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Trigger transformer circuit for xenon flash lamp is unusual...why?

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grizedale

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Hello,

Why ar C6 and D1 used in this flashlamp's trigger transformer circuit?

https://i49.tinypic.com/21d3jon.jpg

-they make absolutely no difference to the function or waveforms, so why are they in the circuit.?
 

I remember you asked a very similar question recently. In that case it turned out you didn't have the full schematic, and the extra components were for when a second lamp was added. Perhaps it's the same with this circuit.
 
Hello.

C6 and D1 are protection devices to protect M1 against negative pulses. At the time, when M1 switch off the inductivity of L1 produce a reverse pulse. This pulse will be clamp by C6 and D1 to the max supply voltage (220V).

Regards

Rainer
 
rfredel....

yes but D1 alone can do that..............so why is C6 needed?
 

FvM,

are you suggesting what i think, ...that the use of C6 is a complete waste of time?
 

Hello,

Sure D1 can do it alone as a freewheeling diode.

Without the knowlegde of all datas of the devices, you can not say, why they use C6. It can be a resonant circuit with the trigger transformer.

That is, why I ask for the complete circuit and not parts of it, without any details.

Regards

Rainer
 
are you suggesting what i think, ...that the use of C6 is a complete waste of time?
Click to expand...
I say that the combination of C6 and D1 is useless, because C6 is only charged once. You would need to add a resistor to make the circuit work as a RCD snubber.

In addition, I don't see a reasonable purpose of a RCD snubber or freewheeling diode in a trigger circuit.
 
sorry i cant give circuit as id be sacked.

Its basically a "capacitive external trigger" flashlamp circuit as on page 4 of this........
we used a buckboost converter to charge the tube cap and our thyristor is referenced to ground, the tube is referenced to vin, but otherwise its the same.




So C6 really is useless.......a waste of time, just as i thought.
 

Pretend that the FET is a thyristor, because it usually is in trigger circuits.................


Well then surely you cannot say that D1 is useless?
I can appreciate that C6 needs shorting out..........but D1 removed?...surely not.

You have an inductive current flowing in a loop which includes the thyristor.eventually the inductive current will be broken when the thyristor turns off.................so a diode to the high voltage rail sounds like a good way of clamping the voltage.

Having siad that, the frequency of the current in the primary LC circuit is about 150KHz, and that is so fast that the thyristor would not commutate off at its zero crossing since the current builds up again very quickly after the zero crossing as it oscillates at such high frequency, and , after all, the reverse recovery of a thyristor such as BT168GW is long.

So in fact, i am puzzled as to the presence of a clamping circuit (D1), but our contractors have used it..why have they used it?
 

FvM:
I don't see a reasonable purpose of a RCD snubber or freewheeling diode in a trigger circuit.
Click to expand...

...i agree that an RCD snubber is not needed....

...............but an inductive current is being broken by the switch off of the thyristor, and as rfredel says, you need a freewheeling diode to clamp the voltage to the rail?
 

No you don't need it. The voltage at the transformer primary follows a characteristic dampened oscillation waveform with a Vpeak of about twice the capacitor voltage. A freewheeling diode would cut it after the first halfwave, surely not the operation mode specified by the trigger transformer manufacturer. The well known standard circuit without a snubber or clamping diode has been used since more than 50 years, I wonder why it must be revised now?
 
FvM

Here is a voltage spike at the primary of the trigger transformer caused by the trigger FET turning off while there was current flowing in the trigger transformer primary….

..i think this concludes that a clamp diode is definetely needed.?


https://i50.tinypic.com/2940y6a.jpg

Its from a simulation of this in LTSpice…

Here is the Ltspice simulation circuit.
https://i49.tinypic.com/2qkpfv4.jpg

Here is the LTspice simulation file (attached and also below)

