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LT3750 Cap Charger

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GhostNull

Mon Apr 05 2010, 10:14AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Hi guys

I'm making a capacitor charger based on the LT3750 and yes i know you can build the flybacks and boost converters but i already have all the parts for it and it has a good efficiency and charge time.

Some specs.

With 12v Vtrans, 300Vout and in a 3amp peak configuration:
- Can charge a 100uF 300v cap in approx. 0.4 sec.
- A claimed efficiency of approx. 90% (see datatsheet)
Capable of charging at up to 300v (with my components)
Capable of charging at up to 9a (peak current)
3v to 24v input range (much better efficiency with higher input)
Very compact (with my newb PCB design skills and using the biggest smd compents I could find it fits onto 3" x 1")
Done output
I'll be using it in the 3amp configuration

I've attached the datasheet for the LT3750 3750fa.pdf (contains alot of info), design notes dn405f.pdf (by LT), more design notes, a PNG of my PCB design and an eagle file of my PCB (in the .zip)

Wow thats alot of attachments, well in the PNG all the vias go to the bottom layer (my ground plain) apart for the input of the voltage regulator, which goes to the Vin pour (at the top). I've tried to use pours to increase the conductive area.

This is my first PCB i have designed so I hoped you guys could check it and give me pointers before I go ahead and etch it.

Thanks!
]3750fa.pdf[/file]
]dn405f.pdf[/file]
]ltmag-lt375.pdf[/file]

---------------------------------------UPDATE 3 May 2010------------------------------------------
Okay, so now I have changed my design slightly, the output voltage is now selectable, no more voltage regulator (no need for one) and more compact. I etched the board using toner transfer method and sodium persulphate (came in a PCB etching kit) but since I used a hack saw to cut it out it was a bit wonky but still bigger than the PCB. Solder was *fun*

it was my first time soldering SMT and I didn't do the best job

soldering the tiny MSOP-10 package was a challenge; it might have improperly soldered it. I made the enclosure pretty well and I have attached some pics, sorry for bad quality, I took the photos with my webcam. I choose to power it from a 12v 6a/hr SLA. I wired a mosfet wrong (this should be fixed in the pcb I have attached) and a resistor burnt almost started a fire

but it didn't so I rewired the mosfet, replaced the resistor (the pcb underneth was burnt

) and powered it up again, no smell of burning plastic, no components on fire or any visual signs that anything is wrong. Then a few days later I took it over to my neighbour's (he's an experienced retired electronics teacher at the local TAFE). So powered it up. This time connected by spade crimps. Again no buring plastic or any visual signs of problems. So I got out my 350dc 2900uF computer grade 'lytic, some solder tags attached to the terminals served as connections for a DMM put the charge pin high by connecting it to the + terminal of the battery. the voltage climbed to about 34v and stopped. So we cut the power disconected everything and discharged the cap. (with my 22k 20watt (50watts heat sinked) resistor) and watched the voltage drop to zero. An inspection of the PCB revealed a tiny speck of solder (probaly due to my uber soldering skills!grr) shorting out some legs of the ic so I cleaned it up. So then set everything up again, connected battery, capacitor etc. initiated charging and watched the voltage expontentially climb to 200v and climb and climb 0.0 soon it reached 305v (it had been set to charge to 250v). So we cut the power, disconnected everything and the discharged the cap with a 100w incandescent. with a bright flash from the light bulb the voltage droped down to 24v and I contected my 22k resistor to do the rest.

So at this point it mostly works but it charges beyond the set output voltage.

Any comments, tips, advice constructive criticism would be much appreciated.

!IMPORTANT EDIT: I forgot to mention that the pad for the flyback transformer on my PCB is the wrong size; still possible to solder correctly but wrong size!

GhostNull

Mon Apr 05 2010, 10:15AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Sorry forgot to attach .zip
]lt3750.zip[/file]

GhostNull

Mon Apr 19 2010, 08:47AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

UPDATE: Looks like i'm not getting help with this.

I have reveiwed my design and done some changes and etched the board I'll post the new design if anyone is interested. All I have to do now is solder the thing together. It'll post the results soon...

GhostNull

Mon May 03 2010, 11:21AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

UPDATED mainpost
]lt3750_v2.zip[/file] contains: pictures of enclosure, new schematic, eagle files and .png of pcb

killerbanjo

Thu May 27 2010, 02:32PM

Registered Member #2881 Joined: Wed May 26 2010, 09:55PM
Location: IO70FC
Posts: 8

Good looking conveter man - I cant help though im affraid :(

Mantis

Tue Jun 08 2010, 03:31AM

Registered Member #2843 Joined: Sun May 02 2010, 03:40PM
Location: King George, Virginia, U.S.A.
Posts: 4

Here is my version of the LT3750-based capacitor charger. I basically took the first design example in the datasheet (page 12) and added an optocoupler interface. So far, it works great! It charges up my 210uF capacitor to 300V in less than 0.5 seconds.

