The DC blocking capacitor: SSTC or DRSSTC by value alone

There is a capacitor between the bridge and the primary on an SSTC too. Its value, and nothing else, decides whether you have built an SSTC or a DRSSTC. People find this out by accident, and the way they find out is that the transistors explode.

What it is and why

A bridge does not switch perfectly symmetrically, so a DC component appears. To a coil, DC is very nearly a short circuit, and the sequence from there is short: a DC offset unbalances the core, the unbalance runs away, the core saturates, the current goes up, and the transformer and the transistors die together.

So there is a capacitor in series with the primary, to keep DC out of it. On a full bridge, in series. On a half bridge, in series, or as two capacitors forming a divider with the primary to their midpoint.

What you decide

There is one decision and it is the whole article, which is why this page is short.

Whether it resonates with the primary

A capacitor in series with a coil is a resonant circuit. If its resonance comes anywhere near the working frequency then you have unintentionally built a DRSSTC, and a DRSSTC without overcurrent protection destroys itself.

Criterion, for it to stay an SSTC: the capacitor has to behave like a piece of wire.

X_C below 2 ohms at the working frequency
the primary circuit's resonance several times below the working frequency

Worked, on two 0.68 µF films in parallel, which is 1.36 µF:

X_C = 0.47 ohm at 250 kHz            practically a link
primary resonance about 43 kHz       six times below the working 250

The author of that design justified the margin as five times below the resonant frequency being far enough that the DRSSTC condition cannot appear.

What sort of part

Very low ESR, which means the same MKP film parts that go into a tank bank. This capacitor carries the full primary current and there is no version of it that is a general-purpose component.

What will get you

Three sums before the first power-up, and there is no fourth outcome where it does not matter:

1. X_C at your working frequency. It should come out below 2 Ω.
2. The resonance of your primary inductance with that capacitor. It should be several times below the working frequency.
3. If the second fails, either increase the capacitance, or accept that you are building a DRSSTC and fit current protection.

The numbers

• X_C to stay a link: under 2 Ω at the working frequency.
• Margin on the primary resonance: several times below the working frequency. Five is the figure the worked design used; the example achieves six.
• The worked case: 1.36 µF gives 0.47 Ω at 250 kHz and a primary resonance near 43 kHz.

What goes wrong

• Transistors explode on a coil that was designed as an SSTC. The blocking capacitor resonates with the primary somewhere near the working frequency. Do sum two.
• The current is far higher than the primary's reactance allows. Same cause. On a true SSTC the primary's own reactance limits the current, and that is the only thing limiting it.
• The core saturates and the current runs away over seconds. DC got through, which is this capacitor failed or was never fitted.
• The capacitor gets hot. ESR. It is carrying the primary current, so it wants a film part, not whatever was in the drawer.

Where next

• What the bridge does, which is what this sits between.
• Sizing a tank capacitor by RMS current, the same component chosen the other way.
• Overcurrent is not a setting, which you need the moment sum two fails.

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The SSTC department diagram draws the tank across the middle of the bridge with no capacitor in it, which is the whole difference between that department and the next. On the QCW page you can take the capacitor out yourself and watch what happens to the arc.