Power Electronics

SSPCs Handle Heavy Loads With Foldback Current Limiting

Turning on a power source into a highly capacitive load can wreak havoc on circuit elements and protection devices. Foldback current limiting allows circuit elements to withstand transient currents without over-designing for maximum load conditions.

Solid-State Power Controllers or SSPCs replace clumsy electromechanical circuit breakers that trip when their coil's I2R heating is excessive, which prevents system wiring and components from overheating (Fig. 1). Like most circuit breakers, SSPCs operate in accordance with the current trip curve referred to in MIL-STD-1760 as the I2t curve, where I is the load current and t is time. This curve (Fig. 2) plots the allowable safe operating region for energy dissipated in a conductor before damage by I2R heating occurs. The larger the overload means the shorter the time allowable.

SSPCs do their jobs well, but controlling the power applied to a large capacitive load requires special design considerations. To address this problem, we must first understand the SSPC and its capabilities.

SSPCs provide intelligent, computer-controlled management of power electronic systems. They conserve power by switching subsystems online only when required. SSPCs enable a reduction in the wire gauge (and subsequent weight) of conductors in aircraft in two distinct ways. Using them in a power management role allows the total load of a particular power bus conductor to be time-shared, reducing the total wire gauge requirement. For example, if the total bus load for a wire is 50A, but only 25A is required simultaneously, you can automatically switch off the unnecessary loads, maintaining the load at continuous 25A or less. The other mechanism for wire weight reduction is that a SSPC can be located near the load with only light gauge signal wires, or the existing data bus controlling them from the cockpit or control panel. Elimination of these heavy conductors over many feet can save hundreds of pounds in aircraft wiring.

Analog SSPCs have been and are being used to turn on power sources into large capacitive loads with some limitations. The controlled turn-on time of a typical 270V SSPC design, in concert with the high surge current or “instant trips” rating, can reliably turn on into a capacitance of 150 µF to 200 µF. Load capacitance higher than this requires special techniques.

A traditional method for a large capacitive load is to use a “ratchet-up” technique. If the capacitance is so large such that the energy supplied to it during turn on does not fully charge it, then the SSPC trips. The ratchet-up circuit senses this trip, together with the rising output voltage, and turns the SSPC back on. This sequence is repeated until the required capacitance is fully charged, at which time the SSPC remains on. Unfortunately, this technique doesn't work if, while attempting to charge the large capacitance, full load current is present. The full load current rapidly discharges the capacitor during the SSPC's off time. Unless the restart delay is less than the SSPC's on time, the capacitor will not charge up completely.

Another approach for switching a large capacitive load on and off is straight current limiting in which the SSPC supplies a constant current, well in excess of its rating, to turn on the capacitive load. The problem with straight current limiting is that while the current supplied to the load is constant, the power dissipated in the SSPC switching element is not. The resulting very high power dissipation during turn on requires larger and more expensive switching elements. Although this is possible, it significantly increases size, weight, and cost.

A new technique for dealing with the driving of a large capacitive load include using a digital SSPC (DSSPC) with foldback current limiting. The simplified block diagram of a DSSPC (Fig. 4) consists of a digital control ASIC (DCA), smart switch ASIC (SSA), series pass element, and shunt resistor. The DCA employs a data bus to receive commands from and supply status and other information to the system processor. Algorithms in the DCA provide the I2t, foldback current limiting, safe operating area rating (SOAR), and status functions. The SSA implements the instant trip feature for foolproof self-protection. DCA flexibility allows customization of the standard I2t curve to virtually any requirement. In addition to the simple status information available from an analog SSPC, the DCA in the DSSPC can also supply load current, peak overload current, and more complete status information. By monitoring load deviations from stored profiles, the DSSPC can implement advanced features, such as failure prediction.

Foldback Limiting

To overcome the capacitive load problem, we implemented a foldback current limiting technique. This actively controls the current supplied by the SSPC during turn on, making it inversely proportional to the voltage across the switching element(s). This technique supplies energy to the load limited only by the SOAR of the switching element(s).

