Electronics > Projects, Designs, and Technical Stuff
SMPS for vacuum tube power amplifiers.(status: back at it)
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SK_Caterpilar_SK:
Hey Tim, or anyone really who knows the answer to this.

How do I figure out what kind of capacitance I need on the input side of my powersupply (mains side).

Assuming ill be operating in DCM at 100kHz with peak current of 2.5A
Min voltage 120V DC up to 377VDC. What capacitance do I need after the mains rectifier?
MagicSmoker:

--- Quote from: SK_Caterpilar_SK on September 16, 2019, 06:49:57 pm ---Hey Tim, or anyone really who knows the answer to this.

How do I figure out what kind of capacitance I need on the input side of my powersupply (mains side).

Assuming ill be operating in DCM at 100kHz with peak current of 2.5A
Min voltage 120V DC up to 377VDC. What capacitance do I need after the mains rectifier?

--- End quote ---

At it's simplest you can rearrange/modify the capacitor energy equation to solve this problem:

C = [(2 * Pin) / (Vpeak2 - Vtrough2)] * tdischarge

Where tdischarge is the period of half a line cycle (e.g. - 10ms in most of the world, 8.33ms in North America), Vpeak is the max voltage the capacitor charges to each half cycle and vtrough is the minimum voltage the capacitor discharges to each half cycle. A typical value for vtrough for full-wave rectified single-phase mains is 75% of Vpeak. Note that while the capacitor size is reduced as Vtrough is reduced, the peak current in the switch goes up. No free lunches and all that.

For example, if you have 220VAC nominal mains and 500W input power then you only need 240uF to keep peak to peak ripple under 25%. That said, keep in mind that electrolytic capacitors have some of the loosest tolerances and lowest lifespans of any electronic component and you might find you need to use a lot more capacitance just to handle the ripple current and/or to achieve a reasonable life.


EDIT - I haven't read all of this thread.
SK_Caterpilar_SK:
So

2x 56uF 450V and one 10uF low ESR all in parallel (perhaps a SMD high voltage cap to bypass it a little better. I dont know if that makes it any better but I expect it will ;D )

Max output power I expect is 70W peak 50W nominal.
T3sl4co1l:
One 100-220uF snap-in type will suffice.  They have surprisingly excellent performance.  Even a smaller value, give or take ratings (it may be conventional can style; always check the datasheet, in any case!).

These flyback supplies use the same range, for 120V input (the one on the left has a 400V cap so should be able to run from a 240V input as well; or, I forget if the transistors were 600V+ rated, if they're only 250 or 400V, that's a bit of a problem with that).
https://www.seventransistorlabs.com/Images/100WPowerSupplies.jpg
So at 240V only, even half capacitance (47-100uF) would be acceptable.

Tim
SK_Caterpilar_SK:
I'm using snap ins for the output filtration so I indeed plan on using snaps at the input side to keep the low profile look with them. Also I wanted two 56uF to give me total 112uF and another 10uF low impedance just to give it some extra security of it not failing in short order. The load would split evenly across the caps hopefully not stressing them too much.
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