Brochure

ESG
8.628.62
8.628.62
8.62
Paralleling SSRs
(Fig. 19) (Fig. 19)
(Fig. 19) (Fig. 19)
(Fig. 19)
SSRs with MOSFET outputs are self balancing and easily paralleled, whereas most others with bipolar or thyristor
outputs require special attention. Ideally, the forward voltage drops should be matched to achieve thermal balance
and lowest dissipation; alternatively, balancing resistors (Rx) are used to force current sharing as shown, For examp-
le, with 40 amps allowed through SSR1, SSR2 must carry 32 amps.
Assuming V1 = 1,3 volts and V2 = 1,5 volts (worst case):
R
X
=
Δ V
Δ I
R
X
=
=
V2 - V1
I1 - I2
=
1,5 - 1,3
40 - 32
=
0,025 Ohm
Thus producing a total voltage drop of 2,3 V.
For zero voltage turn-on thyristor types, either one of the SSRs must be capable of handling the initial full load surge
alone because of a possible half cycle mismatch. Thyristor SSRs have additional turn-on problems that can prevent
paralleling.
Input control
(series or parallel)
Out
SSR1
(40 A max.)
In
+–
+–
V1
Out
SSR2
(40 A max.)
In
+–
+–
V2
I1
R
X
R
Load
72 A
I2
R
X
AC or DC
power
supply
Fig. 19: Paralleling SSRs