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Application Brief

Application Brief

Fast Power Fault Detection by Monitoring 24V AC
Systems Using TPS37x Voltage Supervisor

Ben ChanThis technical note discusses a novel implementationof combining both wide voltage alternating current(AC) and direct current (DC) monitoring, with
programmable sense and reset delay features of
the TPS37x, to create a unique approach tovoltage monitoring. Designing the TPS37x to detect
undervoltage (UV) and overvoltage (OV) systemfaults into industrial and factory applications provides
innovative solutions.
The TPS37x incorporates the sense delay which isthe delay time for which the sense voltage needs tobe under the voltage threshold in order to assert areset. On previous TI voltage supervisors this delay
was small and fixed, however, on the TPS37x, sensedelay time can be programmed from several hundred
microseconds up to several seconds via an external
capacitor.
There are many applications for which a sense delay
would be useful. One application of sense delay is
the sensing of the slowing of the operating frequency
of a wide voltage AC voltage rail in an industrial
setting. By detecting the AC operating frequency of an
AC power rail, the speed of fault detection is greatly
improved to prevent crucial downstream systems frombeing damaged. Another application for sense delay
is preventing false resets on noisy power voltage rails.

it may not be fast enough to sense the changes of
the AC input. One approach to increase the speed
detection is to directly monitor the AC power input.
The Full-Wave Diode Bridge DC Rectifier
Approach: Slow but Proven Effective
The full-wave diode bridge DC rectifier circuit,illustrated in Figure 1, has been proven in multiple
circuits to effectively convert an AC voltage into a DCvoltage. Many systems today continue to implement
the rectifier circuit to generate a DC voltage such thata voltage supervisor can be used to indirectly monitor
the AC power rail. One drawback of using the fullwave diode bridge rectifier circuit is the response timeof the DC voltage when the AC power rail experiencesa change of operating frequency or voltage amplitude.
Due to the output filter of the full-bridge rectifier,the detection of a change in voltage or operating
frequency may require many AC cycles before thevoltage supervisor alerts a fault condition.

+VOD1
C1

How to Combine AC and DC Sensing to Detect
System Faults
In many industrial and factory automation
applications, there are multiple alternating current(AC) and direct current (DC) power rails that power
various subsystems within the application. Someof these power rails include 24 VAC source witha known operating frequency that requires a fullbridge rectifier and capacitors to convert its signal
into a DC voltage. Monitoring the DC rectified
voltage ranges, with a voltage supervisor, is vital tomaintaining and supporting the power requirement
to prolong the health of the system operation.
Although the described conversion is well established,it has significant speed detection drawback. Whilemonitoring the DC rectified voltage is an easier way,
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R10V
GND

Figure 1. Classic Full-Wave Diode Bridge DC
Rectifier

Fast Power Fault Detection by Monitoring 24V AC Systems Using TPS37x
Voltage Supervisor
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The Half-Wave AC approach for Fast Transient
Response
One unique approach to detect a faster fault
condition; whether the AC power rail has a change
in operating frequency or voltage amplitude, is shownin Figure 2. The circuit allows direct monitoring ofan AC source by using a topology called a “resistivedrop” where the passive circuit elements are used
to provide a fast-transient fault detection. During thepositive phase of the AC voltage, the voltage going
into UV SENSE2 is a sinusoidal voltage that is divided
down and its voltage value is compared to the factory
programmed threshold voltage of the TPS37A. The
negative phase is excluded or omitted by R3, C1,and D1 resulting in only positive phase voltage at theUV SENSE2 input. Because the TPS37A is directly
monitoring the AC voltage and eliminating the delay
inherited with full-bridge rectifier, the fault detection
time is substantially shorter.
DC Supply(Rec ed)2.7 V to 65 V

VPull-Up"Resisve-Drop"supply topology circuit

VDD
OV SENSE1

TPS37AR1

UV SENSE2

24 VACRESET1

RPull-UpRESET2

R2
CTS1
R3C1

GND

RPull-Up
DC Fault Flag
AC EarlyFault Flag

CTS2

D1

* The circuit soluon is not isolated and one must take into accountthe use of high voltage when planning to use in high power systems.

Figure 2. Half-Wave AC Circuit on UV SENSE2
Input
Combining the Best of Both Sensing Schemes:
DC + AC Fault Detection Using TPS37A
Figure 2 shows one of many ways in which theTPS37A can be used to monitor an industrial orfactory AC power rail. Input OV SENSE1 is setup
to sense the DC signal from the rectification ofthe 24 VAC source and monitors the AC sourcefor overvoltage events due to a change of voltageamplitude. The UV SENSE2 is setup to monitor theAC undervoltage (UV) positive half-wave signal thatis generated from the “resistive-drop” supply topology
circuit. The resulting outputs from the TPS37A is a DC(OV) fault and a fast-transient AC (UV) fault signal.
Combining both monitoring schemes (DC and AC),the TPS37A offers engineers the ability to implementa unique “window operating” voltage sensing solution.

Speed Up Fault Detection by AC Sensing
The “resistive-drop” circuit in Figure 2 resistively
divides the AC voltage signal and provides only aperiodic positive half-wave signal into UV SENSE2
input. The signal does not go through any output
filtering and hence any change to the AC voltage
or operating frequency can be rapidly detected.
Given the operating frequency of the AC source
and converting the frequency to the time domain, asense time delay is determined to monitor a change
in operating frequency. The sense time delay isprogrammed by placing a capacitor on the CTS
pin and its value sets the sense time delay or
the detection time. For example, Figure 3 shows aperiodic positive half wave cycles with a frequencyof 800 Hz or 1.25 ms. Each full cycle is 1.25 ms or
0.625 ms for a half cycle. When the half wave voltage
amplitude falls below the SENSE2 threshold voltage,
the programmed sense time delay capacitor begins
to charge while monitoring the AC source. If the next
voltage amplitude positive cycle exceeds the SENSE2
threshold voltage, the SENSE2 programmed timedelay capacitor, CTS, will reset and the pin will notindicate a fault. Conversely, if the voltage amplitudeof the positive cycle does not reach the SENSE2
threshold voltage within the programmed time delay oftCTS, a fault will occur. In an event where the operating
frequency from the AC source decreases (half cycle >0.625 ms), the AC voltage amplitude may not crossthe SENSE2 threshold voltage at the programmed
time delay tCTS resulting in a fault condition.
Full CycletFC = 1.25 ms

Half CycletHC = 0.625 ms

Half Wave (SENSE2)
AC Early Fault Flag (RESET2)tCTStCTS ≤ tHC

Figure 3. Timing Diagram
Conclusion
The flexibility of wide-voltage range monitoring
ensures that critical systems in an industrial andfactory settings are operating within the safety
limits of the workplace environment. Wide-voltage
range supervisors are able to monitor various
types of DC and AC voltage power rails with afew passive components, offering engineers many
benefits, including an effective and novel way toprovide a direct rapid AC sensing method. Monitoringa voltage rail with over and under voltage capability
on each SENSE pin provides redundancy to create ahighly reliable system without false system resets.

*Note that this design solution is not isolated and one must consider the dangers of high voltage when planning
to use in high power systems.

2Fast Power Fault Detection by Monitoring 24V AC Systems Using TPS37x
Voltage Supervisor
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