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Thermoelectric cooling (TEC) elements, or a Peltier device, require a H-bridge to control the direction of current flowing through the device. As current flows through the device, one side of the device becomes cooler while the other side becomes warmer. If current is reversed, the cool side heats up while the warm side cools down. Using current control, a precise temperature can be achieved for a variety of applications.
H-bridges. common in motor drive products. are ideal to allow PWM control of the output voltage and the ability to reverse direction of the current similar to changing the direction of a brushed DC motor.
Motor drivers like the DRV8412, DRV8432, DRV8874, DRV8873, and DRV8256P are commonly considered for these applications depending on the system needs. These are high current H-bridge drivers, up to 24A peak, that can be PWM controlled to precisely control the current in the TEC element. If you need a lower current, lower voltage option for your TEC then the DRV8210 might be a good option.
One consideration in selecting the best device for your system is the PWM switching frequency. TEC elements are primarily resistive and require a low ripple current. Therefore, a LC filter is used to filter the PWM output before the TEC element to minimize ripple and improve efficiency. It is the same tradeoff that is made with switch mode power supplies. With a higher switching frequency, smaller inductors and capacitors can be used in the filter. Devices like the DRV8412 and higher current variant, the DRV8432, can switch up to 500kHz.
Another consideration is minimum PWM on time. All the above devices include short circuit protection that will disable the device if a short is detected on the output. Devices like the DRV8874 have an over-current protection deglitch timer that is typically 3 micro-seconds and the maximum switching frequency is up to 100kHz. If it is desired to switch up to 100kHz, the minimum on time of 3 microseconds must be met in order to detect a short at the output. This equates to a minimum on-time pulse that is equal to a 30% duty cycle when considering a period of 10 microseconds. If this on-time is not met, shorts at the output will go undetected. For the DRV8412 and DRV8432, this specification is defined as the Overcurrent response time in the datasheet and is specified at 250 nanoseconds.
Closing the loop on current is another key consideration for choosing TEC devices. The DRV8873, DRV8874, and DRV8256P can achieve this without the need for an external current shunt resistor. In the DRV8873 and DRV8874, an internal current mirror is used to monitor the current in the H-bridge and this information is available on an external pin for a micro-processor to monitor and adjust the PWM accordingly. Additionally, the DRV8874 and DRV8256P devices can regulate the current internally by providing an external voltage reference to the device to adjust the current regulation trip point. The current loop would then be closed within the H-bridge itself. The DRV8873 can also regulate current in this manner with 4 discrete current levels selectable via a SPI bus. For the DRV8412/32 and DRV8210, external shunts can be placed on the low-side ground return to monitor current using an external amplifier.
One final consideration is diagnostics and the availability of automotive qualified devices. The DRV8873 and the AEC-Q100, DRV8873-Q1, both other SPI variants and provide advanced diagnostics and also open up other features like slew rate control and the ability to daisy-chain multiple devices to reduce the number of general purpose IOs in the micro-processor.
These are just a few of the many options from Texas Instruments to consider when driving TEC elements. If this FAQ did not completely address all your concerns, please post on the forum and you will get a quick response to any question you have on the devices above or other devices that interest you. Please be sure in your post to mention some of the considerations you have from the topics mentioned above and we will help direct you to the right device.
Motor Drive Application Manager
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