Electronics > Projects, Designs, and Technical Stuff
SMPS for vacuum tube power amplifiers.(status: back at it)
Circlotron:
On your schematic the TL494 outputs are open emitter so you need to put a pull down resistor. Probably something like 470R, someone could suggest a value?
T3sl4co1l:
Gotta have a pull-down for that to work! The "emitter" outputs are just that, pull-ups only (or, well, pulling to the respective "collector" for the most part, but as shown, yeah).
Flyback isn't very good for TL494, and especially not voltage mode. Why not another equally classic, easy-to-use controller like UC3842?
Tim
SK_Caterpilar_SK:
--- Quote from: T3sl4co1l on April 28, 2019, 03:57:41 am ---Gotta have a pull-down for that to work! The "emitter" outputs are just that, pull-ups only (or, well, pulling to the respective "collector" for the most part, but as shown, yeah).
Flyback isn't very good for TL494, and especially not voltage mode. Why not another equally classic, easy-to-use controller like UC3842?
Tim
--- End quote ---
I did actually make a SMPS based on the UC3842, but the output was rather noisy, very noisy electrically and audiably. I think it is due to my design but for the intended use it was not a big deal. With the TL494 I had so far only positives compared to everything else. I dont really like the 3842 or any 384x because I dont really see the function of a current mode controller. Its also non ideal for light loads. At least thats what I can tell from my findings.
I SAID ITS NOT COMPLETE :D Yeah I gotta look at it a couple more times before im sure that everythings as suposed.. Would 3k be enough for the pulldown?
T3sl4co1l:
The fundamental reason for current mode, is because the inductor's state variable is current.
Control the current, and you never have to worry about exploding the switch, overheating anything (maybe), or fusing the input or output, because current is limited, period.
Then, you control that current, to regulate voltage. There's no LC filter (as in a voltage mode forward converter) to make your loop impossible to stabilize, compensation is straightforward. Instead, the L is in the current loop, and the current loop and C is in the voltage loop.
The biggest drawback to voltage mode controls is exploding transistors due to unlimited transient response.
Tim
SK_Caterpilar_SK:
--- Quote from: T3sl4co1l on April 28, 2019, 05:09:21 pm ---The fundamental reason for current mode, is because the inductor's state variable is current.
Control the current, and you never have to worry about exploding the switch, overheating anything (maybe), or fusing the input or output, because current is limited, period.
Then, you control that current, to regulate voltage. There's no LC filter (as in a voltage mode forward converter) to make your loop impossible to stabilize, compensation is straightforward. Instead, the L is in the current loop, and the current loop and C is in the voltage loop.
The biggest drawback to voltage mode controls is exploding transistors due to unlimited transient response.
Tim
--- End quote ---
"The biggest drawback to voltage mode controls is exploding transistors due to unlimited transient response."
That statment makes sense...actually its what im experiencing with my converter. I threw out the 3842 as a no go because I have used it in the past and wasnt really the best ever I seen. I have blown so far 5 fets on my pre pre pre prototype converter. It was getting as hot as the blazes of hell maybe even hotter (MrCarlson). Then that got fixed by puting it on a beefy heatsink and still died after a couple hours. The only transistor that survived all so far is the G30N60. IGBT. And that does heat up but but not so dramatically and id rather blame my PCB for that heating than anything.
Im also not as familiar with the UC3842 as with the TL494 but I guess a look at the datasheet can change a lot of things withing a couple of minutes. The biggest problem I see with the UC3842 is current function. This may not be a problem with the mains powered PSU, but it sure is with the 12-24 range. There are simply too high currents for a standard resistor to sense the current. I supose I could calculate resistance to get 70W max out of the controller and get the resitance and create a resistor on the PCB by calculating the trace resistance to create a quite precise shunt. Just have to remmeber not to solder coat it :D . I dont have a lot of the same switching transformers so I can make only 3 more prototypes, one is the TL494 one other one could be the UC3842 and I can decide wheter to fit it with the GA3459 that has a slightly higher inductance and get 20A peak currents at 50kHz at 5V or the GA3460 that has a lower inductance and gets 50A of peak current at 50kHz at 12V.
Yeah I will look into the UC3842 and try that one as well.
One thing I cant understand at first glance is a lot of the schematics I see on the interwebz like the one I attached dont have direct feedback. Its totally feedbackless? I guess it knows the voltages by the sensing the transient? That would be my only guess. And the losses within the transformer are low enought to not disturb any other rails?
Now seeing how it kind of works, I can see what the problem could have been with my original design. Tho I was 14 at the timne of designing that 10-24 input to input voltage-60V 50W converter ^-^ . It has a lot of flaws xD , but interestingly I do remeber that the transistor was not heating as much as the diode did, so the fet did give a damn unlike the diode, that was burning hot without a heatsing and even with the heatsink it gets to a modest temp. Im getting off track ;D .
PS: I just noticed one thing in that schematic, the voltage is hard set I guess? How can I vary the voltage because the output should be settable anywhere between 240V up to 500V?
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