AVR Freaks

I've just finished fixing a few problems with someone else's design on a  product that does just this. It controls a bock converter using PWM.  Interestingly the previous engineer used only the Integral term in PID control to control the PWM and it took absolutely ages to start up (About 2 seconds). I added some Proportional signal which made an order of magnitude difference but 'I couldm't control the overshoot.

I shall watch this thread with interest in your progress.

<edit>Silly typo: Changed "could control" to "couldn't control". Meaning I could not eliminate it completely.</edit>

Back 20 years ago I made 12V just for fun with a 8515 with a ferrit bead as a transformer, and I was surprised that I actually could make 12V 5ma with just 2 5V port pins.  

But nowadays you can just be lazy and pick a cheap boost controler like the sx1308.

Basically like the circuit in post #8 but instead of a MOSFET, you get MOSFET + control logic in a single package. I've tested this chip with crap components from Aliexpress and it works fine for 150mA or so (5V -> 12V), more than that and the poor quality inductor I'm using saturates and starts to heat up. For UPDI it should be more than enough.

Ok, here we go

Energy stored in an inductor:

So substituting the equation

assuming I₀ is zero, that is, the inductor gets fully discharged at each cycle, the energy stored in our inductor is

where t_HIGH is the time the MOSFET is on (HIGH part of PWM wave)

During the discharge part, this energy is consumed by the load. The energy dissipated in our resistive load is:

where t_LOW is the time the MOSFET is closed (LOW part of PWM wave)

Now, considering the duty cycle formula is:

and the frequency of the PWM wave is

If I didn't mess up somewhere, the duty cycle is given by the formula

where

Let me exemplify with f=1MHz, L=1uH, R=1 kohm, V_IN=5V and V_OUT=12V:

k=173.6 gives d aprox. 10%

With a heavier load of 50 ohm, we get k=8.7 so d is aprox. 38%

Most would call that a small ferrite toroid.

Interesting, to me the idea of using an MCU to run a converter has merits.

Good to see people still investigate circuit operation by experimenting; I call it "Live" SPICE.

Find that it works best as it takes all possible practical parameters into account and due to its ad hoc/experimental nature gives one a much better chance to discover better solutions in terms of design and price.

Recently ran a few experiments with higher current (amps) boost, buck and buck/boost converters using the CTC on the 328 and found the mentioned resistor can be helpful.

The CTC of course can run things without processor support and checking it every few seconds or so is all that is needed for battery charging from a solar source.

I used an efficient inductor and used a 1.5" yellow/white core from a SMPS with 25 turns of 1mm enamelled copper wire, some 50uH.

(Great to be able to use that inches again, its illegal here)

Was able to get some decent currents and efficiency of over 80% with out to much effort.

So think a decent inductor, if you have space, is better than trying to scrimp on it.

Wikipedia's page on switch-mode converters says that as long as the converter operates in "continuous" mode, the output voltage is determined by the input voltage and the duty cycle.

I interpret "continuous mode" means that the current is always being limited by the inductance - no zero-current initial state, no steady state at the end, no saturation effects, no imperfect components.

Getting that to work in practice means picking appropriate inductances, frequencies, paying attention to the output current, and so on...

Now, the needs of UPDI are a bit "interesting" WRT this problem.  Efficiency is unimportant, and the load current is going to be very low.  Also, you need to switch the signal between the 12V value, and a regular 5V (or lower) logic signal level.  Preferably without frying the pin that the logic signal is coming from.

Yeah, if the frequency is just 100kHz, you will need a sizeable inductor. Normally if you want to use small inductors, F should be a lot higher, like 1MHz or so.

The original cap booster idea was dismissed because of the internal pin clamping diodes, but seems to be easy to overcome in various ways (diodes).

Some advantages are it uses common parts and its quite hard to get into trouble. Diode selection can be used to tweak max voltage, cap values can be tweaked as needed also. The output can be clamped, but with component value selection one could probably eliminate the need (or maybe use a resistor to create a minimum output load). Not much thinking required on the software side, either.

This version uses a complementary output pwm (or similar), but adding another diode and cap would then need only a single pwm output. I'm sure there are a thousand and one variations.

I used a max232 type chip for 12v when I made my avr 'Fuse Buster'-

http://www.mtcnet.net/~henryvm/4AvrFuseBuster/4avrfusebuster.JPG

but I also had an old style r232 port so had that available anyway.

edit-

also added an npn version, and changed the diodes to bat41 type to make up for the losses.

>What keeps the voltage at 12V?  Prob unregulated & just happens to land near there, depending on load...Your other circuit had zeners!

Will probably get lucky if it makes it much over 12v, and if it does I doubt the other end will care with only couple milliamps available at most (which also drops the voltage as it ramps up the very little available current it has). I think the datasheet says max 13v on the reset pin, and I think I read somewhere its probably happy with its '12v pulse' somewhere around 2xVcc, so an exact 12v is probably not that necessary and could also get by with 'normal' diodes and stay in the 11-12v area without any problems.

I would like to see the result of whatever happens here, but I have given up caring about using a updi pin for gpio use. If I need to use all the available pins I will move up to the next model with more pins to keep the programming pin free. The big advantage is there is only one in this case. Maybe when using an 8pin tiny it makes more sense, but an 8pin soic vs a 14pin soic is not much difference in price or size. When you move into the higher pin counts you also have the smaller sized packages available.

I am following this as a particularly interesting thread, don't see where any critique has been made.
avrcandies method is the same as I used, earlier on I called it "live spice", which is a term I phrased some 20 years ago.
Got it all working fine, using it in a solar charging device.

The control loop is usually slower than the pwm freq. Reason being the capacitance on the output forms a pole so the output voltage can only change at a slower rate.