Gate supply for long bangs: amps and millifarads

On an ordinary DRSSTC the gate supply is a detail. On a QCW it is not, and this is where people get it wrong repeatedly.

The reason is the length of the bang. The bridge switches for ten to twenty five milliseconds without a break, not for a few hundred microseconds. The charge accumulates.

Q per ramp = Q_g × (devices) × (transitions per cycle) × f_working × t_ramp

The three factors people leave out

1. Count the charge over the full swing. On a bipolar ±24 V supply the gate travels from −24 to +24, not from 0 to 24. The datasheet's Q_g is quoted for a stated swing and has to be scaled to yours.
2. Both devices in a leg. While the supply is charging one gate from −24 to +24, it is discharging the other from +24 to −24.
3. Twice per cycle, not once.

The order of magnitude

For a bridge device at about 400 nC over the full swing, running at 400 kHz with a 25 ms ramp. Note this is a smaller part than the buck's own switch, which is nearer 2.8 µC:

• counting only the charging of both gates: 8 mC per ramp, 0.32 A average, 4 mF for a 2 V sag;
• counting charging and discharging: 16 mC, 0.64 A, 8 mF.

And what a rail that sags actually does

The arithmetic above stops at a number of farads, which makes it look like a component-selection problem. It is not. Follow it one step further and it turns into the failure people actually see.

dV = Q / C

Take the 16 mC and put it against a reservoir chosen for a different job. At 4 mF that is a 4 V droop over the ramp; at 1 mF it is 16. The gate rail does not fall off a cliff, it sinks steadily for twenty five milliseconds.

And nothing about that is gentle when it arrives, because the driver is watching that rail. UVLO sits a few volts below the nominal, so a supply sagging by more than that does not degrade the edges, it switches the driver off part of the way through the bang — and it does so mid-ramp, at the point where the arc is longest and the bus is highest. The bridge stops driving while the tank is still full.

The reservoir sizing above is therefore not "enough to keep the edges pretty". It is enough that the droop stays under the UVLO's margin for the length of the longest ramp you intend to fire — which is a different and larger number, and it is the one to design to.

Local beats large

A local electrolytic, something like 470 µF, plus a fast ceramic bypass right at the pins, delivers current before the supply cable's inductance has time to notice. A big bank at a distance does not save you, because the cable is in the way.

And the buck's own switch

The buck's gate supply has two separate traps of its own, and both are quiet killers.

There is a third, at the switching frequency: the average gate current is Q_g · f and grows linearly, so at 30 kHz a two watt isolated module is at its limit and at 60 kHz you want five. That one is with the choke, because it is the hidden price of the frequency knob.

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The gate transformer on the QCW diagram has the driver coming up into it from below and the gates coming down from above, and this is what has to be behind the driver's own supply pin for a ramp that lasts twenty five milliseconds.