Re: gEDA-user: Switching regulator question

[Dave McGuire <mcguire@xxxxxxxxxxxxx>]

On Jun 16, 2007, at 7:43 PM, Randall Nortman wrote:
> Anybody here knowledgeable in the field of switching power supply
> design?  I'm designing my first one, to knock 24VAC down to 5VDC/0.5A,
> so it's a fairly small supply, but I don't think I'd want to do that
> much of a drop with a linear regulator.  I'm settling on the TPS5420
> integrated switcher (controller with on-chip MOSFET) from TI, which
> has a switching frequency of 500kHz.  (Mostly chosen because of its
> large input range -- 24VAC RMS means 34V peak, plus I have to assume
> that it might be as much as 10% above nominal, then subtract the diode
> drops from the rectifier, so I'm designing for 36V peak.)
>
> My question (one among many) is how much do I need to filter the
> ripple coming out of the full-wave rectifier?  Given a switching
> frequency of 500kHz, I would think that 120Hz ripple on the input
> would not bother the thing, even if it's large ripple, so long as the
> voltage never drops below the minimum required to still enable a 5V
> output (i.e., about 8V).  It seems that the lower the average input
> voltage, the higher the average efficiency of the regulator is going
> to be, so I would ideally aim to have the largest input ripple
> possible, which coincidentally allows me to choose a smaller, cheaper
> input capacitor.  That also means lower peak currents through the
> rectifier and a better power factor.

   You will definitely need *some* filtering...when each half cycle  
drops to zero, you'll be below the regulator's minimum input voltage,  
of course.  The two concerns I can think of offhand are keeping the  
input voltage above the regulator's dropout voltage, and the response  
time of the regulator's feedback system.  Switching regulators tend  
to draw current in spikes, so a low-ESR capacitor placed very close  
to the switching regulator is important...don't skimp there.  Since  
those current spikes have fast rise times, you'll need to minimize  
trace inductances along the current path from that input capacitor to  
the regulator...it will act as a reservoir to satisfy the regulator's  
short-term current demands when the controller turns the FET on.

   You are correct in your assessment of the 120Hz ripple frequency  
vs. the 500KHz switching frequency, but you'll still need to look at  
the regulator's line regulation specifications with an eye toward time.

            -Dave

-- 
Dave McGuire
Port Charlotte, FL




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