Download Temperature Control with SCR

Temperature Control with SCR.
Roberto Fernandez, Telecommunication engineer.
In high power A.C. applications, the use of thyristors and triacs to control resistive loads
is very common. However, sometimes it is not possible to control these loads through a
triac since their current rating may not be enough to cover the rating needed by the load.
In these cases, it is recommended to connect two thyristors in inverse – parallel form
since this arrangement allows driving bigger level of power as shown in Fig. 2. The
power delivered to a load may be regulated by SCR power controllers using either the
zero-cross (synchronous) voltage switching or the phase-angle (asynchronous) control
mode. Synchronous control mode of SCRs not only insures lower generated noise
(EMI) and surge currents into resistive loads but it also provides high noise immunity
for the detection circuit. Three phase loads can be controlled in line or in phase, in all
the lines or phases or in two lines or phases. In our case, we used the in - line control of
two lines of a balanced delta resistive load, according to assembly criterions, in spite of
the line current is bigger than the phase current, the third line was connected directly as
shown in Figure 1.
Figure 1.- Electric Schematic Diagram of the system.
At first, the temperature controller was working with contactors but the controller uses a
PID – fuzzy algorithm, this situation lead to a very frequent commutation when the
process value is near to the set point and, because of that, the contactors often were
broken – down. A solid state switching was the solution of this problem. To solve this
situation, we got 4 thyristors SKT100 and two ASPF240D3.
The phase current of the delta circuit is:
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U phase
I phase 
R

220
 44 A
5
the line current under control is:
I line  I phase 3  44 3  76 A
Fig.2.- Power Block.
Bearing in mind that current through thyristors is half wave, then the value will be
i(t ) 
1 
1
1
I pick (  cos t  cos 2t  cos 4t........)

2 4
3
15
2
where
I pick  I RMS 2  76 2  107 A
the mean value of the current through a thyristor is:
I TAV 
I pick


107

 34 A
the RMS value of the current through a thyristors is:
I TRMS 
I pick
0.707
I TAV 
 38 A
0.637
2 2
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Voltage in ON state between cathode and anode of thyristor SKT100 is 1.75 V therefore
the maximum power dissipated is:
PD  I TRMSVT  38 *1.75  66.5W
The maximum junction temperature of this thyristor T j is 130 ºC. In order to calculate
the heat sink thermal resistance T j = 70 ºC.
The thermal resistance of the heat sink may be calculated by using the next equation:
T j  PD ( RJC  RCS  RSA )  TA
where T j 
junction temperature,
PD  power dissipation
RJC  semiconductor thermal resistance (junction to case)
RCS  interface thermal resistance (case to heat sink)
RSA  heat sink thermal resistance (heat sink to ambient)
TA  ambient temperature
for this thyristor
RJC  0.31º C / W
and
RCS  0
because there is not interface.
Then
RSA 
TJ  T A
70  30
 RJC 
 0.31  0.29º C / W
PD
66.5
To achieve this value, a aluminum heat sink of a cylinder head of a motorcycle was
used and a blower was added. Thyristors were assembled directly using heat sink
compound to increase the temperature transference. The system is working very well;
the maximum temperature reached by thyristors is less than 55 ºC. This solution is a
first step to prove this idea in order to use this circuit to drive bigger currents.
ISIS and ARES programs of PROTEUS v6.6 was used to design some printed circuits
and to do a simulation of some parts.
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