Electronics > Beginners

MC34063 high voltage dc-dc boost converter

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dazz:

--- Quote from: Siwastaja on January 26, 2020, 03:59:30 pm ---Get an oscillosscope, sorry but you just can't do switch mode converters without one. A cheap Rigol will do fine.

--- End quote ---

Yeah, I know, I'll see what I can find. Thanks Siwastaja

BTW, guys, I sort of got it working. Sort of.
I replaced de 1N5817 diode with a proper rectifier diode and it's now outputting hundreds of volts. How many? Not sure because the hysteresis is insane! With a 100uH toroidal inductor, and the pot set to the minimum output voltage that should produce some 100V, it jumps to 320V, Vref to 3.5V, then the IC apparently turns off and the voltage drops to 100V, Vref to 1.2V, and back again to the massive spike.

I then replaced the inductor with a 470uH one, and as a result, I get 460V at the output and 5V at Vref. It stays like that, as if the IC never stops switching. Had to stop the test because the output cap is rated at only 450V. Also the inductor gets scorching hot, which I believe is to be expected if it's conducting all the time.

MagicSmoker:
Put a load on the output. Running a boost (or flyback) converter without a load is just asking for trouble.

And yeah, get a scope. Heck, even one of those janky little DSO Nano's is better than nothing.

dazz:

--- Quote from: MagicSmoker on January 27, 2020, 05:14:15 pm ---Put a load on the output. Running a boost (or flyback) converter without a load is just asking for trouble.

And yeah, get a scope. Heck, even one of those janky little DSO Nano's is better than nothing.

--- End quote ---

Oh, I thought it was only flybacks that needed a load to be stable. Right now I have a tiny 680K load, that's less than 1mA, will try that, thanks.

And I'm already shopping around for a scope  :-+

MagicSmoker:

--- Quote from: dazz on January 27, 2020, 05:19:54 pm ---Oh, I thought it was only flybacks that needed a load to be stable. ...

--- End quote ---

Basically, any switchmode converter in which power is supplied to the load during the switch off time is going to be unstable without a load, and the boost converter does just that, same as the flyback.

dazz:

--- Quote from: MagicSmoker on January 27, 2020, 06:22:33 pm ---
--- Quote from: dazz on January 27, 2020, 05:19:54 pm ---Oh, I thought it was only flybacks that needed a load to be stable. ...

--- End quote ---

Basically, any switchmode converter in which power is supplied to the load during the switch off time is going to be unstable without a load, and the boost converter does just that, same as the flyback.

--- End quote ---

It's the sudden current spike at the inductor when the magnetic field collapses that does this, right? I noticed the effect got progressively less noticeable as I decreased the inductance of the coil. I'm at 33uH right now.

Building a constant current load with a mosfet now to test it under load. What kind of load current should I expect to make it stable? Because at 300V even a few mA will mean a few watts of wasted power and a pretty large dummy load resistor. I guess I'll find out soon enough  ;D

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