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

author: Epsilon
author_rank: Newbie
timestamp: June 22, 2023, 08:02:18 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: Weston
author_rank: Regular Contributor
timestamp: June 22, 2023, 08:52:04 pm
content: I cant say if they are causing your problems, but there are some issues with your schematic:You are missing the decoupling capacitor on VREFYour voltage divider for the output feedback could possibly damage the chip if adjusted incorrectly. The maximum voltage on the FB pin is 6.3V, but if R1 is adjusted to the end of the range the division ratio for feedback will be ~1, thats a minimum voltage of Vin minus a diode drop for a boost converter.The layout is also not great. The long wires contribute inductance which can cause damaging voltage spikes or lower efficiency.Debugging is a search process. Try starting by checking the pins for some basic functionality to narrow down what might be wrong:Is the VREF pin at 5V like it should be?Is the chip oscillating? (is there a sawtooth waveform on RT/CT pin?)What are the voltages at the FB and COMP pin? Is the error amplifier working as it should? (is your feedback voltage divider set wrong?)What is the voltage at the current sense pin? Is the shunt connected correctly?

author: CountChocula
author_rank: Unknown
timestamp: June 22, 2023, 08:57:00 pm
content: Possibly a silly thing because it's a bit hard to tell, but in the photo your IC seems to be off the socket by one row (e.g.: pins 1 and 8 look like they're just dangling outside the socket and not actually connected to anything). Again, it could just be a trick of the eye, but I figured I'd mention that, just in case.—CC

author: Epsilon
author_rank: Newbie
timestamp: June 22, 2023, 10:20:35 pm
content: I might have damaged the chip because I adjusted it from min to max resistance. I think the long wires are indeed problematic. I changed the chip and I get 5v at Vref, No oscillation at RT/CT pin, FB/COMP around 100mV. How do I know error amplifier is working? Thank you, this is some dense stuff and I only heard many of this today so sorry if I don't understand too much.

author: Weston
author_rank: Regular Contributor
timestamp: June 22, 2023, 11:04:40 pm
content: Before you do any more testing you should add a ~15k resistor or so between VOUT and R1 so you can not damage the UC3842 with too high a voltage on the FB pin. You should also add a decoupling capacitor on VREF. The lack of that capacitor could cause the chip to not work correctly.The error amplifier for the UC3842 compares the internal reference voltage, which is 2.5V, and the voltage on the feedback pin. The voltage at the COMP pin controls the peak current. If the voltage at the FB pin is less than 2.5V the voltage at the COMP pin will be at the maximum, which is ~5V.Its a bit perplexing you are not seeing any oscillation on the RT/CT pin and both the FB and COMP pin are at a low voltage. Whats the voltage at the RT/CT pin then?The 5V output is gated by the UVLO, so if 5V is present on VREF it should be oscillating. Are you sure there are no faulty solder joints or incorrectly connected wires?