using a synchronous rectifier in a boost circuit
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From: Joerg on 4 Jul 2010 18:22 Fred Bartoli wrote: > Joerg a �crit : >> Jamie wrote: >>> On 6/28/2010 6:41 PM, Joerg wrote: >>>> Jamie wrote: >>>>> Hi, >>>>> >>>>> I am making a high power boost circuit (500watts+ with 12VDC input and >>>>> 13.5VDC output). The boost diode dissipates more than 30watts so I am >>>>> looking for a circuit to replace this diode with a synchronous >>>>> rectifier. Are there any boost PWM controller IC's out there that can >>>>> control an external sync rectifier (n-mosfet) to replace the boost >>>>> diode? I have found many buck IC's with sync rectifier control but >>>>> haven't seen one for the boost configuration. I have tried rigging up >>>>> my own sync rectifier with a bit of success but it would be nice to >>>>> find >>>>> a boost IC that has this function built in! :) >>>>> >>>> See if this one fits your needs, comes with LTSpice simulator model and >>>> the whole nine yards: >>>> >>>> http://cds.linear.com/docs/Datasheet/3813fb.pdf >>> Thanks Joerg, that IC looks great! I have a sim of it running with >>> 12VDC input and 13.3VDC output at about 750watts and 97% efficiency >>> measured in ltspice. I would be happy with anything above 90%, but this >>> makes it a lot easier to heatsink the sync rectifier when it dissipates >>> 10watts instead of 30watts+ for a diode. >>> >> >> Play with the Rdson of the sync rectifier FET versus PWM frequency >> versus gate drive capacitance, might be able to push the dissipation >> down some more. >> >> Thing is, you'll have to obtain models for more modern FETs. The ones in >> the LTSpice selection table are a bit long in the tooth. Or just whip up >> a prototype. >> >> Another thing that helps is to zoom in and look at when exactly the body >> diode comes on and when the FET channel starts conducting. The FET >> channel conduction window must be inside the diode conduction window, >> fully. Now two things happen that don't go in your favor: >> >> a. The body diode behaves like a Si-diode, meaning a corresponding >> voltage drop. This hits when the channel hasn't come on yet or has >> already been turned off. >> >> b. The body diode is kind of sluggish. >> >> Not much you can do about b but you can do something about a: Connect a >> big Schottky in parallel if the BOM budget can stomach it. >> > > > Joerg, > > I've a 1A output current linear amplifier with a mosfet output stage > where the supply voltage is servoed 1V above/below the output voltage. > > Question: do you know of a better mosfet (with lower rdson) that will > lower the power dissipation... > Sorry for the late reply, was on a business trip. A fast amplifier where the output is dynamically adjusted to the need? That only gets the heat off the chip and sinks it elsewhere, at the FETs. There's plenty of those, I'd tell your friend to check EU-stock because I don't know what you guys an buy or not. IRF would be a good place to start, or Vishay. > > Hey, not my question but rather the one that my customer's young analog > "specialist" dared to ask. > > Pretty sad times... > Yeah, you'd think he'd figure this stuff out on his own :-) If he wants to really get rid of some dissipation he'd have to servo with two switchers. Or do class-D and forget the servo. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 4 Jul 2010 18:23
JosephKK wrote: > On Tue, 29 Jun 2010 10:04:27 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> WangoTango wrote: >>> In article <vtij2694kou6ap4e7j17kraavkq7ifk56b(a)4ax.com>, spam(a)spam.com >>> says... >>>> On Tue, 29 Jun 2010 11:29:05 +0300, Paul Keinanen <keinanen(a)sci.fi> >>>> wrote: >>>> >>>>> On Tue, 29 Jun 2010 01:02:31 GMT, Jamie <jmorken(a)shaw.ca> wrote: >>>>> >>>>>> I am making a high power boost circuit (500watts+ with 12VDC input and >>>>>> 13.5VDC output). >>>>> That is more than 42 A. >>>>> >>>>> Are you sure that the ATX power supply will deliver that amount of >>>>> current _at_ 12 V ? >>>>> >>>> Paul makes a good point. >>>> >>>> It's usually the 5V rail thats good fo 30 to 50A the 12V is usually >>>> only good for 12A to maybe 20A. At least the ones I've seen. >>>> >>>> Oh and I wouldnt count on the supply to be able to delivery it for >>>> extended periods of time. >>>> >>> Or do it at all with no load on the +5V >>> >> It'll cost efficiency but I agree. The 12V rail diodes in most PC >> supplies are sort of skimpy. Probably because 12V only has to deliver >> peaks and once the hard disks or CD have spun up consumption goes back down. >> >> Of course Jamie could install bigger diodes on the 12V side but then he >> might as well swing the regulation loop over to 12V, disable everything >> else and set the new voltage divider resistor so that 13.5V comes out. > > The very winding on the transformer won't take it either. Install an ice cube dispenser above it :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |