Brochure

ESG
8.518.51
8.518.51
8.51
Leakage from the drive source
The off state leakage current in the driving semicoon-
ductors shown in Figs. 6 to 8 is significant, just a few
microamperes, which could not possibly turn on the
SSRs. However, the off state (output leakage current of
any packaged solid state driving device (e.g. temperature
controller, etc.) should first be checked for compatibility
with the SSR.
One method is to multiply the maximum leakage
current (amps) by the maximum input impedance
(ohms) of the SSR. This should result in a voltage that is
less than the specified turn-off voltage. If it is not, a
resistive shunt across the SSR input may be required.
Thermal considerations
One of the major considerations when using a SSR, is
that an effective method of removing heat from the SSR
package must be employed. SSRs have a relatively high
"contact" dissipation, in excess of 1 watt per amp. Usual
methods for heat dissipation are cooling by flowing air
or forced airflow around the SSR or the application of
heat sink.
With loads of less than 5 amps, cooling by free flowing
air or forced airflow around the SSR is usually sufficient.
At higher currents it will become necessary to make sure
the radiating surface is in good contact with a heat sink.
Essentially this involves mounting the base plate of the
SSR onto a good heat conductor, usually aluminium;
good thermal transfer between the SSR and the heat
sink can be achieved with thermal grease or heat sink
thermal resistance (RθCS) is reduced to a negligible value
of 0,1°C/W (celsius per watt) or less. The simplified
thermal model in Fig. 9 indicates the basic elements to
be considered in the thermal design. The values that are
determinable by the user are the case to heat sink
interface (RθCS), as previously mentioned, and the heat
sink to ambient interface (RθSA).
Thermal calculations
Fig. 9 illustrates the thermal relationships between the
output semiconductor junction and the surrounding
ambient. TJ -TA is the temperature gradient or drop from
junction to ambient, which is the sum of the thermal
resistances multiplied by the junction power dissipation
(P [watts]).
Hence:
whereas:
TJ = Junction temperature, [°C]
TA = Ambient temperature, [°C]
P = Power dissipation (ILOAD x EDROP), [W]
(RθJC) = Thermal resistance, junction to case, [°C/W]
(RθCS) = Thermal resistance, case to sink, [°C/W]
(RθSA) = Thermal resistance, sink to ambient, [°C/W]
To use the equation, the maximum junction temperature
must be known, typically 125°C, together with the actual
power dissipation, say 12 watts for a 10 amp SSR, assu-
ming a 1,2 volt effective (not actual) voltage drop across
the output semiconductor. The power dissipation (P
watts) is determined by multiplying the effective voltage
drop (EDROP) by the load current (ILOAD).
Assuming a thermal resistance from junction to case of
say, 1,3°C/W and inserting the above typical values (RθCS)
into the equation, solutions can be found for unknown
parameters, such as maximum load current, maximum
operating temperature, and the appropriate heat sink
thermal resistance.
Where two of these parameters are known, the third can
be found as shown in the following examples:
Fig. 9: A simplified thermal model
T
J
- T
A
= P (R
θJC
+ R
θCS
+ R
θSA
)