Leakage will increase dramatically as the diode warms up. 0.5mA seems very high - is this at ambient or max temperature? Richard P Hi Russell, 1Khz etc was from the data sheet of the inductor, we are running with a 220pf cap which on a CRO gives 75khz frequency. circuit current is around 150ma.. would I be right in then saying.. For 220 uH, 0.15A & 12-3.3 = 8v say. t = 220E-6 x 0.15/8 = 4.125 usec Does that sound better? The catch diode is 10BQ030 Vf = 0.3V The data sheet on the LT device talks about reverse leakage current having a large effect on efficiency at lower currents, ie the diode I am using say typical 0.5mA leakage, and the LT data sheet recommends one with a uA leakage (MBR0540). Do you think the diode I have selected is not that good? Thank you very much for your help :) Learning all the time.. as you do in this industry :) Cheers, Peter -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Russell McMahon Sent: Tuesday, 1 June 2004 6:14 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [EE]: DC:DC regulators - what effects efficiency? > Inductor we are using is Tyco brand 220uh shielded ferrite core > style pn 3631b221k, > value 220uh 10% > max RDC 0.460ohms > max IDC 0.70A > LQ test frequency 1Khz > > I have built another version using a LM2674, and on vero > board it performs similar to the MC33063, so guess with a > good layout and better selected components will get even > better. Yes. I think that's rather low for the LM2674. If by LQ test frequency you mean the switching fequency of the converter, then I suspect that the 1 KHz frequency is far too low for the inductance. Lets see: I =~ t.V/L or t = LI/V For 220 uH, 0.7A & 12-3.3 = 8v say. t = 220E-6 x 0.7/8 = 20 usec !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Your inductor is being well and truly saturated if you have a 1 kHz duty cycle, unless you have a suitable cycle by cycle current trip operating (which the 33063 supports). Try a substantially higher frequency for the 33063. *** You don't say what catch/flywheel diode you are using. It MUST be a Schottky if you wish to minimise losses. The 2764 looks after its own frequency. They claim around 87% efficiency at 500 mA , 3V3 out and 12v in so over 80% at 150 mA is expected. If available I'd try another inductor of same or similar inductance but rated at higher current just to see what effect it had on efficiency. FWIW they recommend 100 uH in your application but 220 uH should be OK. Russell McMahon > > I also have a Linear LT1934 sample on order - I think this > one will suit even better as its quiescent current is much > lower than the LM or MC parts, which means i should be able > to get much better eff at my low current levels. > > Regards, > Peter > > -----Original Message----- > From: pic microcontroller discussion list > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Russell McMahon > Sent: Monday, 31 May 2004 10:24 PM > To: PICLIST@MITVMA.MIT.EDU > Subject: Re: [EE]: DC:DC regulators - what effects efficiency? > > > > Trying to get my head around DC:DC buck regulators.. > > Have a circuit that generals runs from 12V input > > and has 3.5V output at 150ma. > > At present am using an ON Semi MC33063 chip. Although > > cheap it uses transistor switches inside, and would like > > to be able to have a bit more head room with a FET style > > unit.. > > > > Anyway, at present with 12Vin, 3.5Vout@ 150ma I am > > getting 69% efficiency. > > > > What components most effect this efficiency number? > > I like the 33063, but it is very old and clunky BUT very very flexible - > excellent building block if effiiciency is not the main aim. > The 33063 switch is usually a non saturating darlington (unles you take > special & annoying design care to make it saturate) so takes 1v plus across > it which is about 10% plus loss at 12v. > Use an external Schittky flyback diode for lower losses there. As this is in > the 3.5v path a diode drop of 0.5v loses ~~~ 0.5/(3.5+0.5) = 12%+ > efficiency. > Inductor can be nasty - 150 mA isn't much BUT be CERTAIN that the inductor > is not saturating at your design current and frequency. Inductor needs to be > rated at about ~~ 400 mA in that application. A saturating inductor will eat > as much of your lunch as you will let it have. What inductor are youi > using? - type number and / or specs? > > > Also, it seems that the DC:DC solution likes a bit of > > current going through it, as when I switch off things > > my efficiency drops to 48% (13ma output current). > > Should I do things differently? > > Operating cuttent taken by IC becomes a larger % of total as load current > goes down. > If your application is well defined (as it seems to be) then there are any > number of modern ICs (Maxim, LT, natsemi, ...) that will give you 80% to > over 90% at the higher end of your range. Things like FET switches (rather > than non saturating bipolar), synchronous rectification (gold standard) or > schottly flyback diodes (pretty good), higher frequencies and more help to > improve on 33063. It's an extremely useful chip, but can't compete in niche > applications nowadays. > > > > Russell McMahon > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.