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Transformers
When switching transformers, consider the characteristics of the secondary load�
These characteristics reflect the effective load on the SSR� In addition, voltage
transients from secondary load circuits can act as transformers and impose on the
SSR�
Transformers present a special challenge: Depending on the transformer flux state
at turn-off, the transformer may saturate during the first half-cycle when voltage is
next applied� This saturation can impose a very large current (10–100 times the rated
typical current) on the SSR, which far exceeds its half-cycle surge rating� SSRs with
random turn-on may have a better chance of survival than a zero-cross turn-on device,
since they commonly require the transformer to support only a portion of the first half-
cycle of the voltage� On the other hand, a random turn-on device will frequently close
at the zero-cross point, and then the SSR must sustain the worst-case saturation
current� A zero-cross turn-on device has the advantage that it turns on in a known
mode and will immediately demonstrate the worst case condition� The use of a current
shunt and an oscilloscope is recommended to verify that the half-cycle surge capability
is not exceeded�
As a general rule, when applying an SSR to a transformer load, select an SSR having
a half-cycle current surge rating greater than the following:
(maximum applied line voltage) ÷ (transformer primary resistance)
The primary resistance is usually easy to measure and can be relied on as a minimum
impedance limiting the first half-cycle of inrush current� The presence of some residual
flux, plus the saturated reactance of the primary, will then further limit, in the worst
case, the half-cycle surge safely within the surge rating of the SSR�
Switching Devices
The power family of semiconductors consists of several switching devices� The
most widely used of this family are metal-oxide semiconductor field-effect transistors
(MOSFETs), silicon controlled rectifiers (SCRs), TRIAC, and Alternistor TRIAC� In
many applications, these devices perform key functions, so you must understand
their advantages as well as their shortcomings to properly design a reliable system�
Applied correctly, SSRs are an asset in meeting environmental, speed, and reliability
specifications which their electromechanical counterparts could not fulfill�
• MOSFET
A power MOSFET is a specific type of metal-oxide semiconductor field-effect transistor
(MOSFET) designed to handle large amounts of power� It is a vertical-structured
transistor capable of sustaining high blocking voltage and high current� Power
MOSFETs are used in DC switching applications� Care must be taken to ensure proper
polarity for all DC ports� Failure to do so can lead to permanent device damage�
• TRIAC
A TRIAC is an electronic component approximately equivalent to two silicon-controlled
rectifiers joined in inverse parallel (paralleled but with the polarity reversed) and with
their gates connected together� This results in a bidirectional electronic switch that can
conduct AC current only� The TRIAC is ideal for switching non-reactive loads�
• Alternistor TRIAC
The Alternistor is specifically designed for applications that switch highly inductive
AC loads� A special chip offers performance similar to two SCRs wired in inverse
parallel (back-to-back), providing better turn-off behavior than a standard TRIAC� The
Alternistor TRIAC is an economical solution, ideal for switching inductive AC loads�
• SCR
The SCR (silicon-controlled rectifier) acts as a switch, conducting when its gate
receives a current pulse, and continuing to conduct as long as it is forward biased�
The SCR is ideal for switching all types of AC loads�
Application Data (continued) Legacy Solid-State Relays