Four transistors, or two, arranged so that they can connect the primary across the supply one way round and then the other. That is the whole idea, and everything else in the machine exists to serve it.
What it is and why
The secondary is a resonator. It rings at its own frequency, and to build a voltage on it you have to push it in time with that ringing, the way you push a swing. Push always in one direction and nothing happens.
So the bridge takes a direct voltage from the bus and turns it into an alternating one at the coil's frequency. That is all an inverter is, and this one just runs at a few hundred kilohertz instead of fifty.
How the four are used. Label them by leg: two on the left, high and low, and two on the right. To push one way, the top-left and the bottom-right turn on together, so the primary sees the bus in one polarity. To push the other way, the other diagonal turns on: bottom-left and top-right. The two diagonals take it in turn, and the primary sees the bus reversed each half cycle.
What it is pushing into is not a resistor. The primary, and on a doubly resonant coil the tank capacitor in series with it, form a tuned circuit, and how that circuit behaves depends on whether the bridge is running above, below or exactly at its resonance. That is why so much of coiling turns out to be about timing.
What you decide
Half bridge or full
The criterion is current, because current is the expensive one: conduction loss goes as its square.
A half bridge has one leg instead of two, and the primary connects between that leg and a midpoint made by two capacitors. That puts half the bus across the primary, so for the same power it needs twice the current.
- Devices: two instead of four, so roughly half the cost of the power section.
- Arc: about two thirds. A quarter of the energy per bang, and length goes roughly as the cube root of energy, which comes out at 0.63.
Nearly every serious coil is a full bridge. The half is a sensible first machine and an unsatisfying second one.
Which devices go in it
A decision of its own, and it turns on frequency more than on current: IGBT or MOSFET.
What will get you
The heatsink temperature is not the temperature that kills the device. The sink is a baseline and the die swings above it. The size of that swing tears bond wires over thousands of cycles, long before anything on the sink reads near a rated figure.
The numbers
- A half bridge across the primary: half the bus, so twice the current for the same power.
- A half bridge's arc: about 0.63 of a full bridge's. Derived from a quarter of the energy and the cube-root law, not measured.
- Devices: four against two.
What goes wrong
- Both devices in a leg destroyed instantly on first power-up. Shoot-through. Check the dead time, and check that nothing has inverted one gate signal relative to the other.
- Devices die after minutes rather than instantly, with the heatsink barely warm. Die temperature swing, not average heat. Look at the bang rate before you look at the cooling.
- Devices die at one particular tuning and nowhere else. The switching instant relative to the current zero, which is what a phase lead sets.
- Everything looked right on the scope with the bus off. It would. The Miller plateau does not appear without bus voltage, so the gate waveform you set the gate resistor on was half the story.
Where next
- IGBT or MOSFET, which device and why.
- The bridge in detail, layout and the failure mechanisms.
- The gate transformer, how the signal reaches the floating half.
The bridge is drawn out rather than boxed on every department diagram, because whether it is a full one or a half is the first decision anybody makes, and the toggle beside the drawing shows what the arc does about it.