AN5369 Datasheet, PDF (1/3 Pages) Dynex Semiconductor – Selection Of SCRs For Parallel Operation

Replaces September 2000 version AN5369-2.0
AN5369 Application Note
AN5369
Selection Of SCRs For Parallel Operation
Application Note
AN5369-2.1 July 2002
Selection of Thyristors (and Diodes) for Parallel Operation
with non-reactor/resistor sharing.
When thyristors are intended to be used in parallel without the
use of reactors or resistors to force current sharing then the
thyristors must be chosen in such a way that they will share the
current in relation to their forward voltage drop at the operating
current.
This Application Note gives a simple way of doing this from the
data presented in the device data sheet, based on a number of
simplifying assumptions. The problems of differing turn-on
performance, finger voltage etc. have not been considered here
but are addressed in Application Note AN4999 “Turn-on
Performance of Thyristors in Parallel”. The treatise below is
presented in terms of the fully turned on phase of a thyristor
conduction cycle. The methodology can equally be applied to
rectifier diodes in parallel.
Dynex Semiconductor will append a 4 digit selection number to
the device type to specify your particular sharing criteria.
Consider the case of two thyristors in parallel with voltage V₂
across them. In this situation the total current is divided between
the two SCRs such that SCR₁ is carrying I₂ and SCR₂ is carrying
I₁. At the current I₁ SCR₁ has a forward voltage drop of V₁.
It is assumed here that all the variation in Forward Voltage Drop
between the thyristors is due to variation in the slope resistance
and that the knee voltage V₀ is the same for both thyristors.
SCR₂ is assumed to be the data book limit case device and V₀,
rt are the figures printed in the data book.
2
Now V = V + I .rt for SCR
2
0 12
2
and V₂ = V₀ + I₂.rt₁ for SCR₁
i.e. I₁.rt₂ = I₂.rt₁
or rt₁ = I₁.rt₂/I₂ ..............................................................(1)
At the current I₁ we have:
V₁ = V₀ + I₁.rt₁
V₂ = V₀ + I₁.rt₂
∴ V₂ -V₁ = I₁.(rt₂-rt₁)
Substituting for rt₁ from (1) we have
V₂ -V₁ = I₁.(rt₂ - I₁.rt₂/I₂)
i.e. ∆V = I₁.rt₂.(1 -I₁/I₂).......................................................(2)
SCR₁ (V0, rT₁)
SCR₂ (V0, rT₂)
I₂
I₁
V0
V1
V2
Instantaneous forward volt drop - (V)
Fig. 1 Total current is divided between the two SCRs
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