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PLL around Resonant LC Circuit

K

Klaus Kragelund

Jan 1, 1970
0
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

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

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.

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.

Has anyone of you guys tried anything simelar - a locked phase circuit
like this?

Thanks

Klaus
 
Klaus said:
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

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

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.

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.

Has anyone of you guys tried anything simelar - a locked phase circuit
like this?

Only at a much lower frequency - around 100Hz - to drive a solenoid
being used to excite a reciprocating stirrer at its mechanical
resonance.

It worked, but the loop didn't settle on the frequency which gave the
maximum mechanical displacement, which didn't correspond to a 0 degree
phase shift.

I hypothesise that if I'd chosen to lock at a different phase shift
(and I could have varied that in steps of 3 degrees) I could have got
the loop to settle very close to the point that did give maximum
amplitude, but I was on holiday in Australia when the circuit was being
tested on real live stirrers, so I didn't get the chance.
 
P

Phil Hobbs

Jan 1, 1970
0
Klaus said:
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

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

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.

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.

Has anyone of you guys tried anything simelar - a locked phase circuit
like this?

Thanks

Klaus

You can adjust the coupling coefficient by adjusting the Q of the
resonance. At the peak, the coupling will go through a maximum and then
start to decline, like a double-tuned IF transformer. If you make the
coupling a shade too strong in the normal position, that'll make the
coupling nearly independent of distance over a reasonable range. How
far were you planning to go?

Cheers,

Phil Hobbs
 
J

Jim Thompson

Jan 1, 1970
0
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

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

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.

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.
Why?

And
if this scheme will work at-all.

It should. Sounds like a fun project.
Has anyone of you guys tried anything simelar - a locked phase circuit
like this?

Thanks

Klaus

Edge-matching? Since Ron Treadway and I invented the scheme I DO
think it will work ;-)

...Jim Thompson
 
T

Tim Wescott

Jan 1, 1970
0
Klaus said:
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

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

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.

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.

Has anyone of you guys tried anything simelar - a locked phase circuit
like this?

Thanks

Klaus
1. Your PLL idea should work.

2. Why not an oscillator with the coil as the frequency-determining
element?
 
M

Mike

Jan 1, 1970
0
Klaus,

If the receive pcb energy requirement were low enough you might get
away with a high-efficiency LED/photodiode transmission arrangement.

Stepping down in frequency, how about a microwave transmitter driving
pcb (tx/rx) antennas?

At 10Mhz you might consider using diode phase detectors rather than the
4046's exclusive-or device.

Good luck,
Mike
 
J

Jim Thompson

Jan 1, 1970
0
1. Your PLL idea should work.

2. Why not an oscillator with the coil as the frequency-determining
element?

Klaus is trying to find the "sweet spot" for best power transfer.

...Jim Thompson
 
C

Chris Jones

Jan 1, 1970
0
Klaus said:
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
 
J

Joerg

Jan 1, 1970
0
Hello Klaus,
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

The usual way this is done is series resonance. Put a cap in series with
the primary coil and then ride on the resonance. This cap should be able
to stomach the expected maximum RF currents. I had a ceramic cap turn
into green glass on a project like that, so not all of them like that.

I don't think the 4046 is going to cut it at 10MHz but you could use
other chips. Precise measurement of V&I going into the coil would
require an RF bridge, basically a couple of little toroids. Or only one
and a resistive divider for V. All you'd really need is reflected power
which would be minimized, unless you also want to know how much the
remote board is drawing.

There are some TI chips (the former UC Unitrode series) that have the
series resonant control guts in them but you can't directly use their
outputs. They are meant to be used in series resonant power converters
around 100kHz. But maybe they could be 'abused' here.

Please mind the legal stuff, you don't want to have the laws come in and
shut it down because you were outside ISM (10MHz is outside). Trouble
is, the next ISM band at 13.56MHz is too narrow to do series resonance
sliding.

Regards, Joerg
 
K

Klaus Kragelund

Jan 1, 1970
0
Phil Hobbs skrev:
You can adjust the coupling coefficient by adjusting the Q of the
resonance. At the peak, the coupling will go through a maximum and then
start to decline, like a double-tuned IF transformer. If you make the
coupling a shade too strong in the normal position, that'll make the
coupling nearly independent of distance over a reasonable range. How
far were you planning to go?

I'm planning to transmit power over a barrier of approx. 10mm

Thanks

Klaus
 
K

Klaus Kragelund

Jan 1, 1970
0
Jim said:

Well, beause I'm moving through the resonant point and the phase is
then in once instance postive and in the other negative. I'm just
worrying about the lock of the PLL circuit when the phase is negative
and the function of the phase comparator in that case
It should. Sounds like a fun project.


Edge-matching? Since Ron Treadway and I invented the scheme I DO
think it will work ;-)
What is "Edge-matching"?

Thanks

Klaus
 
K

Klaus Kragelund

Jan 1, 1970
0
Mike said:
Klaus,

If the receive pcb energy requirement were low enough you might get
away with a high-efficiency LED/photodiode transmission arrangement.

Stepping down in frequency, how about a microwave transmitter driving
pcb (tx/rx) antennas?

At 10Mhz you might consider using diode phase detectors rather than the
4046's exclusive-or device.
I should have said that from the get-go - the barrier is through a
plastic cabinet - so no LED option :-(

Thanks

Klaus
 
K

Klaus Kragelund

Jan 1, 1970
0
Chris said:
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?

