HEALTH MANAGEMENT STRATEGY FOR ELECTRONIC POWER DISTRIBUTION SYSTEMS.pdf

8/10/2019 HEALTH MANAGEMENT STRATEGY FOR ELECTRONIC POWER DISTRIBUTION SYSTEMS.pdf

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HEALTH MANAGEMENT STRATEGY FOR ELECTRONIC POWER

DISTRIBUTION SYSTEMS

Sonia Vohnout, Justin Judkins, James Hofmeister, and Ronald Carlsten

Ridgetop Group, Inc.

6595 N. Oracle Road, Suite 153B

Tucson, AZ 85704

Tel: 520-742-3300

[email protected]

Abstract: We present an indirect and non-invasive prognostic and health management

strategy for fault-tolerant electric power systems. Switch-mode power supplies (SMPS)

have become ubiquitous in electronic modules and systems, delivering a regulated DC

voltage over a power bus or to a specific module. Often, the power supply has the highestfailure rate within the electronic system making it the most important factor in reliability

and asset readiness. This paper will show how we use prognostic sensor modules for DC

power supplies to monitor degradation signatures and extend the operating life of a

critical power system by dynamically reconfiguring the loads. Sensor modules are placed

on the power distribution system external to the power supply and poled at regular

intervals. The operating life of a critical system can be extended by dynamically

distributing the load based on the degradation signatures being monitored. This modular

approach is simple and can be implemented on existing systems with minimal redesign. It

offers the advantage of increased reliability at low cost, is vendor independent, scalable,

and is applicable to many non-prognostics enabled power supplies.

Keywords: Electronic prognostics; Health monitor; Optical coupler; Switch-mode

power supply

Introduction: A leading cause of failure in its electronic systems is a failure of the power

components, such as power supplies and actuator drives. The consequence of the loss of

the power electronics can range from mission failure to making critical assets unavailable

when they are needed. Doctrine advises a frequent maintenance schedule to ensure

reliable systems, but given the logistics cost of carrying spare parts, this is not alwayspractical. The solution is to augment schedule-based maintenance with condition-based

maintenance, as supported by electronic prognostics. Anticipating electronic failures

before they occur can reduce scheduled maintenance and associated costs, improve

mission success and reliability, enhance system readiness, increase safety, and reduce

overall lifecycle cost in advanced electrical systems. The emerging field of electronic

prognostics and health management (ePHM) is becoming a key enabler of cost-effective

reliable, available, and robust electronic systems with a long service life. Electronic


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prognostics allows the detection of impending solder joint failures in package

interconnections, monitoring of degradation signatures in ceramic and electrolytic

capacitors in power supplies, detection of broken bar faults in squirrel cage induction

motors, and health monitoring in network servers ([1] -[4]).

Switch-mode power supplies (SMPS) have become ubiquitous in electronic modules andsystems, delivering a regulated DC voltage over a power bus or to a specific module. As

with all electronic subsystems, the SMPS is prone to wear out and eventually fail. Often,

these power supplies have a higher failure rate than the downstream components, making

them the weak link in a system. Common fault modes that are present in most SMPS

include degradation of the output capacitor, failure of the power metal oxide substrate

field effect transistor (MOSFET) switch and diodes, failure of the control IC (integrated

circuit), degradation of the opto-isolator, and a variety of electro-mechanical issues

including printed circuit board (PCB) delamination and failure of interconnections [5].

SMPS manufacturers have been reluctant to add prognostics to their supplies because of

the trade-off between fault coverage and reliability; adding more sensors increases thedetection resolution but also makes a circuit inherently less reliable. A sensor failure

within the circuit may impact the electrical performance in an unpredictable way. Hence,

adding sensors within the power supply lowers its reliability. In some cases the benefits

of this capability do not justify the cost and reduction in reliability. It is always desirable

when attaching a sensor to the system that it is as non-invasive as possible, cost effective,

and a reliable prognostic solution.

