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Satellite and launch vehicle applications traditionally use electromechanical relay (EMR) technology in many of its control systems. Electromechanical switches contain inherent reliability issues and may not perform correctly under severe environmental conditions, such as the vibration profile during launch, or at low temperatures. Clearly, a more desirable solution is required. Recent generations of IR's radiation-hardened MOSFET technology packaged and configured as a solid-state relay (SSR) has enabled designers to implement increasingly complex electronic control schemes for power management. Packaging multiple rad-hard MOSFET devices into a single package reduces the amount of interconnections, parasitic inductance, and capacitance, thus increasing system reliability.

During the harsh shock and vibration environment of a satellite launch, the EMR is prone to false operation. An EMR switch is built within a metal enclosure with the electrical connections made on the bottom of the can. The electrical connections are soldered firmly in place. This aspect ratio of being taller than it is wide and firmly held in place at the bottom results in a natural resonance that works to tear the switch from its mounting location. Switch movement conditions (see below) such as bounce and chatter are also an inherent part of the electromechanical switch. These issues need to be addressed by the designer. The typical approach is to add a cushioning layer to dampen the shock and vibration pulses and to mitigate the effects on the components to be protected. The top of the electromechanical switch package must be clamped firmly in place so it has little movement during vibration. SSR's allow the designer to simply place the component on a printed circuit board (PCB).

EMR switches also have a physically moving part called the armature. The armature is moved between the electrical contacts to make or break an electrical connection. Physically moving parts are subject to wear and tear of normal day to day operation. If load currents are high enough, a spark is created during switching. This causes a burning of the contact and accelerated degradation of the contact area. Another limitation is that moving parts in contact with one another need lubrication for continued smooth operation. The lubrication material has to perform in the cold environment of space. The materials chosen have been subject to a phase change (liquid to solid) in the cold space environment, inhibiting the movement of the parts and resulting in failure.

The electromechanical device's armature is physically moved by inducing a current in a coil. The magnetomotive force (MMF) moves the contact from one switch position to another creating the open or closed condition. The magnetic field of the coil induces unwanted noise in the system. The equipment that the electromechanical switch is turning on or off must be immune to the induced current and additional filtering is required. Since there is no coil in the solid-state switch, this is not a problem.

The basic operation of SSRs is simple. As shown in Figure 1, as the name implies the device is solid as there are no moving parts to break down, wear out or fail. MOSFET switches block and conduct current to supply power to the device being controlled. The MOSFET is actuated by a photovoltaic isolator (PVI). The PVI is one or more LEDs and a photovoltaic array. As current is passed through the LED, the emitted photons generate a voltage in the array. This voltage charges the gate of the MOSFET. In the case where N-channel enhancement-mode FETs are used, the switch transitions from the “normally open” condition to the closed condition or the “on” state. The physical separation of the LED and the array will provide 1 kV minimum isolation between the actuation current and the controlled current. When the device is switched off the opposite effect occurs, except that additional gate discharge circuitry is used to assure an “off” state is achieved and done so quickly. This scheme is designed for a dc current.

If ac control is required, as is an ac motor, a second MOSFET is used connected in a common source configuration. Both MOSFET gates are controlled by the array (Figure 2).

Figure 2 has described a normally open switch. Some switch schemes require a “normally closed” contact. This is achieved using an N-channel depletion mode MOSFET. SSRs can also contain a latching function to remain in the switch position commanded until it is reset.

All satellite system electronics are operated via a solar cell array in conjunction with a battery to power the electrical systems of the satellite with a regulator limiting the battery charge. This regulated voltage is conditioned using a dc-dc converter and fed to each system as required. A centralized power management controller will enable SSRs to switch on and off the equipment and instruments, as they are needed. Connecting and disconnecting equipment from the power bus can cause unwanted system noise and a costly EMI conformance specification imposed on the dc-dc converter. SSRs can be turned on slowly reducing the inrush current. This leads to a “quiet” power bus supply and alleviates the need for conducted susceptibility (CS) filtering within the dc-dc converter. In addition, all of the subsystems can be controlled smoothly using power only when needed. The SSR can be used with a current limiting feature and shut down in the case of a system failure to assure protection of the main bus.

