50 - 300 Volt step down converter - Page 1
Forum section: electronics / beginners

author: Jamadir
author_rank: Newbie
timestamp: December 10, 2018, 03:31:59 am
content: Hello,I am currently working on an automated solar Pump controller which should work with > 40 < 300 Volts depending on the Number of attached solar Panels.I already have a Driver that just works with Ac and has a built in power supply for Ac.I want to be able to let it run off-grid, so I need a way to get the 12Volts for Logic and the Relay.I already searched for a pre made Buck converter that can handle this Voltage but the highest I found was up to 100V and needed at least 30V.So I was Wondering if someone of you knew a way how i could build a converter by myself. The Relay and Logic should be fine with 0.5 Amps.Thanks for your time.

author: mariush
author_rank: Super Contributor
timestamp: December 10, 2018, 05:41:28 am
content: So you're asking for 6w. that's kind of a lot when you also want 40v..300v dc input, to a low output voltage.You should check if you can use a relay rated for higher voltage. I don't know what kind of relay you need, but if they're generic general purpose ones, then you will easily find 36v or 48v relays.Most 36v relays will trigger from around 27v and the 48v ones will turn on at 33v or more.You could use a switching regulator IC like LTC7138 .. it's 40v .. 140v input and can do 36v up to 0.4A (varies with input, min 120mA with 40v input)They're kinda expensive at 8$ a piece for the LTC7138 but if you don't need isolation, these can save you custom transformers, the datasheet even has an example circuit :A 36v or 48v relay will only use around 10mA, so you'd be left with around 3w for your logic... if you can optimize the power consumption on the rest (maybe use a bunch of capacitors to handle bursts of power consumption if the idle consumption is low)36v to 3.3v..5v for your logic can be done with any 0.5$ switching regulator.So you could have a DC in jack for 40v.. 120v DC which goes to your swiching regulator, and you could have a second 100v ... 300v DC / 80v .. 260v AC input that could go to any generic ac to 36v / 48v dc adapter.... or a ac/dc converter like Recom RAC20-K series :https://www.recom-power.com/pdf/Powerline_AC-DC/RAC20-K.pdfA potential workaround for having a second high voltage psu could be this, if you get get the total power below around 2.5..2.8wThere are linear regulators like LR8K4 which can work with up to 440v DC and will output up to 20mA and (Vin-12v) output voltage. So you could set the output to 140v and the regulator will regulate and output up to 20mA ... that's 140v x 0.02 = 2.8 watts. With 300v in, you'll dissipate around 3.5w in heat ... which isn't a huge amount for a linear regulator, cooling is doable.Link to regulator:https://uk.farnell.com/microchip/lr8k4-g/linear-volt-reg-0-02a-440v-to/dp/2448524

author: ogden
author_rank: Super Contributor
timestamp: December 10, 2018, 05:59:08 am
content: You shall look at flyback topologies, do not limit search to non-isolated supplies.Few you may like:http://www.ti.com/tool/PMP10200http://www.ti.com/tool/TIDA-00583There's more:http://www.ti.com/power-management/reference-designs.html

author: David Hess
author_rank: Super Contributor
timestamp: December 10, 2018, 02:55:33 pm
content: I would also tend to use a flyback topology for galvanic isolation between the input and output but the disadvantage is leakage inductance in the primary which will require a snubber and even higher voltage transistor. A part like the LT1725, LT1737, or LT3748 is suitable.A 4:1 input voltage range is common for wide input range converters but higher is possible. The problems include:1. The power switch must be able to sustain both high current at low input voltage and high voltage at low input current.2. The duty cycle will vary more. For a buck converter, this will be 30% at low input voltage but 4% at high input voltage which is marginal suggesting that a transformer or tapped inductor should be used.3. The inductance will need to be sized for the maximum input voltage so larger than necessary at lower input voltages.4. The above compromises will lower efficiency so require better heat sinking of the power switch. The larger power switch will be more expensive.A buck converter is the simplest but provides no isolation and complicates driving the high side switch.A SEPIC converter has the advantage of providing DC isolation from input to output if the power switch fails but not galvanic isolation. Driving the power switch is easy just like with a flyback converter but two inductors or a double inductor is required. The grounded switch of the SEPIC and flyback also make primary side peak current monitoring easy which may be important for reliability.Powering the switching controller during startup needs some attention. The power up circuit needs to handle the full input voltage range. After starting, the non-isolated designs can use the 12 volt output for power but the isolated flyback design would usually have a third winding on the power transformer.This is not a project for a beginner. A buck converter like this is something I might do just for a challenge.