FAQ
TL;DR: Need an LTspice IGBT model for a Tesla coil? Use a voltage‑controlled switch with ≈420 pF output capacitance; dv/dt at turn‑off can average ~4300 V/µs. “IGBTs are forgiving of high current.” [Elektroda, DAVID CUTHBERT, post #21670036]
Why it matters: This FAQ helps hobbyists and students simulate, size, and protect IGBTs when a vendor SPICE model isn’t available.
Quick Facts
- Typical IGBT limits in this thread: fast turn‑on (~50 ns), turn‑off tail (~10%), plus higher Vce(sat) and thermal impedance. [Elektroda, DAVID CUTHBERT, post #21670028]
- Practical LTspice proxy: voltage‑controlled switch with ~420 pF across it (180 pF Crss + 240 pF Cout). [Elektroda, DAVID CUTHBERT, post #21670036]
- Example device data point: at 600 V, 40 A, avg dv/dt ≈ 4300 V/µs; Esw ≈ 5.8 mJ per cycle. [Elektroda, DAVID CUTHBERT, post #21670034]
- Clamp/diode choice matters: 1N4007 is slow; use ultra‑fast diodes for flyback/clamp paths. [Elektroda, DAVID CUTHBERT, post #21670046]
- Thermal pointer: MUR460 ~70 °C/W; keep ≲2 W in free air unless heatsinked. [Elektroda, DAVID CUTHBERT, post #21670054]
How do I simulate the NGTB40N120FLWG in LTspice without a vendor model?
Use a voltage‑controlled switch as the IGBT and place ~420 pF across it to emulate output plus reverse‑transfer capacitance. This lets you explore primary energy transfer, clamp behavior, and dv/dt. It is not identical to an IGBT, but it’s good enough to validate topology, stresses, and timing before hardware. Start conservative on voltages and duty cycle, then iterate. “The switch is not quite the same as an IGBT but it gives you a way to experiment.” [Elektroda, DAVID CUTHBERT, post #21670036]
What limits should I expect for IGBT switching in a Tesla‑coil driver?
Expect fast turn‑on (~50 ns), but turn‑off includes a tail current about 10% of peak, extending for a few hundred ns. These behaviors raise switching loss and affect primary ring‑down. Plan your frequency and duty cycle with this in mind, and ensure strong gate drive for clean transitions. High forward drop and die thermal impedance further limit continuous current. [Elektroda, DAVID CUTHBERT, post #21670028]
What dv/dt and switching loss numbers should I design around?
A referenced data point is ~4300 V/µs average dv/dt at turn‑off with 600 V and 40 A. Total switching loss about 5.8 mJ per event under those conditions. At 1 kHz this alone is ~5.8 W; add conduction loss Vce(sat) × I × duty. Example: 3.5 V × 40 A × 10% ≈ 14 W, totaling ~20 W. [Elektroda, DAVID CUTHBERT, post #21670034]
Can I stack IGBTs in series for higher voltage?
It’s possible but not beginner‑friendly. Stacked devices need isolated, high‑current gate drives for fast OFF transitions and reliable dynamic voltage sharing. Passive RC sharing helps only at low speed. A flyback topology with a proper clamp is a simpler, safer start than series stacks. [Elektroda, DAVID CUTHBERT, post #21670036]
How should I drive the IGBT gate—can a 555 or op‑amp work?
You need a low‑impedance gate driver that can source and sink high peak current to charge and discharge gate charge quickly. A simple 555 or op‑amp output is insufficient without a push‑pull buffer or a dedicated driver IC. “Driving an IGBT fast takes much less gate drive than a MOSFET,” but you still need current. [Elektroda, DAVID CUTHBERT, post #21670028]
What is dv/dt, and why does it matter here?
dv/dt is the rate of voltage change. High dv/dt at IGBT turn‑off stresses clamps, diodes, and insulation, and can couple into controls. At 600 V and 40 A, an average ~4300 V/µs was cited, so layout, diode speed, and snubbing are critical to avoid overshoot or avalanche. [Elektroda, DAVID CUTHBERT, post #21670034]
How do I size a flyback clamp for the primary?
Use a fast diode stack to a clamp capacitor and bleed resistor. Choose C so that dumping the primary’s energy raises its voltage by ~10% per event. Example: 20 A in 100 µH stores 20 mJ; 1 µF would rise about 200 V. Select R to hold the desired DC bias (e.g., ~800 V). [Elektroda, DAVID CUTHBERT, post #21670044]
Which diodes should I use for the clamp and series blocking?
Avoid 1N4007. It has slow reverse recovery and even slow forward recovery (~1 µs), causing reverse conduction and losses. Use ultra‑fast diodes like MUR‑class parts for both the clamp and series blocking paths. Parallel identical diodes can share without ballast if kept cool. [Elektroda, DAVID CUTHBERT, post #21670054]
Do I need a series blocking diode with the IGBT?
Yes, in a flyback‑style primary drive. After turn‑off, the IGBT node flies above the supply and then below ground. A series blocking diode prevents reverse voltage across the IGBT, while the clamp stops overvoltage. Push‑pull drivers self‑clamp but need floating high‑side drive. [Elektroda, DAVID CUTHBERT, post #21670046]
How do I account for primary LC current during gating?
Use V = L di/dt to estimate ramp during ON time. With a parallel tank capacitor, initial charging current can be very high unless limited by series resistance or finite turn‑on speed. Add a series resistor to move heat out of the IGBT during charge. [Elektroda, DAVID CUTHBERT, post #21670044]
What’s a simple way to start up and tune the clamp safely?
Three‑step bring‑up: 1) Start with reduced HV and monitor the clamp node with a DVM. 2) Adjust the bleed resistor to reach target clamp voltage while observing collector waveform. 3) Run briefly, then thermal‑check components and iterate. [Elektroda, DAVID CUTHBERT, post #21670054]
Can large electrolytics work as the clamp capacitor?
Yes, but beware voltage rating and total energy. Using larger C reduces voltage rise per pulse, but increases stored energy and fault stress. Series electrolytics need balance and clearance. Microwave oven capacitors can work if their ESR and voltage ratings fit your pulse duty. [Elektroda, Cody Gass, post #21670053]
How hot is too hot for parts in a hobby Tesla coil?
For non‑product builds, pushing parts is acceptable with caution. Running semiconductor junctions near 150 °C, resistors at rated power, and capacitors with minimal derating can be acceptable if you monitor temperature and provide airflow. This is an experimental tradeoff. [Elektroda, DAVID CUTHBERT, post #21670051]
Why did my LTspice show diodes conducting negative current?
Slow diodes exhibit reverse recovery; they conduct briefly in reverse when switching, which appears as negative current in simulation. This can overheat parts and distort waveforms. Replace with ultra‑fast diodes to reduce reverse recovery charge and ringing. [Elektroda, DAVID CUTHBERT, post #21670046]
What’s a practical audio/modulation approach for musical coils?
Modulate primary current rather than trying to linearize voltage. Sense current with a small shunt and use it in a control loop, or convert audio amplitude to pulse width so higher amplitude yields longer ON time. This avoids complex loop stability issues. [Elektroda, DAVID CUTHBERT, post #21670046]
What is a flyback vs. push‑pull primary drive in this context?
Flyback charges primary current then transfers energy on turn‑off; it needs a clamp and blocking diode but a simpler supply. Push‑pull alternates two switches, is self‑clamping, but needs higher supply voltage and a floating high‑side driver. Choose based on skills and parts. [Elektroda, DAVID CUTHBERT, post #21670046]
