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Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
yea, but thats assuming 100% efficiency. for something that is just thrown together with not too much engineering i expect something like 40-60%. i have been going through spice models with the lt1070. i have played with everything but the inductor, and the power looks ok. 20W is easily achievable in the sims. in building the real circuit you may need an external fet. the datasheet shows 150uH inductors, but in spice, power is greatly improved using larger(i just input 300uH) inductors.
i have never really used spice much, but based on the simulations, i will definitely buy a few of these chips. thanks for the help. so far the only thing people have done here is criticize my first draft driver board
actualy, do you know of any equivilants(exact same internals) rated for a lower power? i saw the price on digikey, and as this will not be my only expense on this project, i can't spend 30$ for three of them.
real data here: P= C*V^2*f using a 7.44nF gate at 15v at a muximum of 60khz P= 7.44e-9 * 225 * 60e+3 = 0.1W
two supply windings driving two driver boards = .2W the purpose is for an induction heater using two igbt's in a half bridge.
not much power going on here, but i would like at least 5W just in case i want to drive anything else off of the core.
You know at 0.2W, even my VFD driver could deliver that. ;)
The output is a sine wave at 2.17Vrms (at about 80kHz). You can play with the turns ratio to get the right voltages and increase that 0.1uF cap to change the resonant frequency. That values are what I measured on my home made coils.
I dropped the load resistance from my simulation so that it draws 0.213W. The two transistors power dissipation are at 54mW so even the SOT23 I have would work fine. Got to use good quality film caps for that 0.1uF as it does get warm as lot of circulating current (500mA) with the LC. I am using PNP as I also used the circuit as source for charge pump for the -30V supply to run my VFD.
Power input: 0.486W Power at resistor: 0.213W
So efficiency = 43.8%. In case you are wonder why the efficiency is so low... It is the inductor internal resistance 0.1 ohm. I had to make up a number as I can't measure accurately below that.
Attachment: LTSpice file Also picture of that power supply for my VFD.
]vfd_-_pnp.zip[/file]
As for the LT1070, it is for a very low frequency 40kHz switching which is perfect for your CRT flyback core. A higher inductance allows you to put in more power before the switch current limit kicks in.
The LT1071 on the same datasheet is 1/2 the current and there might be others. Maxim also at one point make/ask LT to make their versions. You have to check to make sure that the switching frequency isn't too high for your core. National (now part of TI had some simple switchers) Not sure if their older 52kHz / 100kHz parts would have the isolated mode.
I don't keep track of these parts as I have not use them for last 10 years. It is just your application, they seem to be perfect fit.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
ok, thank you. i have ordered two from lt, and i hope i don't end up breaking them! i agree that these are ideal for my application, and i think i will get better results with this ic than my uc3842 30% efficiency attempt. i will post if i have any more questions when building this. i am new to the fine points of fly back converters.
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decent 20W isolated power supply
