Wednesday, 8 February 2012

Synchronous Buck Converter Circuit

Synchronous Buck Converter Circuit

This is a design for synchronous buck converters. A synchronous buck converter consists of a high side and a low-side MOSFET, which is placed in place of the conventional buck converter catch diode to provide a lower loss path for the load current. Shoot-through leads to current spikes at the switching instants and manifests itself as a decrease in the efficiency of the converter. A current probe cannot be used to measure it because the inductance of the probe significantly affects the circuit operation. An alternative way to detect shoot through is by looking for spikes on the gate source voltages of the two FETs. (The gate-source voltage of the top MOSFET can be monitored differentially). This is the figure of the circuit;

One approach is to employ a controller IC with a “fixed dead-time,” which ensures that there is a delay after the top MOSFET is turned-off before the lower MOSFET is turned on. This approach is simple, but has to be implemented carefully. If the dead time is too short, shoot-through may not be averted. If it is too long, the conduction losses increase because during the dead time the body diode of the bottom FET is on. Because of the conduction of this body diode during the dead-time, the efficiency of the system when using this technique depends somewhat on the bottom MOSFET’s body diode characteristics.

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