MC34063 - Problem with output voltage under load - Page 1
Forum section: electronics / projects

author: chupocro
author_rank: Contributor
timestamp: June 05, 2022, 10:50:56 pm
content: Hi all, my first post here.TLDR (very short description):I am trying to modify the powerboard of a device programmer for which is known its powerboard has to be modified to make it work well. I did some modifications but there is a problem with the output voltage of MC34063under load. The input voltage is 5V and the output voltage is 10V or 12V selectable by turning ON the MOSFET connecting the additional resistor. The circuit should work well under the load of 100 Ohm.The voltages without the load are correct but when 100 Ohm resistor is connected at the output of the MC34063 the problem occurs as in the picture.Maybe someone would recognize what is causing the problem right from the oscillograms. Is such a behaviour maybe sign of too highRscor toohigh resistor going to the Drive Collectorpin of the MC34063?Detailed description:The powerboard shown in the picture is the power source for the iProg device programmer which can program many EEPROMs and microcontrollers. The programmer consists of the main unit containing the powerboard and many adapters. It has DB44 connector for attaching various adapters and is used by running various scripts. There are hundreds if not thousands of scripts for programming different devices.Since some devices require 10V or 12V it has MC34063 working in step-up configuration. The microcontroller can select the output voltage of the MC34063 by turning ON the MOSFET (Q2 in the schematics).There is a script for testing if every voltage is correct and there should be a 100 Ohm resistor connected to the output of the MC34063 when using the script. The script first enables 10V at the output and measures the voltage with AD converter and then after 0.2 seconds it enables 12V and repeats the measurement. The voltages reported by the script (and measured with the oscilloscope) are correct only when there isn't any load connected to the output of the MC34063 but after connecting the load the voltages are too low becausethe output voltage quickly drops after reaching the maximum and then slowly rise.The original Rsc was 13 Ohm in parallel with 1 Ohm but in that case both 10V and 12V measured by the test script were the same meaning the current limit was too low. After replacing these resistors with 3x1 Ohm in parallel (0.33 Ohm) the voltages measured by the script are now not longer the same but they are still too low because they quickly drop upon reaching the maximum. Unfortunately I don't have the oscrillograms from before replacing the Rsc.There is a huge thread on another forum(I am not sure if it is OK to post the link)about modifying that powerboard and most of users reported the problem was solved by replacing theRscwith 0.33 Ohm.I replacedRscwith 0.33 Ohm and I replaced the BSS84 MOSFETs with AO3401 MOSFETs which have much lower Rdson but the device still couldn't pass the tests and now I am searching for the cause of the problem.Note:There are many revisions of the PCB and the markings in the schematics donotcorrespond to the labels on the PCB. For example,Rscin the schematics is R33 in parallel with R36 but these are R30_1 in parallel with R32_1 on the PCB. The diode I marked as removed can be ignored, it is not in the schematics and it was meant to be used only with the external power supply but it shouldn't be there.My next step would be to replace Rsc with 0.22 Ohm as in the MC34063 datasheet and to replace the Drive Collector resistor with 180 Ohm(it is 390 Ohm at the moment) but before doing further modifications I wanted to ask someone who is familiar with MC34063.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2022, 11:53:39 pm
content: What is the input voltage? Source? Is it drooping during test?What do the waveforms show (oscilloscope)?Does the load actually have to pass 100 ohms? If this is doing the sort of thing I think it is (Vpp for EPROMs, etc.?), it isn't actually going to be much, isn't it?No idea what the "removed" diode is, if designators between SCH and PCB are unrelated.Q1 is apparently backwards, type also not shown. (Also odd: R76 is "1500" even though R55, etc. are "1.5K", or C25 etc. "100 uF x 16V" but C22 etc. "47uF 25V", the formatting is all over the place.)Tim

