The bridge switches the bus onto the primary. Everything else in the coil exists to serve it, and most of what goes wrong with a coil goes wrong here.
Half or full
A half bridge puts half the bus across the primary, so for the same power it needs twice the current, and current is the expensive one: losses go as the square of it. That is why serious coils are nearly always full bridges. A half bridge also needs midpoint capacitors, and they are not decorative: the whole primary current goes through them.
Two devices against four, half the cost, half the voltage on the tank, a quarter of the energy and about two thirds of the arc.
Voltage headroom
On a 650 V bus, which is what a mains doubler gives you, fit 1200 V devices. Five hundred and fifty volts of margin is reasonable under soft switching.
Layout, in the order that matters
- Minimum inductance between the high and low sides at the output node. Under hard switching the current commutates between one side's transistor and the other's diode at a large di/dt, and that produces the overshoots you see on Vce. This has been tested the wrong way round: moving the transistors further apart made it worse.
- Overlapping copper planes for the power rails.
- Local snubber capacitors at each leg, surface mount, right at the pins.
- Generous creepage. At high frequency and high voltage, surface flashover does not behave the way it does on DC.
- No copper under the bleeder resistor.
- Gate traces thin and side by side, so the loop is small.
- No ground plane under the GDT. It makes a parasitic resonator.
The bleeder across the output is a power part
Not the one across the bus bank, which is the safety item on the bank's own page. A second one goes across the inverter output, and what it is there for is the charge left sitting on the tank capacitor when a bang ends.
Two independent builds arrive at the same fix by different routes: 6 kΩ across the inverter output, made as three 2 kΩ resistors in series, and separately 10 kΩ across a half bridge's output to discharge the MMC.
The number that makes it real is the wattage: those 2 kΩ parts are 75 W each. This is not a decorative bleeder soldered to a tag strip. It is a power element, it wants mounting and air like one, and it is why the layout list above has "no copper under the bleeder resistor" on it.
What it does to the rest of the machine, and where our reasoning starts rather than the sources': the driver is the part that suffers, directly and by measurement. The gate drive transformer suffers nothing on its own account, but it passes what the driver sends, so a glitchy output arrives at the gates as glitchy gate drive and the fault will be looked for at the transformer, which is the wrong end of it. And the timing: a bang that begins on a charged tank does not begin at the current zero the phase lead was set against, so the first cycles are switched somewhere other than where they were aimed. That last step is ours, not a measurement.
More than one bridge
When the current will not fit in one bridge, people add another. You cannot simply wire the outputs together: a bridge's output impedance is negligible, so one of them ends up doing all the work and the other fighting it. Something has to force them to share.
- Transformers. Each bridge drives its own primary, and the secondaries go in series. Since the secondaries are in series the currents must be equal, which is the whole point. Steve Ward describes it as putting the inverters in series rather than in parallel.
- A split MMC, a section per bridge in an equi-drive arrangement. Steve Ward's own note on it is that it is not as fool-proof as the transformer scheme: if a leg fails you get shoot-through between bridges.
Failure spreads by plasma
The lesson from a real post-mortem is that the damage is always wider than the part that failed, because plasma from the failed device reaches the ones beside it. Which gives three practical rules:
- Space the transistors out and give each its own heatsink.
- Use materials that take the hit. Industrial modules are potted in silicone gel for a reason.
- Consider an IGBT as a bus breaker. It opens in about a microsecond. It needs snubbers, or opening under current gives you an overvoltage of its own, and a high gate resistance so that it opens slowly.
What actually kills a bridge
The heatsink is a baseline and the die swings above it. On most coils the transient rise is larger than the heatsink's rise, so a sink that measures cool tells you very little.
And it is not only the peak temperature. The size of the swing, dT, tears bond wires and die attach over thousands of cycles, long before anything reaches Tjmax. That is the likely mechanism behind bridges that die for no visible reason weeks into their life.
The bridge is drawn out rather than boxed on every department diagram, because whether it is a full one or a half is the first decision anybody makes. Figures here are from published work and datasheets, not measured on this bench.