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> Boost converter capacitor charger, A circuit evaluation if you please.
MrHeckles
  Posted: July 31, 2005 03:47 am
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Hi everyone.

I have been trying to design a ciruit for a boost converter in order to charge up a big capacitor. I have come up with a schematic. I would welcome feedback on this circuit, specifically will it work, and what adjustments/additions would be needed to make it work (or work better).

user posted image

The idea of the circuit is to use the 555 timer to proivde the oscillation (at about 48kHz) for the Mosfet driver. The LED between the IC's will be on to indicate charging. The transistor on the right side should activate when the voltage entering the large capacitor hits around 48v. In this case the right LED turns on the the 555 timer is switched off by the reset pin (number 4).

The thing I am most unsure of is the values of the components and whether I got them correct.

Thanks for your help.
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Nettron
Posted: July 31, 2005 04:15 am
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The schemy shows high voltage output , wouldnt the output be less than 12 volts with that setup ? there should be a voltage doubler or tripler stuck in there somewhere.


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MrHeckles
Posted: July 31, 2005 05:23 am
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The high voltage output will be connected to some other stuff with a switch.

I was under the impression that when the high voltage capacitor (100v 6600uF) is charged and then connected to something else, it will discharge with the full 100v (or in this case 48v) into the other circuit (not this one because of the diode) in a very short space of time, which of course is what I'm after.

If the capacitor was not charged, and the follow on circuit was activated, it would only deliver 12v (which is this circuits supply).

I could be entirely wrong, in which case, it would be good if someone could point this out to me biggrin.gif
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Geek
Posted: July 31, 2005 10:41 am
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48V, eh?

Check the schematics for some auto power amps - they run about 50V Vdd. They are also prone to going *BOOM* with no minimum load attached.

Have you considered a push-pull circuit with a TL494?


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Nettron
Posted: July 31, 2005 01:45 pm
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QUOTE
I could be entirely wrong, in which case, it would be good if someone could point this out to me 


Charging a capacitor rated for 50 WV or 1000 WV using 12 volts will charge it to exactly 12 volts no matter how long you leave it charging. You need a voltage multiplier.


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govjohn
Posted: July 31, 2005 08:10 pm
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Hi MrHeckles, the circuit you have shown is a flyback converter and should work fine in priciple but a few points come to mind. the inductor must be fed from a low impedance decouple the LH side with a 10,000F capacitor, also the inductace of 100H seems a bit low what are its characeristics? Will it saturate during the on time of the transistor? Finally, a 1N5401 is a bit slow a BYD33M would be better but appart from that it should work OK. SAIS. John.
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Nettron
Posted: July 31, 2005 11:56 pm
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Inductor ? oops !


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MrHeckles
Posted: August 01, 2005 05:27 am
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Govjohn - I looked up decoupling. I am not sure where the decouple capacitor is supposed to be, alot of places say one for each IC and the like. I have redone the design a little taking into account your suggestions. Take a look.

user posted image

As you can see I have stuck in the decouple capacitor across the battery, and marked it as 1000uF. From what I understand the bigger the better but I had trouble find a 10,000uF like you suggested that was cheapish where I live.

I also changed the diode to a MR856 fast recovery one. The local parts store didnt have the one you suggested but this one looks good.

You'll also note now the inductor is 330uH. I am not sure if it is going to saturate. Information I could get on inductors didnt have a saturation current as such. This is the info I do have for this new one.

user posted image

And lastly I decided to put a voltage regulator in to stabilise the voltage going to the IC's.

Comments or more suggestions?

Note to Geek - I didnt really understand push-pull stuff before and avoided the transformer side of things, which is why I ended up with a boost converter design.

