Re: PLL around Resonant LC Circuit
- From: Chris Jones <lugnut808@xxxxxxxxxxxxxxxx>
- Date: Thu, 17 Nov 2005 22:32:50 +0000
Klaus Kragelund wrote:
> Hi
>
> I'm working on a transformer to supply power from one PCB to another
> PCB. The transformer consists of spiral-traces in the PCB and likewise
> on the receiver PCB. The optimum frequency of this transformer is just
> below the resonant frequency (~10MHz) where maximum power is
> transferred
Are you sure that it is not better to put a capacitor across the primary
winding and then run it at the resonant frequency? If not, why not? If
you have found a frequency better than the resonant frequency, then how
much better is it? Would it be tolerable to just run it at the resonant
frequency?
> I need to have a circuit that dynamically changes the frequency so that
> if the two PCBs are moved apart the frequency is changed accordingly
Ok, but according to what?
> My thought is to measure the current into the primary winding and
> relating this current to the voltage accross this winding. When these
> vectors are in phase the load should be resistive and thus in resonance
> and optimum power is acchieved.
Ok, this makes sense to me, I would run it at resonance by making a LC
oscillator around the primary. This doesn't seem consistent with what you
said before, but it seems more sensible.
> I have been looking at the 4046 PLL to create a loop to maintain 0
> degrees phase in regulation. The type I phase detector of the 4046 will
> output (after LP filtering) 0*VDD at 0 degrees phase difference. But
> I'm worried about operating at this low range of phase difference. And
> if this scheme will work at-all.
If you want it to lock at 0 degrees, you need the phase detector with two
flip-flops, but it seems too complicated for my liking. Perhaps it will
work but first I think you should try an oscillator. I suggest the
following MOSFET circuit, it oscillates particularly well:
Sorry about the drawing, I don't have software to do this for me.
+ = junction
--+-- 5V
|
.| .
nnnnn.nnnnn Centre-tapped inductor
| |
+---| |---+ Cap, or can use two caps to positive supply instead,
| |
+------ |
| ---|---+
| | | |
D | | D Small n-mos fets, try to get ones with low capacitance
G-+ +--G
S S
| |
+----+----+
|
|
(
( Optional choke, not really needed for basic operation
(
|
\
/
\ Optional resistor or current sink, if you want to control
/ the amplitude of oscillation, otherwise short to gnd
|
GND
> Has anyone of you guys tried anything simelar - a locked phase circuit
> like this?
Not me.
One thing I would worry about, either with a free running oscillator or with
a PLL circuit, is that the variable frequency might cause the device to
fail EMC testing. I would be inclined to try forcing the thing to run at
13.56MHz or whatever the ISM band is around there. That way you would have
less difficulty in getting it approved I would think. You might have to
tune the capacitance on the primary of the transformer to keep it
resonating at 13.56MHz under all circumstances, but that might not be too
hard. One option might be to make a self-oscillating VCO, and then phase
lock it to a 13.56MHz crystal oscillator. That way it would be both (a) at
13.56MHz and (b) at resonance all the time.
Chris
.
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