PrathikP

modified 6 years ago

300V to 12VDC Flyback SMPS

836

12:42:03

This is one monster project that I spent sooo many hours on and its finally working. I just had to change the startup resistor value from 1M to 100K to make it work. So here it is, a discrete flyback SMPS. Big circuit, hard to explain so here is the working in brief: this circuit consists of a relaxation Oscillator, whose signal is fed to a gate driver. There is a latch at the input of the driver that is triggered if either the switch current limiting is reached or the output voltage limit is reached. The circuit is initially powered through the now 100k startup resistor, after which the auxiliary winding is used for powering the control circuit. The output is just a single diode rectifier and capacitor.

published 6 years ago

hurz

6 years ago

im also working on this/your version to get rid of CFS. lets see if we can at least dramatic reduce this nasty effect. 1M down to 100k i haven't tried. not sure if this helps.

PrathikP

6 years ago

1M to 100K helps. It's what I've done. Ofcourse, I added snubbers to but Changing the startup helped

PrathikP

6 years ago

And I also realised that, due to the high current draw of the control circuit, it would never startup with a 1M resistor.

hurz

6 years ago

diodes are nonlinear components and in case of high current they cuz problems. Tune them to 5nF of for less current just 500pF this help for little longer. 100k to start quicker, idk.

hurz

6 years ago

zener as well. maybe only 500pF

hurz

6 years ago

snubber with 100k in parallel to the cap is to high in resistance need to be lower to quicker burn power

hurz

6 years ago

100Ohm parallel to the opto LED is much to small and avois the LED to glow, while the V2C converter can be lower not so much current per volt.

hurz

6 years ago

680uF at output can be much smaller, it runs with 100kHz and 47uF shall work fine, maybe with 10uH in front as low pass

hurz

6 years ago

The 1nF across the transformer for current alignment from oposite mains wire can be for example much smaller. 30pF.

hurz

6 years ago

give it a try

PrathikP

6 years ago

Thanks, I'll take a look.

PrathikP

6 years ago

Oh and about the led glowing, even a few uA of current makes the opto send current into the latch, a few uA, which is enough to trigger it. That's why the opto led never glows, and non because of the parallel resistance. I've seen small values like 68ohms before. It's also why my output voltage is 5V, even though it should be 5.1V + 1V = 6.1V or thereabouts.

PrathikP

6 years ago

And I chose 680u capacitance to be able to draw higher load current, not just 500mA, but I guess for this specific load I can reduce it.

PrathikP

6 years ago

Changes diode capacitance and the sim doesn't crash.

PrathikP

6 years ago

Startup resistor back to 1M

PrathikP

6 years ago

10u inductor again results in CFS. I'm using 3.3u

hurz

6 years ago

10uH is critical and cause again CFS for the price of better filtering. Sure

hurz

6 years ago

Back to 1M bootstrap resistor, ok, you came with 1M, why not.

hurz

6 years ago

Issue crash caused by diodes/zener, you see a step ahead and you are happy? Fine!

hurz

6 years ago

680uF, sure the bigger the LPF-cap the lower the filter corner frequency. The question is what capacitance you need to serve your switching frequency you target. Yours is not audible for a human above 20kHz but below 100kHz. because of regulating pulse drops its around 50kHz or lower. But this is for a SMPS already quite high and can work with very low caps. tau=R×C for the discharge case 1/[(680u×12Ω)/10]=1.2kHz for a exceptable low ripple. This is factor 10 or even 20 too much capacitance and 22uF should work as well as yours is not running at 1.2kHz its 40 times faster switching.

hurz

6 years ago

Opto, with 100 Ω parallel to low in impedance, the current is bypassed by the resistor and not going to the opto. The LED is only working like a higher current limiter only in case its needed. The resistor must be 100 times higher (10kΩ) to cause some opto current. The LED threshold can be IR realistic set to 1.4V and sure is just cosmetic. 68Ω before means in series, but not in parallel ! The VCCS must have more gain. While writing this i see you have lowered the output voltage, doesn't matter. I worked for the 12V and saw it ramping till this voltage and for a while it does work fine!

hurz

6 years ago

Here my adaptions schematics you can take to double check. I forgot many changes. sorry http://everycircuit.com/circuit/5368607754944512

PrathikP

6 years ago

Ok I checked the parallel resistance again. It's 680ohm and not 68ohm.

PrathikP

6 years ago

Series resistance is 59ohm

PrathikP

6 years ago

Oh and about the opto led diode I have an opto with a diode voltage of 1.01V (@1mA+-0.6)

Fabritren

5 years ago

Awesome!

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