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I agree with Bob's comments here and in his next related email 
(wonders never cease :-).

I'd like to add a little more.
I may add a few parts from Bob/Mohit's next interchange.

IF you don't need the Peltier to be electrically grounded then you can 
use an N Channel "low side" FET switch which generally gives you 
better specs (especially Rdson) per $.

> I doubt very much this circuit will work at 80KHz.
> You'll need much stronger turnoff than a 10K resistor
> on the gates of the MOSFETs.

>> What do you suggest? Should I be opting for
>> dedicated MOSFET drivers?
>
> Either a much lower resistance(470 ohms), a push-pull
> circuit, or a driver IC.

Even 470 ohms is a bit high if you were using a resistive only drive.
Without looking up FET spec, gate capacitance is liable to be in the 
nF range so the gate time constant per nanofarad would be about 0.5 uS 
meaning actual switching time could be 1 uS + per nF. This is too much 
to degrade the circuit by when there are other problems which are 
harder to address.

A very simple and cheap gate driver is as per attached gif.
This shows an N Channel low side drive but would be OK for the P 
Channel circuit you are using. The high side drive is via an emitter 
follower that does not saturate and low side drive is via a saturating 
open collector with less pull down (here = turn on). For more low side 
drive D1 can be replaced by a PNP transistor with base to Q8 base, 
emitter to Q8 emitter and collector to ground. A very low cost, mind 
bogglingly simple but effective circuit.

The gate-source zener diode shown here is very highly recommended in 
real world designs (although some competent designers will swear they 
are unneeded and others will tell you that they can cause oscillation 
problems. I have had total success using them. Locate very close to 
FET with as short a lead length as possible.

> Also I question your use of the entire N-channel part
> of the circuit - throw it out and use a single
> schottky rectifier there.

I agree.
For REALLY high current systems - say 100A up, a synchronous FET as 
shown here will outperform a Schottky if you are willing to spend the 
effort of designing it to do so. For currents around 20A you need a 
really good FET to do better than the Schottky AND you must pay 
exquisite care to your drive timings and levels so that it's a diode 
when you want it to be and not one when you don't. The saving grace is 
(or may be) the FET body diode which "steps in" if you get it wrong, 
or id switching fails for any reason, but with much higher losses both 
due to voltage drop and charge storage issues during switching.

In most real world situations up to tens of amps a TO220 Schottky 
diode of appropriate rating is liable to be cost competitive with a 
FET of equivalent capability and have none of the switching and 
control issues.

If you do insist on using a synchronous FET rectifier as shown then 
the gate drive circuit will need much improvement. It is essential 
that the FET is turned on and off reasonably precisely. Unless you 
went severely out of your way to non-overlap the switching waveforms 
the 10k pulldown shown here would leave your FET on or partially on 
when your pass FET is turned on and magic smoke can be expected to be 
released.

Using a copy of the attached gate driver circuit here also would work 
wonders. Cost is very very low.

If you must go to 80 kHz or above (and I'm not sure why you'd need to) 
watch skin depth in the inductor winding. Use of a "Litz wire" winding 
(simply multiple turn s in parallel) will help deal with skin effect 
and make winding easier. Even copper crow bars are quite hard to wind 
coils with :-).

You mention difficulties sourcing parts. From your ad I assume that 
you are in India. Digikey has reasonable prices for high capacity 
Schottky diodes and other FETs but importing these may be problematic. 
Digikey purport to operate an Indian front end 
http://www.digikey.co.in/ but I suspect this may be about as real as 
their Taiwanese equivalent :-(. Certainly, any component searching 
takes you directly to a suspiciously US looking page in $US.

If you do find it advantageous to pursue a synchronous rectifier then 
you will need a superior driver plus having a lower rated Schottky 
there as well (as I think someone else suggested) to handle the 
switching edges if you don't get things quite right. This almost 
constitutes diode abuse :-) (thump the Schottky solidly at each 
switching edge, let it recover for the majority of each cycle) but 
seems to be what other people do. [I have a cheapo Chinese sourced 
laptop 12V-16V converter and that's what they do too][It died, but not 
the diode].

Core selection and inductor design is a far blacker art than may be 
apparent if you want to do it efficiently. The alternative is to use a 
far larger than needed core that always runs cool with large diameter 
wire. If you want an optimally sized and priced core it's not quite so 
easy unless you know secret short cuts. Such as ...

I recommend that you look at the Micrometals website 
www.micrometals.com and download their core calculator. It deals with 
nice things like partial core DC saturation effects on inductance, 
core thermal lifetimes (a non trivial matter), skin effect, core 
materials and size and more. You can arrive at some quite sobering 
realisations using this calculator. Designing a semi saturated core, 
as most buck regulators will have, is a far less trivial task than 
might be expected, except when you use this calculator :-). 
Micrometals cores are amazingly cheap for what you get and have 
superior thermal performance to wannabe clones of theirs. uM have 
trademarked the core colour codes but that just gives the Asian clone 
makers something to make their products look more authentic :-). [[I 
have no financial arrangements whatsoever with uM except as a happy 
customer]]. Their cores are powdered iron (not ferrite) and have some 
amazingly good characteristics per dollar and per size compared to 
what one may expect from ferrite in many cases. [[One day I hope to 
make some of my own using local beach ironsand "just for fun", but 
until then I'm very happy with their products.]]

> Next is your output filter - there's way too much
> capacitance. You may not need very quick reaction
> time, but it's a waste either way. Try 22 uH and 2.2
> uF. Then you can use a ceramic capacitor. Your ripple
> will be low enough. Use a bobbin ferrite inductor -
> shielded preferably.

I imagine the over filtering may be aimed at a low ripple for the 
Peltier. If you care muchly then a multistage filter may help. Again, 
a micrometals (or other) powdered iron core can do this well AND the 
toroidal core is inherently shielded (although perhaps not quite as 
well as a homogeneous ferrite core is for low mu cores).

> Use a P-channel MOSFET with lower resistance. You
> don't need the 100 volt rating of the IRF9540n.

Agree.
But, as above, if you can run the Peltier above ground an N Channel 
device will be even better.

I believe (and may be wrong) that the main reason for running Peltiers 
on low ripple is that the device is effectively thermally cycled at 
each current cycle and the thermal stresses lead to degradation of the 
material. If this is so, then it follows that a ripple frequency well 
above any thermal response frequency, may be OK. But, don't take my 
word for it.

Ripple can be greatly reduced at a small cost in efficiency and $ by 
running a tracking linear regulator on the buck regulator output. This 
can be as simple as a FET and probably a very cheap opamp which 
derives a reference point just below the lowest ripple voltage and 
removes any voltage input above that point. Half an LM358 and a 
suitably current rated FET with low Vdsmax should do. (The FET floats 
and never "knows" what the actual supply voltage is. Having enough 
gate drive voltage may be an issue if Vout is close to Vin.

Enough for now :-)



        Russell


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