Code: 
Version 4
SHEET 1 1224 1016
WIRE 176 -96 -368 -96
WIRE 176 -64 176 -96
WIRE -368 16 -368 -96
WIRE 176 96 176 16
WIRE 208 96 176 96
WIRE 336 96 336 64
WIRE 336 96 272 96
WIRE 400 96 336 96
WIRE 576 96 576 64
WIRE 576 96 528 96
WIRE 656 96 576 96
WIRE 656 128 656 96
WIRE 400 160 400 96
WIRE 528 160 528 96
WIRE -368 352 -368 96
WIRE -144 352 -368 352
WIRE 336 352 336 320
WIRE 336 352 -144 352
WIRE 384 352 336 352
WIRE 400 352 400 240
WIRE 400 352 384 352
WIRE 528 352 528 240
WIRE 528 352 400 352
WIRE 656 352 656 192
WIRE 656 352 528 352
WIRE 176 512 176 96
WIRE 384 528 384 352
WIRE 128 592 16 592
WIRE 176 608 176 592
WIRE -144 624 -144 352
WIRE 16 656 16 592
WIRE -368 736 -368 352
WIRE 16 768 16 736
WIRE -368 896 -368 816
WIRE -192 896 -368 896
WIRE -144 896 -144 688
WIRE -144 896 -192 896
WIRE 16 896 16 848
WIRE 16 896 -144 896
WIRE 176 896 176 608
WIRE 176 896 16 896
WIRE 384 896 384 592
WIRE 384 896 176 896
WIRE -192 928 -192 896
FLAG -192 928 0
FLAG 336 64 L1_c
FLAG 336 320 L1_r
FLAG 576 64 sec
SYMBOL cap 272 80 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 47n
SYMATTR SpiceLine Rser=0.01
SYMBOL res 160 -80 R0
SYMATTR InstName R1
SYMATTR Value 440k
SYMBOL nmos 128 512 R0
SYMATTR InstName M1
SYMATTR Value STW11NM80
SYMBOL voltage -368 0 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 124 Left 2
SYMATTR SpiceLine Rser=0.01
SYMATTR InstName V1
SYMATTR Value 220
SYMBOL ind2 384 144 R0
SYMATTR InstName L1
SYMATTR Value 20µ
SYMATTR Type ind
SYMATTR SpiceLine Rser=0.325
SYMBOL ind2 512 144 R0
SYMATTR InstName L2
SYMATTR Value 18m
SYMATTR Type ind
SYMATTR SpiceLine Rser=230
SYMBOL cap 640 128 R0
SYMATTR InstName C2
SYMATTR Value 1p
SYMATTR SpiceLine Rser=0.01
SYMBOL voltage 16 752 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
WINDOW 3 -123 183 Left 2
SYMATTR Value PULSE(0 10 0 1u 1u 13u 800m)
SYMATTR InstName V2
SYMBOL res 0 640 R0
SYMATTR InstName R2
SYMATTR Value 1
SYMBOL voltage -368 720 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 124 Left 2
SYMATTR SpiceLine Rser=0.01
SYMATTR InstName V3
SYMATTR Value 12
SYMBOL cap -160 624 R0
SYMATTR InstName C3
SYMATTR Value 47µ
SYMATTR SpiceLine Rser=0.1
SYMBOL cap 368 528 R0
SYMATTR InstName C4
SYMATTR Value 220n
SYMATTR SpiceLine Rser=0.01
TEXT -208 1000 Left 2 !.tran 0 1000m 0 startup
TEXT 416 128 Left 2 !K L1 L2 1
TEXT 696 56 Left 2 ;1p imitates the capacitance\nat the secondary.
TEXT 600 -72 Left 2 ;ZS1052 SUL(H) V02\nTrigger\nTransformer.
TEXT 816 208 Left 2 ;Xenon flash\ntube would \nbe here.



Here is the circuit with the clamp diode added...which i am sure all believe now is needed?

https://i47.tinypic.com/v6kprc.jpg
 

Attachments

  • Trigger transformer without diode.txt
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The simulation shows a point I didn't think about, because I've been exclusively using SCR trigger circuits, that have a different behaviour, because they can't interrupt a positive current through the switch. As an additional point is that you have an unrealistic low windings parallel capacitance in your simulation circuit, so the real waveform will look different. On the other hand, the trigger transformer involves a considerable leakage inductance.

Due to this simulation uncertainties, I have difficulties to make suggestions based on the simulation circuit. You'll also notice that you can cause the voltage peak to vanish by slightly adjusting the on-time...

One point is sure however: If the triggering is expected to occur during MOSFET on-time, there's nothing against a simple freewheeling diode between in parallel to the charging resistor.

Also the second point: A CD snubber without R doesn't work, except for the first pulse.
 
FvM
there's nothing against a simple freewheeling diode between in parallel to the charging resistor.
Click to expand...

But to get the best protection for the primary winding we need to put the clamp diode from the primary to the rail as follows?

https://i47.tinypic.com/v6kprc.jpg


Also, SCR's turn off so slowly that it can't really cause an overvoltage spike (is this true?).

...but i think due to the noise in the trigger circuit, sometimes the SCR does indeed turn off quickly, and so, after all, the clamp diode is needed. do you agree?
 
