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I want to build a bench power supply. My basic reqirements are:

  1. 0 - ~30V adjustable output voltage
  2. 0 - ~2A adjustable output current limit
  3. input from mains

I found this design by EEVblog guy loosely fitting my requirements, except for point 3. So my idea was to insert isolated switchmode converter beetwen the mains and said circuit like this: Swithing converter would output voltage needed at the output + linear regulator's dropout voltage. The converter would: convert 230V to lower voltage without the need of bulky transformer, isolate output from mains and reduce power dissipated on linear regulator - without converter in the worst case scenario power on the regulator could be as high as 60W in my case, while with converter outputing voltage 2V higher then output voltage maximal power on regulator equals 4W.

So I started looking for the switching converter IC that satisfies these conditions:

  1. Isolated
  2. Mains input voltage
  3. 0-30V adjustable output voltage
  4. 2A output current

I searched mouser, digikey and tme and didnt find single IC that satisfies both 2 and 3. Am i expecting too much from a single IC, or am i just bad at searching?

If such ICs really aren't available, would you advise me to achieve my goals in 2 separate converters (one isolated non-adjustable converter lowering voltage to <50V and second adjusting the voltage to the needed value), or rather to design my own DC-DC converter control circuit(my knowledge about switchmode power supplies is very limitted)? Or maybe the whole idea is ill-formed and the best way is to use a 50Hz transformer, large heatsink and fans?

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  • \$\begingroup\$ Since you want to design the DC/DC part, I would recommend you to buy a AC/DC module that had the specs you need for you DC/DC converter section. There are many affordable AC/DC modules out there that will give you a fixed DC voltage with high efficiency and very good power factor. Besides, if you are not very experienced with AC/DC conversion, buying an isolated AC/DC module would make your work much safer. \$\endgroup\$ – Lucas Sep 9 '17 at 16:14
  • \$\begingroup\$ Besides, if you have an isolated AC/DC, you can use a non-isolated DC/DC topology. Make sure to always work safe and add necessary(ies) fuse(s) to your design. \$\endgroup\$ – Lucas Sep 9 '17 at 16:16
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I'd use a 36-48V SMPS (or a good old transformer) followed by one of these which (on paper) should be the best option for you.

Layout needs to be perfect.

If you want to adjust the output of a DC-DC, simply inject a current into the feedback node, via a current source, or a resistor to a DAC outputting a control voltage. This keeps all feedback parameters constant, compensation is always the same. Do not use a digipot, as this will give variable gain, and thus the best compensation values would be output voltage dependent.

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  • \$\begingroup\$ This isn't the cheapest chip i've seen. Isn't there some 2A cheaper version of this? Other than that I think I'll go with this soultion. Thanks. \$\endgroup\$ – ja2142 Sep 9 '17 at 16:41
  • \$\begingroup\$ I think there's a smaller one, yeah. LT8640S LT8609S LT8643S beware of the packages though... \$\endgroup\$ – peufeu Sep 9 '17 at 16:52
  • \$\begingroup\$ Only QFN? Im not quite comfortable soldering this. Aren't there some devices as user friendly as this but in package easier to solder? \$\endgroup\$ – ja2142 Sep 9 '17 at 16:59
  • \$\begingroup\$ Yeah, it's full QFN, because it gives lowest lead inductance and highest performance. If you don't have the gear for QFN, you can input your requirements here linear.com/products/step-down_(buck)_regulators and it will deliver a nice list ;) Also check TI and others, but I like Linear, as they provide Spice models and a nice simulator. \$\endgroup\$ – peufeu Sep 9 '17 at 17:05
  • \$\begingroup\$ Synchronous internal switch buck will be the easiest to layout, less headache with MOSFETs, for your 2-3 amps it'll be OK. But pay attention to the output voltage range, some stop at low voltages... \$\endgroup\$ – peufeu Sep 9 '17 at 17:06

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