Rockwell Automation Trusted TMR 24V DC, Оригинальное руководство пользователя

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Chapter 2 Description
Rockwell Automation Publication ICSTT-RM451D-EN-P - March 2021 15
The upper switches in shown in Figure 3 are denoted as N.O. (Normally
Open), and are controlled by the FIU on which they are physically resident
1 .
The lower switches are depicted as N.C. (Normally Closed), and are controlled
by the “upstream” neighboring FIU
2 .
Tip: In this context, N.O. is defined as being in the off state in the absence of control signal power,
and similarly, N.C. is the on state in the absence of control signal power. These switches are
constructed from enhancement mode MOSFETs and are both guaranteed to be off in the absence of
Module power to create gate voltage signals to bias them on
3
(unlike electromechanical relays for
example).
The reason that the lower switches are specified to be on in the absence of
control signal power is to allow two channels to power the load should an
entire slice fail. Even if an entire slice fails, the surviving output circuits
carries the necessary control.
The structure of each FIU output is shown in this diagram:
Figure 4: Simplified Switch Circuit Diagram
A resistor provides a means of continuously monitoring the switch current. A
signal transistor is used to drive the gate of Switch 2. It provides Switch 2 with
a negative gate voltage, to minimize its on resistance, and serves to hold
Switch 2 on in case the secondary gate control loses power.
The Zener diode between the gate of Switch 2 and source is only required to
protect the gate from large voltage spikes on the drain that might capacitively
couple through when Switch 1 and Switch 2 are in the off state.
1 Their “home” FIU.
2 The home FIU, supplies an independent control signal for the “downstream” FIU
FSS.
3 For an unfaulted transistor.