Diagnosing a gate waveform: symptom, cause, cure

Three different faults look similar on a scope and their cures are mutually exclusive. Without knowing which is which you turn the resistor and make it worse, and the usual first move is the wrong one.

What you are aiming at

flat top and bottom · steep edges · small overshoot at the transitions
NO RINGING AFTER THE TRANSITIONS · rise and fall under 200 ns

What goes wrong

• Low frequency ringing. Too much leakage inductance. Rewind the transformer. A damping resistor does not fix this, and reaching for one is the usual first move.
• High frequency ringing. Not enough damping. Add a series damper and work on the leakage.
• Overshoot at the transitions. Not enough damping. Increase the resistor in small steps.
• Rounded edges, slow rise. Too much damping. Decrease it.
• A shark fin. Either over-damped, or the driver is too weak for that gate capacitance. Reduce the damper or get a bigger driver.
• Sag on the top or the bottom. Not enough primary inductance: too few turns. More turns, or a core with a higher A_L. Up to about 2 V of sag is acceptable.
• A loud buzz from the transformer. The core is saturating, and the risk is at the low frequency end, not the high one. See the numbers below.

What will get you

Thirty to fifty volts is the minimum for the plateau to show up; a hundred or more for the overshoots to be realistic. Probe through an isolated supply, and note that the Miller step on Vge is normal, not a fault.

What you change, and in what order

The transformer, before the resistor

• Ratio 1:1 usually, sometimes 1:1.5 or 1:2.
• Under thirty turns per winding: fewer turns couple better.
• An ungapped ferrite ring with enough A_L to reach the turns you need in one layer, and roll-off to spare at your frequency — which is a calculation and not a permeability to buy against, and it is the transformer's own page that does it. Powdered iron is worse than ferrite; nanocrystalline alloys are better.
• Each winding in a single layer.

Wind it as tightly twisted pairs, each secondary wire paired with its own primary. Separate windings give high leakage and overshoot. But twisting past the point where the pair holds together increases leakage again, because the wire gets longer. Tight enough to stay together, no more.

The inductance of a GDT's own leads is usually far larger than anything on the board, so this is a transformer construction problem and not a routing problem.

Then the damping

Diodes across the gate resistors give asymmetric drive and a fast turn-off. Ferrite beads on the twisted pairs deal with high frequency oscillation. Why the order matters, and what value the resistor should be, is the gate loop is an RLC.

And the things that quietly spoil it

• No ground plane under the GDT. Capacitance to the switching node plus the inductance of the ground wire makes a parasitic resonator.
• A non-1:1 ratio loads the driver as the square of the ratio. 1:1.5 is more than twice the load.
• Solid versus stranded wire makes almost no difference. Twist and lead length do.

Three resistors near the gate drive that are not part of it

They are here because this is where a repair happens, and a repair is exactly when they get confused with the gate drive's own resistors. None of these changes the waveform you are reading.

• The current transformer burdens. Two of them, different values, adjacent on the board, and swapping them changes what the driver believes about the tank rather than anything about the gates. A clean gate waveform and a coil that behaves strangely is this, not the transformer. The values and what they are for are with the driver.
• The bleeder across the bus bank, a safety item and mandatory. Nothing to do with the gates at all; listed so it is not mistaken for something optional.
• The bleeder across the inverter output, which is a power part: 6 kΩ as three 2 kΩ of 75 W each. This one does reach the gate waveform, but from the far end — residual charge on the tank gets back to the driver and comes out as glitchy switching, and the gate drive is where people then go looking, which is the wrong end of the machine. It lives with the bridge.

The numbers

• Edges to aim at: under 200 ns.
• Acceptable sag: up to about 2 V.
• Bus voltage for a meaningful measurement: 30 to 50 V minimum for the plateau, 100 or more for realistic overshoots.
• Saturation: the core's V·µs rating must exceed the largest applied V·t, and V·s per turn = core area × B_sat. Worked: 95.8 mm² × 0.22 T is 21 µV·s per turn; at a 2.1 µs half cycle that allows up to 10 V per turn, which is where a 4:5 winding with margin comes from.

And on a QCW, the gate supply is a power supply

For long bangs the gate charge has to be counted over the full rail-to-rail swing, for both devices in a leg, twice per cycle, across the whole length of the bang. That comes out in amps of average current and millifarads of capacitance, which is not a decoupling problem any more, and it is its own piece.

Where next

• The gate loop is an RLC, which is where the resistor value comes from.
• What a gate transformer is for, if the question is why there is a transformer at all.

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The GDT is drawn as one wound core per leg on the department diagrams, so two on a full bridge and one on a half. That is what a freewheeling driver wants; how many a bridge actually needs depends on how it is modulated. The gates come down into it from above and the driver comes up from below, and nothing is threaded through them: a line drawn through both would be a skewer, not a winding.