AN5951 Datasheet, PDF (1/5 Pages) Dynex Semiconductor – Estimation of turn-off losses

Introduction:
The total power losses in a thyristor are
comprised of off-state losses, switching losses
and conduction losses. The off-state losses
are the steady state losses as a result of
blocking voltage and current (leakage
current). The switching losses are the dynamic
losses encountered during the turn-on and
the reverse recovery phases of the thyristor.
The conduction losses are the steady state on-
state losses during the conduction phase of
the thyristor. In the majority of the phase
control thyristor applications the conduction
losses are the dominant power losses
compared to others. Therefore it is often
sufficient to design thermal circuit using just
the conduction losses with some safety
margin. To help towards this process Dynex i²
phase control thyristor datasheets give charts
of power dissipation under the commonly
encountered waveforms such as sine wave
and the rectangular wave for different
conduction angles.
The switching power losses are the function
of the repetition frequency and the
commutating di/dt. Therefore these losses
become significant at higher frequencies and
for high di/dt. For high voltage applications
the contribution made by the reverse
recovery losses can no longer be ignored. The
reverse recovery energy is given by:
𝐸_{𝑟𝑒𝑐} = 𝐼_{𝑟𝑒𝑐} 𝑡 × 𝑉_í µí± (𝑡)𝑑𝑡
(1)
To calculate the energy loss as per equation
(1), detailed knowledge of the reverse
AN 5951
Estimation of turn-off losses
in a thyristor due to reverse
recovery
Application Note
AN5951-3 January 2010 LN27002
Authors: Dinesh Chamund; Colin Rout
recovery current and voltage waveforms is
required. This is usually acquired through
actual measurements in the real circuit.
However for initial design purposes and
dimensioning of the device, a quick method of
estimating the recovery losses is desirable. In
this Application Note a method of estimating
power losses due to reverse recovery is
outlined.
Fig. 1 Thyristor Turn-off waveforms
Approximation of reverse recovery
waveforms:
Fig. 1 shows the current and voltage
waveforms observed during the turn-off
phase of a thyristor. The charge stored during
the conduction phase is extracted as reverse
Page 1 of 5