I'm designing an industrial control system that incorporates several peltier devices.

As I understand it, these devices are more efficient when driven with a constant voltage, vs being cycled every few seconds.

What I would like to do is drive the peltiers using PWM in the kHz range, and run the power through an inductor to limit the current ripple.

The issue is that my go-to device for high current control is an SSR like this: AD-SSR6M12-DC-200D Afaik, these will not work at kHz frequencies; and at lower frequencies the inductor becomes excessively large.

So here's what I'm wondering. Is there an industrial type device similar to a SSR that will work at kHz frequencies? Ideally I would like to keep the isolation feature as well.

I know I could build my own MOSFET circuit to accomplish this, but a reliable off the shelf device would be preferable. Does such a thing exist?

My peltiers, and the PLC outputs are all 24 volts. The peltiers draw 4.2A at 24V

Link to Peltier Assembly.

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    \$\begingroup\$ Hi Drew, I know this is a pet peeve of mine, but: you've found several ways of capitalizing kHz, none of which were correct. It's small "k" for kilo (not capital for Kelvin) and "Hz" for Hertz. \$\endgroup\$ – Marcus Müller Feb 9 '19 at 10:07
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    \$\begingroup\$ @MarcusMüller If you don’t have that pet peeve, you’re not a true engineer. \$\endgroup\$ – winny Feb 9 '19 at 11:37
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    \$\begingroup\$ Well, let's say I'm a recovering engineer. I can tolerate Khz. But the 433 mhz ISM band really grinds my gears. \$\endgroup\$ – Marcus Müller Feb 9 '19 at 12:29

What you're describing is an adjustable switch-mode buck voltage supply. These exist. As a matter of fact, you're quite possibly using multiples of them to access the internet :D

And: whilst 4.2 A isn't "small" current, it's also not heavy duty, so you'll be able to design one with the online power designers that e.g. TI.com has. I don't know what your maximum operation voltage should be, but it's probably easiest to use an off-the shelf 48V supply and design a buck converter with Vin=48V, Vout=24V, Iout=5A (headroom is always nice), and then adapt the control loop to be adjustable to your needs (often, you can "hijack" a current limiting input by adding a constant voltage e.g. from a DAC to the feedback voltage).

You'll notice that you can pick topologies that are isolated; however, that's typically pretty complex. One could argue that your 48V supply should be the one that's isolated together with the control interface, and that thus you don't need isolation in your adjustable power supply.

Something that'd, in my book, work would be:

TPS based SMPS

Since you want adjustability, you'd remove the connection between the Rfbt/Rfbb voltage divider and the FB pin, and replace it with an opamp adder, where you add the voltage divider voltage to a 0-0.8V voltage generated by a DAC. Since generation of reference voltages is a very PLC thing to do, chances are you've already got an isolated DAC available (you might need to use a voltage divider to adjust it to your 0-0.8 V range).

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  • \$\begingroup\$ Thanks for the response Marcus. I am familiar with the circuit you're describing. I have these on several PCBs I've designed. I think it's even overkill. All I really need is D1 and L1, and I can replace the regulator IC with a simple mosfet. The issue if I do that is 1. Dev time, and 2. This will be used industrially and subject to abuse, so I'd be taking on a the burden of making sure it will be reliable, which is not trivial. \$\endgroup\$ – Drew Feb 9 '19 at 20:55
  • \$\begingroup\$ no, you're approaching this from the wrong side: an SSR is not what you want, but a SMPS is – so, your approach will end up in way more complications than just building a power supply. \$\endgroup\$ – Marcus Müller Feb 9 '19 at 22:45
  • \$\begingroup\$ Ideally I would like to build neither. \$\endgroup\$ – Drew Feb 10 '19 at 4:53
  • \$\begingroup\$ Well, then get an off-the-shelf adjustable power supply that you can control remotely. Again, switching something on and off quickly + inductivity = buck power supply. \$\endgroup\$ – Marcus Müller Feb 10 '19 at 8:44
  • \$\begingroup\$ That would be an example of what I'm looking for. I'm not aware of any industrial type power supply that meets those criteria. \$\endgroup\$ – Drew Feb 10 '19 at 22:32

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