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I am currently working on an LLC resonant converter. While reading an application note from ST microelectronics, I found something about using different rectifiers for different purposes.

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I understand that we would have half the voltage in a center-tap rectifier. But I don't understand why center-tap rectifiers are preferred for high current. Also, if we want less voltage, we can reduce the number of turns of the secondary windings and use a full-bridge rectifier.

Why is the voltage/current rating for these rectifiers different?

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Note this is true in general, not specific to LLC converters.

The RMS current in each winding is 71% that of the output. This reduces stress on the windings themselves*, on the rectifier components, and costs fewer voltage drops (while either polarity is conducting, FWB has two diodes connected in series).

*Note that the transformer needs to be slightly larger (more VA capacity), due to each winding being in use only half the time. That is: for a given output, the FWCT winding has twice the output voltage, but needs more than half the current rating, of the FWB case. So slightly more wire must be used.

This makes push-pull (or FWCT) preferable at low voltages and high currents (generally: low switching impedances).

We may also prefer push-pull because its (N-type) switches are ground-referenced. Synchronous rectifier controller ICs are readily available in this configuration, but in relatively short supply for bridge configurations.

Conversely, half-bridge (or full-wave doubler rectifier) topology is preferable at high voltages and low currents (generally: high switching impedances). The rectifier delivers a DC output of the peak-to-peak voltage, saving on winding length and stray capacitance. Or in reverse: the half-bridge inverter delivers an average magnitude VIN/2 to the load, so needs to deliver twice as much current for the same output capacity (VAs).

Full bridge is most practical for middling impedances, and where the difficulty of high-side drive is acceptable (e.g., industrial inverters, where isolated gate drivers must be used for safety reasons anyway).

Indeed, even more generally speaking: these circuits [PP and half bridge] are series-parallel transformations of each other, differing by a factor of 2 in their impedance (relative to the full-bridge case, which can also be seen as the combination or hybrid case of both). And they can be extended even further still, as in current/voltage multiplying rectifier circuits, or multi-level inverters. The inverters have the added feature that, the legs don't all have to switch at the same time, so a smoother output can be generated, such as for reduced harmonics in motor drives.

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Half-bridge rectification is popular in LLC topologies because only two diodes are needed. Furthermore, as you can see below, synchronous rectification is easy to implement considering ground-referenced control:

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The voltage stress for a diode with this configuration is \$2V_{out}\$ while it is only \$V_{out}\$ in a full-bridge rectifier. The full-bridge synchronous rectifier would however require a more complicated drive circuitry and is rarely seen in commercial products.

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