using a synchronous rectifier in a boost circuit
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From: whit3rd on 29 Jun 2010 03:08 On Jun 28, 8:22 pm, Jamie <jmor...(a)shaw.ca> wrote: > >> I am making a high power boost circuit (500watts+ with 12VDC input and > >> 13.5VDC output). > I am using an off the shelf ATX power factor corrected 750Watt > powersupply as the 12VDC input, and then boosting this to ~13.5VDC for > charging lead acid batteries at 600watts+ Two things are wrong here. Firstly, the 'powersupply' that is your input has low ripple and excellent regulation, both of which are not required for charging lead-acid batteries. Second, your 'boost circuit' just adds a second layer, altering the 12VDC power setpoint would have the same effect. There's already a heatsinked rectifier or two for the +12, stacking a second rectifier in series is ... wasteful of energy. I'd either get a 'brute force' 13.5V power supply, or a lead-acid charger instead of doing this kind of modification. I hope, too, this is outdoors? Six hundred watts of heat will likely make a sulphuric mess if your charger stays connected too long. Commercial chargers do sensing and foldback limiting to avoid that.
From: Paul Keinanen on 29 Jun 2010 04:29 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 ?
From: Hammy on 29 Jun 2010 06:36 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.
From: WangoTango on 29 Jun 2010 11:43 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
From: Joerg on 29 Jun 2010 12:49
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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |