Technical information
Cautions for Use of SSR
11
ds_x61_en_ssr_technical_information: 011212D
Snubber Circuit
Thermal Design
Protection Circuit
1. Reduce dv/dt
An SSR used with an inductive load can
accidentally fire due to a high load
voltage rise rate (dv/dt), even though the
load voltage is below the allowable level
(inductive load firing).
Our SSRs contain a snubber circuit
designed to reduce dv/dt (except AQ-H).
2. Selecting the snubber constants
1) C selection
The charging coefficient tau for C of the
SSR circuit is shown in formula 1
=(R
L+R) C ------------1
By setting formula 1 so that it is below
dv/dt value you have:
C=0.632V
A/((dv/dt) (RL+R)) -----2
By setting C = 0.1 to 0.2 F, dv/dt can be
controlled to between nV/s and n+V/s
or lower. For the condenser, use either an
MP condenser metallized polyester film.
For the 100 V line, use a voltage between
250 and 400 V, and for the 200 V line,
use a voltage between 400 and 600 V.
2) R selection
If there is no resistance R (the resistance
R controls the discharge current from
condenser C), at turn-on of the SSR,
there will be a sharp rise in dv/dt and the
high peak value discharge current will
begin to flow. This may cause damage to
the internal elements of the SSR.
Load
power
supply
Inductive
load
Snubber circuit
SSR
VA
RL
R
C
Therefore, it is always necessary to insert
a resistance R. In normal applications, for
the 100 V line, have R = 10 to 100 and
for the 200 V line, have R = 20 to 100 .
(The allowable discharge current at turn-
on will differ depending on the internal
elements of the SSR.) The power loss
from R, written as P, caused by the
discharge current and charging current
from C, is shown in formula 3 below.
For the 100 V line, use a power of 1/2 W,
and for the 200 V line, use a power above
2 W.
------------ 3
f = Power supply frequency
Also, at turn-off of the SSR, a ringing
circuit is formed with the capacitor C and
the circuit inductance L, and a spike
voltage is generated at both terminals of
the SSR. The resistance R serves as a
control resistance to prevent this ringing.
Moreover, a good non-inductive
resistance for R is required. Carbon film
resistors or metal film resistors are often
used. For general applications, the
recommended values are C = 0.1 F and
R = 20 to 100 . There are cases of
resonance in the inductive load, so the
appropriate care must be taken when
making your selections.
P=
C × V
×
f
2
2
A
SSRs used in high-reliability equipment
require careful thermal design. In
particular, junction temperature control
has a significant effect on device function
and life time. The rated load current for
board-mounting SSRs is defined as the
maximum current allowable at an
ambient temperature of 40C (30C) and
under natural cooling. If the ambient
temperature exceeds the SSRs derating
temperature point (20C to 40C,
depending on SSR), load current derating
in accordance with the load current vs
temperature diagram becomes
nesessary. If adjacent devices act as heat
sources, the SSR should be located more
than 10 mm away from those devices.
SSRs with a 5 A rating or more must be
used with the dedicated heat sinks listed
in Table 1 or equivalents. To ensure
adequate thermal conduction, apply
thermal conductive compound (Toshiba
silicone YG6111, TSK5303 or alternate)
to the SSR’s mounting surface. For
information on external heat sinks for our
SSRs and their mounting method, refer to
“Data and Cautions for Use for respective
relay”.
Table 1. Dedicated on-board heat sinks
*It is possible to mounting on the DIN rail.
Load current Type Heat sink
to 10 A AQ10A2-ZT4/32V DC AQ-HS-5A
10 A AQ-J (10A)
AQP-HS-J10A
AQP-HS-J10A (for AQ-J)
AQP-HS-SJ10A (for AQ-J)*
AQP-HS-SJ20A*
15 A AQ-A (15A), AQ-J (15A)
AQP-HS-J10A
AQP-HS-J10A (for AQ-J)
AQP-HS-SJ10A (for AQ-J)*
AQP-HS-SJ20A*
20 A AQ-J (25A)
AQP-HS-J10A
AQP-HS-SJ10A (for AQ-J)*
AQP-HS-SJ20A*
25 to 40 A AQ-A (25A) AQP-HS-30/40A
25 A AQ-J (25A) AQP-HS-J25A
40 A AQ-A (40A) AQP-HS-J25A
High-reliability SSR circuits require an
adequate protection circuit, as well as
careful study of the characteristics and
maximum ratings of the device.
1. Over-Voltage Protection
The SSR load power supply requires
adequate protection against over-voltage
errors from various causes. The methods
of over-voltage protection include the
following:
(1) Use devices with a guaranteed
reverse surge withstand voltage
(controlled avalanche devices, etc.)
(2) Suppress transient spikes
Use a switching device in the secondary
circuit of a transformer or use a switch
with a slow opening speed.
(3) Use a surge absorption circuit
Use a CR surge absorber or varistor
across the load power supply or SSR.
Special care must be taken so power on/
off surges or external surges do not
exceed the device’s rated load voltage. If
a surge voltage exceeding the device’s
rated voltage is anticipated, use a surge
absorption device and circuit (e.g. a ZNR
from Panasonic Electronic Devices Co.,
Ltd.).