Rockwell Automation T8444 Trusted TMR Pulse Generator and Monitoring Module - Manual (Page 15)

Trusted
TM
Module T8444
Issue 09 Apr 10 PD-T8444 15
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 minimise it’s on
resistance, and serves to hold Switch 2 on in the event that 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.
The resistors in series with the gate of Switch 1 and the signal transistor serve to protect the drive logic
in the event of a malicious switch failure. The pull-up resistors define the gate voltages in the absence
of power.
1.9.1. Switch Diagnostics.
During normal operation, Switch 1 and Switch 2 are maintained on. In this state, Switch 1 and Switch 2
exhibit less than 0.5L of resistance each.
To determine the ability of the system to control the load via Switch 1 and Switch 2, their gate voltages
are modulated, one at a time. As the gate voltages are modulated, the monitoring signals
synchronously change in a predictable fashion. The local DSP analyses the relative amplitude and
phase of these small AC signals, to determine the on resistance and threshold voltages of each switch.
The current to the load does not need to be completely interrupted in order to obtain a level of
confidence in the ability of the transistors to turn off. For the TMR switch configuration in the on state,
only one fail-safe switch at a time needs to be modulated, while the other two bear the load current.