I'm trying to get some advice on boosting two 7.4v LiPo bats run series (14.8v) to charge two 400v 1500uf caps to 95% in less than 0.0 625sec. The bats I have are for RC application but have the highest output I have been able to find at 65c. The ones I have are 5300mAh but that could be changed if needed. The objective is to pulse an electromagnet 16 times a sec. The final issue its that the components need to be more or less compact. 3"x2"x1" would be the max total size of the charging circuit. I'm pretty new to electronics and this will be the first circuit I am building, so the more info the better.
Charging caps quick
boostcapacitorcharging
Related Solutions
I started this as a comment but the diagram was needed and you asked for critique as part of the answer
Place a ban on Olin coming within 10 miles of your diagram or he will kill you/it.
Your circuit diagram has much unnecessary non-functional mess on it that makes it much harder to read than is necessary. There are little marks and annotations everywhere that have no meaning to most readers and no place on a diagram that is meant for others to inspect for circuit design analysis purposes.
Consider this extract. Most messy stuff is marked. R25 value is very hard to read/ Large circles at component ends are very detrimental to appearance. Dot where vertical line from C3 meets grounds is OK and others should be like that or disabled for presentation purpises. It is fine for a drawing to look like this when you are working with it as long as it is not allowed to escape from captivity.
Depletion mode FETs have their place but it is very uncommon to need them and you should try hard not to do so. They are relatively old tech in most cases and have poor specs (as you found). As you note, a P Channel FET will do what you want about as well in most cases and biasing an N Channel on at all times as its default setting via a whisper of gate drive is acceptable if needed.
A boost converter may never have Vin > Vout + 1 diode drop unless you are happy for Vin to flow to Vout via the output diode. You may be.
If you rely on a processor to terminate a LiPo charge cycle, when the uC crashes the battery will too in due course.
Using a regulator's inbuilt current limit as a max chg regulator for a LiPo is OK only if allowable max I is >> Ireg max as reg max I out varies widely.
1A charge - what size cells?
More can be said.
Olin may choose to say some of it.
Flame suits on :-).
You are totally correct on the voltage front, polarized should be fine, i wasn't looking at the diagram correctly. That being said when using polarized capacitors a 50% safety margin is recommended because they tend to fail in very spectacular ways, both aluminum and tantalum. Please be careful.
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Best Answer
To charge one of your capacitors as specified requires an energy of:
$$ \require{cancel} \begin{align} W &= \frac{1}{2}CV^2 \\ &= \frac{1}{2}1500 \mu F (0.95 \cdot 400V)^2 \\ &= \frac {1}{2}0.001500 \frac{J}{\cancel{V^2}} 144400 \cancel{V^2} \\ &= 108.3J \end{align}$$
You want to do this to two such capacitors, 16 times per second. The required power is then:
$$ 108.3J \cdot 2 \cdot 16/s = 3465.6W $$
That's a lot of power for a couple batteries. Whatever this thing is, it's not going to be small.
If you demanded this power from a 14.8V power source, the current required would be:
$$ 3465.6W / 14.8V \approx 234 A $$
Expressed as a C-rate, this is:
$$ \frac{234A}{5300mA} = 44.15 $$
This is within the specified capabilities of your batteries, but not by a large margin. Consider that we have thus far assumed that we can convert the battery's energy to the capacitor's energy with 100% efficiency. That's not going to happen in practice.
Say you can convert the power with 80% efficiency:
$$ \begin{align} P_{out} &= 0.8 \cdot P_{in} \\ 3465.6W &= 0.8 P_{in} \\ 3465.6W/0.8 &= P_{in} \\ 4331.25W &= P_{in} \end{align} $$
The difference is the power lost to inefficiencies, which will be heat:
$$ P_{loss} = 4331.25W - 3465.6W \approx 856W $$
You are not going to dissipate that much heat in a 3"x2"x1" box. No way. To give you some idea, think of how hot a 100W light bulb gets. Now put 8.5 of them in the same space.