Failure model and detecting method for MOSFET degradation in DC-DC power converters

MOSFET is the most commonly used devices in DC-DC power converters, and its performance is important to the prognosis and health management of power. The paper proposes a degradation analysis model for MOSFET in DC-DC power converters. A method for detecting the degradation of MOSFET is also introduced. Simulations have shown that the method can predict deterioration in the performance of MOSFET. The simulation results are good agreement with the theory.


INTRODUCTION
DC-DC power converters are widely used in telecommunications equipment, electrical equipment, control equipment, and so on, because they are light, small, thin, and low-cost (Rosas-Caro et al., 2010;Julio Cesar Rosas-Caro et al., 2012).Moreover, DC-DC power is a main component of electronic systems.However, the use of DC-DC power restricts the reliability of electronic systems and directly affects the working status of the entire electronic systems in other aspects such as accuracy and stability.According to statistics, electrolytic capacitors and MOSFETs have a higher degradation rate than other components in DC-DC power (Pang and Pong, 2010).In-depth studies have been conducted on the failure of aluminum electrolytic capacitors, discussing such problems as early failure, the failure mode and the failure mechanism, and the course of deterioration.
Considering the impact of the parameters of the working environment (such as temperature, voltage, current, frequency, etc.), some researchers have analyzed the impact of the main parameters of aluminum electrolytic capacitors (such as capacitance, loss angle, leakage current, ripple current, etc.) on its life (Braham et al., 2010;Abdennadher and Venet, 2010;Buiatti et al., 2010;Sun et al., 2010).In the case of MOSFETs, several methods have been proposed to detect faults in MOSFET, for example, short-circuit and open-circuit faults.Previous work on MOSFETs has focused primarily on reliability designs (Azoui et al., 2011), predicting the remaining useful life of MOSFETs using off-line accelerated aging tests (Smet et al., 2011), and developing degradation models based on accelerated life tests (Oukaour et al., 2011).*Corresponding author.E-mail: wooleef@gmail.comAuthor(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License  (Ginart et al., 2008).An online non-intrusive method of obtaining the degradation state of MOSFETs based on the Volterra series has been proposed (Wu et al., 2014); however, these methods are complex.In this paper, a failure model for MOSFET is presented and a simple method for estimating degradation of MOSFET in DC-DC power converters is proposed.

MODEL
The equivalent circuit diagram for a DC-DC Buck converter is shown in Figure 1.To achieve the ideal state (as shown in Figure 1(a)), it is possible to obtain transfer functions between the input and output of the converter (Chen, 2010).
The transfer function is as follows: where frequency, and damping ratio, respectively: The MOSFET equivalent circuit consists of an ideal switch connected in a series with a resistance (Ron), as shown in Figure 1(b), while Figure 1(c) is the Buck converter large signal averaged model.The ideal switch and diode are replaced by a dependent current and voltage sources, respectively.The current of g i can be expressed as Equation ( 5) when the MOSFET is turned on.
The effective value of g i is: The power loss of MOSFET when turned on is obtained using following equation: Therefore, it is well known that the equivalent resistance (Ron) is Ron/D, which is equivalent to DRon in the inductor branch based on the energy conserved, according to Equation ( 7).

Methods
According to Figure 1(d), the small signal equivalent circuit diagram of the non-ideal Buck converter in the s domain has been established (Figure 2).
It is well known that the transfer functions can be expressed as follows: where 1 K , 0  ,  represent the amplification, corner frequency and damping ratio, respectively: Now, adding an excitation signal, the output will change.While the MOSET degradation occurs, the resistance Ron will increase, causing the output to change.The state of MOSFET through the output can be obtained.
The u(t) can be obtained using the Laplace inverse transformation, which is shown as: where d  ,  ,  can be expressed as: Comparing Equations (2 to 4) and Equations (9 to 11), it is clear that these parameters will change with the degradation of MOSFET.Therefore, it is clear that the state of MOSFET can be obtained by detecting the transient response signal.From the response signal, the Ron is gained.According to the relationship between the degradation state (useful life) and the Ron (Celaya et al., 2012), the state can be detected.The method for detecting the degradation of MOSFET is shown in Figure 3.

RESULTS AND ANALYSIS
In order to verify the validity of the method, MOSFETs in different degrees of degradation are selected to be set in the Buck converter, and the output signals are shown in Figure 4.The results indicate that the Ron increases with the degradation of MOSFET, causing the transient response signal to change.Ron1 expresses the state before degradation, and Ron8 expresses the failure state.From the information of the output signals, including the amplification, corner frequency and damping ratio, the degree of degradation of MOSFET can be judged.By monitoring the changes of the feature signals, it is easy to indirectly measure the degree to which MOSFET has deteriorated.The relationship between the max amplitude and the Ron can be expressed, as shown in Figure 5.According the relationship shown in Figure 5, the Ron can be gained, and the degradation of MOSFET can be predicted by taking advantage of the relation of L=g(Ron) (Celaya et al., 2012).

Conclusions
The simulation and the test results indicate that degradation process of MOSFET leads to an increase in the resistance.The change in the Ron is difficult to measure; it is easy to obtain the change in the response signal caused by the Ron.The degree of degradation is determined by analyzing the response signal collected in the test.Based on the trend of the changes, the remaining useful life of MOSFET in a DC-DC converter can be accurately predicted.

Figure 1 .
Figure 1.Equivalent Circuit Diagram of a Buck Converter: (a) ideal, (b) non-ideal, (c) large signal averaged model, (d) small signal circuit model.

Figure 2 .
Figure 2. Small signal equivalent circuit diagram of the non-ideal Buck converter in the s domain.

Figure 3 .
Figure 3. Chart on the method for detecting the degradation of MOSFET.

Figure 4 .
Figure 4.The transient response signal with the Ron increase caused by degradation of MOSFET.

Figure 5 .
Figure 5.The relationship between the max amplitude and the Ron.