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© Copyright 1999 – 2004, Analog Technologies, Inc. All Rights Reserved. Updated: 8/25/04
3
TECEV103
Analog Technologies, Inc.
the set-point temperature TMPS just a small amount,
simulating a step function. At the same time, connect
an oscilloscope at the VDR test pin (on the right side
of the evaluation board), set it to a scrolling mode (0.2
Second/Division or slower) and monitor the waveform
of VDR as TMPS is fed by a step function signal. The
circuit in the compensation network is shown in
Figure 3 below.
−
+
−
+
TMPS
TMPO
VDR
Cd
Rd
Rp
Ri
Ci
Cf
Figure 3 Compensation network
The transfer function of the compensation network, defined
as H(
ω)=VDR(ω)/TMPO(ω), is shown in figure 4.
H(
ω )
Gp
ω
Gd
ω
1
ω
3
ω
2
ω
4
1.41Gp
0.71Gd
Figure 4 Transfer Function of the Compensation Network
In principle, these are the impacts of the components to the
tuning results:
a.
Rp/Ri
determines
the
gain
for
the
proportional
component of the feedback signal which is from the
thermistor, Gp = Rp/Ri, in the control loop, the higher
the gain, the smaller the short term error in the target
temperature (which is of the cold side of the TEC)
compared with the set-point temperature, but the higher
the tendency of the loop’s instability.
b.
Rp/Rd
determines
the
gain
for
the
differential
component, Gd = Rp/(Rd//Ri)
≈ Rp/Rd, where symbol
“//” stands for two resistors in parallel, since Ri >> Rd,
Rd//Ri = Rd. The higher the gain, the shorter the rise
time of the response, the more the overshoot and/or the
undershoot will be.
c.
Ci*Rp determines the corner frequency,
ω
1= 1/(Ci*Rp),
where the integral component starts picking up, as the
frequency
goes
down.
It
determines
the
cut-off
frequency below which the TEC controller will start
having a large open loop gain. The higher the open loop
gain, the smaller the tracking error will be.
d.
Cd*Ri determines the corner frequency,
ω
2=1/(Cd*Ri),
where the differential component starts picking up (see
Figure 4), as the frequency goes up.
e.
Cd*Rd determines the corner frequency,
ω
3=1/(Cd*Rd),
where the differential component starts getting flat. It
determines the cut-off frequency above which the TEC
controller will give extra weight or gain in response.
f.
Cf*Rp determines the corner frequency,
ω
4=1/(Cf*Rp),
where the differential component starts rolling down.
Since this frequency is way higher than being needed for
controlling the TEC,
ω
4 does not need to be tuned. The
capacitor is built into the TEC controller module, not
the evaluation board.
To start the tuning, turn off the differential circuit by setting
Cd Open. Turn W1 quickly by a small angle, back and forth,
approximately 5 seconds per change. Set Ci to 1uF, set Ri to
1M, and increase the ratio of Rp/Ri as much as possible,
provided the loop is stable, i.e. there are no oscillations seen
in VDR. Then, minimize Ci as much as possible, provided
the loop is stable. The next step is to minimize Rd and
maximize Cd while maintaining about 10% overshoot found
in VDR. Optimum result can be obtained after diligent and
patient tuning. The tuning is fun and important.
When the TEC controller is used for driving a TEC to
stabilize the temperature of a diode laser, there is no need to
turn on the laser diode while tuning the TEC controller. To
simulate the active thermal load given by the laser diode,
setting the set-point temperature lower than the room
temperature is enough.
For a typical laser head used in EDFA’s or laser transmitters
(found in DWDM applications, for instance), Ri = 1M
Ω, Rp
= 1M
Ω, Ci = 680nF, Cd = 1.5µF, and Rd = 250kΩ. These
values may vary, depending on the characteristics of a
particular thermal load.
To be conservative in stability, use larger Ci and larger Ri;
To have quicker response, use smaller Rd and larger Cd.
The closer to the TEC the thermistor is mounted, the easier
to have the loop stabilized, the shorter the rise time and the
settling time of the response will be.
3. After tuning, the values of the capacitors for Cd and Ci
can be read off the capacitor selection switches. The
values of the resistors, Ri, Rd and Rp, can be measured
by an Ohm-meter by connecting to the resistor pins. As
seen in the photo of Figure 2, Ri can be read off between
TMPO and CMPIN test points; Rd can be read off
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