How to lower losses of no chip flyback power supply  - Page 1
Forum section: electronics / projects

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 06, 2023, 09:19:37 am
content: Hello i am interest in building one of there no chip flyback power supply. Does anyone know typical range of one these circuits? Since there is a BJT i guess frequency would not be much high.schematic diagram:https://danyk.cz/impulz2_en.htmlthanks

author: SeanB
author_rank: Super Contributor
timestamp: January 06, 2023, 12:02:07 pm
content: Using a BUT11 frequency will vary with load, anything from 10kHz to 40kHz, depending on how the transformer is wound, and the load applied. It will also likely oscillate at around 50-400Hz at zero load, depending on that, as the feedback turns off the oscillation when the output voltage turns the optocoupler on hard, and the power supply hiccups as it only starts up when the voltage drops and the power supply restarts off the start up resistor. Will also be very audible with a shorted output, though there it will also likely blow up as the transistor current rises to high levels during the on time.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 06, 2023, 02:52:15 pm
content: 10Khz ? But only if you count blocking via photocoupler? i am interest more in self oscilating frequency which i think is regulated by 2n2 capacitor.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 08, 2023, 09:21:45 pm
content: Does anyone know how to make this design more efficient? I just measured that NPN transistor is turning on 4,2us! Thats so slow and losses are so high =/. So i have changed 1M base resistor to 390k and lower it by 1,2us. So its 3us now. When i load it with 8W NPN gets quite hot. 100C measured. (NO heatsink).In datasheet turn on time is 0,5us. I still can lower base resistor from mains but i dont have much space ath the moment...

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 09, 2023, 03:40:55 pm
content: So ive just changed base resistor to 200k and transistor is gone... Base current was only 1,6mA. What happened? With 1M and 400k its was working fine.Base to emitor is shorted right now and its transistor itself.

author: twospoons
author_rank: Frequent Contributor
timestamp: January 10, 2023, 03:31:56 am
content: Too much bias current from R1 for the feedback coil to overcome, and turn off the transistor. You would likely need to make the feedback capacitor bigger to make this work with 220k. Or add more turns to the feedback winding.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 10, 2023, 06:56:56 am
content: Range with respect to what? Frequency? Conduction and switching loss? Power capacity (do you want more watts from a given transistor or transformer)?Reduce or even remove the 390R, it's killing turn-off speed especially. Or I suppose a diode across it could maintain fast turn-off while limiting forward current.Consider adding a peak current sense: add a small NPN, wire its:- collector to BUT11 base,- base to 100R to BUT11 emitter,- emitter to -V;then add a current shunt between BUT11 emitter and -V.The control method will be frequency modulation in DCM: the damper (100R+220p) shall be designed so that the switching waveform after one pulse settles quickly (the 2n2 and 390R also need to be adjusted to give single pulses rather than bursts of pulses (squegging)). Varying current into the opto just changes the frequency, and the 1M 1W needs to be set so that, at Vin(min), the PRF (pulse repetition frequency) is high enough to deliver full load current, while at Vin(max), it doesn't freaking blow up (it may go BCM or CCM but check it to be sure).The biggest danger is if output voltage overshoots too high, and the feedback winding sees current draw on the back swing -- oh; remove the 1N4148, that's fatal! You very specifically DO NOT want the 2n2 recharged on the backswing, that forces the transistor on immediately as voltage swings back around, putting it at 100% throttle regardless of the 1M's current flow.Anyway, without that diode, the remaining danger is if BUT11 E-B avalanche occurs, which should have a comfortable margin (~10V?), but again, do check it at Vin(max) and output voltage overshoot conditions (worst case should be load step from 100-0%). If it's getting close, remove a turn of feedback. Conversely if you have headroom to spare, add another turn to increase drive strength and therefore improve switching speed.If you're just going for hysteretic control (notice TL431 is not compensated so will turn on and off suddenly around threshold), I'd just put in a zener diode proper instead of TL431 and settle for the modest output regulation.With TL431, probably the 150R should be increased, to 1-2.2k say, and a 1k wired in parallel with the opto diode to set TL431 minimum bias current. Then an R+C between TL431 K and R.More reflections on as-shown:Hmm, ah yes, it might be workable still, the problem is: it's operating CW (continuous wave), there's no way to prevent the transistor turning on again after every pulse, or almost none. The opto can only work by shunting some of BUT11's base current, which can be quite large compared to the opto's current handling ability (maybe this is why the 150R is so small?), so will mainly work to shorten the pulse width. But the flyback pulse then shortens too, so it increases Fsw. Which does reduce output power. But the transistor can only go so fast (low MHz?), and switching losses go up with frequency! So, this is not a very good control method.If the 4N35 can drive a secondary transistor (something around 2N4401 should be big enough?) to amplify its current capacity, then that should be enough to strong-arm the BUT11 off, and get a gated-pulse control method. It'll still be frequency modulated while oscillating, but there ought to be some way to adjust how high it goes before it finally turns off, so switching losses at light load can be controlled. I think this [maximum frequency during a burst] depends on the 2n2, 390R, feedback turns, transformer inductance, and BUT11 characteristics (particularly fT and t_stg, hFE, Ccb). Well... depends on most things, needless to say...To power anything additional on the primary side, you'll probably need a rectifier from the feedback winding, to make an aux supply. Filter with a small electrolytic (say 47uF 16V?) so it can stay active while the oscillator is gated off.(This can still be combined with the peak current turn-off mechanism mentioned earlier.)You can see why I suggested more drastic changes earlier; the CW operating mode I think is much messier and more likely to fail. The blocking oscillator in single-pulse mode (hence, "blocking" until it hits threshold again, neat huh?) is far easier to control, albeit at expense of more bias current (the 1M might have to be more like 100k instead).Tim