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