The bus bank: sizing the reservoir

Two numbers decide this part, and neither of them is the capacitance somebody else used. Most of the page below is those two numbers and what they cost you; the rest is the handful of things about a charged bank that are worth being frightened of.

What the bank is for

The mains delivers in bursts, on the tops of the sine wave. The bridge draws whenever it is switched on. Between the two sits a bank of capacitors, and its job is to answer while the mains is not looking.

On a DRSSTC the bang is short and the bank barely notices it. On a QCW the ramp runs 10 to 25 milliseconds, which is long enough to pull the bus down while the arc is still growing. The last centimetres of an arc are the dearest ones to buy, so that is the worst possible moment to run short.

Everything the coil throws was sitting in this bank a moment before it left.

What you decide

Whether to fit a doubler

The criterion is not volts. It is what fraction of the bank's stored energy one bang consumes.

E = C·V² / 2

Doubling the voltage quadruples the store at the same capacitance. One 406 J bang against a 9900 µF bank:

• 325 V, a plain bridge on 230 V mains: the bank holds 523 J and the bang eats 78 per cent of it.
• 424 V, an intermediate bus rather than a topology of its own: 890 J, and 46 per cent.
• 650 V, a Delon doubler: 2091 J, and 19 per cent.

Below about a fifth the ramp holds its shape. Near half it visibly sags, and past that the large bank has stopped being large.

A doubler also takes both half cycles, and it pulls the bus practically to zero every cycle, so no residual voltage is left to branch the arc at the start of the ramp.

What it costs is higher-rated parts down the whole chain, and one that is easy to miss: the isolation rating on gate drive DC-DC modules. A module rated 400 V on a 650 V bus is a blocker nobody notices until something fails. How the three arrangements are built is on the rectifier's own page.

How much capacitance

The criterion is the sag over one bang.

dV = (I_avg × t_ramp) / C

At 200 mA over 22 ms, which is 4.4 millicoulombs:

• 470 µF sags about 9 V, and the top of the ramp collapses.
• 2200 µF sags about 2 V.
• 4700 µF sags about 1 V.

What it costs is ripple current. The bank delivers in bursts and refills in bursts, and both heat it through its ESR. On electrolytics that is a rated parameter and it is not one to exceed, so the part that satisfies the sag has still to satisfy the heating.

Whether the source behind it is stiff

The criterion is how fast the bank refills between bangs, which is not the same question as whether the supply can carry the average.

The mains through a doubler refills willingly: the mains impedance is a fraction of an ohm. A transformer's leakage inductance limits the rate. On average heating a transformer is comfortable, because a QCW's average power is low, and that is exactly what makes the trap: the number that looks fine is not the number that matters.

How the bank gets charged in the first place

A bank of thousands of microfarads at switch-on is a short circuit across the mains. A resistor in the AC line before the rectifier, shorted out by a relay after a few seconds.

What will get you

For indication, a bright LED with a dropper resistor, which bleeds the bank slowly at the same time, and a neon as backup. The neon goes out below about 90 V, and ninety volts across thousands of microfarads will still hurt you badly, so it cannot be the only indicator.

The wire element inside a precharge resistor weighs about three grams and jumps three hundred kelvin instantly. Only afterwards does the heat spread into the fifty gram body, for a ten degree rise. Repeated jumps fatigue it, so leave a minute between starts.

And the big bank on its own will not feed the bridge. The inductance of the power cable will not let it deliver quickly, so the bridge needs capacitors of its own beside it. The same goes for driver supplies: a local electrolytic plus a fast ceramic bypass right at the pins.

The numbers in one place

• Bus from 230 V mains: about 325 V through a plain bridge, about 650 through a Delon doubler.
• Bang energy as a fraction of the bank: under about a fifth and the ramp holds its shape; near a half and it visibly sags. Derived from the worked cases above, not measured.
• Sag to aim at over a ramp: 1 to 2 V.
• A bank that achieves it: 4700 to 9900 µF at the currents above.
• Precharge time constant behind a doubler: twice R·C.
• Precharge element: about 3 g, and about +300 K per start.
• Neon extinguishes: about 90 V, which is not a safe bank.

Figures here are from the component notes this site is built on, and the arithmetic on them is ours.

What goes wrong

• The top of the ramp is flat, or collapsing. Bus sag. It looks exactly like a tuning problem and it is a supply problem, which is why it is at the top of this list.
• The arc branches in the first milliseconds of the ramp. Residual bus voltage left from the previous bang. A doubler pulls the bus to zero every cycle; a half wave supply does not.
• A thump of inrush when the precharge relay closes. The delay was set for a plain rectifier and there is a doubler behind it. Size it on the doubled time constant.
• A precharge resistor fails well inside its rating. The rating was continuous. The element inside jumped three hundred kelvin.
• The bank runs hot with nothing else wrong. Ripple current exceeded. It heats on the delivery and on the refill both.
• The bus does not recover between bangs on a transformer supply. Leakage inductance. The average heating is fine and the refill rate is not.
• Gate drive dies as soon as the bus is raised. The isolation rating on the DC-DC modules.

Where next

• The rectifier, which fills this bank: the three arrangements and how to pick the diodes.
• The buck modulator, which is what draws from it.
• A high bus and a low bridge, on using 650 V without putting it on the devices.

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The bus is the third box on every department diagram, and the one everything else is drawn downstream of.