### Switch Protection Design - Fast-Recovery Diodes

**Abstract**

The number of fast recovery applications in high power systems

continues to grow leading to various dynamic constraints and

hence different diode designs and behaviours. Along with

conventional RC (“SCR-type”) and C (“GTO-type”) snubber

conditions, snubberless conditions in both IGBT and IGCT

applications are gaining ground at ever higher currents and

voltages (presently 6 kV). Within these two groups, the further

distinctions of “inductive” and “resistive” commutation di/dt must

be made for an optimal diode design. Diodes capable of high

reverse di/dt and dv/dt can today be realised thanks to controlled

life-time profiling which will be described here with both measured

and simulated results. As will also be explained, such “robust”

designs, though essential for snubberless operation, may be “less

robust” under snubbered conditions so that a clear understanding

of the application (Snubber, Free-Wheel, Clamp, Resistive or

Inductive di/dt) is required for the correct choice or design of a fast

recovery diode. The different diode commutation conditions will

be described and categorised and the optimal diode design

identified with supporting measurements and simulations.

Fig 2 “Inductive” commutation circuit fitted

with snubber and clamp

Traditionally the diode under consideration (in this case a

Free-Wheel Diode (FWD)) is fitted with a snubber and may also

be fitted with a clamp as shown in Fig. 2. Thus for the inductive

commutation circuit, we can define the additional sub-conditions

consisting of permutations of the snubbered/unsnubbered &

clamped/unclamped conditions whereby the snubber controls

the Duet’s dv/dt whereas the clamp controls its peak voltage.

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with snubber and clamp

Traditionally the diode under consideration (in this case a

Free-Wheel Diode (FWD)) is fitted with a snubber and may also

be fitted with a clamp as shown in Fig. 2. Thus for the inductive

commutation circuit, we can define the additional sub-conditions

consisting of permutations of the snubbered/unsnubbered &

clamped/unclamped conditions whereby the snubber controls

the Duet’s dv/dt whereas the clamp controls its peak voltage.

More pdf

Labels: Fast-Recovery Diodes, Snubber