But to get the best protection for the primary winding we need to put the clamp diode from the primary to the rail as follows?
Click to expand...
No, I don't think so. With a the freewheeling diode connected to the drain, no voltage peak occurs. Another option is to increase MOSFET on-time, open it after mosr of the energy has dissipated. A diode directly at the primary winding will possibly attenuate the trigger pulse.
...but i think due to the noise in the trigger circuit, sometimes the SCR does indeed turn off quickly, and so, after all, the clamp diode is needed. do you agree?
Click to expand...
As said, a SCR doesn't turn off while the current is positive, thus there's no voltage peak.
 
I think there's two key points here:

FvM said:
The voltage at the transformer primary follows a characteristic dampened oscillation waveform......
Click to expand...
Yes, so by the time the trigger circuit switches off, the amplitude is very much reduced. However in the simulation, the oscillation is undamped and still at maximum amplitude when the trigger circuit switches off. So in this respect the simulation is misleading.

FvM said:
As said, a SCR doesn't turn off while the current is positive, thus there's no voltage peak.
Click to expand...
I believe the actual circuit does use an SCR. It's only in the simulation that he's using a MOSFET, and switching it off while current is flowing. Again, the simulation is completely misleading.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The important thing to bear in mind is that we are not looking at the circuit diagram of the actual circuit. The circuit shown here contains a diode and capacitor that may serve no purpose, but that does not mean the actual circuit contains a useless diode and capacitor.

As mentioned in post 2, Grizedale showed a similar (incomplete) circuit with an apparently useless diode and capacitor a couple of weeks ago. In that case he later learned that the diode and capacitor did serve a useful purpose in the actual circuit.

I don't see any point in showing incorrect incomplete schematics here, and expecting anybody to give sensible explanations.
 
As mentioned in post 2, Grizedale showed a similar (incomplete) circuit with an apparently useless diode and capacitor a couple of weeks ago. In that case he later learned that the diode and capacitor did serve a useful purpose in the actual circuit.

I don't see any point in showing incorrect incomplete schematics here, and expecting anybody to give sensible explanations.
Click to expand...

At some point, I was reminded to a fake and original picture puzzle. Or probably a new variant with two fakes where you have to guess the original.

To perform a meaningful simulation, I would start to determine the trigger transformer parameters that are not given in the datasheet, SRF (respectively secondary winding capacitance) and coupling coefficient.
 
Actually i have here the trigger circuit schematic, and its more realistic because i managed to find a 600V SCR model

https://i45.tinypic.com/2vi1s91.jpg



This is really it, and do you think that D1 is best placed where it is?................it only operates if the trigger transformer primary voltage goes above the rail voltage, which it doesnt normally do.

The SCR is triggered from a microcontroller, and there is an enormous current pulse when the scr is triggered..i believe that this makes the ground at the SCR "bounce" down (compared to the ground trace local to the microcontroller) and this can sometimes switch off the SCR prematurely and swiftly, ....causing the voltage spike which damaged my R1 and Trigger transformer some time back (it was either that, or our contractors sabotaged the trigger transformer because they didnt want us to succeed where they failed)

The SCR is triggered by the microcontroller.




