Many components in a laser cavity such as the pump diode or the gain crystal require careful and stable temperature control. Where water cooling is not feasible, thermoelectric coolers or “Peltier Elements” are the method of choice.
For simple applications, one could drive the Peltier cooler using a MOSFET, whose gate is controlled by a PWM signal. When working with long time constants, e.g. in the order of seconds, this approach might work. But when higher switching frequencies are used, the pulsed signal will lead to a rapid decrease in cooling efficiency and could even damage the device.
Additionally, the required temperature stability demands a sophisticated control loop, often implemented using a PID controller. These kinds of circuits can easily become quite complicated, and therefore a solution in form of an Integrated Circuit can be very convenient.
After a bit of research, my decision fell on the LTC1923 by Linear Technologies. The main reason is that the thermoelectric coolers which I will be using require several amps at a fairly low voltage (2-4V), and other controllers have integrated MOSFETs which are only rated for some hundred milliamps of continuous current. Secondly, the documentation on LT’s website is exemplary, they even provide detailed schematics and board design guidelines.
In a first version, I designed the circuit to be capable of providing higher voltages, so that I can also drive the TEC1270X devices sold on various online marketplaces. Unfortunately, this requires the use of external high-speed gate drivers, as the LTC1923 can’t be powered with more than 5.5V.
I drew the board in EAGLE, closely following the recommended design outlines which can be found in the datasheet. (“Right Click” -> “Open image in new tab” for Hi-Res pictures)
As can be seen in the schematic, the circuit essentially consists of the following subunits (left to right):
- H-Bridge with gate drivers: The two dual N- and P-Channel MOSFETs (IRF7343) are driven by dual high-speed gate drivers (LTC1693) and control the current flow as well as its direction.
- LTC1923: This is the main integrated circuit which includes the error amplifier and controls the gate drivers.
- LTC2053: An additional Rail-to-Rail Instrumentation Amplifier is used to achieve a higher setpoint stability of up to a theoretical maximum of 0.01°C.
- MCP4725: An 12-bit I²C Digital-to-Analog Converter with nonvolatile memory allows us to easily set the temperature setpoint using e.g. a microcontroller.
The temperature feedback is provided by a thermistor which has to be in close thermal contact to the device that is to be cooled/heated. A jumper decides whether the temperature setpoint is controlled via an analog trimmer potentiometer on the board, or via the I²C DAC.
The prototype then went to iteadstudio, who manufactured the board for me. As I chose the cheapest manufacturing options and hadn’t ordered any custom PCBs before, I wasn’t sure how the final board would turn out. When it finally arrived, the result surpassed all my expectations: The green solder mask is applied uniformly, texts on the silkscreen can easily be read and the HASL finish is of high quality. For better solderability, I could as well have chosen an ENIG gold finish, but this option would have cost disproportionately more and soldering problems weren’t observed during assembly.
Assembly was pretty straightforward, as no excessively fine pitched packages were used. However, it still took about three hours of focused soldering to mount the many small capacitors and resistors.
A first test verified that the board is operative and no traces are shorted out. I am currently building a simple test stand to monitor the behavior in a real environment, as the component values critical to the feedback loop have to be determined experimentally. Afterwards, I can give a statement about the controller’s actual performance.
> TO BE CONTINUED: FIELD TESTS <
I am planning to design a new version which is intended to work with supply voltages of 2.7V to 5.5V and therefore without additional gate drivers. The layout will fit a 5x5cm area for decreased manufacturing costs.
: Schematic (PDF)
: Top Layer (PDF)
: Bottom Layer (PDF)