UC3843 boost converter (boost from 3.7 to 80v) - Page 1
Forum section: electronics / projects

author: TuralH
author_rank: Newbie
timestamp: November 18, 2024, 08:05:38 am
content: I want to assemble a dc-dc converter that will increase the voltage from 3.7 volts to 80 volts/I need to increase it using uc3843, but due to the fact that this PWM controller operates from 12-28 volts, I assembled another boost converter on mc34063 increased it to 27-30 volts and using LM7812 supplied 12 volts, in Ltspice the circuit works and the expected voltage is obtained at the output, but in reality and in practice will it be possible to increase the voltage using uc3843 and external NMOS from 3.7 to 80?The duty cycle is: D≈83.76% simulated in LTspice and with 91khz of frequency. Of course there are ready-made DC-DC converters that can do this, but I would like this circuit to work too.I will be glad for your help. Photos of the simulation are attached below.

author: ftg
author_rank: Frequent Contributor
timestamp: November 18, 2024, 04:40:42 pm
content: I did 74volts with LM78S40 on a bread board in collage, does that count?The LM78S40 seems reasonably similar to the MC34063 and this onsemi appnote seems to agree.http://www.intusoft.com/onsemipdfs/AN920-D.pdfBut I'd presonally look for something more modern that's explicitly meant for that.MC34063 is slow and rather ancient.

author: tural_h
author_rank: Newbie
timestamp: November 18, 2024, 08:00:43 pm
content: I use mc34063 as power supply for uc3843 and I increase the voltage through uc3843 from 3.7 to 74-78 volts, but I was wondering if it was possible to increase the voltage in stages, if so, how? Are there any diagrams, because on the Internet there is only a theoretical component and there are no practical applications, I mean a cascade increase in voltage

author: David Hess
author_rank: Super Contributor
timestamp: November 18, 2024, 11:47:44 pm
content: How much current at 74 to 78 volts do you need? At low currents a boost converter can directly drive a voltage multiplier as shown below.Will an MC34063 run on 3.7 volts? I guess it will. I have started using the LM3578 for general purpose stuff.Quote from: ftg on November 18, 2024, 04:40:42 pmThe MC34063 is a 78S40 without the uncommitted operational amplifier and power diode, so essentially identical.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 19, 2024, 12:24:18 am
content: Consider using a flyback topology to generate your 80V, rather than boost topology.See figure 27, page 30 for an example.https://www.onsemi.com/pub/Collateral/AN920-D.PDFIf you require high current @ 80V, you must use an external NPN transistor (or N-ch MOSFET)See figure 10, page 6 for an example.https://www.onsemi.com/pdf/datasheet/mc34063a-d.pdfExternal transistor/MOSFET may be needed anyway, because 80V in either circuit topology may generate a reverse voltage that exceeds the internal switch.

author: TuralH
author_rank: Newbie
timestamp: November 19, 2024, 06:00:16 am
content: Quote from: David Hess on November 18, 2024, 11:47:44 pmthanks for the advice and method, yes, my main goal is to achieve 74-80 volts of output voltage, and charge a capacitor with a nominal value of 1000uF and 80V, transfer 3.2J about 80-100mA in output it means 2-2.5A input

author: TuralH
author_rank: Newbie
timestamp: November 19, 2024, 06:01:26 am
content: Quote from: Andy Chee on November 19, 2024, 12:24:18 amThank you very much for the documentation, I’ll definitely take a look at it

author: David Hess
author_rank: Super Contributor
timestamp: November 20, 2024, 09:54:19 pm
content: Quote from: TuralH on November 19, 2024, 06:00:16 amThat seems like a low voltage photoflash supply. They use flyback transformers but at 80 volts, I would consider using a boost converter with external transistor. This avoids having to deal with flyback transformers and leakage inductance.