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Infineon Technologies RS Components (Opens in A New Window)

- A solid state relay (SSR) uses an LED, photovoltaic unit, and MOSFETS to switch loads on and off without mechanical parts. A current passing through the LED causes the photovoltaic unit to output 15V to turn on the MOSFETS. - SSRs come in different package types and configurations to operate as single or double pole relays, normally open, normally closed, or changeover functions. They also have a wide range of voltage and current ratings. - Information on SSRs can be found on manufacturer websites like Infineon Technologies or components suppliers like RS Components.

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72 views1 page

Infineon Technologies RS Components (Opens in A New Window)

- A solid state relay (SSR) uses an LED, photovoltaic unit, and MOSFETS to switch loads on and off without mechanical parts. A current passing through the LED causes the photovoltaic unit to output 15V to turn on the MOSFETS. - SSRs come in different package types and configurations to operate as single or double pole relays, normally open, normally closed, or changeover functions. They also have a wide range of voltage and current ratings. - Information on SSRs can be found on manufacturer websites like Infineon Technologies or components suppliers like RS Components.

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Question

A typical circuit of an SSR is shown in Figure 2. A current of about 20mA through the LED is sufficient to
switch on the output MOSFETS. The (infra red) light from the LED falls on the Photovoltaic unit, which
comprises 25 silicon diodes. Each diode produces 0.6V to give the 15V gate potential needed to turn on
the MOSFETS. Figure 2 represents a basic example of a SSR, in this case the Siemens LH1540T SSR,
manufactured as a 6 pin DIL package. Many more complex chips are available that act as double pole,
Normally Closed (NC), Normally Open (NO), and Changeover relays with a wide variety of extra
facilities. SSRs are also manufactured in a range of output voltages and current ratings, with a range of
package types ranging from small surface mount components through complex multi pin chips and large
heavy current examples, for rack mounting in electrical control cabinets. Information on SSRs can be
found by searching for Solid State Relays on manufacturers websites such as Infineon Technologies or
at semiconductor suppliers such as RS Components (opens in a new window)

Snubber circuits are used for semiconductor switching devices such as


transistors, SCRs, etc. When a switching device changes its state from ONstate to OFF-state, the impedandce of the device abruptly jumps to a very
high level, blocking the current. But the current still tends to keep flowing
through the switch, which induces a high voltage across the switch. The faster
the current decreases, the higher the induced voltage becomes. It may reach
to sufficiently high level to destroy the switch. If the switch is unable to
withstand the high induced voltage, it will be destroyed, and can no more
block the current as an OFF-state switch. To avoid this, an auxiliary network is
connected across the switch that prevents the induced voltage from going too
high. The network is called a snubber. Conversely, when a switching device
changes its state from OFF-state to ON-state, the impedance of the device
abruptly goes down to nearly zero, allowing current to flow freely. The problem
in this case is that the current does not start to flow uniformly through the
switch. This is because the switch does not recover its conductivity over the
whole its cross-sectional area simultaneously. Some part of the area may be
first open, and the current flow is concentrated to this relatively small portion
of the switch, which may cause overheating and eventually failure. Thus some
semiconductor switches should be protected from too fast increase of current.
This is another function of a snubber. In view of above discussion, the snubber
circuit is someting like a "bumper." It alleviates rise of voltage when the
switch is turning OFF, and alleviates rise of current when the switch is turning
ON, to a safe level.

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