sepic not much good for that high power level (250W).
Sepic rings too much when you switch on at that high power level.
use transformer isolated smps
For this application it might be fine to operate it in DCM, meaning you don't need such high inductance. So long as you can tolerate the higher ripple currents on the input and output it should be fine. And you don't absolutely need a coupled inductor. In fact I'd be surprised if you could find one that would work at such high voltages.
DCM just means that the inductor current falls to zero during a switching cycle. DCM/CCM aren't really "set up." They just occur depending on the various operating conditions and component values in the converter:How do i set it to work in DCM mode and not CCM ?
I am using the UC3842 chip.
what does the high ripple currents actually mean ?
DCM is associated with higher ripple current, which is generally associated with lower efficiency, but not always. In an application like yours where you're operating at relatively high voltages and low currents, and you're operating under a wide range of line/load conditions, avoiding DCM in all cases is pretty much impossible without having enormous inductors or operating at very high frequency (both of which could easily kill efficiency).what consequences will it have on the input and output ?
A lot of those simple calculators are only useful for DC/DC converters with relatively fixed line/load conditions. And they generally assume you want to operate in CCM all the time. But that's just not true for your case (unless you really like the idea of having huge 6mH inductors). So your approach should be a bit different.How do i start here ?
seems like i have too many variables, that can change during operation.
Vin can change, Vout can change, I out always changes because i am charging a big capacitor....
I am totally confused. the calculators sometimes give me 6000uH and sometimes 400uH. all with possible scenarios.
It increases the RMS ripple current in the converter. That's... just what ripple current is. I'm not sure how to simplify it any more. It influence the losses in all the capacitors, and also increases the peak currents the inductors must withstand.Thanks I got it.
so if i understand right, i CAN use a lower inductor, say 470uH (just throwing a number), and then it will be in DCM, which is not too bad in my case...
what effect do have higher ripple currents ? i still do not understand that.
The capacitance should be enough so that the resonant frequency it forms with the inductors is much lower than the switching frequency (by about a factor of ten or more). It must also sized in order to take the required ripple current in the converter. Generally when you satisfy the ripple current condition, you'll also satisfy the minimum capacitance requirement (because higher ripple current generally requires higher capacitance). At 20KHz, and L=470uH, 4.7uF might be enough. For the voltage rating, it should be greater than the maximum Vin. However under some conditions that's not enough. Like if the converter suddenly stops operating when at a high load, the circuit may ring with a large amplitude, potentially greater than Vin. This situation should be simulated to see its severity.also, I have an understanding that the capacitor size between the two inductors is setting the converter current.
and this capacitor has an average voltage of like Vin (say 320v) - but what should be the MAX voltage rating for it ?
and it has to be a low ESR type to avoid heating, so i thought of using a CBB Film capacitor rated at 400v (is it enough?)
any idea what size of capacitor i need ? are wqe talking about 4.7uF or 47uF ? (a ~47uF Film thing at 400v is huge and expensive)
Depends on what charging profile you want. If you charge it with constant current, then your power will range from about 0W to 700W within a charging cycle. But constant current isn't absolutely necessary...1.
I will be charging a 3000uF capacitor (for the bigger model) to 600 or 700 volts,
the capacitor starts at 0 volts because the appliance wasn't connected to power and the capacitor is empty
this is 700 Wattsecond energy, which i would want to charge in 2 seconds say, which is 350 Watts to the capacitors. (does it count as 0.5A at 700v ?)
Shutting off the converter will take some care. It should be allowed to reach a steady state before discharging the capacitor.2.
The discharge phase: in about just a few miliseconds the capacitor will be discharged, until it reaches about 80 volts. which means the next charge cycle starts from ~80v in the capacitor.
Here i need to integrate a system that will totally disable the SEPIC converter during the short discharge, to prevent power supply during the discharge, otherwise the discharge device won't turn off.
You could just have a switch on the input which changes your rectifier between a full bridge (for 220Vin) or voltage doubler (for 110Vin). This is done in lots of instruments, and it works fine so long as the switch is correctly set...i really CAN accept a "110vac" and "220vac" versions for the circuit, if it makes things simpler, so the voltage input range would be reduced significantly (300-350v for 220vac and say 140-180v for 110vac)
Shutting off the converter will take some care. It should be allowed to reach a steady state before discharging the capacitor.
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