here is the LTspice simulation file

Version 4
SHEET 1 2312 3172
WIRE 176 -96 -368 -96
WIRE 416 -96 176 -96
WIRE 176 -64 176 -96
WIRE 416 -32 416 -96
WIRE -368 16 -368 -96
WIRE 176 96 176 16
WIRE 208 96 176 96
WIRE 336 96 336 64
WIRE 336 96 272 96
WIRE 400 96 336 96
WIRE 416 96 416 32
WIRE 416 96 400 96
WIRE 576 96 576 64
WIRE 576 96 528 96
WIRE 656 96 576 96
WIRE 656 128 656 96
WIRE 400 160 400 96
WIRE 528 160 528 96
WIRE -368 352 -368 96
WIRE -144 352 -368 352
WIRE 336 352 336 320
WIRE 336 352 -144 352
WIRE 384 352 336 352
WIRE 400 352 400 240
WIRE 400 352 384 352
WIRE 528 352 528 240
WIRE 528 352 400 352
WIRE 656 352 656 192
WIRE 656 352 528 352
WIRE 176 528 176 96
WIRE 384 528 384 352
WIRE 128 592 16 592
WIRE -144 624 -144 352
WIRE 16 656 16 592
WIRE -368 736 -368 352
WIRE 16 768 16 736
WIRE -368 896 -368 816
WIRE -192 896 -368 896
WIRE -144 896 -144 688
WIRE -144 896 -192 896
WIRE 16 896 16 848
WIRE 16 896 -144 896
WIRE 176 896 176 592
WIRE 176 896 16 896
WIRE 384 896 384 592
WIRE 384 896 176 896
WIRE -192 928 -192 896
FLAG -192 928 0
FLAG 336 64 L1_c
FLAG 336 320 L1_r
FLAG 576 64 sec
SYMBOL cap 272 80 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 47n
SYMATTR SpiceLine Rser=0.01
SYMBOL res 160 -80 R0
SYMATTR InstName R1
SYMATTR Value 440k
SYMBOL voltage -368 0 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 124 Left 2
SYMATTR SpiceLine Rser=0.01
SYMATTR InstName V1
SYMATTR Value 220
SYMBOL ind2 384 144 R0
SYMATTR InstName L1
SYMATTR Value 20µ
SYMATTR Type ind
SYMATTR SpiceLine Rser=0.325
SYMBOL ind2 512 144 R0
SYMATTR InstName L2
SYMATTR Value 18m
SYMATTR Type ind
SYMATTR SpiceLine Rser=230
SYMBOL cap 640 128 R0
SYMATTR InstName C2
SYMATTR Value 1p
SYMATTR SpiceLine Rser=0.01
SYMBOL voltage 16 752 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
WINDOW 3 -123 183 Left 2
SYMATTR Value PULSE(0 5 0 1u 1u 13u 800m)
SYMATTR InstName V2
SYMBOL res 0 640 R0
SYMATTR InstName R2
SYMATTR Value 1K
SYMBOL voltage -368 720 R0
WINDOW 123 0 0 Left 2
WINDOW 39 24 124 Left 2
SYMATTR SpiceLine Rser=0.01
SYMATTR InstName V3
SYMATTR Value 12
SYMBOL cap -160 624 R0
SYMATTR InstName C3
SYMATTR Value 47µ
SYMATTR SpiceLine Rser=0.1
SYMBOL cap 368 528 R0
SYMATTR InstName C4
SYMATTR Value 220n
SYMATTR SpiceLine Rser=0.01
SYMBOL Misc\\SCR 160 528 R0
SYMATTR InstName U1
SYMATTR Value mcr8sn
SYMBOL diode 432 32 R180
WINDOW 0 24 64 Left 2
WINDOW 3 -33 97 Left 2
SYMATTR InstName D1
SYMATTR Value UPSC600
TEXT -208 1000 Left 2 !.tran 0 1000m 0 startup
TEXT 416 128 Left 2 !K L1 L2 1
TEXT 696 56 Left 2 ;1p imitates the capacitance\nat the secondary.
TEXT 600 -72 Left 2 ;ZS1052 SUL(H) V02\nTrigger\nTransformer.
TEXT 816 208 Left 2 ;Xenon flash\ntube would \nbe here.
TEXT -1256 -312 Left 2 !.SUBCKT mcr8sn anode gate cathode PARAMS:\n**************************************\n* Model Generated by CZ LAB *\n* August 27, 2001 *\n* Copyright(c) On Semiconductor *\n* All Rights Reserved *\n*Commercial Use or Resale Restricted *\n**************************************\n*Silicon Controlled Rectifier\n*MODEL FORMAT: PSpice\n+ Vdrm=960v Vrrm=960v Idrm=10u\n+ Ih=0.5ma dVdt=1.5e7\n+ Igt=0.025ma Vgt=0.7v\n* Vgt must be greater than 0.65\n+ Vtm=1.25v Itm=16\n+ Ton=2u Toff=15u\n* Where:\n* Vdrm => Forward breakover voltage\n* Vrrm => Reverse breakdown voltage\n* Idrm => Peak blocking current\n* Ih => Holding current\n* dVdt => Critical value for dV/dt triggering\n* Igt => Gate trigger current\n* Vgt => Gate trigger voltage\n* Vtm => On-state voltage\n* Itm => On-state current\n* Ton => Turn-on time\n* Toff => Turn-off time\n*-------------------------------------------------------------------------------\n* Library of Thyristor (SCR and Triac) models\n* This is a reduced version of MicroSim's Thyristor components library.\n* You are welcome to make as many copies of it as you find convenient.