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In the handful of transistors I've looked at so far, the frequency is too higih and drive current too low causing the transistors to become excessively hot.
What frequency is to high... the switching frequency? What PWM are you using? What MOSFET's have you looked at? Are you talking about "switching losses" being to high causing the MOSFET to get hot?
The controller IC? Or the driver IC? I haven't decided on a controller yet and I'd only be using a driver IC If I was making everything from scratch with an FPGA. But if I did do that, the driver IC I would be using would be an IRS2186. The controller ICs I'm looking at right now are the bq24620/bq24600, bq24747, and 1759fs. I'm not sure why they seem to specify an 8A-10A charge current maximum though. Seems to me you could just scale the current sense and limiting resistors, though the ICs do seem to have many current -related resistors and there might be some relationship in there that I am overlooking that would not allow simply scaling the current sense.
Stay away from voltage mode control if you can... it will have the dreaded right-half-plane zero, which is a pain to deal with. In current mode control the output inductor looks like a current source and the double pole formed by the LC goes away. The dominant pole is from the output C and load R. The zero is from the ouput C and it's ESR. BTW- in current mode there are 2 loops... an "outer" voltage loop and a "inner" current loop.
What are you talking about? The buck does not have a right hand plane zero, that's the boost (and possibly others). Were you thinking of a boost instead of buck? The buck is fairly easy to stabilize.
Also, there is no way he can "stay away from voltage mode control" because that's one of the requirements of the Li-ion charger.
dk:
There are chips with two modes of control build right in. Perhaps you can check them out. That will get you both voltage and current control.
From my understanding there is some difficult with compensating a buck converter that is using voltage-mode control compared to current mode control due to that zero (or some other sentence that sounds like that).
From what I've been reading current mode controllers do have a voltage feedback loop, but do not control output voltage directly. So I'm rather puzzled if you can actually use it for charging batteries. At first glance I think no, but then I think "well, is is used to voltage regulators so why wouldn't you be able to use it to charge batteries?"
I've got a bunch of ebooks and printed out a bunch of pages to see if actual texts will explain things better than the glossing they tend to do on the web.
I'm staring at these pages and I sort of get what's going on but I've been looking for transistors too.
I've found an simpler IC where I understand what's going on inside enough to see that even though they spec a 10A current limit I can just use a smaller current sense resistor to enable higher charging current. And I've also found some transistors that can switch fast and are heatsinkable so it should be workable if I had in a fan. They're over the temperature limit in my calculations a bit, but that's for worst case conditions which is for fast charging batteries twice as large as any I currently have. The IC runs at 300kHz and I can't change it which is too bad since running at 100kHz would eliminate all temperature problems even without a fan.
No problem...same thing happened to me a while back except they were talking about boost and i was talking about buck. Took me about 2 posts to realize we were not on the same page <chuckle>.
From my understanding there is some difficult with compensating a buck converter that is using voltage-mode control compared to current mode control due to that zero (or some other sentence that sounds like that).
From what I've been reading current mode controllers do have a voltage feedback loop, but do not control output voltage directly. So I'm rather puzzled if you can actually use it for charging batteries. At first glance I think no, but then I think "well, is is used to voltage regulators so why wouldn't you be able to use it to charge batteries?"
I've got a bunch of ebooks and printed out a bunch of pages to see if actual texts will explain things better than the glossing they tend to do on the web.
Yeah that's the boost or something else, the buck is almost like a regular linear regulator, in fact the averaged model is almost exactly like a linear regulator except it's got a switching element of course. The switching element works very closely like a simple gain so there isnt usually too much of a problem with compensation. The boost does have the right hand plane zero as the control switch acts like a negative gain for some time...it's a very strange setup and we're lucky it works at all as boost can be very handy sometimes
About the different chips...
Take a look at the chips with part numbers like TL494 and similar. These are a bit older type chips but they are quite simple in theory and work pretty well. Im pretty sure they make a MOSFET drive version too, i'd have to look up the number but it's similar to that number.
You can make push pull out of those chips or even buck if i remember right. The duty cycle might be limited to 95 percent though, but that's not too bad as we need some dead time anyway,
and the frequency is variable depending on a small capacitor value.
The nice thing is that they have two error amps built in, one you can use for voltage feedback and the other for current feedback...one loop per feedback, and they are already OR'd together.
As i was saying, the voltage feedback measures output voltage and the current feedback measures output current. If the voltage tries to go over the set point (4.2v) the voltage cuts back the pulse width, and if the current tries to go over that set point (say 1amp) the current feedback cuts back the pulse width. It's not hard to understand really. Take a look at those chips and see what you think. Even if you dont use them you might gain some insight.
In any case, i hope i have been able to help in this thread.
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