MC34063 dc/dc 5 to 400v won't start - Page 1
Forum section: electronics / projects

author: pikey
author_rank: Newbie
timestamp: January 11, 2018, 11:14:10 pm
content: Hello EEVblog community!I've build DC/DC Coverter based on others found on Internet, in theory it should output something close to 400V.It uses MC34063. Output voltage from this puppy right now is about 3.3V and the frequency from SWE is +- 1kHz. Power consumption is 6mAIt looks like boost converter do not starts correctly, but I've no idea what is wrong.The PNP transistor is BC557CThere is a circuit:Any help very appreciated

author: floobydust
author_rank: Super Contributor
timestamp: January 11, 2018, 11:48:15 pm
content: There are many losses in your IRF840 MOSFET driveStarting at 5V, you lose about 1.4V in theMC34063Darlington output stage. Then theBAS40loses another 0.4VThe end result estimate is 3.2V drive to theIRF840which is not a logic-level MOSFET. VGS(th) is 2-4V for it, so the part is pretty much asleep, not turning on much.

author: pikey
author_rank: Newbie
timestamp: January 12, 2018, 12:05:07 am
content: Ok thanks I'll change it to logic level tomorrow. Also I didn't have 32nF cap instead of I've used 22nF which I think can also be a problemWys?ane z iPhone za pomoc? Tapatalk

author: pikey
author_rank: Newbie
timestamp: January 12, 2018, 07:19:20 am
content: Quote from: floobydust on January 11, 2018, 11:48:15 pmAhh, there is no logic-level Mosfets rated for 400V+

author: NiaDebesis
author_rank: Frequent Contributor
timestamp: January 12, 2018, 07:35:14 am
content: Quote from: pikey on January 12, 2018, 07:19:20 am

author: mikerj
author_rank: Super Contributor
timestamp: January 12, 2018, 12:13:50 pm
content: You might want to rethink the 1N4007 as well, it's quite a slow rectifier for a switch mode regulator.

author: pikey
author_rank: Newbie
timestamp: January 12, 2018, 04:22:53 pm
content: There is some progress, I've noticed that BAT42 I've got is different series that I though is was. It was reversed. Now I have 24V output and MC34063 makes "peep" sound, so I'll try to change 1N4007 to Schottky as You all suggest.

author: pikey
author_rank: Newbie
timestamp: January 12, 2018, 05:28:13 pm
content: Quote from: NiaDebesis on January 12, 2018, 07:35:14 am

author: ThomasDK
author_rank: Regular Contributor
timestamp: January 12, 2018, 10:25:16 pm
content: Quote from: pikey on January 12, 2018, 05:28:13 pmThe datasheet says that gate threshold voltage ismin2V and max 4V.The gate threshold is where the FET starts to conduct, it is not fully on. Looking at the graphs in the datasheet you need at least 7-8V to turn it completely on (and all ratings are at 10Vgs)

author: Andreas
author_rank: Super Contributor
timestamp: January 13, 2018, 06:21:50 am
content: Quote from: pikey on January 11, 2018, 11:14:10 pmHello,really 400V from only 5V? (without transformer?)usually the step-up ratio is limited to a factor far below 80.with best regardsAndreas

author: not1xor1
author_rank: Frequent Contributor
timestamp: January 13, 2018, 06:39:51 am
content: Quote from: ThomasDK on January 12, 2018, 10:25:16 pmYes, that is one of the most common error of beginners (besides not reading carefully or not understanding fully data sheets).The gate threshold voltage is specified for currents of the order 100s of microA...To get a low drain-source resistance ordinary mosfets need more than 9-10V while logic level ones can work with 4-5V or even less in some special cases, but in exchange of that, those mosfet can withstand just few 10s of V as drain-source voltage.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 13, 2018, 09:31:19 pm
content: For that ratio, a flyback or "flyboost" topology is the way to go.

author: pikey
author_rank: Newbie
timestamp: January 14, 2018, 09:08:41 pm
content: Quote from: Andreas on January 13, 2018, 06:21:50 amYep, why not. There is no other way if you want to carry pocket geiger muller counter in your pocket powered from batteryWys?ane z iPhone za pomoc? Tapatalk

author: mikerj
author_rank: Super Contributor
timestamp: January 14, 2018, 09:37:22 pm
content: Quote from: pikey on January 14, 2018, 09:08:41 pmYes there is, a flyback converter as NiHaoMike mentioned. A simple boost converter requires high duty cycles to achieve high voltage gains, and this significantly lowers the efficiency of the converter.

author: pikey
author_rank: Newbie
timestamp: January 16, 2018, 12:04:44 am
content: Quote from: mikerj on January 14, 2018, 09:37:22 pmOk so I've changed diode to faster one, UF4007 and finally I'm able to achieve 406V on output. There are 2 major issues, the Mosfet temperature because of too low gate voltage (I could not find any matching transistor locally so for now I still use IRF840) and second issue is the noise I don't know what makes it. It could be an inductor or MC34063 itself, still I'm really happy it works. Thanks to all for help. I'll try to build flyback converter soon in order to learn more.Wys?ane z iPhone za pomoc? Tapatalk