Hot swap controllers (next get power_dist part 3)

This is one of a series covering the new mjbots power_dist board. See part 1 and part 2 for more context.

As mentioned previously, hot swap controllers are primarily used to allow a card to be inserted live into a server backplane, while minimizing disruption to the primary power bus while doing so. Additionally, they often implement protection features like over-current and short-circuit protection, and some support energy monitoring.

Typical topology

A typical hot-swap topology looks like:

Here, the high side FET is used for two purposes. During the “pre-charge” phase, the FET is operated in the linear regime, with a large voltage across it until the load capacitance is fully charged, at which point the FET reaches its “fully on” state and operates with its stated Rds on resistance. Notably, this means that the entire pre-charging energy is dissipated in those FETs, as opposed to the r3.1 design where a power resistor serves that purpose.

To accomplish the protection functionality, the current shunt (R1) is used to measure total current moving through the primary FET. Depending upon the particular chip, this could include over-current monitoring, where the FET is brought back into the linear operating region to limit the current, or short-circuit protection, where the FET is immediately turned off. Additionally, the resistor divider (R2/R3) can be used to program an undervoltage threshold.

Design constraints

The biggest challenge that a design faces with any hot swap controller is selection of the primary FET. Because it has to dissipate the entire pre-charge energy in a short time window, the device becomes very stressed. This limits the total capacitance that can be charged, the maximum voltage, and somewhat non-intuitively, the current that can be drawn during this startup window. For a design like the power_dist, where there is no “power good” signal distributed to downstream loads, they can draw what are effectively constant-power loads soon after the bus voltage reaches a valid intermediate state. Given that this can occur when there is still a large voltage across the high side FET, it can add a lot of energy dissipation.

To make this even harder, the design constraints are such that during the critical pre-charge window, FETs cannot be naively be placed in parallel. When operating in the linear regime, minute differences in device characteristics and temperature can cause drastic load imbalances. Thus controller design equations only allow parallel FETs for the purposes of increasing steady state on current, not increasing power or capacitance during the pre-charge window.

For a given FET, it will have a “safe operating area” plot like the below (this from the PSMN3R7-100BSE used in the soon-to-be-released power_dist r4):

This shows how much energy can be safety dissipated over different time windows. When designing the pre-charge system, it is then a balancing act of getting things to charge as quickly as possible, while not violating the SOA. Because of the details, going faster or slower can be problematic. Faster can be an issue because the energy may violate the short term peak energy, slower may be the culprit because the initial load current spends a longer time moving through the FET when it has a large voltage across it.

Next steps

In the next post, I’ll cover my various iterations, and where they fell short.

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