UC3842-Based Boost Converter I made is not working. (Many beginner questions) - Page 1
Forum section: electronics / repair

author: Epsilon
author_rank: Newbie
timestamp: June 22, 2023, 07:51:29 pm
content: ignore the wrong position of the IC when I took a photoI have based my design from here:https://www.pcbway.com/blog/technology/DC_to_DC_Boost_Converter_using_UC3843.htmlUC3842 Datasheet:https://www.mouser.com/datasheet/2/149/uc3843-309322.pdfThe problem is the output (15.7V) is just below the input voltage (16V), only change by the diode drop (0.3V).The out pin of the IC also do not have any pulse.This is my first time doing analog stuff but I know the basic operation of a boost converter.I don't understand exactly how the chip works.I keep it at 16v as it is the UVLO on.My goal is to have the output be greater than the input (16v) and my life is complete.I have checked: every connection, values and no shorts, voltage coming to the IC, etc...My components are limited only to what I salvaged, that's why I have different values.But I have 10 UC3842 I bought online.What do you think is the problem?Is the shunt resistor in series with Mosfet too high?Is the Mosfet model incompatible?Is the capacitance critical at Isense pin?Are these UC3842 chips dead? (I've swapped them, same results)I was surprised there is a shunt in the high-current Mosfet (efficiency?), does it need to be high power resistor like I used?Is the board type inappropriate for my application?Bonus question:Do current mode controlers do calculus? They seem to react with slope of current vs time from what I've read. Can't understand it tho.What I have:Lab PSUAnalog scope 60Mhz2 DMMs1st year EEI'm already 4 days tried everything in my mental capacity. Any advice/tips or theory what's wrong?THANK YOU SO MUCH!

author: Slh
author_rank: Regular Contributor
timestamp: June 22, 2023, 09:37:12 pm
content: The UVLO is not guaranteed to be 16V. That's the typical value. You should ignore it and only use the min/max values. In this case, the maximum UVLO is 17.5V so make sure that your input voltage is more than 17.5V.Once that's sorted, you might run into issues with the feedback. The control loop tries to keep the voltage at the FB pin at 2.5V. if the voltage is less than 2.5V it will enable the PWM as that hopefully will increase the voltage. If the voltage is greater than 2.5V then it can only reduce it by stopping switching. With the feedback potentiometer where it is there's going to be a lot of it's range where it's trying to maintain less than the input voltage so it won't run. For your first one I'd suggest replacing it with a fixed resistor so you always know what the output voltage should be. 9k1 should give around 25V out.After you've got those issues sorted it may start running and them you'll want to sort out the issues below.You appear to be targeting running >100kHz. That's going to be hard to get it to work well without good layout. Also, my experience of the yellow cored inductors is that they're not great when run at that frequency. It will probably run hot. I think that need to find a higher value inductor and run at a lower frequency to start with.You want to reduce the length of as many of those wires as possible. The inductor doesn't matter much but the MOSFET and capacitor want to be very close to the IC.

author: Epsilon
author_rank: Newbie
timestamp: June 22, 2023, 09:41:19 pm
content: I have taken note of all your comments. I did not think of any of those but everything makes sense. Are there any general rules with this types of power supplies? Thank you for the knowledge.

author: Epsilon
author_rank: Newbie
timestamp: June 22, 2023, 09:56:01 pm
content: Based on information you provided, the voltage at feedback is too high since the pot is in higher potential, can I destroy the chip because of that? Can lower frequency be more workable? I don't care much about efficiency right now, just want it to work but do inductor colors matter actually? What frequency you suggest? The wires are actually very long you are right..