воскресенье, 22 ноября 2015 г.

More volts!

I've got a very interesting hobby project but I need a DC-DC boost converter for it. I plan to use 7.4V Li-Po battery as a power source but I have to boost it up to 80-120V. Let's say 100V. The load is a coil with 1.5 kOmh active resistance.

I dicided to make a boost converter myself. I tried different inductors and frequences but my best result is only 27V. Something's wrong with my boost converter.


Here's the schematic:



I don't have a signal generator, so I use an Arduino Uno with this simple sketch:

void setup() {
  tone(8, 4000);
}

void loop() {
}

I checked the signal on oscilloscope. It's fine  4 kHz square wave.

Here is the output:

I measured this signal on L2 coil and D2 diod. There's a tiny 0.2V fluctuation with 4 kHz frequency. I think that's normal and not dangerous.

But why can I not boost the voltage higher? Is something wrong with my transistor switch? Yes, I think I've found something strange. But I can't explain it. Take a look at the signal on transistor's drain. I tried different frequences: 2kHz, 4kHz and 8kHz. The multimeter near oscilloscope shows the output voltage (on L2 inductor).




It is obvious that on 8kHz and 4kHz when the transistor is opened I've got my true zero volts. But when the transistor is closed I expected 20 and something volts. Well, I'll be happy if it is 100V, but ok, let it be 20 for now. The problem is that during the first quarter of period it has true 20V, but in the second quarter I've got voltage reduction and some high frequance signal.

On 2kHz the signal is more complicated but I think it has the same nature.

What is this high frequency noise? What's going wrong?