Thanks
MrHeckles
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govjohn
Posted: August 02, 2005 01:55 pm
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Hi MrHeckles, The inductor you show above should be fine, the on time of the FET is 15S so the peak current through the coil is about 0.545A this well within the capabilitys of the inductor shown. Using a 12V regulator from a 12V supply will not work as you need at least 2V headroom for these regulators to work OK. You could change it to 9 or 10V but using a regulator just to feed a 555 is a bit overkill. Feed the 2N3904 and IC2 direct from the 12V supply and feed the 555 through a 1K resistor decoupled with 10F, that should do, if you like you could put a 9.6V zenner across the capacitor but it doesn't realy matter as the 555's timing is independent of the supply voltage. My suggestion of 10,000F was a bit OTT 1000F will be fine but connect it directly between the LH end of the coil and the bottom of the O/P capacitor, which in view of the PRF could also be 1000F. The connections in this area should be as short as possible and direct. Incidentally, what O/P current are you expecting to get. John.
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OutToLunch
Posted: August 02, 2005 06:41 pm
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This is the same reply that I posted to your same question at another forum...

1. You have a 12V source feeding an LM7812 - you're attempting to produce 12V from 12V. I believe the LM7812 has a minimum dropout of around 2V, so you need at least 14V for it to regulate 12V. What is the point of this linear regulator?

2. It looks as though this boost converter is running at a fixed switching frequency with no feedback control. The only control shown is through the 2n3904 that I am assuming is meant to shut down the pulse train from the 555 when the output voltage reaches a limit value that can be set by the variable resistor. If you pursue this method of control, I would suggest using a window comparator instead of just a single trip point. The single trip point will be turning the 555 on and off like crazy just from output capacitor leakage. You can setup a window comparator to turn off the pulse train when the output rises to a certain level. The 555 won't be re-enabled until the output falls below a lower threshold. This will give a window of regulation and might be a bit more stable.

3. With or without a window comparator, this regulator will not be able to react when load is applied and removed. As the load current is increased, the output voltage will droop because the duty cycle is not being increased to accomodate for the higher current. This could be perfectly acceptable if the maximum load will not drop the output too terribly much. Don't expect great regulation once loaded though.

4. There are two LEDs on the circuit - the first one shows the pulse train from the 555 and the other lites up when the pulse train is disabled. There is no functional purpose to either of these LEDs - they're just wasting power. If you feel you need them to indicate status to you then you can remove one of them - when one is on the other is off so there is a bit of redundancy there.

5. There are boost converter ICs available - why not just find and use one of them?
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MrHeckles
Posted: August 05, 2005 08:26 am
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Hey guys,

I have made some more changes. Hooray! Here's the current incarnation.

user posted image

Right, I chose to put a voltage reg in (you'll see it's an 8v one now) because it will be running off a car battery and chances are the battery will be run down (due to a pile of other things running of it too) at some point and the voltage will get a bit variable (13.2v down to who knows what). I figured the voltage reg was a good way to stabilise the voltage to the IC's, and I'm all for overkill on stability.

I hope now that I have the decoupling capacitor in the right place.

The idea of the circuit is to charge up the main cap C1 (which I will probably change later) and then discharge it into another circuit in one go. It will be used to drive a big juicy solenoid for a one off big push/pull, then it can be recharged again at leisure. Consequently I was trying to go for a circuit design that could charge up to 400 or 500v (hence the ratings on the mosfet and diode). I am starting out with a nice respectable 48v (because I have some 50v capacitors) to see what happens. If it all works nice, I theoretically should be able to JUST change the capacitor and readjust the regulator pot.

Because of these above reasons, I am not concerned by output current because it doesnt really matter how long it takes to charge the cap. But it should be around 0.5A at 48V charging I think. Also to note, there should be effectively no loading on the main cap until another circuit is tripped, then its everything at once. I was thinking that when turning on the other circuit I would also disconnect the charging circuit. Keeps things tidy I think.

As far as the feedback system goes (with the single trip transistor), I understand that the window comparator would be quite nice. However for this it is more important that the cap be charged up "tip top" all the time. I do realise that the 555 will be on and off like a mad thing, so I stuck in C6(1uF) to smooth out the reset for the timer. Will this work (even just a little bit)?

OutToLunch > I did have a look at the boost converter IC's but the ones I can get over here easily are really only good to about 65 volts. I needed a circuit to go much higher. Plus this kind of thing is interesting :-)

Oh, and I also took out one of the LED's. :-)

How does it look now anyways?

Thanks guys
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