Imagined Baxandall photomultiplier power supply

This is an LTSpice-only study for a possible photomultiplier power supply.

L8 and L10 are realistic - you should be able to buy them from Farnell. The transformer L1, L2, L3, L4, L5 could be wound on an EPCOS ETD 29 core - the B66358-G-X187 (Farnell part number 1781876) is the cheaper N87 option. The centre-tapped primary - L1 and L2 - would be bifilar wound as a single layer of 10 turns of a twisted pair 0.85mm diameter enameled copper wire - plus grade 2 enamel, it's maximum total overall diameter is 0.937mm, so 20 turns would fit as a single layer on the EPCOS flat-on-the-board B66359W1013T001 former. The MOSFET-driving centre-tapped secondary - L3 and L4 - would be wound on top as 7 turns of a twisted pair of much lighter wire - 0.2mm would do.

The output winding would be six or seven layers of 0.1mm diameter copper wire on top of that - 0.129mm max OD including Grade 2 enamel. The LTSpice model assumes 1000 turns, but it looks as if I'd need 1100 turns to get the maximum output voltage - 1.65kV - I've got in mind.

There's room for a thickish spacing layer between each of the 6 or 7 layers of 0.1mm copper on the former, but to get the 1.5pF effective parallel capacitance shown in the L5 data, the winding would probably have to be "banked" - built up as three or four 7-layer thick "banks" along the former, so that the capacitance between each layer was smaller (because the banks are narrower than the former) and there would be three or four times as many inter-layer capacitors connected in series. Even that might not work well enough.

The other option is to get the high DC voltage out of a couple of stages of Cockroft-Walton multiplier - the photo-cathode end of a photomultiplier dynode chain doesn't draw much current.

The unidentified diodes - D1, D2, D3 and D4 - would probably be SM3FG parts, but they aren't critical.

Version 4 SHEET 1 1992 864 WIRE -816 -288 -896 -288 WIRE -752 -288 -816 -288 WIRE -320 -288 -752 -288 WIRE 304 -288 -320 -288 WIRE 960 -224 880 -224 WIRE 1024 -224 960 -224 WIRE 1152 -224 1104 -224 WIRE 1248 -224 1216 -224 WIRE 1344 -224 1248 -224 WIRE 1376 -224 1344 -224 WIRE 304 -208 304 -288 WIRE 880 -144 880 -224 WIRE 960 -144 960 -224 WIRE 1248 -144 1248 -224 WIRE 1344 -144 1344 -224 WIRE -64 -96 -112 -96 WIRE 96 -96 16 -96 WIRE 608 -96 96 -96 WIRE 720 -96 688 -96 WIRE -816 -32 -816 -288 WIRE -672 -32 -816 -32 WIRE -560 -32 -608 -32 WIRE -416 -32 -480 -32 WIRE 64 -32 -416 -32 WIRE 96 -32 96 -96 WIRE 96 -32 64 -32 WIRE 960 32 960 -80 WIRE 1248 32 1248 -80 WIRE 1248 32 960 32 WIRE -416 48 -416 -32 WIRE -112 80 -112 -96 WIRE -32 80 -112 80 WIRE 48 80 -32 80 WIRE 608 80 112 80 WIRE 720 80 720 -96 WIRE 720 80 608 80 WIRE 1248 80 1248 32 WIRE -416 160 -416 128 WIRE -112 176 -112 80 WIRE 720 176 720 80 WIRE 1248 208 1248 160 WIRE -16 256 -64 256 WIRE 128 256 64 256 WIRE 304 256 304 -128 WIRE 304 256 208 256 WIRE 320 256 304 256 WIRE 384 256 320 256 WIRE 544 256 464 256 WIRE 672 256 624 256 WIRE -416 288 -416 224 WIRE 320 304 320 256 WIRE 464 304 320 304 WIRE -320 336 -320 -288 WIRE -320 336 -352 336 WIRE 320 336 320 304 WIRE -896 368 -896 -288 WIRE 1056 384 960 384 WIRE 1248 384 1248 288 WIRE 1248 384 1056 384 WIRE -416 400 -416 384 WIRE -416 400 -576 400 WIRE -576 416 -576 400 WIRE -416 416 -416 400 WIRE 320 448 320 416 WIRE 320 448 144 448 WIRE 144 480 144 448 WIRE 464 496 464 304 WIRE 320 512 320 448 WIRE 1248 512 1248 384 WIRE -416 528 -416 496 WIRE 80 528 -416 528 WIRE 960 528 960 384 WIRE -416 560 -416 528 WIRE -896 672 -896 448 WIRE -576 672 -576 480 WIRE -576 672 -896 672 WIRE -416 672 -416 640 WIRE -416 672 -576 672 WIRE -112 672 -112 272 WIRE -112 672 -416 672 WIRE 144 672 144 576 WIRE 144 672 -112 672 WIRE 320 672 320 592 WIRE 320 672 144 672 WIRE 464 672 464 560 WIRE 464 672 320 672 WIRE 720 672 720 272 WIRE 720 672 464 672 WIRE 880 672 880 -80 WIRE 880 672 720 672 WIRE 960 672 960 592 WIRE 960 672 880 672 WIRE 1248 672 1248 592 WIRE 1248 672 960 672 WIRE 1344 672 1344 -80 WIRE 1344 672 1248 672 WIRE 1376 672 1344 672 WIRE -896 704 -896 672 FLAG -896 704 0 FLAG -32 80 tank- FLAG 608 80 tank+ FLAG 64 -32 ct FLAG -752 -288 +12V FLAG 1056 384 Out SYMBOL ind2 -80 -80 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 0.22m SYMATTR Type ind SYMATTR SpiceLine Rser=0.022 SYMBOL ind2 592 -80 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L2 SYMATTR Value 0.22m SYMATTR Type ind SYMATTR SpiceLine Rser=0.022 SYMBOL cap 112 64 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 46 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 10p SYMBOL ind2 -576 -16 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L10 SYMATTR Value 1m SYMATTR SpiceLine Rser=1.8 Cpar=63p SYMATTR Type ind SYMBOL voltage -896 352 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 132 Left 2 SYMATTR SpiceLine Rser=1 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL ind2 1120 -240 R90 WINDOW 0 4 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName L5 SYMATTR Value 2200m SYMATTR Type ind SYMATTR SpiceLine Rser=10 Cpar=1.5pF SYMBOL res 1232 496 R0 SYMATTR InstName R3 SYMATTR Value 3300k SYMBOL FerriteBead -640 -32 R90 WINDOW 0 -16 0 VBottom 2 SYMATTR InstName L6

SYMATTR SpiceLine Ipk=0.2 Rser=1 Rpar=785 Cpar=535f mfg="Wurth Elektronik eiSos" pn="742 792 79" SYMBOL res 1232 64 R0 SYMATTR InstName R1 SYMATTR Value 2.2k SYMBOL cap 944 528 R0 SYMATTR InstName C2 SYMATTR Value 10n SYMATTR SpiceLine V=3k SYMBOL ind2 480 272 M270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L4 SYMATTR Value 0.108m SYMATTR Type ind SYMATTR SpiceLine Rser=0.022 SYMBOL ind2 224 272 M270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L3 SYMATTR Value 0.108m SYMATTR Type ind SYMATTR SpiceLine Rser=0.022 SYMBOL res 288 -224 R0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL nmos 672 176 R0 SYMATTR InstName M1 SYMATTR Value AP9465GEM SYMBOL nmos -64 176 M0 SYMATTR InstName M2 SYMATTR Value AP9465GEM SYMBOL res 304 496 R0 SYMATTR InstName R4 SYMATTR Value 1.3k SYMBOL res -432 32 R0 SYMATTR InstName R5 SYMATTR Value 8.2k SYMBOL diode -432 160 R0 SYMATTR InstName D5 SYMATTR Value 1N4148 SYMBOL pnp -352 384 R180 SYMATTR InstName Q1 SYMATTR Value 2N3906 SYMBOL res -432 400 R0 SYMATTR InstName R6 SYMATTR Value 100k SYMBOL cap -592 416 R0 SYMATTR InstName C3 SYMATTR Value 10n SYMATTR SpiceLine V=3k SYMBOL res -432 544 R0 SYMATTR InstName R7 SYMATTR Value 6.8k SYMBOL npn 80 480 R0 SYMATTR InstName Q2 SYMATTR Value 2N3904 SYMBOL diode 1264 -80 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMBOL diode 976 -80 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D2 SYMBOL diode 864 -144 R0 SYMATTR InstName D3 SYMBOL diode 1328 -144 R0 SYMATTR InstName D4 SYMBOL cap 448 496 R0 SYMATTR InstName C4 SYMATTR Value 100n SYMBOL res 304 320 R0 SYMATTR InstName R8 SYMATTR Value 820 SYMBOL res 80 240 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R9 SYMATTR Value 47 SYMBOL res 640 240 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R10 SYMATTR Value 47 SYMBOL FerriteBead 1184 -224 R90 WINDOW 0 -16 0 VBottom 2 SYMATTR InstName L7

