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Tube use? 24->300V large cap charger with 300V 150mA reg
Tube use? 24->300V large cap charger with 300V 150mA reg
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Old 11th April 2021, 12:26 PM   #141
NickKUK is offline NickKUK  United Kingdom
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Join Date: Dec 2019
So been finalising a little.

1. The booster maths ties Ipeak * frequency * L = C * duty cycle
2. The inductor maths 1/Ltotal =1/L1 +1/L2 + ... for inductors in parallel. (ie Ltotal = L1*L2/(L1+L2) )

The previous PCV2 inductors coped with high current, providing 560uH each with little saturation impact, the down side is they have zero EMI shielding and act as BBC Crystal Palace radio mast transmitters.

The switch to shielded AGP-series inductors sees a lower 460uH and 30% inductance drop at about 4A, so although they can take 12A each their inductance would drop considerably at that point.

Doing some quick booster maths - the previous setup
75kHz, 24Vin, 260Vout, 600mA and 0.1Vripple gives us a 90% duty cycle, with 23uH minimum with a requirement of 13Apeak and a minimum of 80uF Cout. The diode seeing a good 13+Amps. I Use 260Vout to give some room for regulators but so far we've been seting this to 230V for all the sims..
The PCV2 inductors max was 7Arms. Hence two in parallel may drop the inductance to 290uH total, the current handling is 14A. Hence two were chosen originally.

The AGP sees both lower 'usable' average current and lower inductance - 470uH which in parallel is 235uH, so either peak current has to increase (nope!) or frequency (easier).
If we can target a max working current of 2x4=8A (with 24A peaks acceptable) then we can use two AGPs. The inductance still exceeds the minimum required (235uH vs 80).

So pushing up the frequency to 95kHz with a 90.7% duty cycle, we see the minimum inductance required (L) drop to 17uH for 13A. With our relationship from point 1 at the start.. we can that for Ipeak * L *C = C, we don't worry about C, so if we want to drop Ipeak we need to increase L. So if we have 235uH vs 17uH minimum, we're 10x that so we can reduce the current. 1.3A in an ideal world so it's likely to be more.. in reality that's going to be higher but the simulation shows it's very happy running at 4A per inductor - 2x is 8A. So we're dropping only a small amount of peak.
Now plugging the frequency back into the calculations.. we see we only need about 60uF minimum Cout for 0.1V ripple.. So happy if that's 100uF.

The AGP series plots show it's very happy up to a good 300kHz before it slowly starts increasing, so it's stable at 95kHz.

So the simulation for 95kHz and a max of 8A on the current sense limiting sees the system apply push the desired 230V 600mA giving the 3080 regulator space for 200V 1mVripple I want.


For giggles I already know that given the duty cycle, freq that the system is probably balanced for the choice of components and current limits. When you look at the maths, adding another inductor won't work, adding more frequency helps a little but in the end because duty cycle is Vin/Vout pushing 400V from 24V means a 94% duty cycle regardless of frequency with 20A.. >50% duty cycle is a danger zone from a stability perspective and we're already implementing slow compensation. At this point, going high in voltage/current from 24V is probably worth switching from a simple Booster to a different topology. This aligns with the industry norms, where we see switching to half-bridge with active secondary rectification etc.
It's possible to switch to 48V for 400V, the duty cycle naturally drops to 88%, and additional work on the IC side so a boost could work. All depends on the inductor voltage and insulation ratings.
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Old 14th April 2021, 10:04 AM   #142
NickKUK is offline NickKUK  United Kingdom
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Join Date: Dec 2019
So the coil craft inductors should arrive today Seems stocks of caps are scarce etc..

I've got a plan B - I have a 5A-9A 13.8V power supply (a maplin cheapie) from years ago. I had a look inside and it's transformer delivers around 18-22Vac but doesn't have EMI filtering. A simple NPN & op-amp acts as a regulator down to 13.8V.

This could, with a voltage doubler provide 600mA of 200V based on tapping the transformer - enough to get the amp build moving, along with a SMPS 5A supply that could be used for the heaters.

