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China Battery Charger / Boost-Up Converter 3,6V to 5V

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MRacerxdl

Sun Aug 09 2009, 03:38AM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Hi, recently I bought that thing here :

When I received, I noticed that weight was too low for 8.000mAh, and when I opened, puff, it wasnt even a 2000mAh battery (as expected by the price), but it works very good, and charges my Nintendo DS very fine once.

Ok, I wanted to increase the battery pack, but when I put more cells (Only Cells not the entire battery circuit) and some things get worsed and smoked out (as expected too)

Now I trying to figure out how the circuit works, to remake it with a better version. I studied a few Li-Ion Battery Chargers and Boost Up Converters and I made a few tests. The Battery charger is easy, Philips recomends constant voltage/constant current charge, at about 0.7 * Battery Capacity. Made a constant current/voltage circuit, and poof, it charges with no problems!

Ok, tryed to made a Boost-Up Converter with an simply circuit that I have found on the internet, here is it:

(With a Led on the output to identify that is turned on)

It works without load perfectly, with load it works only if the battery levels are high than 3.5V, if they are lower, I get an strange oscilation at the output levels (5V to 8V before the 7805, and 4,1 to 5V after the 7805), so my DS stops to charge when it happens.

After that I resolved to see how the original circuit works, but I didnt make to get the entire circut so I need help =)
Here is the board layout that I have made from the board (Soon I will take a pic from the board):

Some details that I have:
SW1 is that switch turns on the 5V output.
R1 is the big resistor on board, as I remeber is 10R
R2 is 100R
R3 is 220R
R4 and R5 is a 510R
The LED is Bidirectional (in the working circuit, it are green when output is on, and red when its charging)
The 5V output is showed.
The Input far as I know was on the X Point with the +5V and common ground with the 3,7V Battery Module.

Ok My notes:
First I think that was two transistors on the output, but I was wrong, the lower as described as U2 (The board doesnt have U1 -.- ), and probally is a 5V regulator.
Tryed to made the circuit on the paper, but I didnt make to understand it.

If someone can help me, I will be grate =)

Thanks

Nicko

Sun Aug 09 2009, 04:49AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

No disrespect, but this is a pretty rubbish design - it has no feedback to the 555, so it relies on generating a big enough voltage for the 7805 to perform the regulation it could so easily do itself. The 7805 has a nominal dropout of 2V, so realistically, for it to work to generate 5V out, it needs at least 7 volts in. Its all hugely inefficient - the last thing you need from a battery-driven unit.

Second point is that when designing boards for any SMPS, layout becomes pretty critical. Your layout is tortuous - a good starting point is to look at the logical flow of the circuit and to position the components on the board in the same basic layout. Keep high current traces short & fat, make sure than any feedback traces are kept away from the main power runs. Use an earth plane if you can, or at the very least a "star-earth" layout - remember that in boost converter all the switching current in the +ve rails through the inductor & diode is reflected in the GND path too...

What frequency is this supposed to run at - with the values for the 555 in astable mode that you have, it would seem to run at about 430kHz - way way too fast - you should be aiming for about 60kHz.

Some thing like the circuit here is the general idea - note the feedback to regulate the output and that it runs at about 30kHz. Even so, at best, it'll only be about 70% efficient if you are lucky. Yes, I know that this example generates 180V out, but you adjust that by changing the feedback divider network.

Use a FAST recovery diode, not some random stuff, and a low RDSon (on resistance) & low Qg (gate capacitance) logic FET (need a logic type FET else it won't switch on fully with your low input voltage) instead of the switching transistor. Alternatively, for a circuit like this which is never going to be very efficient, just use a reasonably fast NPN transistor that will switch 500mA or so.

Cheers

MRacerxdl

Sun Aug 09 2009, 05:25AM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Yeah I know that I could make better circuit =P That was only for testing purposes, and that layout was from china board, not mine.
The 555 wasnt wired like that (I see that I moded after puting it onboard) it was oscilating below of 30Khz, because when it starts the "strange oscillation" I can hear the inductor screaming.

I think about a mosfet, but I cant find a cheap logic mosfet model =/

My plan was to made a cheap Charger/Inverter to substitute the china one, I know that I cant get the ~US$14.

Also, I will study the site that you have send, it seens to have very usefull infos. If you know some common Logic FET model, please tell me, I didnt find anything that I can find relative easily here.

Thanks =)

uzzors2k

Sun Aug 09 2009, 09:19AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I've built a few different USB chargers, and found them inadequate for actual use, though you might have more luck. Instead of hacking a 555 into a PWM chip, buy a dedicated IC designed for this purpose. Your electronics supplier should have all sorts in stock, and the datasheets often provide examples showing exactly how to wire them up as 5V regulators.

1245524874 95 FT0 Soldered Travel Charger

If they can actually supply the power you need is another question. The charger above seems capable of charging an ipod nano, but the chip gets very hot as it contains the switch and has to pass the load current. Useing an external transistor would be a good idea.

Btw, look at BJ-transistors instead of mosfets for your switch.

MRacerxdl

Sun Aug 09 2009, 05:01PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

I was looking for relative high current (2 ou 3 amps) low Saturation Voltage BJT for that use, I see that TIP122 have a high voltage saturation if you consider that the battery is only 3,7V, BC547 have low saturation voltage but doesnt handle so good the current.

I sampled one IC from texas for testing, but when it comes, I saw that minimum input voltage was 4V :S

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