MC34063A for MPPT Project

[TheInfernoMan]

Hello,

for a MPPT Project for a fuel cell, I will build a DC to DC converter with possability to boost and buck. My idea was a seperate boost and a seperate buck converter in series.
For the boost part I tried with a MC34063A but it don't worked at all. I used the original example schematic but with external MOSFET to increase the Current.

My specs are:
MOSFET: IRFZ44N
L=10µH
Cout=400µF
Diode:SB330 3A
Rsc=0.1 Ohms
Ct=330pF

If I increase the voltage at the Output (to 24V from 5V Input) with nearly no load (only 4k7 Ohms minimal load) the output voltage increases slightly (regarding the Vin) and then the IC begins to let the MOSFET continuously on and so the Current Protection of my Power Supply kicks in. It was a little bit better with more output capacity but it should be possible to output 24V 1.5A which seems impossible at the moment. What I am doing wrong with the MC34063A?

And is there a better way to make a digitally controllable DC-DC Converter? It must not be perfect it should work in first part...
Thanks in Advance 

Sorry for my bad english, I am a student with few english experience and few experience in analog electronics...

[Buriedcode]

There are many many devices that are much better than the MC34063.

Clearly state your input and output requirements, min/max current load, and I'm sure we can help you along.

I realise you've already got the MC34063, but honestly, unless its just for a quick cheap step up/step down, it'll be more hassle than its worth, for poor efficiency.  There could be any number of things wrong, but by far the most common is the choice of inductor, or for external switches, problems driving the gate at the specified frequency (the MC34063 doesn't have a push-pull output).

[TheInfernoMan]

Thanks for your answer @Buriedcode,

my requirements are:

Input: ~4-24V
Output: 12-14V with adjustable current (for charging a lead acid battery)

Current would be good if over 1 Amp at the output side would be possible...
And of course I want to control the DC-DC Converter with an AVR running the MPP Alogrithm.
Thanks for your help in advance...

I used the MC34063 Output to connect the MOSFET Gate to 12V and added a 4k7 Ohm PullDown at the gate... Don't know if that is the problem :/

[TheInfernoMan]

Should I choose a different IC which is easier to control digitally and more efficient?
Or should I build a DC-DC Converter with a MOSFET, Diode and Capacitor from Scratch?

Thanks in advance...

[David Hess]

The problem with the MC34063 is that it lacks a pulse width modulator.  It works fine as a switching regulator but is not amendable to the kind of outside control that you want.

As far as your current problem, the 4.7k pull-down resistor for the external MOSFET is not going to work with any switching regulator because it allow switching times which are too slow.  That would be a problem with any switching regulator and not just the MC34063.

If your input voltage range is 4 to 24 volts, then you will need to take steps to protect the MOSFET gate from excessive voltage and some MOSFETs are not suitable for operation at 4 volts unless the gate voltage is boosted.

[Buriedcode]

It takes more than a MOSFET, Diode, and Capacitor to make an SMPS    But generally, controller IC's require them as external components, some can integrate either the switch (MOSFET), diode, or both.

4-24V input is  very wide range.  With 12-14V output being a rather narrow range.  The options for this sort of setup would be Buck-boost, Sepic, Zeta, or some hybrid type converter, which either acts as a buck or a boost and tries to 'seamlessly' switch between them (often called a "non-inverting buck-boost converter").  Cascaded setups - as in boost then buck, or buck then boost tend to be rather inefficient, and also cost more since you effectively have two converters.

Sepic and zeta generally (but not necessarily) require coupled inductors, that is an inductor with two identical windings.  These are available off the shelf, although not as common as stock single winding ones. A flyback converter can also have an output that is either higher or lower than the output, but this would require a customer inductor.

With 4V in, and 14V (min in, max out) out thats 14/4 = 3.5 boost factor.  With 24V in and 12V out (max in, min out) thats 0.5.

https://www.ti.com/seclit/ml/slup346/slup346.pdf
http://www.ti.com/ww/en/simple_switcher/design-tools.html  - originally national semiconductor, but since TI took over, you have to sign up to use this sadly.  Doesn't seem to work properly for me.
http://www.linear.com/product/LT1513  - lead acid battery charger SEPIC
http://www.ti.com/lit/ds/symlink/lm5118.pdf - buck boost


I believe there are application notes for MCU controlled battery chargers that use the microcontroller itself as the power supply controller.

http://www.st.com/resource/en/application_note/cd00116930.pdf

I couldn't find the Atmel one, although I vaguely remember seeing one about years ago.

That should be enough to get you started.  Given its a battery charger, you shouldn't have to worry too much about output noise/ripple, or extreme efficiency (as in 95%+) which makes life easier - you can even do it "from scratch" as you mentioned, although this is very difficult, and would be much easier/safer to get a working power supply first, then integrating that into your charge controller rather than have the MCU do everything.

[soubitos]

I dont have experience with back-boost topology but for a simple boost design i have used MT3608 with success. It has a few glitches but it's cheap and effective.

[TheInfernoMan]

Thanks for your helpful answers. I agree this sort of setup (first buck then boost) is not perfect and should be made in a different way.
The problem with the MC34063A and the external mosfet with pulldown is now clear to me, thanks @David Hess.

Thanks Buriedcode for the in depth answer... These types of alternative converters seem a little bit difficult to me in first sight for a this simple task (in my eyes as a beginner (any voltage to 12-14V)).
You mentioned that I have to make the SMPS first and then implement it to the MCU. But how can a purely analog IC be controlled by the MCU?

Isn't there any off the shelf IC which is easy and popular which can handle this task? I am a little bit overwhelmed in this huge amount of possible IC's and types...

[Buriedcode]

Thanks Buriedcode for the in depth answer... These types of alternative converters seem a little bit difficult to me in first sight for a this simple task (in my eyes as a beginner (any voltage to 12-14V)).
You mentioned that I have to make the SMPS first and then implement it to the MCU. But how can a purely analog IC be controlled by the MCU?

Well, you can switch in/out resistors in the voltage feedback network to change the output voltage to fixed presets, or sometimes you can control the current limit.  I am assuming this is for a lead-acid battery, so you could set the voltage to 13.8V for float charging, or higher voltage for bulk charge.  Normally however, charging lead acid batteries is done with a constant current source - where the current is regulated, but the voltage is not.  What I meant before is that, the MCU can control battery charging by monitoring the battery voltage, and current, and turning on/off parts of the power supply.  This means the 'analogue stuff' does what it does best - regulating voltage or current, with the MCU telling it what preset voltage/current output do use just using IO's.

When implementing the whole thing in the MCU, it is the microcontroller, with PWM, ADC, comparator etc.. that forms the heart of the power supply.  This is much harder, and also, the feedback loop tends to be slow.  I was just saying it is often easier to have a dumb power supply 'block' that provides the voltages/currents you require, with a few control inputs that the MCU can use.

Also, MPPT's aren't simple devices.  I assume you have some idea of the algorithm and just posted about the power supply part?

Isn't there any off the shelf IC which is easy and popular which can handle this task? I am a little bit overwhelmed in this huge amount of possible IC's and types...

Totally understandable.  These days there are so many parts, even when you marrow down the search by package, input voltaqe, output voltage, one is still left with hundreds of choices.   This is especially daunting for a beginner.

I will try to find a part that only requires a few external parts, doesn't cost much, and isn't in a stupid package (no-one likes soldering QFN).  I posted some links, although I imagine those two were overwhelming.  It might be worth googling for similar designs.