Fig. 5 shows the smart switch control ASIC, pass element(s), and a shunt for a typical DSSPC with foldback current limiting. The DCA receives load current, pass element, and status information (on the left of Fig. 5), then computes and controls the maximum current supplied to the load. Also, based on an instant trip during turn-on, it can automatically restart the Smart Switch in its foldback current limiting mode. In this mode, the output current starts rising at approximately 20% of its rating when the full line voltage appears across the pass element(s), increasing to more than 150% of its rating as the pass element(s) approach saturation.

When commanding this mode of turn-on, the DCA uses its resources to continuously compute the energy dissipated within the pass element(s). It utilizes this information to turn off the pass element(s) when they approach the limit of their safe operating area. Power MOSFETs were chosen for this application because of their inherent ruggedness when operating in the linear region.

Fig. 6, on page 29, shows the oscilloscope traces demonstrating the actual output current and load voltage of a current foldback limiting output stage successfully driving a capacitance of 1500 µF in parallel with a 27Ω load. After a narrow initial current spike due to circuit response times, the current climbs steadily as the pass voltage drops from the initial 270V to the final saturation drop of less than 2V. The total time to charge the 1500-µF capacitor, in this configuration, was less than 130 ms.

Operating Considerations

In any power-switching configuration, it's necessary to guard against voltage transients. We equip our SSPCs with an internal shunt diode to suppress negative transients on the load side and a catch diode to absorb inductive kick voltages above the line voltage during turn-off. The protection afforded by these diodes is adequate for most applications. On the line side, the system should employ transient voltage suppression. Any positive transients at this terminal add to the line voltage.

To prevent voltage breakdown on the switch elements, use transient suppression devices of proper voltage and energy rating. To reduce any inductance that might reduce their effectiveness, devices should be installed as close as possible to the SSPCs.

During turn-on, capacitive loads cause high surge currents limited only by wire impedance. If the capacitance value is too great, then the instant trip circuit might trip the SSPC, requiring a restart. Controlled or soft turn-on of the MOSFETs or IGBTs reduces this effect. However, system designers should examine their system to ensure there's a reasonable resistance in the wiring to limit high surge currents or use the foldback current limiting.

Applications

Space — SSPCs are being used on the International Space Station. Their small size, light weight, and high reliability — combined with their remote command capability — make them ideal for this application.

Aircraft — The use of SSPCs in aircraft permits remote or automated power management, and allows reduction in wire gauges and consequently aircraft harness weight.

Battery-Powered Vehicles — The low forward voltage drop makes SSPCs an ideal power management device to conserve battery power in electric vehicles by shutting down unneeded subsystems or devices.

Industrial Power Control — SSPCs are excellent intelligent remote power switches for turning subsystems on and off while providing surge and overload protection.

References:

  1. “New Solid State Power Controller Integrated Design Improves Efficiency and Reliability,” Kevin A. Mussmacher, P.E.; pp. 432-438, Power Electronics 2001, Proceedings, September 2001.

Solid-State Power Controllers

A typical analog SSPC (Fig. 3, on page 24) consists of the following circuit functions and elements:

  • Input control circuits that condition the input voltages and commands for use in other parts of the SSPC.
  • Isolation circuits that isolate the line and load voltages to protect the low level logic circuits.
  • Series pass element that use SCRs for ac versions and IGBTs or MOSFETs for dc versions. This switching element controls the turn-on and turn-off of the line voltage.
  • Driver, protection, trip, and status circuits that provide the I2t control, driver circuits for the status signals, and the decision logic to generate and interpret the status and command signals.

Analog SSPCs sense the load current by measuring the voltage developed across a small resistor or shunt in series with the load. In severe overloads, this voltage exceeds the instant trip level (set by programming resistors external to the SSPC) and the SSPC trips, removing power from the load. Less severe overload conditions depend on the time in overload and the degree of overload as seen by the I2t circuitry monitoring the sense voltage. If these levels exceed a predetermined (I2t) level set by programming resistors external to the SSPC, the SSPC trips and removes power from the load.

The difference between traditional analog SSPCs and DSSPCs is flexibility. In the analog SSPC, trip characteristics, such as the instant trip and I2t curve, are set in physical circuitry. This means that changing instant trip points requires the changing of a programming resistor. Tailoring the trip time, even slightly, requires internal SSPC modifications. Also, most analog SSPCs usually only provide the associated system with rudimentary status information.

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