A satellite system will run on its battery power during the hours not exposed to the sun and deplete its charge. Conversely, when operating in sunlight, the solar array supplies power to the system. Several solar array panels on a satellite can be switched in connection with the battery or the electrical bus as needed as a method of regulation. An example would be when the satellite emerges from darkness, the battery will need recharging at a faster rate, so all panels can be switched in connection with the battery, while later in the day, most panels can be removed so that only a trickle charge remains.

When using an electronic switch to control solar array charging of the battery, the tendency is to employ a blocking diode, as shown in Figure 3.

However, the circuit in Figure 4 is more efficient as it still blocks current both directions when off, but it also has less loss when charging because a FET has less loss than a diode.

These on/off switches work with other protection circuitry to limit di/dt and dv/dt. In addition to the protection features, the system can contain on/off status and load current telemetry features to be used in the master control unit. These circuits can be fabricated on a single chip application-specific integrated circuit (ASIC) providing the control and sensing functions. The ASIC and power MOSFETs can be mounted on a thick film substrate and placed in a metal hermetically sealed package. The metal package will provide additional protection against the harmful effects of radiation in a space environment. This approach will reduce the size and weight of the components used and result in a cost savings to the project.

Most satellites employ the use of heater coils to keep sensors and/or circuitry at a defined operating temperature. This is especially true in the case of a scientific satellite on an interplanetary mission. There is a significant amount of research being performed on the planet Mars where the temperature at night is -96° C. When switching the heaters on and off, the gentle switching characteristics of the SSR will minimize the level of noise induced into the system. The heater is a coil that, during on and off switching, will produce an inductive kick. The robust nature of SSRs will absorb this transient voltage without additional suppression circuitry.

Reaction wheel motion control can also be performed with an SSR. The very fast and accurate turn on and turn off times allow for precision actuation of the reaction wheel and torque rods used to position the satellite in space. Feedback from the star-locating cameras to the centralized command unit and subsequent actuation of the switches controlling the wheel assures the proper location of the satellite.

Planetary exploration vehicles such as the Mars Rover that landed on Mars in December 2003 move around the planet to collect data in different locations. This vehicle has six traction wheels whose movement can be controlled using a dc motor. A possible application would be to use solid-state switches to supply current to the motor windings, with polarity reversal for forward and reverse direction. This concept is common in commercial applications and the radiation hardening of the SSRs makes the technology a good fit for planetary exploration.

In order to use an electromechanical system, the designer must incorporate methods to accommodate its weaknesses in order to make the parts useful. In doing the design for the padding, design time to determine how the device fits into the vibration profile as compared to the natural frequency of the device and its inherent susceptibility must be budgeted. This goes away with the SSR choice. The hermetic package with reliable gold-plated leads makes circuit assembly fast and simple. Internal silicon interconnections are made using wire bonds in order to provide the highest level of reliability and lowest failure rate. The small size and weight of the SSRs reduce the system cost.

IR offers a range of SSRs to meet designer applications. MOSFET voltages are available in 60 V, 100 V and 150 V breakdown voltages. Depending on the bus voltage, MOSFETS can be chosen to give the greatest efficiency and lowest power consumption. The MOSFETS used offer Rds (on) values as low as 80 milliOhm. And they come in all configurations normally associated with EMRs. Circuit configuration consists of SPST/DPST NO or SPDT, with input buffering of 3.3 V or 5 V as can be seen in Table 1. Plus, the company can package a number of devices in a single package. To meet space-screening applications, such parts are assembled in hi-rel facility that is certified in space-level manufacturing assembly process and control.

SSRs are a highly reliable way to manage power distribution within a satellite. SSRs are a direct replacement for EMRs without the associated problems and additional design considerations that must be implemented. This article has provided the designer with examples of possible uses or “design concepts” that are intended to stimulate thought for actual design-in applications. As future satellite design mandates a reduction of failure modes while simultaneously reducing the overall cost and size of the system, radiation-hardened SSRs meet and exceed this requirement.

Alan Tasker is solid-state relay product line manager for International Rectifier's Hi-Rel Group in Leominster, Mass. Tasker received his B.S. and M.S. degrees in Electrical Engineering from Northeastern University in Boston. Prior to this, he worked in the power design area, including developing SSRs for the F-16 aircraft.

Michael Toland is marketing/applications engineer in International Rectifier's Hi-Rel Group in Leominster, Mass. Toland joined IR in the fall of 2004. He has 25 years of experience in packaging of application-specific semiconductors for the military/space and commercial market segments.