Specifications of Switches

There are many parameters that are important to the switches. The most important among these are :

• Voltage ratings: Forward and reverse repetitive peak voltages and an on-state forward voltage drop.
• Current ratings: Average, root-mean-square (rms), repetitive peak, non-repetitive peak, and off-state leakage currents.
• Switching speed or Frequency: Transition from a fully non-conducting to a fully conducting state (turn-on) and from a fully conducting to a fully non-conducting state (turn-off) are very important parameters. The switching period TS and frequency fS are given by
  fS​ = 1/Ts​ = 1 / td​+tr​+ton​+ts​+tf​+toff
   where t_off is the off time during which the switch remains off.
• di/dt Rating: The device needs a minimum amount of time before its whole conducting surface comes into play in carrying the full current. If the current rises rapidly, the current flow may be concentrated to a certain area and the device may be damaged. The di/dt of the current through the device is normally limited by connecting a small inductor in series with the device, known as a series snubber.
  
• dv/dt Rating: A semiconductor device has an internal junction capacitance CJ. If the voltage across the switch changes rapidly during turn-on, turn-off and also while connecting the main supply the initial current, the current CJ dv/dt flowing through CJ may be too high, thereby causing damage to the device. The dv/dt of the voltage across the device is limited by connecting an RC circuit across the device, known as a shunt snubber, or simply snubber.
• Switching losses: During turn-on the forward current rises before the forward voltage falls, and during turn-off the forward voltage rises before the current falls. Simultaneous existence of high voltage and current in the device represents power losses. Because of their repetitiveness, they represent a significant part of the losses and often exceed the on-state conduction losses.
  
• Gate drive requirements: The gate-drive voltage and current are important parameters to turn-on and turn-off a device. The gate-driver power and the energy requirement are very important parts of the losses and total equipment cost. With large and long current pulse requirements for turn-on and turn-off, the gate drive losses can be significant in relation to the total losses, and the cost of the driver circuit can be higher than the device itself.
  
• Safe Operating Area (SOA): The amount of heat generated in the device is proportional to the power loss, that is the voltage-current product. For this product to be constant, P = vi and equal to the maximum allowable value, the current must be inversely proportional to the voltage. This yields the SOA limit on the allowable steady-state operating points in the voltage-current coordinates.
  
• I²t for fusing: This parameter is needed for fuse selection. The I²t of the device must be less than that of the fuse so that the device is protected under fault current conditions.
  
• Temperatures: Maximum allowable junction, case, and storage temperatures, usually between 150°C and 200°C for junction and case, and between –50°C and 175°C for storage.
  
• Thermal resistance: Junction to case thermal resistance, Q_JC case to sink thermal resistance, Q_CS and sink ambient thermal resistance, Q_SA. Power dissipation must be rapidly removed from the internal wafer through the package and ultimately to the cooling medium. The size of semiconductor power switches is small, not exceeding 150 mm, and the thermal capacity of a bare device is too low to safely remove the heat generated by internal losses. Power devices are generally mounted on heat sinks. Thus, removing heat represents a high cost of equipment.
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