1275966098 2843 FT86875 Capacitorcharger

I did run into a bit of "trouble" when the power supply I used was mindlessly set to a current limit of 0.25A. In this case, the LT3750 will hold the NMOS on and wait for the primary side of the transformer to reach a current of 3A. But since the power supply limits the current to 0.25A, the 3A target current is never reached and the LT3750 just holds the NMOS on indefinitely. Of course, this issue goes away when the power supply is set to allow the required 3A current draw.

Another thing I've noticed is that the voltage drops off at roughly 2V/s after the target voltage is reached. I believe this is NOT because of the voltmeter I have hooked up across the 300V capacitor, and here is the justification:
Q = CV
d/dt( Q = CV )
I = C dV/dt
V/(RC) = dV/dt
So if we take R as the internal resistance of the voltmeter (measured as 11 megs), then the voltage should drop at dV/dt = 0.13V/s when the capacitor is at 300V. And if we turn that expression around and solve for R when dV/dt is 2V/s, then it looks like there is a mysterious 714k resistor in parallel with the output capacitor! I suspect it's the internal leakage of the capacitor.

Sulaiman

Tue Jun 08 2010, 04:21AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Mantis
I just want to draw attention to the 100 uF capacitor across the supply.
look at it's datasheet to find it's ripple current rating ! ... This cap will not last long.
I'd put a 10 uF ceramic across the supply
(because I have stock, similar values would be ok)
These small aluminium electrolytics are very unreliable,
Personally I'd add a regular large size radial electrolytic as near to the pcb as practical.
Make sure it's a low esr type e.g. Panasonic FC FK FM series.
I would not use the small smd electrolytic capacitors for anything!
Tantalum and ceramic rule !!! (and large electrolytics)
Always check the current ripple rating of capacitors for suitability in your circuit.

300V/715K = 0.42 mA leakage = Capacitor leakage plus Diode reverse leakage.

GhostNull - similar comments apply, sorry no one had time to check your design.

GhostNull

Tue Jun 08 2010, 08:59AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Nice work Mantis!

The board looks very professional =)

My design has run into some trouble and now I am making a new board setup for and decided to go for broke with a 9A setup. I got the design done and now all I need to do is make the PCB and solder it up. I also got the stuff for the LT3468, which is another cap charger but much lower power and component count, only 5 components min (excl. IC). I don't think optocouplers are need for the CHARGE pin, since it appears to be a high impendance logic level input and it goes through an internal one shot but the done pin does need an optocoupler in this case and I suppose its good practice to use optocouplers. In my design I'm geting the done pin to un-drive (I don't know the proper term =S) a mosfet gate with a pull-up, do you think that would be okay?

@Sulaiman
In the design there is already a couple of X7R 10uF ceramics on the Vtrans side of the flyback near the IC, the 'Lytic is there for providing bulk current and I'm using High Quality low ESR ESL Panasonic capacitors. Each one of mine is 300mA ripple rated and I got a couple is parallel :3

Mantis

Wed Jun 09 2010, 01:17AM

Registered Member #2843 Joined: Sun May 02 2010, 03:40PM
Location: King George, Virginia, U.S.A.
Posts: 4

@Sulaiman

The 100uF SMD capacitor is a Vishay 150 CLZ series capacitor (Newark part number 29M7234). The datasheet claims that there is no peak current limitation. I'd be interested in your opinion of that claim.

Anyway, I selected this capacitor because the reference design in the LT3750's datasheet calls for a 56uF Sayno 25SVP56M (Newark part number 98K8455), which is rated for 3800mA RMS ripple current. I noticed that most of the Panasonic FC/FK/FM series capacitors that you suggested can withstand >3000mA ripple current, but you would need to go into the 1000's of microfarads (and increased real estate) for that.

I am, however, a fan of tantalum capacitors too. In fact, the blank spot next to the 100uF capacitor is for an EIA 7343-31 capacitor. I happen to have stock of Kemet 22uF tantalums (Digi-Key part number 495-2274-1-ND). But the charger seems to work just fine without it (for now at least, knock on wood).

@GhostNull

Thanks for the compliments.

I'm not sure that parallel capacitors rated for 300mA of ripple current will do it. I would try to get capacitors rated for >3000mA ripple current. As for the MOSFET circuit you described, do you mean a MOSFET inverter, like this?
]mosfet_inverter.pdf[/file]
As you know, you have to be careful not to exceed the maximum current limit of the ~DONE pin, which, for both the LT3750 and LT3468 is +/- 1mA (not sure what that translates to for the maximum instantaneous current you get when driving a MOSFET).

GhostNull

Wed Jun 09 2010, 08:08AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Well heres my schematic, and i'll see if I can get a better cap. Is a cap with so much ripple current really needed?

Thanks

1276070898 2648 FT86875 Capacitor Charger

EDIT: Looks like all farnell sells is "NICHICON - PCF1D101MCL1GS - CAPACITOR, 100UF, 20V"

3200mA rated ripple looks okay but it costs $4.30 a piece

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