Solid state power controllers (SSPC) are semiconductor devices that control power (voltage and/or current) supplied to a load. They perform supervisory and diagnostic functions in order to identify overload conditions and prevent short circuits.

SSPCs are similar to electronic circuit breakers that will protect loads from dangerous faults, but because they are more reliable and faster at switching the power off, they are used in more critical kinds of power systems such as aircrafts. SSPCs are smart controllers that can control outputs to critical loads to ensure proper operations. Some contain microprocessors that can be programmed to increase controlling characteristics.

Unlike circuit breakers, SSPCs provide true I²T protection. An SSPC cuts power when there is too much energy transfer where a circuit breaker will trip only when the current gets to its trip point. If a short is about to happen in a system and the current slowly gets higher instead of instantly reaching the trip point, the I²T protection of an SSPC will turn the output off. This functionality is demonstrated in the equation and graph below.

Image Credit: University of Kentucky

Types of Solid State Power Controllers (SSPC)

There are several basic types of solid state power controllers (SSPC).

• AC controllers are designed to switch alternating current (AC) voltages.
• DC controllers are designed to switch direct current (DC) voltages.
• AC/DC controllers are designed to switch both AC and DC voltages.
• Analog controllers use variable voltage, current, or some other method of analog control.

• Microcontrollers are complete computer systems on a chip. They typically combine an arithmetic logic unit (ALU), memory, timer/counters, serial port, input/output (I/O) ports, and clock oscillator. Microcontrollers require programming from external devices.

• Programmable solid -state power controllers (SSPCs) can be programmed by a computer, or by a specialized or proprietary programming method.

Specifications

Input Specification

• Dropout voltage is the voltage applied to the input at or below where the output is guaranteed to be in the 'off' state. It is also known as the must-release voltage or turn-off voltage. This voltage is considered to be the "noise immunity" level of the SSPC. Dropout voltage is important for selection because it does not have to be zero in order for the outputs to be zero. The device must have an appropriate dropout voltage to turn the output on or off if there is a voltage applied but the unit does not provide output until that voltage is increased over this dropout voltage or if there is 'brown out' voltage.
• Input current is the range at which the device is designed to operate.
• Input voltage is the range of voltages which, when applied across the input terminals, will maintain an 'on' condition across the output terminal.  This parameter is also used to define the absolute limits of input voltage that the SSPC can withstand without damage. Also known as 'must operate',  'pick-up', or 'turn-on'' voltage.
• Load voltage (maximum switching voltage, line voltage) is the range of output supply voltages over which the SSPC normally operates.
• Maximum load current (maximum switching voltage range, maximum switching current) is the maximum continuous current allowed across SSPC output terminals under specified heat dissipation and ambient-temperature conditions.

Turn On Switching

• Instant on —Turns on the load immediately when the pickup voltage is present. Used to control inductive loads.
• Peak switching —Turns on the load when the control voltage is present and the voltage is at the peak. Used to control heavy-duty inductive loads where a higher inrush current is needed.
• Random turn-on —Random switching is particularly important for inductive loads (transformers, etc.).
• Zero switching —Switches at the point at which the AC load voltage or current passes through zero as it changes polarity. Used to control resistive loads such as heating elements and lights to prevent large current inrushes.

Output Specifications

• Load voltage is the constant voltage the SSPC will provide to the load. Improper voltages to the load will cause operational problems or even damage. SSPCs use switching to regulate output voltage so output voltage is constant and power transmission-to-output is as efficient as possible.
• Maximum load current is the maximum amount of current that can be applied to the load. The larger the load the more current it will need to operate at maximum capacity. And larger current draws will cause the SSPC to switch off for fault protection.
• Device used to switch the output.
  • Metal-oxide field-effect transistors (MOSFET) —A field effect transistor in which the insulating layer between the gate electrode and the channel is a metal oxide layer. MOSFETs are used because they dissipate very little power making them most efficient
  • Bipolar transistor (BJT) — A three terminal device in which the emitter-to-collector current is controlled by base current. BJTs are predecessors to FETs or field effect transistors such as a MOSFETs and are still commonly used because they have greater linearity and are less expensive than MOSFETs due to easier manufacturing.
  • Silicon-controlled rectifiers (SCR) — An SCR is a three terminal active device that acts as a gated diode. The gate terminal is used to turn the device on, allowing current to pass from cathode to anode.
  • Triac — A triac is a bi-directional gate controlled thyristor. It is similar to an SCR, but it can conduct in both directions. It provides full wave control of AC power.

Standards

Solid state power controllers must adhere to certain standards to ensure proper design and functionality. For example, BS ISO 8816 describes the general requirements for solid state power controllers in aircrafts and ISO 27027 describes general performance requirements for the areospace industry.

Image Credit:

Data Device Corporation (DDC) | Silicon Labs