Elegant Startup Solution for BCMs Powering High Capacitance Loads

Some applications, such as radar systems and LED displays, require the connection of a large capacitance at the load input to accommodate a high pulsed power demand. This can present a problem to some power systems during startup if the input capacitance of the load exceeds the specified maximum effective output capacitance of the upstream converter. The high inrush current required to charge the capacitance will trigger the short circuit protection of the upstream converter, potentially disabling the module and preventing startup.

A major advantage of the BCM’s Sine Amplitude Converter topology is that bulk output filtering capacitance is not required to ensure proper operation. The transformer-based topology allows large capacitances typically connected at the converter output (i.e. at the load input) to be more densely and efficiently provided at the BCM primary. However, in applications where the load capacitance cannot be relocated to the input of the BCM, a simple precharge circuit can be used to allow the BCM to start up properly when the required load capacitance far exceeds the specified limit.

Simple BCM precharge circuit

Simple BCM precharge circuit to delay connection of COUT and limit inrush charging current (click to enlarge)

By allowing the BCM to first reach steady state operation, the capacitor charging can be treated as a load transient event. Increasing the charging time of the capacitance also reduces the pulsed load current requirement on the BCM, ensuring that the module does not go into the short circuit protection mode. This can be accomplished by using a MOSFET to keep the output capacitance disconnected during startup.  A resistive divider connected to the MOSFET gate sets the BCM output voltage threshold for connecting the load capacitance. An additional gate capacitor delays the turn on of the MOSFET by a user defined tCap_Delay.

Conceptual BCM precharge circuit waveforms

Conceptual BCM precharge circuit waveforms

The capacitor charging inrush current is limited to a manageable level below the maximum continuous secondary output current, ISEC_OUT_DC, by the slow turn on transition of the MOSFET. This simple and elegant solution ensures that applications with high capacitance loads can still benefit from the performance of the BCM. More information about the operation and design of the precharge circuit is available in our application note.

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