SYMATTR SpiceLine Ipk=0.2 Rser=1 Rpar=785 Cpar=535f mfg="Wurth Elektronik eiSos" pn="742 792 79" SYMBOL ind 1232 192 R0 SYMATTR InstName L8 SYMATTR Value 47m SYMATTR SpiceLine Rser=52 Cpar=37.5p TEXT -680 744 Left 2 !.tran 0 20m 0m 10n TEXT -680 776 Left 2 !.ic I(L4)=-0.00 I(L5)=-0.000 I(L3)=-0.00003 I(l6)=0.00003 I(L1)=0 I(L2)=0\n.ic V(tank-)=0 V(ct)=1.5 V(tank+)=3.0 TEXT -680 848 Left 2 !K L1 L2 L3 L4 L5 0.999

I can't help thinking that it would make more sense to get the non-overlapping MOSFET drives out a digital driving scheme - clocked at about 20MHz from a 74HCT4046 VCO, divided down to 70kHz in a PLD and locked to the actual resonance of the tank circuit by a PLL.

Tricky to model in LTSpice. gEDA's gnucap is claimed to do better.

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Bill Sloman, Sydney
Reply to
Bill Sloman
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[snip]

What I did was to make a Cockcroft-Walton multiplier with one more stage than the number of dynodes, and with each dynode connected to a tap on the C-W multiplier.

There is a considerable power saving by not biasing the higher-current dynodes from a resistive divider.

If you run the PMT with the anode end near ground, then the Cockcroft-Walton multiplier has the pleasant characteristic that the taps that are able to supply less current are connected to dynodes that require less current, which helps with the required capacitor sizes.

I found that dual diodes in SOT-23, and ceramic capacitors in 1206 package were adequate in order to build the C-W multiplier. The transformer only had to produce about 100V p-p into the multiplier. This meant that winding capacitance was not as much of a problem as with multipliers having fewer stages.

I did add a 1GOhm resistive divider from the high voltage end of the C-W multiplier, and connected the cathode to a tap very near the high voltage end of this divider, because the cathode required a voltage that was not quite a multiple of the increment between dynode voltages. The cathode draws little current so this divider did not need to waste much current. The earthy-end of the divider was tapped to provide sensing and closed-loop control of the supply voltage, by controlling the input to the transformer.

Chris

Reply to
Chris Jones

ly.

It's a known solution. The last divider chain I set up had a pair of Zener diodes to fix the voltage drop between the photo-cathode and the first dyno de at close to the permissible maximum (which maximise the gain at the firs t stage, and minimises the multiplication noise and the electron transit ti me across the photo-cathode to first dynode gap (which tends to be big).

It also had a Zener diode from the last dynode to ground. The voltage drop across this stage doesn't affect the gain of the tube - there may be electr on multiplication at the anode - but the extra electrons all end up back at the anode, and using a Zener diode here stabilises the voltage drop over t he rest of the tube against changing output current. Lush pointed this out in 1965

H.J. Lush "Photomultiplier linearity" J.Sc. Instrum. vol 42 pp 597-602, 196

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Bill Sloman, Sydney
Reply to
Bill Sloman

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)=3.0

Is the PMT running in the continuous or pulsed mode ? Also, the basic principle is to have increasing positive voltages on the gynodes to accelerate the electrons faster and faster. The design includes inductors and MOSFETs - how do they come in ?

Reply to
dakupoto

ly.

Continuous.

The circuit is intended to generate up to -1.65kV which was the maximum all owed voltage across the last PMT I used - a Philips XP2982 fast focussed tu be.

The idea was to have a basic circuit that I could elaborate to the point th at I could generate lower output voltages efficiently - the tube involved u sually ran with something like -600V across it, finely adjusted to get the gain we needed. The transformer ended up less realistic than I like.

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Bill Sloman, Sydney
Reply to
Bill Sloman

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