My SSTC's IGBT's blow up when toroid is added

[Frantisek]

Hello, I've finally finished building my first Tesla coil. Here are some shots.
But I unfortunately have a few problems with it. When I add my big Toroid on top, my transistors blow up.

Source project is on Instructibles: https://www.instructables.com/Building-the-Ultimate-Solid-State-Tesla-Coil-MUSIC/

Impedance should be over 6Ohms, which in my case definitely is. My parameters for JavaTC are following:
units=1,
ambient=1,
s_ws=1,
s_Al=0,
p_ws=1,
p_Al=0,
p_ribbon=0,
temp=20,
g_radius=0,
w_radius=0,
ceil_height=0,
s_radius1=10,
s_radius2=10,
s_height1=0,
s_height2=25,
s_turn=750,
s_wd=0.032,
p_radius1=15,
p_radius2=15,
p_height1=1,
p_height2=12,
p_turn=5,
p_wd=0.1,
p_vwidth=0,
p_rthick=0,
Cp_uF=0,
Lead_Length=0,
Lead_Diameter=0,
desired_k=0,
t.inner=8.5,
t.outer=31,
t.height=25,
TT=true,
TG=false,
x_Vin=0,
x_Vout=0,
x_Iout=0,
x_Hz=0,
x_Vadjust=0,
x_ballast=0,
rsg_ELS=0,
rsg_ELR=0,
rsg_rpm=0,
rsg_disc_D=0,
rsg_ELR_D=0,
rsg_ELS_D=0,
stat_EL=0,
stat_EL_D=0,
stat_gap=0,
SPE=true,
RGE=false

If you have any suggestions, please let me know.

Happy sparks,
Frantisek

[Mads Barnkob]

I can not tell you why your switches are blowing up, but I can tell you about the differences of no-topload and topload operation, which should makes ud able to understand why the switches blow up.

No-topload
- High resonant frequency, higher switch losses
- Low "secondary load" as less charge can be stored

Topload
- Lower resonant frequency, lower switch losses
- High initial "secondary load" as its harder for the inverter to build up the potential difference. Initial start conditions with a empty topload capacitance, could be what blows the switches.

Check impedance matching between inverter and secondary system

[Frantisek]

Hello,
thanks for quick answer. Impedance without the top load is about 9.6 Ohms (based on JavaTC and some basic calculations). With top load, it's about 7.0 Ohms.
Based on LabCoatz design, my coil should work perfectly fine (Impedance should be a bit over 6 Ohms, resonant frequency under 400 kHz).
I'm still not sure why it doesn't work. Antena connected to the PCB is about 15 cm long, and it's end is close to the primary, so that should be fine. Also the whole PCB is grounded with aluminum foil underneath it.
Also, LabCoatz (the main creator) has resonant freq of secondary around 240 kHz, so maybe I'll be forced to cut down my secondary from 25 cm to around 18 cm (so from 750 windings to around 500) to get around these 240 kHz without any top load attached.
I'll also try adjusting my antena, changing it's length and stuff. I will update this post after testing.
PCB schematic:


Happy sparks,
Frantisek

*after some thinking, my primary coil impedance is about 6,7 Ohms with toroid attached (should be over 6 Ohms for my IGBT's). Which could be the case. Also now the primary coil has impedance around 9 Ohms without any top load. I'll try primary with at least 1 more turn (which will cause the impedance to be around 10 Ohms or higher).

[AstRii]

Antenna feedback can be difficult to get right, have you tried using a transformer to sense secondary voltage instead?