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Posted on 25 May 2019

Rated Current Load for Rectifier Diodes

 

 

 

 

 

 

 

The basis of any software tool used for component design and dimensioning is mathematical equations, diagrams and characteristics.

The maximum permissible current load for rectifier diodes is determined by balancing power losses caused by the current flow P = IF×VF and the maximum power dissipation for a given temperature difference ΔT, P = ΔT/RTH.

Diagrams serve to help users determine the maximum permissible forward current for certain cooling conditions and vice versa to determine the cooling requirements for a given current. To calculate the maximum permissible current load for short-time and intermittent duty, the transient thermal resistance is plotted as a function of time for both the diode (Zth(j-c) + Zth(c-s)) and the heat sink (Zth(s-a)). These two values are then added to obtain the total transient thermal resistance Zth(j-a) for the given period of time. The temperature rise is equal to the product of pulse power and the transient thermal resistance of the pulse duration. The pulse power is limited by the permissible chip temperature and the load cycling capability.

Rated current for higher frequencies

In line rectifiers, switching losses are negligible. At 50 Hz, the switching losses are approximately 1-2% of the conduction losses and are thus covered by the design margin. For frequencies of up to around 200 Hz, the power dissipation curves for direct current load are to be used. In the event of a short circuit, the diode must be able to withstand the surge current resulting from the line impedance and the line voltage. An uncharged capacitor behaves likes a short-circuit load. The surge current ratings apply to sinusoidal half waves, and for durations > 10 ms, as a sequence of 10ms 50 Hz half waves.

The use of equations, diagrams, and characteristics to calculate current ratings help users select the right products for their applications.

 

For more information, please read:

Criteria for Successful Selection of Diodes and Thyristors

Snubber Circuits Based on Silicon Avalanche Diodes

 

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