Designing tool for MC34063A DC-DC converter

[truhlik_fredy]

I tried to search forum if it's already here and still I'm not 100% sure if it's somewhere posted, but I will post it anyway. I looked back on couple older EEVBLOG episodes and one of them was DC-DC tutorial and it started me thinking, there must be some tool already to make easier for you design the 34063, so I started searching and found this:

http://www.nomad.ee/micros/mc34063a/

Perhaps somebody will find it handy.

[Simon]

interesting, but I'd not rely 100% on it - useful for making comparisons though. You still have to bear in mind the chip limits. For example it lets me put in a 16V in and a 14V out but lets me set a frequency of 10 KHz. As we know this is not possible due to the constraints of the on/off times based on Ct. The higher the on time (or rather the lower the off time) the smaller Ct must be pushing up the minimum frequency, in my case 100KHz just does it

[truhlik_fredy]

It's just simple javascript. The upside is that you can easily have off-line version from it and it looks like simple enough that could run even on old PDAs.

Code: [Select]

tontoff=(vout+vf)/(vin-vsat-vout);
sumtontoff=1/fmin;
toff=sumtontoff/(tontoff+1);
ton=sumtontoff-toff;
// calculate components
ct=(4.0*0.00001*ton)*1000000000000;
ipk=2*iout;
if (ipk*1000>1500)
alert("Switch peak current "+Math.floor(ipk*1000)+"mA exceeds 1500mA limit!");
rsc=0.3/ipk;
lmin=(((vin-vsat-vout)/ipk)*ton)*1000000;
co=((ipk*sumtontoff)/(8*vripple))*1000000;
document.forms["input"].elements["result"].value=
"Ct="+Math.round(ct)+" pF\n"+
"Ipk="+Math.round(ipk*1000)+" mA\n"+
"Rsc="+Math.round(rsc*1000)/1000+" Ohm\n"+
"Lmin="+Math.round(lmin)+" uH\n"+
"Co="+Math.round(co)+" uF\n"+
"R1="+r1+"k R2="+r2+"k ("+Math.round(avo*100.0)/100.0+"V)";

In the step down code is just current condition and rest is just math. But if you want and explain me more I could alter the code and add more checks.

[Simon]

yes that's interesting, well really you need to fully read and understand the datasheet including graphs taking note of all limitations. Adding options for an external switch trnasistor would be nice too, like a tick box to say your using an external transistor so that it dosen't flag a warning over 1.5A and you can put in your own saturated voltage figure for the transistor, that way the results will be more acurate and true

[FxDev]

3-4 days ago I tested MC34063A for my Grid Connected DC/AC inverter. I don't like this shit! Its datasheet says I can take 1.5A but I didnt. Of course its efficiency is good but here the results.

Test circuit elements: L=220uH, Co=2200uF


Result ("verim" means effc., "yük" means load)


But then I tested LM2576-5V! It gives what i want.

Test circuit elements: L=150uH, Co=2200uF


And the result:

[Simon]

1.5A is the peak voltage the internal transistor can take, usually this is twice the amperage you are working with. I'd not use that chip with an external switch for more than 500mA. Sure at low wattages you may not get good efficiency

[Zero999]

1.5A is the peak voltage current the internal transistor can take, usually this is twice the amperage you are working with.

Corrected.

[Simon]

oops, yea I'm feeling a bit lousy this afternoon