I'm currently experimenting with DC boost with NE555. Simple schematic, like the one here:
(https://i.stack.imgur.com/bWQlb.png)
I wanted to see what is the difference with various frequencies and on/off rates while switching. At first I tested it with R1=33k and R2=1k, C2=100p. It worked well, and the switching frequency was as expected. Frequency 400kHz and duty cycle 97%. But my MOSFET was hot, so I wanted to reduce frequency and duty cycle, and put R1=2k and R2=68k, leaving C2 the same. That should give me 104kHz with 50% duty cycle, but I get some 2Mz(!!!) on my scope and the damn thing isn't working - output voltage is just 9V. It happened last night, and I went to sleep. Now turned it on, everything works as expected, MOSFET is cool, and output voltage just as it should be, with 104kHz frequency on TLC555 output.
WTF???
Can't see neither any Gate drive resistor nor turn-off accelaration configuration (resistor in series with diode).. :icon_wink:
Heatsink ??
Actualy, my schematic is a little bit different, I just didn't have time to make picture on previous post, so used one similar from the web. This is my circuit:
(https://i.postimg.cc/RNpkpGvc/NE555-boost.png) (https://postimg.cc/RNpkpGvc)
I suppose this is what you have in mind with acceleration (R20, D9, D36 and BC327), or not?
Maybe I have a cold joint somewhere, and moving the PCB on my desk just made better contact, which could explain late night's behavior. Will have to check...
Quote from: Phend on January 20, 2022, 11:00:01 AM
Heatsink ??
I tried without heatsink, because I need only 25-27V in this case. I tried with IRF740 for the first time (which was hot, but most probably due to too high duty cycle), then used IRF3205 - it can handle the voltage I need and Rds(on) is 0.008Ohm, which is negligible compared to IRF740's 0.55Ohm. Now it's running for an hour and a half, voltage is stable and MOSFET is completely cool. No heatsink. I will also try to run this using 2N7000, because it can handle the needed power and using TO-220 can be bulky in a pedal.
Quote from: rankot on January 20, 2022, 11:00:57 AM
I suppose this is what you have in mind with acceleration (R20, D9, D36 and BC327), or not?
Yeapp.. :icon_wink:
(+1 for improved version..)
Quote from: rankot on January 20, 2022, 11:05:19 AM
I will also try to run this using 2N7000, because it can handle the needed power and using TO-220 can be bulky in a pedal.
I wouldn't trust a TO-92 package inside a closed box..
IRF3205 (TO-220AB) has R
θJA about 60
oC/W where 2N7000 (TO-92) has 312
oC/W..
The 2N7000 version might take more work.
The funny thing is the 2N7000 is a smaller device and it might turn out increasing the frequency helps. If the inductor is too small and/or the frequency too low the inductor current could ramp up and peak to a value not favorable to the 2N7000.
Measuring the DC input current with a fixed load is a good way judge how much loss the circuit has and that generally means the components are less stressed.
There's also a point where charge-pump circuits make more sense.
QuoteI tried with IRF740 for the first time (which was hot, but most probably due to too high duty cycle), then used IRF3205 - it can handle the voltage I need and Rds(on) is 0.008Ohm, which is negligible compared to IRF740's 0.55Ohm. Now it's running for an hour and a half, voltage is stable and MOSFET is completely cool.
Compare that to say 1 to 2 ohms (5 ohms max) for the 2N7000 and you can see how things change when you have high peak currents.