In previous research we introduced and described a prognostics health monitoring system

where the crossover frequency is monitored through a voltage regulation feedback loop,

and a fault-to-failure progression model is used to predict the health and remaining useful

life (RUL) of an optical isolator in a SMPS. The approach is indirect, non-invasive, costeffective and applicable to non-PHM enabled power supplies, offering increased

reliability. We also introduced a new non-invasive prognostic sensor for the optical

isolator in a SMPS [5]. We will now show how we use prognostic sensor modules for the

opto-isolator and the output capacitor in DC power supplies to monitor degradation

signatures and extend the operating life of a critical power system by dynamically

reconfiguring the loads. The PHM approach presented for fault-tolerant electric power

systems is both indirect and non-invasive; the prognostic modules can be combined on a

single integrated circuit that monitors only the external terminals of a general purpose

DC-DC converter. This approach is simple to implement, low cost, applicable to many

non-PHM enabled power supplies, and offers prognostic support for an on-board vehicle

health management system. Degradation of the power supply can be accurately tracked

and easily processed to provide advanced warning time to impending failure.

Switch-Mode Power Supply Topology: Our approach relies on our understanding of the

behavior of the SMPS as a system with feedback. The DC power supply chosen in this

paper has a closed regulation feedback loop (see Figure 1). As noted in a previous

research paper [5], changes in the performance in this feedback loop have minimal effect


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on the regulating function of the SMPS, but do cause changes in how the output voltage

responds to dynamic inputs to the load or input.

Figure 1. State Diagram of Switch-Mode Power Supply

The phase margin, the difference in the phase of the loop gain in Figure 1 and 180 o, plays

an important role in the stability of a SMPS. A large phase margin will tend to cause

greater damping and less oscillation. A low phase margin will allow the circuit to ring

for an extended time for the small input perturbations. This can result in a marginally

system. Phase margins of 0o or less result in uncontrolled oscillations and instability,

which is undesirable in a closed control system. In our research, we use an impulse input

to cause the output voltage to ring. The properties of the ringing, such as magnitude,

frequency, and damping) are then related to the loop gain and phase margin. We can

exploit this behavior by detecting and analyzing the transient behavior to infer the level of

wear-out of certain componentswithin the loop, such as diodes, switches, transformers,

and in particular the optical coupler in the isolation stage of the circuit and then decide ifa SMPS is on a failure trajectory while it is still in service and define how soon it will

need replacement or repair. The transient signals are therefore available for diagnostic

purposes if one has an understanding of the correlation between the loop gain and phase

margin.

Fault-to-Failure Progression Model: Acommon component to fail in a SMPS topology

is the optical coupler or opto-isolator that acts as a signal amplifier between the error

signal generation and the pulse-width modulation stages of the power supply. We selected

an optical isolator for this study, based on a GaAsphotodiode and a phototransistor. The

current gain for this device typically ranges from 1.0-3.0 Amp/Amp. This gain, referred to

as the current transfer ratio (CTR), is a multiplier in the loop gain that effectively causes avertical shift in the Bode plot of the feedback loop gain and, consequently, also affects the

cross over frequency. In an earlier paper [5], we showed how the degradation in the opto-

isolator is expected to result in a decrease in ring frequency.


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1000 10000 100000

-40

-20

0

20

CTR = 3.0

CTR = 0.7

Magnitu

de(dB)

Frequency (Hz)

fc

Figure 2. Bode plot of the loop gain showing how a shift occurs in Crossover Frequency

due to a decreased in CTR

The opto-isolator is one of a few high failure-in-time (FIT) rate items in the SMPS. Thedegradation progression for this component is a relatively slow decrease in CTR over

time. So long as the loop gain remains above unity, the diminishing CTR does not affect

steady state operation of the circuit. Only when the CTR falls below a critical threshold

value will the circuit cease to properly regulate the output voltage , allowing it to drift

higher. However, well before this failure point, the health of the optical coupler can be

measured from the observation of the crossover frequency. This frequency will decrease

as CTR is reduced (see

Figure 2 and Figure 3).

-0,2 0,0 0,2 0,4-0,2 0,0 0,2 0,4

CTR = 3.0

CTR = 0.7

Time (ms)

VoltageTransient

CurentImpulse

Figure 3. SPICE simulation output showing voltage transient response to load current

impulse. Damped ringing behavior occurs for two values of opto-isolator CTR.

