Lets get a bit more specific about buck type switching power supply design.
For an interesting web page, see
http://www.smpstech.com/tutorial/t02top.htm#BUCK
Figure 2-8 shows the basic circuit. The value of the inductor:
L = V * Ton / I
For this case, assuming 100 kHz (10 usec) and 30% ontime and 10% ripple
current,
L = 50V * (10 usec * 30%) / (50A * 10%) = 30 uH
So, this supply needs a 30 uH, 60Amp rated choke, sounds like a stack of
toroids wound with several #8 wires in parallel...sort of like a plate tank
coil on a 160 linear.
The biggest problem with overshoot in a buck type switcher comes from the
inductor dumping it's energy when the load goes away. Because an inductor
it want to keep the current flowing and will increase voltage to make it
happen. The simplest solution provides a capacitor to handle the dump
energy. The web page formula for a Cap that limits over shoot to 1.4 times
nominal (70V in this case), (rearranged):
C = L / Zo^2
where Zo = 50V/50A = 1 Ohm
C = 30 uH / 1 Ohm^2 = 30 uF
So the output cap needs to be 30 uF with greater than a 5 Amp ripple current
spec at 100 KHz. This probably requires multiple Low ESR caps in parallel,
but not too many.
The output L/C filter has a cutoff frequency of
F = 1/(2 * pi * sqrt(L * C)) = 5.3 kHz
Which means the control loop can operate at 500 - 1000 Hz without
interacting badly with the filter components...implying a 1-2 ms transient
response time. This seems easily with striking distance for the desired
performance.
jeff, wa1hco.
----- Original Message -----
From: Alex Von Yonkers <avy@qwestonline.com>
To: Radio WC6W <wc6w@juno.com>; <jeff@wa1hco.mv.com>; <amps@contesting.com>
Sent: Monday, March 26, 2001 9:09 PM
Subject: RE: [AMPS] Transformer ++
>
>
> > -----Original Message-----
> > From: Radio WC6W [mailto:wc6w@juno.com]
> > Sent: Monday, March 26, 2001 7:14 PM
> > To: avy@qwestonline.com; jeff@wa1hco.mv.com; amps@contesting.com
> > Subject: Re: [AMPS] Transformer ++
> >
> >
> >
> > The initial power up will have to be somehow soft-started to protect
> > the mains -- so, no overshoot here.
>
> Yes... but how about the subsequential.... like when your VOX is active.
> Remember? The suggestion was to put in "standby" the power drivers leaving
> only the PFC running. So , when VOX said "POWER ON" the drivers become
> active immediately...
> No overshot here??? I think it must be as the feedback loop must stabilize
> over
> a finite time... hence overshot :(
>
> >
> > With just a little bit of care the SPS should run over 90% efficient.
> > Of course, that still means there will be 500W to remove with a decent
> > sized Heatsink & fan arrangement.
> I am not sure I can design a 90% efficiency Switcher... :( Perhaps the
> second generation :)
>
>
> Now... Thermal budget :) Transistors of 1500W dissipation @ 50% efficiency
> (worst case)times 4 is 3000W + 500W from the PS = 3500W of heat .... What
a
> boiler:(!!!
>
>
> Alex
>
>
>
>
>
>
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|