designing around the MC34063 min inductor value formula

[Simon]

Right well, inspired by daves 110 video post i thought I'd run through the formula's for making up a basic converter. The final end for the converter is a stepdown MPPT battery controller for a solar system using an external switch.

Now the datasheets bang on about BJT switches, anything against mosfets ?

I ran the formula for the minimum inductor and was suprised to find that with a 16V in and a 14.4 V out @ 5A (worse case scenario and "by the book") I got a massive... 1.2 uH - wow datasheets go on about 100 uH plus. Am I missing something. This is a 100 KHz design.

Looking at the formula I realized that if my Vin got bigger and my Vout go smaller I'd actually need a larger inductor, making 18V in requires a 3.1 uH inductor. So why is the minimum inductance based on minimum input voltage ? it seems like the higher the voltage difference the more inductance required (and that makes intuitive sense)

what value am I supposed to choose ? How much bigger than the min value can I go ?

[tecman]

The numbers seem OKay by your formula.  Remember you are not doing much conversion with 16 in and 14 out.  Watch 100K on the standard chip.  I find that the chip looses efficiency at those frequencies.  I try to stay at 40-50K.  As for mosfets, same rules apply, just figure the on DS voltage at the current and use it for Vsat.  Also a bigger inductor will usually work just fine, as long as you don't go overboard.

paul

[Simon]

yes i was worried about getting massive values with my "big" current of 5A, I think I will drop to 50 KHz

But see I don't get it, if the smaller voltage step calculates for the samller value of inductance why not use the maximum step scenario ? I could well have up to 20V in and 13 out would the inductor calculated for 16-14.4 still be ok then ?

Can I get away with not using the current sensing resistor ? I mean for 5A I get 3R, that's crazy, I'll have other current controlling circuitry anyhow

[Simon]

hm that being one aspect I didn't think to check, thanks for the heads up

[Simon]

where do I find the maximum Ton versus Toff ? or do i work it out from the max charge/discharge values, or msybe I have a real duff datasheet

[Simon]

ok I totally missed that one, I was looking for a figure or graph of on/off time ratios.

So what exactly does that graph give ? is it telling me the transition time from high to low of the output switch or the "on time" minus the "off time" which in turn will control the duty cycle.

If I'm reading it right I need as high a speed as possible to get higher duty ?

[Simon]

as yes I reread your post and the graph, get it now, I'm a fraction of a uS out (maybe not an issue), looks like I'd be sticking with 100KHz though. looks like I best look into another controller

[Alex]

Simon, have a look at the discussion here as well:

https://www.eevblog.com/forum/index.php?topic=2662.msg36459#msg36459

For your application the duty cycle will be 14.4/16 = 90%.

This controller cannot meet this without additional external circuitry.

Jahonen had posted some waveforms in a different thread, sorry I can't remember the name.

For such small conversion voltages you can consider an LDO; it should give similar effciency.

Alex.

[Simon]

Thanks Alex

Well it will use an external mosfet switch. This is to be the power converting heart of an MPPT controller so it does need to be a SMPS controller and not a LDO linear reg. the points of discussion are the worse case scenarios. Hopefully normally the circuit will not work at the exteme of 90% duty

[Alex]

I see. Since you are not in such a tight budget situation you can consider using a proper PWM controller. Marketing departments have created too much confusion to just give you the name of them. Looking at the internal block diagram of the chip is your best guarantee.

[Simon]

oh right ? so what do they call MPPT controller chips ? how do i recognise one ? my plan was to use a cheap SMPS controller with some nice low Rdson mosfet(s) that had an MCU supplying the feedback signal taking it's cue from a current sense chip (MAX4080). I'd just write a simple program that sends a PWM signal that is low pass filtered to the SMPS controller feedback and varies (dither's) it in order to stay around the maximum current output

[scrat]

The way you describe it is one of the standard ways to implement an MPPT control.
A PWM controller is not an MPPT controller, it simply gives out a PWM modulated signal based on the reference you give it.
Although I think there could be some exotic MPPT controller ICs, somewhere, I think the solution using an MCU is the simplest and cheapest.
You can run a relatively slow MPPT algorithm on the MCU, then you have two alternatives: output the current reference to an external PWM current controller (something like UC2886) via filtered PWM or an external DAC, or take the current control inside the MCU, too. Implementing a good current control inside the MCU usually requires high processing speed, but you could accept a low bandwidth control and update the duty-cycle at each n PWM cycles, while using hardware current protection (comparator).