Firstly, thanks for reading my questions and providing *exactly* what I asked for.
voltage at input side of inductor fig1 with 1v/div and 0.1ms/div.
I presume you're using a 10x probe here? So we're seeing about a 3V drop in the battery voltage as the current increases.
if this is a 1x probe, then you have very little to worry about.
I'm guessing it's a 1x probe because other measurements taken later would not make sense if they were done with a 10x probe (so I'll assume 10x only when you say so).
Was it AC coupled here?, if not it looks like your battery was supplying only about 11 to 11.5V at minimum inductor current.
voltage across inductor fig 2 with 10X probe.5v/div and 0.1ms/div. please note that lower levels are not exactly parallel to reference but has some +ve non zero slope indicating large Rds(on).
And 0.1ms/div, really? That's only 400Hz which is really low for a SMPS.
OK, I'm assuming that it's probably AC coupled again. I can see the voltage across the inductor falling slightly as the current increases, and the sharp spike in voltage during the off period.
Inductance is checked with LCR meter and found to 300uH. also inductor slightly heated up during operation. inductor is wound on ferrite core with 18 gauge wire.
"slight heating" is OK.
How is the mosfet and the schottky diode for temperature?
switching off should be quite faster because of gate being pulled down by npn BJT fig 3 upper trace.
Yeah, switch off is fast and that will contribute to lower power dissipation in the mosfet. The switch on time is slower, but that is not of great consequence.
inductor current measured with 0.1ohm resistance in series fig 4 0.1v/div and 0.1ms/div with 0 line 1 div below reference line.
OK, so we see a number of things here. Firstly the inductor current doesn't fall to zero.
By my calculation, the voltage across the 0R1 resistor varies between 0.06V and 0.5V (I think you must be using a 1x probe).
The second (and important) important thing here is that we don't see the current increasing at an increasing rate at any point, so the inductor isn't nearing saturation.
However it does seem (based on your measurement of Rds(on) or 0R8) like the voltage across the mosfet rises to about 4V during this period, and that is going to be a little wasteful.
It will see a peak current of 500mA, and an average current of about 250 mA during the ON cycle, and let's also say it's an 80% duty cycle. So this means about 0.08W on average being dissipated by the mosfet.
However, since you're only driving a 500 ohm load (that should draw no more than about 50mA), I'm concerned about why the current is so high. I would be expecting an average current closer to 130mA or thereabouts.
Let's assume 8V across the 300uH inductor for 2ms. The current should rise by about 53 amps. The current is rising by about 1% of that, so I would guess that your frequency is actually about 40,000 Hz
total current measured with 0.1ohm resistance in series with battery fig 5
OK, that's a classic shape, but I need to know the scale and where zero is. In any case, it seems fine. It mimics the current through the inductor, which is that I'd be expecting (I presume there is other circuitry that you've not shown us
)
Also i have checked Rds(on) with ohmmeter and found it to be 0.8ohm
please advice
thanks in advance
Everything seems to be going along quite swimmingly, except the current you're seeing is way higher than I'd expect.
Several things come to mind:
1) are you *really* using a 1N5404 diode? to rectify the output?
2) do you have some other (unshown) feedback path to limit the voltage across the load?
If you are really using a 1N5404 diode, then pull it out and get a suitable rated schottky diode. I would expect that if you placed a current sense resistor in series with the1N5404, you would see significant reverse current flowing. This may both increase the input current required, AND increase the current flowing through your mosfet.
If I were to suggest anything, it would be to replace this diode.
You show no feedback to maintain the output voltage at 24V I assume there must be something, otherwise the output voltage will rise significantly, often until something (mosfet, diode, or capacitor) decides to start conducting current when it's not supposed to.