AN 6147 Use of rectifier diodes at elevated temperatures for short term overloads Application Note AN6147-1 May 2014 LN31596 Author: Colin Rout Introduction: Use of rectifier diodes at elevated temperatures Many of the characteristics of a rectifier diode are temperature dependent, but for modest increases in junction temperature many of these changes are insignificant. However the leakage current will approximately double for every 10°C rise in junction temperature as illustrated in figure 1. This increase in leakage current combined with the repetitive voltage VRRM will result in increased power dissipation and potential thermal run-away of the equipment. By reducing the repetitive voltage the power dissipation during reverse blocking can be reduced to an acceptable level and the equipment allowed to ride out the overload without any problems. In this Application Note a method of estimating voltage derating is outlined. 4 3.5 Leakage current normalised to 175°C, - ( pu ) In many applications the rating of the semiconductor is determined by the overload conditions rather than the normal continuous running operation. If the overload condition is of a short duration, of the order of one minute, then rectifier diodes can be operated at above their maximum rated junction temperature as long as the repetitive voltage is reduced. 4.5 3 2.5 2 1.5 1 0.5 0 100 110 120 130 140 150 160 170 180 190 200 210 Junction temperature, T j - ( °C) Fig. 1 Leakage current vs junction temperature Fig. 2 shows the typical effect of voltage on leakage current. The leakage current is reduced by 50% if the voltage is reduced to 75% of VRRM. Thus we can see that if the junction temperature approaches 190°C but we keep the VxI product constant by reducing the voltage to 75% of VRRM, the power dissipation will remain constant. By combining Figure 1 with the inverse of figure 2 we obtain the useful graph given in fig 3. Voltage blocking of 50% VRRM at 200° has been verified in the laboratory. Page 1 of 4 AN 6147 1 Normalised Leakge current to 100% V RRM, - ( pu ) 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 20 40 60 Percentage of V RRM, (%) 80 100 Fig.2 Leakage current vs % of the repetitive voltage VRRM 5 Normalised leakage current, I RRM - (pu) 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 Junction temperature, T j - ( °C ) Required percentage of V RRM Fig3. Required percentage of VRRM for rectifier junction temperatures above 175°C Page 2 of 4 AN 6147 Unfortunately, stored charge is a strong function of temperature as shown in fig.4 so Normalised Stored Charge Value 1.4 1.2 1 0.8 0.6 0.4 0.2 0 100 120 140 160 180 200 Virtual Junction Temperature, Tvj - (oC) Fig 4 Stored Charge against Junction Temperature the reverse recovery losses and the voltage overshoot during reverse recovery will also increase so the snubber design should be reviewed to ensure that the diode is still operating within safe limits. Forward voltage drop may also increase. This can be seen by observing the cross-over point of the VF characteristics in the datasheet compared to the operating current. A linear extrapolation of the interval between the datasheet curves will give a reasonable approximation. These additional increases in conduction and recovery losses may mean that VRRM has to be further reduced to maintain device thermal stability. Page 3 of 4 AN 6147 IMPORTANT INFORMATION: This publication is provided for information only and not for resale. The products and information in this publication are intended for use by appropriately trained technical personnel. Due to the diversity of product applications, the information contained herein is provided as a general guide only and does not constitute any guarantee of suitability for use in a specific application.The user must evaluate the suitability of the product and the completeness of the product data for the application. The user is responsible for product selection and ensuring all safety and any warning requirements are met. Should additional product information be needed please contact Customer Service. Although we have endeavoured to carefully compile the information in this publication it may contain inaccuracies or typographical errors. The information is provided without any warranty or guarantee of any kind. 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