\n* Library of SCR models\n* NOTE: This library requires the "Analog Behavioral Modeling"\n* option available with PSpice. A model developed without\n* Behavioral Modeling was found to be very slow and not\n* very robust.\n* This macromodel uses a controlled switch as the basic SCR\n* structure. In all cases, the designer should use\n* the manufacturer's data book for actual part selection.\n* The required parameters were derived from data sheet (Motorola)\n* information on each part. When available, only "typical"\n* parameters are used (except for Idrm which is always\n* a "max" value). If a "typical" parameter is not available,\n* a "min" or "max" value may be used in which case a comment is\n* made in the library.\n* The SCRs are modeled at room temperature and do not track\n* changes with temperature. Note that Vdrm is specified by the\n* manufacturer as valid over a temperature range. Also, in\n* nearly all cases, dVdt and Toff are specified by the\n* manufacturer at approximately 100 degrees C. This results in a\n* model which is somewhat "conservative" for a room temperature\n* model.\n* The parameter dVdt (when available from the date sheet) is used\n* to model the Critical Rate of Rise of Off-State Voltage. If\n* not specified, dVdt is defaulted to 1000 V/microsecond.\n* A side effect of this model is that the turn-on current, Ion,\n* is determined by Vtm/(Ih*Vdrm). Vtm is also used as the\n* holding voltage.\n* Main conduction path\nScr anode anode0 control 0 Vswitch ; controlled switch\nDak1 anode0 anode2 Dakfwd OFF ; SCR is initially off\nDka cathode anode0 Dkarev OFF\nVIak anode2 cathode ; current sensor\n* dVdt Turn-on\nEmon dvdt0 0 TABLE {v(anode,cathode)} (0 0) (2000 2000)\nCdVdt dvdt0 dvdt1 100pfd ; displacement current\nRdlay dvdt1 dvdt2 1k\nVdVdt dvdt2 cathode DC 0.0\nEdVdt condvdt 0 TABLE {i(vdVdt)-100p*dVdt} (0 0 ) (.1m 10)\nRdVdt condvdt 0 1meg\n* Gate\nRseries gate gate1 {(Vgt-0.65)/Igt}\nRshunt gate1 gate2 {0.65/Igt}\nDgkf gate1 gate2 Dgk\nVIgf gate2 cathode ; current sensor\n* Gate Turn-on\nEgate1 gate4 0 TABLE {i(Vigf)-0.95*Igt} (0 0) (1m 10)\nRgate1 gate4 0 1meg\nEgon1 congate 0 TABLE {v(gate4)*v(anode,cathode)} (0 0) (10 10)\nRgon1 congate 0 1meg\n* Main Turn-on\nEItot Itot 0 TABLE {i(VIak)+5E-5*i(VIgf)/Igt} (0 0) (2000 2000)\nRItot Itot 0 1meg\nEprod prod 0 TABLE {v(anode,cathode)*v(Itot)} (0 0) (1 1)\nRprod prod 0 1meg\nElin conmain 0 TABLE\n+ {10*(v(prod) - (Vtm*Ih))/(Vtm*Ih)} (0 0) (2 10)\nRlin conmain 0 1meg\n* Turn-on/Turn-off control\nEonoff contot 0 TABLE\n+ {v(congate)+v(conmain)+v(condvdt)} (0 0) (10 10)\n* Turn-on/Turn-off delays\nRton contot dlay1 825\nDton dlay1 control Delay\nRtoff contot dlay2 {290*Toff/Ton}\nDtoff control dlay2 Delay\nCton control 0 {Ton/454}\n* Reverse breakdown\nDbreak anode break1 Dbreak\nDbreak2 cathode break1 Dseries\n* Controlled switch model\n.MODEL Vswitch vswitch\n+ (Ron = {(Vtm-0.7)/Itm}, Roff = {Vdrm*Vdrm/(Vtm*Ih)},\n+ Von = 5.0, Voff = 1.5)\n* Diodes\n.MODEL Dgk D (Is=1E-16 Cjo=50pf Rs=5)\n.MODEL Dseries D (Is=1E-14)\n.MODEL Delay D (Is=1E-12 Cjo=5pf Rs=0.01)\n.MODEL Dkarev D (Is=1E-10 Cjo=5pf Rs=0.01)\n.MODEL Dakfwd D (Is=4E-11 Cjo=5pf)\n.MODEL Dbreak D (Ibv=1E-7 Bv={1.1*Vrrm} Cjo=5pf Rs=0.5)\n* Allow the gate to float if required\nRfloat gate cathode 1e10\n.ENDS\n*$
Click to expand...


here is the LTspice simulation as an attachment.


View attachment trigger transformer _SCR 1 text.txt

Here is the model for the mcr8sn SCR

View attachment mcr80.txt
 

grizedale said:
Actually i have here the trigger circuit schematic...
This is really it...
Click to expand...
What happened to C6, the capacitor that caused all the confusion? Suddenly it's gone.8-O

The diode does seem to be in a sensible place now.
 
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