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Registered Member #4537
Joined: Sun Apr 22 2012, 06:59AM
Location:
Posts: 9
Hi everyone !
I am looking for help in deciding and designing a capacitor charger.
what i need: I need a module that will take 12V from a big battery (10~30AH), and charge a high voltage DC capacitor bank. i would want to charge: 500uf capacitor to 700v (120 Joules) in about 1 sec. and in another case, 2500uf capacitor to 700v (610 Joules) which will take 5 times 1 second = 5s 12v peak input current should be about 20~30A maximum (do i need more ?)
option1: design a flyback converter circuit and transformer. using a PWM CHIP with some gate driver and FET (i like the IRF3205) and making a specially winded transformer - i would need help with calculating the transformer details... but the parts count can be low probably with cheap and easily accessible parts.
option2:
the LT3750 capacitor charger. i saw two members doing it here (Here and Here) the questions: can it charge to 700-800v ? all example circuits show only ~350v can it charge faster than what the datasheet advertises (10A) ? (using a different transformer for example or by changing feedback resistors ?
disadvantages: this chip is a tiny grain of plastic. i think i can solder it. but it will still be annoying. and of course i have no idea whether i will be limited to the datasheet's "recommendation" of 10A charging and 350v output...
I'd be so glad if those who can help try to do so, i need some substantial guidance, as i have no experience in SMPS so much. Yes, i have read the basics and workings of flyback converters, but i need to "get physical" now and if someone has some experience i would be HIGHLY grateful !
Registered Member #133
Joined: Fri Feb 10 2006, 10:27PM
Location: Pensacola, Florida
Posts: 47
You will require 120 W to produce 120 J in 1 second. Note that 120 W is approaching the high end of the maximum recommended power capability for a flyback-based design, as 150 W is usually considered the maximum due to lower efficiencies at the higher power levels.
Concerning option 2 (LT3750):
Yes, you may use the LT3750 to charge a capacitor to 700 V.
The 10 A maximum switch current that is referenced in the LT3750 data sheet pertains to the specific recommended transformers in Table 1. You could potentially charge with peak currents of 20 to 30 A, but you will require a transformer other than what is listed in the table. You will likely require a custom transformer. Also: In order to keep the gate drive current within the LT3750's capability, a low gate charge MOSFET should be used.
I would highly recommend a snubber on the primary side to limit the voltage across the MOSFET due to the flyback transformer leakage inductance.
A few of the issues to be aware of if designing for high primary currents: 1) The current sense resistor will need to dissipate several watts in order to produce the required 78 mV to activate the LT3750 current limit comparator at 30 A peak. 2) The MOSFET will dissipate a fair amount power. 3) Proper PWB layout will be mandatory due to noise that may be developed from the high peak currents.
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Help deciding on a capacitor charger - flyback converter
