Jonathan N. Davidson

Improved Bandwidth and Noise Resilience in Thermal Impedance Spectroscopy by Mixing PRBS Signals

This paper presents a method of mixing pseudo-random binary sequences (PRBSs) to form a new signal that can be used to obtain the thermal impedance spectrum of power electronic systems. The proposed technique increases the useful frequency range of a PRBS by mixing two identical sequences at different frequencies. The new signal incorporates the frequency responses of both contributions. Mixing can be performed using a number of mathematical operators and analysis reveals that AND is the operator of choice since it has the lowest average input power for the same effectiveness. The bandwidth, frequency-domain representation, and noise resilience of PRBS signals are also reported. It is shown that the noise floor is significantly reduced under the mixed technique, which allows lower impedances to be measured under noisy measurement conditions. For a typical 8-bit PRBS, mixing reduces the noise floor by a factor of 10.5. Simulated and experimental validation are performed and results show the mixed scheme offers increased bandwidth, reduced computation and improved noise resilience compared to single PRBS techniques.

Real-Time Prediction of Power Electronic Device Temperatures Using PRBS-Generated Frequency-Domain Thermal Cross Coupling Characteristics

This paper presents a technique to predict the temperature response of a multielement thermal system based on the thermal cross coupling between elements. The complex frequency-domain cross coupling of devices is first characterized using a pseudorandom binary sequence technique. The characteristics are then used to predict device temperatures for a known input power waveform using a discrete Fourier transform-based technique. The resulting prediction shows good agreement with an example practical system used for evaluation. To reduce the computational complexity of the initial method, a digital infinite impedance response (IIR) filter is fitted to each cross coupling characteristic. A high correlation fit is demonstrated that produces a near-identical temperature response compared to the initial procedure while requiring fewer mathematical operations. Experimental validation on the practical system shows good agreement between IIR filter predictions and practical results. It is further demonstrated that this agreement can be substantially improved by taking feedback from an internal reference temperature. Additionally, the proposed IIR filter technique allows the efficient calculation of future device temperatures based on simulated input, facilitating future temperature predictions.

Prediction of device temperatures in an electric vehicle battery charger system by analysis of device thermal cross-coupling

This paper presents a practical implementation of the measurement and use of thermal cross-coupling data. Thermal cross-coupling, the thermal transfer impedance between the heat generation of one power device and the temperature of another, is characterised using pseudo-random binary sequences for a typical H-bridge arrangement. Knowledge of the cross-coupling between each pair of devices allows temperature profiles to be predicted for arbitrary input power waveforms. The technique is verified by applying arbitrary power waveforms to all devices in the experimental arrangement and measuring the temperature response. Good agreement is shown between predicted and practical device temperature waveforms, demonstrating the viability of the technique.

Measurement and Characterization Technique for Real-Time Die Temperature Prediction of MOSFET-Based Power Electronics

This paper presents a technique to predict the die temperature of a MOSFET based on an empirical model derived following an offline thermal characterization. First, a method for the near-simultaneous measurement of die temperature during controlled power dissipation is presented. The method uses a linear arbitrary waveform power controller which is momentarily disconnected at regular intervals to allow the forward voltage drop of the MOSFETs antiparallel diode to be measured. Careful timing ensures the power dissipation is not significantly affected by the repeated disconnection of the power controller. Second, a pseudorandom binary sequence-based system identification approach is used to determine the thermal transfer impedance, or cross coupling between the dice of two devices on shared cooling using the near-simultaneous measurement and control method. A set of infinite impulse response digital filters are fitted to the cross-coupling characteristics and used to form a temperature predictor. Experimental verification shows excellent agreement between measured and predicted temperature responses to power dissipation. Results confirm the usefulness of the technique for predicting die temperatures in real time without the need for on-die sensors.

Critical Design Criterion for Achieving Zero Voltage Switching in Inductorless Half-Bridge-Driven Piezoelectric-Transformer-Based Power Supplies

A methodology for predicting the ability of inductorless-driven piezoelectric transformer (PT)-based power supplies to achieve zero voltage switching (ZVS) is presented. A describing function approach is used to derive an equivalent circuit model of the PT operating in the vicinity of ZVS and the subsequent application of the model provides a quantitative measure of a PTs ability to achieve ZVS when driven by an inductorless half-bridge inverter. Through detailed analysis of the analytical model, the limitations of the inductorless half-bridge-driven PT are exposed from which guidelines for designing both the PT and inverter are derived.

Real-Time Temperature Estimation in a Multiple Device Power Electronics System Subject to Dynamic Cooling

This paper presents a technique to estimate the temperature of each power electronic device in a thermally coupled, multiple device system subject to dynamic cooling. Using a demonstrator system, the thermal transfer impedance between pairs of devices is determined in the frequency domain for a quantized range of active cooling levels using a technique based on pseudorandom binary sequences. The technique is illustrated by an application to the case temperatures of power devices. For each cooling level and pair of devices, a sixth-order digital IIR filter is produced, which can be used to directly estimate temperature from device input power. When the cooling level changes, the filters in use are substituted and the internal states of the old filters are converted for use in the new filter. Two methods for filter state conversion are developed-a computationally efficient method, which is suited to infrequent changes in power dissipation and cooling, and a more accurate method, which requires increased memory and processing capacity. Results show that the temperature can be estimated with low error using a system which is suitable for integration on an embedded processor.

Observation of electrolytic capacitor ageing behaviour for the purpose of prognostics

Electrolytic capacitors are an important component within power electronics systems which are known to exhibit poorer reliability compared to other components within the system. In this paper, the changes in electrical parameters (capacitance and equivalent series resistance) which occur as electrolytic capacitors age are characterised at regular intervals over the life of the capacitors. Ageing is observed under three different bias conditions: no bias; constant voltage bias and square wave excitation and at two different ambient temperatures. The data captured within this work presents the changes in capacitor properties from new, reaching to a point which the capacitor parameters have changed sufficiently, such that the capacitor can be considered to have failed. Such data will prove valuable in the development of a system designed to determine the state of health of a capacitor, or could be used to predict its remaining useful lifetime.