Power Design Tips 16: Buffer Forward Converter - Eliminate Buffer Picking Troubles

Photocoupler

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Have you been bothered by how to pick a buffer assembly? Calculating how much capacitance and resistance to add is a challenging task. Here is a shortcut to solve this problem.

The converter is operated by a transformer that couples the input voltage to a secondary circuit that is then rectified and filtered by the secondary circuit. The reflected main voltage and the transformer leakage inductance form a low impedance circuit. When D2 is forced to commutate off through such a resistor, a buffer is usually required. D2 can be a silicon pn diode with a reverse recovery charge that must be depleted before it is turned off. This loads up the excess current in the leakage inductance, resulting in high frequency ringing and excessive diode voltage. A similar situation exists for Schottky diodes and synchronous rectifiers, the former because of its large junction capacitance and the latter because of its turn-off delay time.

In the top trace, you can see that when Q1 is turned on, its drain voltage is reduced below the input voltage, which causes the diode D1 current to increase. If D2 does not have a reverse recovery charging function, the node voltage will rise when the D1 current is equal to the output current. Since D2 has a reverse recovery charging function, the D1 current is further increased, which begins to consume charge. Once the charge is depleted, the diode is turned off, causing the increased junction voltage to increase further. Note that the current will continue to increase until the junction voltage is equal to the reflected input voltage because there is a positive voltage across the leakage inductance. As the current increases, this current will charge the parasitic capacitance and cause more ringing and loss in the circuit.

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