of the input to see the voltage on the gate:

5V on the lower transistor and 300Khz:

http://img.photobucket.com/albums/v222/ahhh/Eletronica/vg.jpg

Why? This is causing my problems now.

The project that I using is this one:


http://www.bineco.dk/ellert/smps.htm


The project is not in english, but I can see the circuti and interpret it...

Hope some help.

Thanks again.

 

You are using a TIP122 for Q2 and a TIP127 for Q3, which are far too slow for this application. The author even said that about the TIP121 and TIP126 on his site; if you use Google Translate to go from Danish to English you will see that.
http://translate.google.com/transla...&tl=en&u=http://www.bineco.dk/ellert/smps.htm

Those TIP122 and TIP127 transistors are pretty much out of gain by 1MHz. Non-Darlington transistors might have a gbw of 300MHz or more; your mileage will vary.

If you want to be serious about this circuit, you will find a much better MOSFET driver.

Using a P-channel MOSFET might seem more convenient, but P-channel gates are ~2.5x as large as equivalent N-ch MOSFETs, thus requiring far more current to charge/discharge the gate as quickly as a comparable N-ch.

 

Thanks for your reply.

I changed the Transistores to a 2n2222 and to 2n2907.

But the result was the same.

Do you have any driver made with discrete components?

Consider that I need to trigger with 5V because I will use a PIC on my simple buck circuit.

bye

 

The regulator the Danish fellow was using had an internal oscillator of 52kHz fixed.

I made some changes to the circuit. Plug them into your simulator and experiment with it. I have it running at 50kHz with a 25% duty cycle.

 

Thanks for your help.

I took your circuit and simplified it to just to test. The ideia is the same:

http://img.photobucket.com/albums/v222/ahhh/Eletronica/buck-1.jpg

So, output with 10% duty and 15Khz:

http://img.photobucket.com/albums/v222/ahhh/Eletronica/out_ok_10_duty.jpg

So, output with 50% duty and 15Khz:

http://img.photobucket.com/albums/v222/ahhh/Eletronica/out_ok_50_duty.jpg

So, output with 80% duty and 15Khz:

http://img.photobucket.com/albums/v222/ahhh/Eletronica/out_ok_80_duty.jpg

Let see the ripple:

http://img.photobucket.com/albums/v222/ahhh/Eletronica/out_ok_50_duty_10m_30m.jpg

High? No, it is not:

http://img.photobucket.com/albums/v222/ahhh/Eletronica/out_ok_50_duty_29m_30m_ripple.jpg

Only 16mV.

So now, lets connect the PIC and try to read the output voltage with a resistor divider and adjust the duty to obtain the Vref of 9V:

Sweet:

http://img.photobucket.com/albums/v222/ahhh/Eletronica/vout_50khz_com_pic.jpg

This last test was with a 50Khz pwm into the lower transistor.

Well, it looks like that is working! Hun?

Coments?