Good point. I have a model of a transformer that transmits power from
one layer in a PCB to another layer. And in that circuit I added a
parallel capacitor on the secondary side to reduce the resonant
frequency to a tolerable level. I will do the same in the this circuit
where the distance is greater - to reduce the frequency down to one
that is manageable by the PLL and associated circuits (bytheway - isn't
there a drawback in adding this capacitance?)
Ok, but according to what?

According the 0 phase difference (resonance point)
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.

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

Very insteresting - I will look into this idea of using a self-tuning
LC circuit
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.

I have thought about this and the conclusion is that for sure I need to
very soon perform a radiated emission test to confirm I don't have a
show-stopper. Other people has made layer to layer PCB transformers
with no shield with both theoretical and experimental results that show
no problems. But these circuits operate with low size, so I might run
into problems. If I do, one thing I looked at was to modulate the
freuquency with a low frequency source to shape the noise outside the
reciever bandwidth of the EMC test reciever (120kHz). I have tried this
in the lab and it can easily give an improvement of 10dB

Thank you very much for your good inputs :)

Klaus
 
K

Klaus Kragelund

Jan 1, 1970
0
Joerg said:
Hello Klaus,


The usual way this is done is series resonance. Put a cap in series with
the primary coil and then ride on the resonance. This cap should be able
to stomach the expected maximum RF currents. I had a ceramic cap turn
into green glass on a project like that, so not all of them like that.

Will look out for that ;-)
I don't think the 4046 is going to cut it at 10MHz but you could use
other chips. Precise measurement of V&I going into the coil would
require an RF bridge, basically a couple of little toroids. Or only one
and a resistive divider for V. All you'd really need is reflected power
which would be minimized, unless you also want to know how much the
remote board is drawing.

I was aiming for a simple voltage measurement of the voltage and a
small current sense resistor in the common ground connection returning
from the LC circuit to measure the current
There are some TI chips (the former UC Unitrode series) that have the
series resonant control guts in them but you can't directly use their
outputs. They are meant to be used in series resonant power converters
around 100kHz. But maybe they could be 'abused' here.

Have checked them out - they are too expensive and too difficult to
modify

Thanks

Klaus
 
J

Jim Thompson

Jan 1, 1970
0
Well, beause I'm moving through the resonant point and the phase is
then in once instance postive and in the other negative. I'm just
worrying about the lock of the PLL circuit when the phase is negative
and the function of the phase comparator in that case

The phase doesn't "snap", it smoothly transitions, tightness
determined by "Q".
What is "Edge-matching"?

PFD... Zero phase at lock, as opposed to XOR which has 90° phase at
lock.
Thanks

Klaus

...Jim Thompson
 
J

Joerg

Jan 1, 1970
0
Hello Klaus,
I was aiming for a simple voltage measurement of the voltage and a
small current sense resistor in the common ground connection returning
from the LC circuit to measure the current

You could. For voltage sensing that's also done in RF gear when the
toroid coupler costs too much. The problem with current sensing is that
even with Schottkys it's tough to demodulate unless you can allow a Volt
or two of drop. Germaniums would be best but can be hard to obtain. Or
you could sample since you'll have a clean reference of the 10MHz going
in. Sometimes stepping up the current signal with an RF current
transformer is easier, it's how I usually do that.

Regards, Joerg
 
M

Mark

Jan 1, 1970
0
Joerg said:
Hello Klaus,


You could. For voltage sensing that's also done in RF gear when the
toroid coupler costs too much. The problem with current sensing is that
even with Schottkys it's tough to demodulate unless you can allow a Volt
or two of drop. Germaniums would be best but can be hard to obtain. Or
you could sample since you'll have a clean reference of the 10MHz going
in. Sometimes stepping up the current signal with an RF current
transformer is easier, it's how I usually do that.

Regards, Joerg


How much power are you trying to tramsit to the other side?

Are both sides shielded?

Sounds like you are making an interference generator.

Mark
 
J

Joerg

Jan 1, 1970
0
Hello Mark,
How much power are you trying to tramsit to the other side?

I don't know about Klaus but I have done contactless power transfer up
to 100W. RF measurements up to a kilowatt.

Are both sides shielded?

At 10MHz hopefully they are.

Sounds like you are making an interference generator.

That's why I hinted that it might be good to move to an ISM band.

Regards, Joerg
 
T

Tony Williams

Jan 1, 1970
0
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
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
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.

Could you get the correct phasing by self-oscillating and,
(depending on the method of driving the tuned circuit),
switch the drive current when the voltage zero-crosses, or
switch the drive voltage when the current zero-crosses?
 
K

Klaus Kragelund

Jan 1, 1970
0
Tony Williams skrev:
Could you get the correct phasing by self-oscillating and,
(depending on the method of driving the tuned circuit),
switch the drive current when the voltage zero-crosses, or
switch the drive voltage when the current zero-crosses?

I have actually though about this. Sort of a short voltage pulse on the
LC circuit and when the current in the inductor rises to a certain
level - switch off the pulse. Then I would wait for the voltage to
cross zero from negative and that would be the point of another pulse.
So effectively just letting the circuit ring at the natural frequency
of the LC resonant point. The reason I haven´t pursued this is that I
worried if the Q is bad that there will not be enough voltage to detect
the zero crossing.

But now you mentioned it - I think I'll give it another shot :)

Thanks

Klaus
 
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