The Picor PI2211 hot-swap controller and circuit breaker doubles the power protection in systems where hot-swap capability is essential. It provides power transient protection during live card insertion and withdrawal from a backplane, as well as when over-current or circuit breaker faults occur during steady-state operation.
The PI2211 also can conserve board space and eliminate the need for external transient suppression components. The accuracy of its digitally based MOSFET control means designers can closely specify and size their MOSFET to its application, instead of having to oversize for an excessive safety margin.
The device fulfils its role so effectively because it easily can be set up to accurately model and respond to MOSFET transient and steady-state thermal behaviour, rather than relying on the less controllable and accurate approach of timer-based constant power limiting. This internal MOSFET model is generic; power designers can configure it for specific MOSFET devices by the addition of three programming resistors.
Designers can obtain values for these resistors by downloading a calculator tool available from the Vicor applications engineering team ([email protected]) and entering parameters from the MOSFET manufacturer’s datasheet, together with other system parameters such as the system bus voltage. The PI2211 is then set up to operate the power MOSFET close to its safe operating area (SOA) limits with a built-in safety margin under different load and transient conditions, without risk of transgressing these limits.
Startup, Steady-State, And Fault Events
We can see how the PI2211 has to control the MOSFET in different modes by reviewing the startup, steady-state, and fault handling process. The startup is initiated when the card with the PI2211, MOSFET, and load is inserted into the system’s live backplane. This creates a startup or inrush current to the load that can cause an electrical disturbance and possibly impose a voltage sag on the backplane power supply.
Therefore, during startup, the PI2211 turns on the MOSFET in a controlled manner, limiting the current to a predefined level based on the value of a user selected sense resistor. Once the load voltage has reached its steady-state value, the device’s power-good pin is asserted “high,” and the startup current limit is disabled.
Once power-good is established, the PI2211 operates the MOSFET in its low-loss RDS(on) region. The device now continuously monitors the load current and MOSFET calculated temperature rise, acting primarily as a circuit breaker with an over-current/over-temperature switching threshold. If a circuit breaker event such as a short circuit occurs, the MOSFET must be turned off. However, this must be a controlled process to avoid voltage ringing on the supply line due to the stored energy in inductive components and copper traces.
The solution is the glitch-catcher mechanism, which initially discharges the MOSFET gate rapidly, then at a slower controlled discharge rate to a full off state. During glitch-catcher operation, the MOSFET is used as an active snubber, allowing the stored energy to pass through the MOSFET into the low impedance or short-circuit load, keeping the bus voltage ringing to a controlled maximum value, well below the avalanche voltage rating of the MOSFET. This active snubber sequence eliminates the need for separate voltage suppression components in the system to protect against the transient over-voltage that a circuit breaker event can create.
The PI2211 can also protect the MOSFET when it is operating at a higher than anticipated load current, but still below the circuit breaker threshold. It constantly calculates junction temperature rise as a function of power dissipation and can shut down the MOSFET, preventing damage. By comparison, a typical hot-swap controller will only fault when a threshold is exceeded and cannot continuously protect the MOSFET during operation.
During these events, the PI2211 has to control the power MOSFET accurately. Preventing any transgression of the MOSFET’s dynamic SOA limits is essential. At the same time, working closely to the limits is desirable as it means a more fully utilised, smaller device can be used.
SOA graphs for any MOSFET device are available from the device manufacturer. These graphs comprise a set of V/I plots showing maximum possible drain current for any drain-source voltage. Each plot represents a different applied voltage pulse duration, say from 100 μs to dc.
Electrical energy is a product of voltage drop, current, and time, so an SOA graph essentially shows how to trade these variables to contain the level of energy dissipation within the MOSFET. This is important because the MOSFET’s junction temperature will rise with energy dissipation, and an excessive junction temperature will destroy the device.
Thermal Model Of Performance
The MOSFET’s ability to dissipate applied electrical energy is influenced by the thermal path from its junction to ambient and by the ambient temperature of its operating environment. The device’s dynamic as well as static performance must be considered as it operates through transient as well as steady-state conditions.
A thermal model that includes capacitance and resistance becomes necessary. The associated RC time constant (τ) indicates how quickly heat can be conducted away from the junction after an electrical power pulse. The device’s junction to case temperature rise depends on this time constant as well as its thermal resistance and the level and duration of the power it dissipates.
The PI2211 emulates and allows for these MOSFET characteristics using its internal MOSFET model (Fig. 1). It comprises two RC stages to represent the total MOSFET thermal characteristic. One stage represents the case to ambient thermal component, and this is fixed. The other represents junction to case, which can be tuned to match the published data for a specific MOSFET device by two programming resistors, RSOAT and RSOAR.
These resistor values can be selected, using the Vicor calculator, to program the PI2211 model for the chosen MOSFET’s thermal characteristics. The model then can calculate the MOSFET’s junction temperature rise for a given transient power pulse. Another resistor, RSOAS, can be programmed to reflect the magnitude of the calculated current through the MOSFET and the power it is dissipating.
SOA Cycling Sequence
The PI2211 continuously monitors the MOSFET’s power dissipation by measuring the voltage drop across it and the calculated current through a sense resistor. The two internal networks emulating the MOSFET’s junction-to-case and case-to-ambient thermal characteristics process this power information. Their two responses are summed together for an accurate junction-to-ambient thermal response, and the junction temperature rise is calculated accordingly. This calculation is repeated every 50 μs.
If the model calculates the junction temperature rise to be 60˚C compared with ambient, the PI2211 initiates an SOA cycling sequence to maintain the MOSFET within its SOA. The device is turned off and allowed to cool while still being continuously monitored.
When the model indicates that the temperature rise has dropped to 21˚C—an estimated drop of 39˚C—the MOSFET is turned on again and monitored. This thermal cycling is repeated 16 times with this temperature range before dropping the cool-down temperature to 3˚C rise. After this, the longer 57˚C hysteresis thermal cycling continues indefinitely, resulting in a long cool-down period.
Example Event Sequence
Figure 2 shows the waveforms associated with an example sequence of events for startup, over-current, and SOA shutdown. The controlled startup as the bus supply rises and stabilises after insertion can be seen. Steady-state conditions persist for a period until a circuit-breaker event occurs, causing the input current to increase sharply until the over-current limit triggers the glitch-catcher shutdown sequence.
After the over-current hold time is completed, the input current starts to rise again. But the short-circuit that caused the circuit-breaker event is still present, so the SOA shuts down and the SOA cycling sequence starts to prevent the MOSFET junction temperature rising by more than 60˚C. After 16 cycles with a 31˚C hysteresis, cycling continues with a 57˚C hysteresis until the fault is removed.
Conclusion
The Picor PI2211 hot-swap controller allows power system designers to ensure safe and reliable operation of their power MOSFET and the system it supplies using a single chip. Design complexity and board space requirements are reduced while protection starts with live board insertion and continues through steady-state operation, over-current, and circuit breaker events. The downloadable calculator allows easy setup of the PI2211 for an application-specific MOSFET device and ambient environment using a few simple external components.