I can then continue with the SMPS but not be completely dependent on it.

I may, with a switch to the amp design, end up needing to split current across B+ and B- rails instead. Eitherway - it's possible todo this with both supplies.

Last edited by NickKUK; 14th April 2021 at 10:12 AM.
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Old 14th April 2021, 11:11 AM   #143
NickKUK is offline NickKUK  United Kingdom
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IMG_8504.jpg

Shiny. Heavy.
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Old 15th April 2021, 07:00 PM   #144
NickKUK is offline NickKUK  United Kingdom
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Join Date: Dec 2019
So a cheeky little test whilst I'm working late.

Switching to 48V input, with the same parallel inductors and max 90% DC. With he same 370ohm load.

Screenshot 2021-04-15 at 19.52.38.png

This is running about 2A RMS per inductor. 4A at 48V total ~192W.... and 155W output.

Just running a test into a 5000 ohm load, I'm interested in seeing the maximum voltage this will do with reduced current...
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Old 15th April 2021, 07:29 PM   #145
NickKUK is offline NickKUK  United Kingdom
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Join Date: Dec 2019
Screenshot 2021-04-15 at 20.23.04.png

48V to 500V 100mA into 5Kohm.. 85W in and 50W out That's with the same current limiting in place.

Now I'm not sure I want 500V anywhere near the inductors! I'll certainly be soldering cables rather than using screw fits!

Coildcraft give no statement on the inductor maximum voltage.. I suspect that it would arc over internally.

Last edited by NickKUK; 15th April 2021 at 07:33 PM.
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Old 16th April 2021, 11:12 AM   #146
NickKUK is offline NickKUK  United Kingdom
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Join Date: Dec 2019
So after more simulations, I'm certainly considering moving to 48V.

The benefit is that for the 90% max duty cycle, it will support a wider range of B+ voltage. The down side is that I have to switch caps on the front end to low ESR 100V (1.40 each but even lower 23mOhm) rather than the 50V. Not a big issue, there's choice to fulfil that role - the IC needs it's own 24V supply from the 48V, the fun there is the supply needs to be stiff enough to stop oscillation as the switching starts as the B+ hits 16V (if it drops below 12V then it shuts down). Putting a resistor works but it needs a cap to then cover the immediate sag. Once the thing is running it's all happy as Larry. Lastly there's the small issue of current inrush I need to sort. I'd assume that SMPS would throw it's toys out of the pram and refuse to start with a 200A peak 20uS possible. Low ESR..

Last edited by NickKUK; 16th April 2021 at 11:16 AM.
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Old 16th April 2021, 01:45 PM   #147
NickKUK is offline NickKUK  United Kingdom
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Join Date: Dec 2019
http://ww1.microchip.com/downloads/e...S50002952B.pdf

This is interesting it's using the same AGP series inductors as differential mode filters. Only they're putting 300-400V through them on each phase. They use a different inductor (40A+) for the boost of each phase. However this really confirms that the inductors will be happy as Larry with 48V and acting as a boost inductor.

Will order the caps and other parts.. along with some amp parts..
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Old 20th April 2021, 09:49 AM   #148
NickKUK is offline NickKUK  United Kingdom
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Join Date: Dec 2019
The Mrs has just bought a Circut Joy so I'm currently investigating if, with a little poking, the little vinyl cutter will work for my PCB creating needs - I can then etch and dremel any through holes and create vias with wire initially.

Only a couple of issues initially (a) the PCB with is limited to 130mm, the width of the device and (b) the device can't handle some aspects of .SVG plot files, this caused part of the print to work but the remainder of the SVG to be essentially piled up at the 0,0 coordinates. I think there is a way to simplify the SVG output by Kicad so that the shape paths don't confuse the little 'joy' (there's a tool in GitHub that can simplify SVGs).

The PCBs quoted for local prototyping were in the region of 250 a pop! So this seems a sensible way to make an initial prototype before ordering from china.. I don't want to fork out for another flatbed cutter/router/plotter.. I have my own gadgets I want
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