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SPICE-modelling and the analysis of the self-excited push-pull dc-dc converter with selfheating taken into account Krzysztof Górecki and Janusz Zarębski Department of Marine Electronics Gdynia Maritime University, POLAND

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2 Outline Introduction The structure of the push-pull dc-dc converter The electrothermal hybrid models of the diode and the BJT The electrothermal model of the transformer Results of investigations Conclusions

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3 Introduction Dc-dc converter RLC elements transformer Semiconductor devices transistorsdiodes

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4 Introduction (cont.) Selfheating – results from changing the electrical energy into the heat and non-ideal cooling conditions Due to selfheating Internal temperatures of semiconductor devices increase Characteristics and parameters values of SMPS change Electrothermal analysis – analysis with selfheating taken into account

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5 In the paper The new electrothermal models (ETM) of: the diode the BJT the pulse transformer dedicated for electrothermal analysis of SMPS in SPICE Measurements and calculations of nonisothermal characteristics of the self-excited push-pull dc-dc converter Why the new models are needed? the literature models are complicated the time duration of calculations with these models is unacceptable long

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6 The structure of the push-pull dc-dc converter The transistors are switched alternativelly the core of the transformer operate with the saturation R1-D – the start circuit

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7 The ETMs of the diode and the BJT The general conception The general conception Only characteristics of the forward biased diode and switched-on transistor operating in SMPS are important Electrothermal hybrid model SPICE isothermal built-in model Controlled sources modelling the influence of selfheating Thermal model

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8 The ETMs of the diode and the BJT (cont.) nonisothermal calculations isothermal calculations measurements

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9 The ETMs of the pulse transformer In the model: –The core without histeresis –Separated temperatures of the core and the windings –The core characteristics dependent on the core temperature –The skin effect in the windings –The mutual thermal interaction between the core and windings Equations – see Proceedings

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10 Results of investigations Transient analysis till the steady-state One non-physical thermal time constant – to short the time of calculations results of calculations results of calculations results of measurements results of measurements

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11 Results of investigations (cont.) results of calculations results of calculations results of measurements results of measurements

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12 Conclusions A very good agreement between the calculated and measured non-isothermal characteristics of the self- excited push-pull dc-dc converter is achieved within a wide range of changes in the input voltage and the load resistance. From the authors’ investigations it results that even at relatively high values of the load resistance the transformer temperature can be high as a result of the energy losses in the core. This temperature is the increasing function of the frequency of the converter operation.

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