A QCW is a coil with a converter bolted between its rectifier and its bridge, and everything the department is named for happens inside that converter. It raises the bus smoothly over ten to twenty five milliseconds. That is the ramp, and the ramp is what turns a branching spark into a sword.
Why it is a whole extra converter
The obvious shortcut is to skip pulses instead: leave the bus alone and walk the current threshold up across the bang. It has been tried three times in public, by Steve Conner in 2011, by Steve Ward on the UD3, and by Chris Dickey. Nobody got a sword out of it.
The reason given is that a branchless arc wants smooth continuous modulation of its power with very little ripple, and you cannot smooth the missing pulses out with the tank's own stored energy unless the tank is monstrously oversized. A buck converter is the reference for that smoothness. It also decouples the switching frequency and the energy store from the resonant tank, which is worth having on its own.
The control loop that is not a loop
duty = V_out / V_inCompute it once per cycle from the measured input voltage and drive the switch with it. That is feedforward: no error amplifier, no compensation, nothing to oscillate. The accuracy is not good, and for a Tesla coil it does not need to be.
The choke decides everything
inductor ripple dI = V_in · D(1-D) / (L · f_pwm)
output ripple dV = dI / (8 · C · f_pwm)
so dV goes as 1/(L · C · f^2)Three knobs, and they do not cost the same:
- Double the inductance. Ripple halves, and so does the discontinuous threshold. Costs size, copper and saturation margin.
- Double the switching frequency. Ripple drops fourfold, the threshold halves. Costs switching loss and gate drive: average gate current is
Q_g · f, and at 30 kHz a two watt isolated supply is already at its limit. - Double the capacitance. Ripple halves, threshold unchanged. Costs pennies.
Frequency looks like the best deal because it enters the ripple squared. It is the one with two prices.
The failure that catches everyone
When the inductor current falls to zero inside a chopping cycle the converter goes discontinuous, and a discontinuous buck does not follow its PWM. The gain rises above the duty cycle and starts depending on the load, and at nearly no load the output floats up towards the full bus at any duty at all.
The threshold is dI/2 and the cure is inductance. When you bench it, use a low resistance dummy load, ten ohms rather than thirty, or you will spend an afternoon chasing a nonlinearity the coil will never show you.
What has to be around it
A freewheel diode. An asynchronous buck does not run without one: when the switch opens the inductor current has nowhere to go. Fast, rated for the full current, and physically against the switch. Fins and air if it runs continuously.
A common trick is a half-bridge module used as a buck, top device switching and the bottom one left open purely for its body diode. Wasteful, convenient, and very common.
Bleeder resistors on the output. Without them the ramp does not come back down, because a buck can only add charge. Two of 2.2k at 200 W is the order of it. That same resistor is the converter's entire load until the arc lights.
The wick
A shelf of about 40 V for the first two milliseconds, so the arc is already alight when the voltage starts to climb.
Set it as a duty cycle and it is not a voltage: the same duty on a 325 V bus and a 650 V bus is two different shelves. Clamp it in the warm-up and in the ramp itself, not one or the other. ZakW removed the shelf entirely and used a non-linear start to the ramp instead, which does the same job without a corner in the curve.
What people actually build
CJ runs 100 µH, 40 µF at 25 kHz: 40 A of ripple, discontinuous below 20 A. gsch.labs runs 210 µH, 13 µF at 60 kHz: 7.9 A of ripple, discontinuous below 4.
CJ's ripple is four times worse and he has 2.4 metres of arc. Watch the discontinuous threshold, not the ripple.
The figures here are from published builds and the arithmetic on them is ours. The rig on the QCW department page draws the buck as what is inside it, because the choke is the reason a QCW has a ramp at all.