This is the big evil, the LC filter for the buck converter. I had really hoped to stay away from assembling a full blown set of drivers to test the filter, but the freq gen I have cant drive the power fet directly. arg.
I assembled the 5V driver I'd worked out before, with a 50% 250Khz source from the frequency gen and started to test, it didn't take long to work out that the 5V drive to the power fet will not work out. And so came a new problem, new power fet driver.
I tried LOTS of fet driver designs, and the result was halarious.
There are two parts to the problem, the pwm needs to be taken from 5V levels to 12V levels, then current amplified to drive the fet at 250Khz, with desired switching times of 0.1us, this isn't easy.
The origional driver I designed just drove the fet at 5V, so I didn't have to worry about stepping up the voltage. My first circuits to step up the votlage were transistor based first bipolar and then fet (2N7000), niether of those yeilded good results (lots of ringing and skew that would not go away.)
So I tried an LM393 for stepping up the votlage, with a bipolar totem for driving the problem was that the bipolar totem would ring badly,
So then I tried a small fet totem pole, which I ran into a new problem, the lm393 can only drive 20mA, and that dosn't work with the 300pf input capacitance of them (AOP605)
So I stood back and thought,
click* a 555 is a 12V capable latched window coparitor with a totem pole output driver thats rated at... 300mA! (the 300pF needs about 30mA to get 0.1us switching across 10V)
However, as it turns out, the output is sufficient to drive the main fet directly, I dont even need an extra current driver.
Pin 8 to +12V, pin 1 to ground,
Tie 2,6 togethor, thats the 5V logic level input.
Tie pin 5 to ground via a 1k resistor, this brings the threshold of the inputs to 5V levels.
pin 4 (reset) gets tied high (pin 8)
pin 3 is output to fet. (I use a series 10-100 ohms resistor)
Its too bad the 555 cant be used to generate the 0-100% duty pwm.
oh yea the fitler. The design looks good, the first stage filters the noise to 0.6V, the second stage takes out everything else.
There are still things that could be improved, for sure if I just dialed the converter freq down to 40Khz or so, I'd have a great circuit. or even 80k, even 160k....
This is how I stay up till 4:30am
Today I worked on it a but more,
There is an almost inpercievable difference between the 555 output running the power fet directly and inserting a bipolar totem pole driver. I was able to improve the switching by inserting a diode across the gate resistor to hurry up the turnoff time, and I was able to quench some of the ringing the power fet generates with a ferrite bead on the Source pin.
Turnon is less than 0.1us and turnoff is about 1.5us
So, while trying to test the filter, I realized a failure point, I put one of the inductor cores I'm planning to use and ran 5A thru it continious, it got WAY too hot. At the same time, I'v worked out that I dont need anything close to the 2uH I was planning on using,, this means I can dial down the turns on the inductor and bring down the resistance, which will help. I'll work this out more with a working pwm genorator.