switcher LC output filter problem

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Driving an LC network as so to generate a sine:

<https://www.dropbox.com/s/tasy5bf92dylhak/Capture.PNG?dl=0

I'd like to calculate some values that make a nice sine into the  
grounded load but also minimize the loss in the switches driving the filter.

I'm not sure this is the topology I want or how to go about picking  
those values in this situation.

Version 4
SHEET 1 940 680
WIRE 160 0 48 0
WIRE 256 0 160 0
WIRE 432 0 336 0
WIRE 656 0 512 0
WIRE 48 96 48 0
WIRE 48 224 48 176
WIRE 176 384 48 384
WIRE 256 384 176 384
WIRE 432 384 336 384
WIRE 656 384 656 0
WIRE 656 384 512 384
WIRE 832 384 656 384
WIRE 832 496 832 384
WIRE 48 512 48 384
WIRE 656 512 656 384
WIRE 656 528 656 512
WIRE 656 624 656 576
WIRE 832 624 832 576
WIRE 48 640 48 592
FLAG 48 224 0
FLAG 48 640 0
FLAG 656 624 0
FLAG 832 624 0
FLAG 160 0 V1
IOPIN 160 0 Out
FLAG 176 384 V2
IOPIN 176 384 Out
FLAG 832 384 OUT
IOPIN 832 384 Out
SYMBOL voltage 48 80 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(12 -25 0 1n 1n 16.66n 50n)
SYMBOL voltage 48 496 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(-12 25 25n 1n 1n 16.66n 50n)
SYMBOL ind2 352 368 R90
WINDOW 0 4 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName L1

SYMBOL ind2 240 16 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 4 56 VBottom 2
SYMATTR InstName L2

SYMBOL cap 640 512 R0
SYMATTR InstName C1
SYMATTR Value 1n
SYMBOL res 816 480 R0
SYMATTR InstName R1
SYMATTR Value 50
SYMBOL res 528 -16 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 0.05
SYMBOL res 528 368 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R3
SYMATTR Value 0.05
TEXT -130 296 Left 2 !.tran 1m
TEXT 240 232 Left 2 !K L1 L2 0.95
TEXT 192 272 Left 2 !.ic I(L1) = 0 I(L2) = 0


Re: switcher LC output filter problem

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You could filter a single square wave and save a bunch of active
parts. Classic bandpass filter, series LC and then a parallel LC
maybe. Maybe notch the 3rd harmonic if you want a super-good sine.

What frequency range do you need? Single frequency?

We had to do that recently to generate a goofy-frequency high voltage
sine to test our capacitive fuel level simulator. Square wave into a
couple of Ls and Cs.

Here's our box.  

http://www.highlandtechnology.com/DSS/P330DS.shtml

It needed about 15 volts RMS at around 20 KHz for testing. We tried
and rejected, or blew up, several commercial RF amp modules.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: switcher LC output filter problem
On 4/9/2020 1:58 PM, John Larkin wrote:
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I'm doing 3-5 MHz at 50-100V P2P, depending.

With the four-banger switching arrangement shown even with dead-time,  
fast FETs with low gate charge and reverse transfer C I'm having trouble  
getting the power dissipation on the higher-voltage FETs below about  
500mW in simulation, for ~6 watts output. Meh....

The LC clearly has influence on what kind of loss there'll be in the  
switches but IDK how to optimize it.


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Re: switcher LC output filter problem
On 4/9/2020 2:10 PM, bitrex wrote:
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I think I need to look more carefully at the gate drive waveforms,  
something's not snapping off fast enough.


Re: switcher LC output filter problem

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How about brute-force RF technology, a mostly linear mosfet driving a
tunable tank? You can get voltage gain from the passives, too.

Or wideband, one biggish linear fet, inductor, 60 volt supply.

You could make a 100v p-p square wave and lowpass filter. Fet
switching losses could be nasty up there, so SiC or GaN would be nice,
but more work.


  
--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

We've slightly trimmed the long signature. Click to see the full one.
Re: switcher LC output filter problem
On 4/9/2020 2:50 PM, John Larkin wrote:
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Client is hoping for more fine-grained control over the output  
frequency, I'm driving this switcher digitally from i2c/clock oscillator.

Quoted text here. Click to load it

I believe I figured out the issue - bad gate driver rail choice. Have to  
be careful about where one derives the boost voltages from when you have  
all N-fets and four rails. I'll post a diagram momentarily


Re: switcher LC output filter problem
On 4/9/2020 1:58 PM, John Larkin wrote:
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OK, I Stopped. Thinked, and think I figured out my issue. The LC I've  
shown will work pretty much OK. The high dissipation problem was with  
the gate drivers. With all four switches being N-fets and four isolated  
driver ICs, two switching nodes and four supply rails have to think  
carefully about how the gate driver rails are connected on the secondary  
side.

The top driver clearly has to be boost diode and cap to drive above the  
most positive rail, and then the other three are your choice. Get it  
wrong and it may work..sort of.



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