Figure 3 shows the results of a Simulation Program with Integrated Circuit Emphasis

(SPICE) simulation for the output voltage for two values of CTR, 3.0 and 0.7. In the

simulation, the load current changes from 5 amps to 10 amps for a duration of 10s and

returns to 5 amps. In the case of CTR=3.0, an oscillation of 20 kHz occurs in the vicinity

of the crossover frequency shown in Figure 4 and is quickly damped. The circuit recovers


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and is again regulating at 5 volts output. Notice that when we shift this curve up or down,

the crossover frequency is also shifting right or left (

Figure 2). The rate of shift is given by the slope of the Bode plot, about one decade per

40dB in level shift.

Figure 3 shows the impulse response to the load transient for a scenario in which the

optical isolation stage has accumulated some amount of damage. The CTR is reducedfrom 3.0 to 0.7, still adequate for steady state voltage regulation. Notice, however, that

the oscillation frequency is reduced. This is a direct result of moving the crossover

frequency in the loop gain curve. An oscillating frequency change of roughly 2 to 1

corresponds to the square root of the relative change in CTR.

This observation suggests the use of resonance measurements as a prognostic indicator in

a regulated SMPS. The relative value of CTR may be tracked as a function of time using

the calculation for the oscillation frequency, and signal averaging or least squares

regression can be used to determine if measurements show a trend toward failure. It has

been demonstrated [8] that the PN junction photodiodes typically used for the optical

emitter exhibit a gradual degradation with time. Such a model can be produced through atest program using highly accelerated life testing.

The Levenberg-Marquardt (L-M) algorithm has proved to be an effective and popular

method for solving nonlinear least squares problems. It is used in many data fitting

applications. We combine the L-M method for fitting the impulse response data and the

degradation rate model presented in [8] to project the wear out time of the opto-isolator

and the eventual loss of the power supply regulation (see Figure 4). Other factors such as

temperature and load conditions change randomly while the device is in the field

environment, and these parameters may also be factored into the degradation model to

improve the accuracy and confidence of the final RUL calculation.

Intermittent faults are another issue which this prognostic addresses. The SMPS is not

considered to have actually failed until the coupling value has decreased below the level

at which the power supply can regulate current. Prior to this point, the power supply may

continue to provide well-regulated voltage output but may fail in certain stress conditions.

Once the supply has been returned from the field and tested with a voltmeter in a

laboratory environment, it is likely to pass Re-test Okay (RTOK) or No Trouble Found

(NTF) unless the conditions for test are similar to the field environment. Recreating those

conditions is not always practical or possible.


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Figure 4. Non-invasive ring frequency detector to monitor degradation and assess RUL

More interesting than the instantaneous measurement is the change or trend in frequency.This scheme may be used to periodically pole the power supply and obtain a running

history of the ring frequency measurement as it shifts over time. By relating this

frequency with a baseline frequency to the CTR shift, we obtain a progression of CTR

versus time.

Data representing CTR shift can thus be obtained by a non-invasive approach. Assuming

that failure of the opto-isolator is the dominant factor in the failure of the regulation loop,

the health of this component and its remaining life can be determined from a CTR physics

model. The confidence of this prediction is based on a number of factors including

measurement noise, noise on the bus, and ability of the model to incorporate all

significant environmental factors in predicting the components failure trend.Nevertheless, the proposed topology provides a significant improvement in the ability to

assess the health of the regulation loop beyond a life model that only considers statistical

data.

Output Capacitor: The primary purpose of output filter capacitor in a SMPS is to

suppress high frequency noise generated by switching in the DC-DC converter. As a

consequence, the output filter capacitor is subject to continuous current oscillation. The

magnitude of the resultant voltage ripple is dependent on Equivalent Series Resistance

(ESR), ambient temperature, output current, and the input voltage of the converter. Stress

can be also applied to the capacitor when a load is removed from the power supply.

Output capacitors fail as a result of high stress electrical bias and/or mechanical failuressuch as cracked internal parts, in which the ESR of the capacitor increases. For high

capacitance of tantalum or ceramic capacitors, the initial value of ESR is small (usually

HEALTH MANAGEMENT STRATEGY FOR ELECTRONIC POWER DISTRIBUTION SYSTEMS.pdf

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