Input  protection setting for DC-DC converter  - Page 1
Forum section: electronics / projects

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: December 21, 2021, 12:07:06 am
content: Hi there,This post is very easy one for you who know better.I am confused to set some values.My DC-DC conveters input is 250-650VDC and output is 24V/5A. Its use for PV battery charger.Take a look here,

author: SuzyC
author_rank: Frequent Contributor
timestamp: December 21, 2021, 02:43:18 am
content: 1,2)R24 and R25 are transient voltage protectors, so additional transient protectors are not needed.3)A fuse will protect against possible fire if somethings shorts out in the convertor.4)These resistor are "bleeder" resistors to discharge the two large 63uf capacitors. Without them a person could have a very dangerous shock if they touched the two convertor's power plug pins after being unplugged.5) Great idea, with a LED indicator on the output leads so one can see that the converter is likely putting out charging power. A 5K resistor in series with a LED would show if any voltage is present at the output (before it is connected to a battery). A 24-V 1W zener in series with a 2K resistor to the LED (with a 100-ohm in parallel with the LED) would show that the voltage is near the proper level, but either way the LED would not show if any current is flowing out to the batteries to be charged.If my zener-LED suggestion for a LED indicator is used, a difference in light output may show charging is occurring when the convertor is connected.Once the output leads are connected to the batteries, the batteries would light the LED even if the convertor/charger power supply is off if the battery has been charged. A more complex current sensing circuit would be needed to show charging current. You would need something like a hall-effect transistor sensor or a meter indicator across a sensing resistor to show actual charging current.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: December 21, 2021, 04:25:17 am
content: Dear Sir SuzyC,Thanks to come up with very impressive feedback.QuoteWhich additional protector do you mean?QuoteDoes fuse is necessary in this case? Its not a bad idea?QuoteDischarging resistor values are ok? Do you want me to see here,http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.htmlQuoteDo you agree with the following circuit?" alt="" class="bbc_img" />

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 21, 2021, 04:56:48 am
content: I'm assuming it's an auxiliary power supply for a much larger system? A 250V string of modern solar panels would be on the order of 2-3kW.My take is that while EMI filtering on the board is probably a good idea, surge and overcurrent protection would probably be better put on the main inverter or charge controller board. It would also be a good idea to have a series diode on the input so the input capacitance doesn't mess with the MPPT circuit.You can add blue LEDs in series with the bleeder resistors to provide a visual indication of voltage. Blue LEDs need very little current to get a visible glow. (A common mistake is engineers running them at the full rated current for indicator use, which makes them obnoxiously bright.) Most common failure mode of LEDs is going short circuit so using multiple in series provides redundancy. (With such a high voltage, it's very likely a LED failing open will quickly turn into failing shorted anyways.)

author: SuzyC
author_rank: Frequent Contributor
timestamp: December 21, 2021, 07:02:34 am
content: Your boss wanted you to add ZNR” Transient/Surge Absorbers, this is what I mean.==A fuse is a good idea, but choose a 'slow-blow' type to avoid fuse blowing out at startup without any real circuit failure.==Discharging resistors are ok as already implemented, just keep your fingers away from the power plug metal contacts soon after you unplug the convertor.==No, circuit would not work but only because you have the Zener diode reversed.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: December 21, 2021, 07:05:22 am
content: QuoteI did not visit their PV installed area, not sure where the MPPT circuit exsits on the system. I usally test circuit with a panel type DC power source where 200-2000VDC is available.QuoteYes, I know blue LED needs little current. Do you agree with the circuit I posted in #3?What else you can suggest?

author: Steffalompen
author_rank: Contributor
timestamp: December 21, 2021, 08:15:51 pm
content: You seem to be talking past each other a lot.Or perhaps you posted ontop of eachother,SuzyC told you the little error in your circuit from post #3.So R24 and 25arethe additional transient suppressors your boss wants?Also, 1M discharge with 800V would dissipate 640mW, which isn't nothing, I suppose that's why there are three.Another aspect is waste, perhaps it's of little importance if it's a huge PV, but 5A doesn't indicate it's huge.And with 130uF it would take a couple of minutes to discharge? (please check my math, seems a lot.)(PS. Ok so it isn't 800V, I just got that from the schematic. But still)

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: December 22, 2021, 06:44:21 am
content: QuoteCan you find a "slow blow" type within my range?What would be your advice for L1? How about this onehttps://www.we-online.com/catalog/datasheet/74482210002.pdf