Why the Picor Cool-Swap PI2211 simplifies power MOSFET designs – even for hot swap applications

May 28, 2012
Picor Cool-Swap

Cool-Swap® – PI2211

The Picor Cool-Swap PI2211 hot swap controller and circuit breaker is a single chip device that allows power system designers to control a power MOSFET accurately and safely, even in hot swap applications such as server backplane architectures.

The PI2211 provides significant savings in PCB real estate as well as simplifying power system design. This is due partly to its single-chip implementation, but also because its Glitch-Catcher™ voltage suppression technology eliminates the need for separate voltage suppression components. Additionally, the True-SOA™ feature allows accurate yet safe MOSFET control, so smaller devices with less safety oversizing can be specified.

The  Glitch-Catcher™, operates when a short-circuit circuit-breaker event calls for the MOSFET to turn off. Glitch-Catcher™ ensures that this turn-off is a controlled process which prevents supply line voltage ringing due to the step release of stored inductive energy. The MOSFET is used as an active snubber, shunting energy into the low-impedance load and maintaining bus voltage ringing to a controlled maximum value.

The True-SOA™ protects the MOSFET if the supply current is unexpectedly increased for any reason. It operates by maintaining the MOSFET within its dynamic Safe Operating Area or SOA. This prevents excessive current and excessive power dissipation within the device causing a junction temperature rise that will destroy it. True-SOA™ is based on a generic MOSFET thermal model built into the PI2211. This uses both transient and steady state device MOSFET responses to calculate junction temperature rise following an electrical power pulse.

During operation, the chip monitors the MOSFET and repeats the junction temperature rise calculation every 50 μS. If this reaches 60 ˚C over ambient, it initiates an SOA sequence of cycling the device off to allow cooling to 21 ˚C over ambient, then on again.  After 16 cycles, the cooling target drops to 3 ˚C over ambient, then cycling continues indefinitely as required.

Users can configure the thermal model for their specific MOSFET by connecting three programming resistors to the PI2211. Designers can obtain values for these resistors after consulting SOA graphs, which are published by every MOSFET manufacturer.

The diagram below shows the waveforms associated with the MOSFET as it progresses through live insertion, steady state operation, a circuit breaker event, and SOA cycling while the fault current conditions persist.

Power MOSFET waveforms during live insertion and fault conditions

Power MOSFET waveforms during live insertion and fault conditions

Useful links:
Vicor PI2211 Data Sheet
Evaluation board user guide


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