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| Microcontroller, DAC and Mosfet to control TEC? |
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| spec:
--- Quote from: Kleinstein on February 02, 2019, 06:12:04 pm --- Even simple linear control with a analog controlled transistor give better results: for the same average current, the PWM case has all the loss is in the TEC and thus about half effectively on the cold side, while linear control has some of the loss in the pass transistor, which does not interfere with the cold side. For low voltage operation (like starting from a 5 V supply) the limitations on linear operation are not that severe, even for more switching type MOSFETs. Unless one uses modern low voltage (e.g. 20 V or maybe 30 V) parts they are probably OK with only 3 or 4 V at the MOSFET. For only 2 A one could use old IRF510 or BUZ10 parts that are specified for linear operation - though might need more than 5 V for the gate. There is still the option to use a BJT (like TIP120) With 1.4 V needed for base to emitter and 2 V for the TEC there is enough headroom from a 5 V source. --- End quote --- These are just general unsubstantiated statements. None of the losses that you mentioned are defined or quantified. Besides which, the losses in a linear approach would be far higher than PWM, so if half of the linear losses were dissipated in the TEC, that would indicate that PWM would heat the TEC less. In fact, the TEC is a resistor and has to have current to operate and that current causes I2R heating, however it is driven. By the way, you can use any MOSFET or any BJT for any function. It is just that some BJTs and MOSFETs happen to be optimized for switching. In all cases a BJT is a BJT and obeys fundamental physical laws and the same for MOSFETs. |
| radar_macgyver:
--- Quote from: spec on February 02, 2019, 05:57:36 pm ---This has often been stated, but I can't see a problem using any MOSFET for linear applications. Could you describe what the problem might be? --- End quote --- Switching FETs don't have an SOA that includes DC (see attached for IRF540), so while the data sheet might say Vdsmax of say 100V and Id of 10A, you can't do both at the same time which is often the case with linear applications. How much you can go is a function of the SOA curve, which in some switching devices does not include DC. This doesn't mean they won't work, they're just not specified and you could potentially end up with thermal runaway. Having said all this, at low currents you could get away with such devices. |
| RoGeorge:
--- Quote from: Kleinstein on February 02, 2019, 06:12:04 pm ---PWM does not work well with TEC: The loss is proportional to I², while the useful cooling effect is proportional to the average current. --- End quote --- This is correct. ^ For the same cooling power, the radiator for the heating face will need to be way bigger with PWM than with a constant voltage. https://electronics.stackexchange.com/questions/28634/how-to-drive-a-peltier-element https://www.meerstetter.ch/compendium/peltier-element-efficiency |
| Kleinstein:
--- Quote from: radar_macgyver on February 02, 2019, 08:11:50 pm --- --- Quote from: spec on February 02, 2019, 05:57:36 pm ---This has often been stated, but I can't see a problem using any MOSFET for linear applications. Could you describe what the problem might be? --- End quote --- Switching FETs don't have an SOA that includes DC (see attached for IRF540), so while the data sheet might say Vdsmax of say 100V and Id of 10A, you can't do both at the same time which is often the case with linear applications. How much you can go is a function of the SOA curve, which in some switching devices does not include DC. This doesn't mean they won't work, they're just not specified and you could potentially end up with thermal runaway. Having said all this, at low currents you could get away with such devices. --- End quote --- In this case we have a rather low voltage. For most MOSFETs thermal runaway is a problem mainly at higher voltage. It is less a problem at low voltage. The missing SOA curve for DC only indicates that they don't bother to specify it. Especially below 12 V linear operation may not be a problem. |
| Ian.M:
From the simulation or modelling viewpoint, a TEC is not a pure resistor. Its a resistor in series with a voltage source that's proportional to the temperature difference across it, representing the thermal EMF. To get it to cool one side and heat the other, one forces current through it in opposition to the thermal EMF. The cooling effect is, over the normal operating range, more or less proportional to the average current but the heating of its internal resistance is proportional to I2. Consider a current of 10% of its rating. Both steady DC and PWM at max current with a 10% duty cycle will have the same average current , but due to the I2 factor, the losses (self heating) during the PWM on time will be 100 times greater, and when you average them over the whole duty cycle for an 'apples to apples' comparison with DC, will still be ten times greater. To correct a few other misconceptions - you need a MOSFET driver between the MCU pin and the gate if you are directly PWMing the MOSFET, because the MCU pin on its own typically can't provide enough current to slew the gate fast enough through its Miller plateau, where the MOSFET is in its linear region and dissipating power like crazy due to its load current multiplied by the Vds drop. across it. For small, low gate charge MOSFETs, and low PWM frequencies (or bang-bang control), you can often get away without a gate driver if the gate drive voltage requirements are within your logic supply rails. If you need more gate voltage than the logic supply one uses a gate driver that includes level shifting. This is usually a one-chip solution that saves a lot of faffing around with discretes. Most gate drivers are not suitable for linear operation, but unless you need a rapid step response, you don't need a gate driver anyway, as the OPAMP output current is sufficient to slew the gate fast enough. You do need a gate resistor to prevent parasitic oscillations and also isolate the OPAMP from the large gate capacitance which could case instability. From the point of view of the TEC, it doesn't matter if the PWM is filtered before a linear controller or in an inductor in series with the TEC, as long as it is filtered so it only gets low ripple smooth DC. Both avoid the evils of directly PWMing the TEC. I don't have a cite for TEC thermal cycling issues. I remember reading it at least a decade back in a TEC manufacture's documentation - either a data sheet or application guide. |
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