author: chupocro
author_rank: Contributor
timestamp: June 06, 2022, 01:18:53 am
content: Quote from: T3sl4co1l onTodayat 01:53:39 amThe input voltage is as I said in th 1st paragraph:<start copy/paste>The input voltage is 5V and the output voltage is 10V or 12Vselectable by turning ON the MOSFET connecting the additional resistor.<end copy/paste>Quote from: T3sl4co1l onTodayat 01:53:39 amThe source is either USB hub powered by the external power supply or 12V external power supply + LM317. The input voltage is stable during the test.Quote from: T3sl4co1l onTodayat 01:53:39 amThe waveforms captured by the oscilloscope are as in the picture I already attachedwhen I posted the question. There are 4 oscillograms with the descriptions in the picture.Quote from: T3sl4co1l onTodayat 01:53:39 amYes, the loadmustpass 100 Ohms test. The device is not doingjustwhat you thing it is doing, it doesmuchmore. Some of the adapters used by the device contain additional voltage regulators and quite a few components, some are used for programming/reading the RFID signals etc. etc. There is the test adapter which has to be used with the test script and the load has to be 100 Ohm.Quote from: T3sl4co1l onTodayat 01:53:39 amAs I said:<start copy/paste>The diode I marked as removed can be ignored, it is not in the schematics and it was meant to be used only with the external power supply but it shouldn't be there.<end copy/paste>Quote from: T3sl4co1l onTodayat 01:53:39 amYes, the source and the drain of the Q1 in the schematics are swapped. The typewasBSS84 but I replaced it with AO3401. It is used only for enabling the 10V/12V output at the connector.You may ignore LM317 marked as U12, that's only when using the external power supply. As I said - the values of the components in the schematics don't correspond with the components on the PCB, the schematics could be used only as a reference.The problem is with the output from the MC34063 which is after D6. The key information is - the voltage without the load is correct but the problem is under load. 100 Ohm at 12V is 120mA which should be below the current limit set by theRsc= 0.33 Ohm. The problem could be the value of Drive Collector resistor - that's what I'd like to hear from someone who is familirar with MC34063.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2022, 05:37:16 am
content: I should be more specific; what do the waveforms show, at the input, current sense and switching nodes?Is this with just the 100R load, or anything else attached -- i.e. do the load(s) ever have  bypass caps of their own and are they connected?And which signals are being switched when? EN_12V I gather, with 10_12V as respective?MC34063 has appalling efficiency at low input, making it particularly ill suited here; even if you're within its current capability, you're easily going to exceed the nominal 500mA capacity of a USB port.Yes, drive resistor must be smaller, that's what, about 4V and 1k so 4mA, and min hFE is 50 so expect it to desat around 200mA? The switch node waveform is diagnostic, it won't be pulling down near GND under load if this is the case. More like 3-5 times more current or 330-220 ohms would be called for, it would seem.If you have the option to change it, NCP3063 is a roughly upgraded equivalent (but actually has worse efficiency from low input voltages), CS5173 even better (a proper "fast" regulator, reasonable performance, still a bit quirkly), AP3012 maybe (a bit low on current, but probably adequate still), etc. And zillions of TPSxxx from TI, etc. The latter being so much faster, significant cost and space savings can be had on capacitors and inductor.Tim

author: Andreas
author_rank: Super Contributor
timestamp: June 06, 2022, 05:41:42 pm
content: Hello,Are you shure that this tiny inductor is able to handle the current without going into saturation.And 100 uH is rather low for typical MC34063 frequencies.I would first check the ripple current and frequency (with a differential probe on the scope) across the limiting resistor and compare that with the datasheet value of the inductor.with best regardsAndreas

author: chupocro
author_rank: Contributor
timestamp: June 06, 2022, 10:29:15 pm
content: Quote from: T3sl4co1l onTodayat 07:37:16 ambypass caps of their own and are they connected?And which signals are being switched when? EN_12V I gather, with 10_12V as respective?MC34063 has appalling efficiency at low input, making it particularly ill suited here; even if you're within its current capability, you're easily going to exceed the nominal 500mA capacity of a USB port.Yes, drive resistor must be smaller, that's what, about 4V and 1k so 4mA, and min hFE is 50 so expect it to desat around 200mA? The switch node waveform is diagnostic, it won't be pulling down near GND under load if this is the case. More like 3-5 times more current or 330-220 ohms would be called for, it would seem.If you have the option to change it, NCP3063 is a roughly upgraded equivalent (but actually has worse efficiency from low input voltages), CS5173 even better (a proper "fast" regulator, reasonable performance, still a bit quirkly), AP3012 maybe (a bit low on current, but probably adequate still), etc. And zillions of TPSxxx from TI, etc. The latter being so much faster, significant cost and space savings can be had on capacitors and inductor.TimI was curious to see what would the output waveform be with the original Rsc (13 Ohm in parallel with the 1 Ohm) as it was when the device arrived. In that case the output is always the same regardless of the state of MOSFET for selecting 10V or 12V. I replaced Rsc with the original one more time to capture the output waveform, then I once again replaced Rsc with 0.33 Ohm and since you said: "Yes, drive resistor must be smaller", I just removed 390 Ohm drive collector resistor and replaced it with 180 Ohm as shown in the MC34063's datasheet and the output is now good :-))The first picture shows the output when running the test script with the factory components, in the 2nd picture Rsc is 0.33 Ohm and in the last picture drive collector resistor is 180 Ohm.By looking at the differences it seems to me the 12V would be even better if Rs was 0.22 Ohm. I don't have resistors smaller than 1 Ohm but tomorrow I'll try to solder one more 1 Ohm resistor in parallel to check the waveform with Rs = 0.25 OhmBut the output is already good enough and the device is passing the test. Although 10V is at the upper margin of tolerance measured as 10.4V and 12V is at lower margin measured as 11.5VThank you very much for your help!!To answer your questions:The load is just a resistor, here is the schematics of the adapter which has to be used with the test script.The signals could be swithed by using the user interface or by running the test script. For example, the user could press12V --> 10Vor12V --> 12Vbutton to select the voltage at the 12V output and then theEnable / Disable 12Vbuttons could be used to enable the output. The fulltest script does all kinds of tests by itself. When testing 10V and 12V the script first sets the signal at the gate of the MOSFET to configure the 10V at the output of the MC34063, then it enables the 10V/12V output, then it measures the voltage at the voltage divider connected to the 10V/12V output which goes to the AD converter of the µC and after 0.2 seconds it sets the signal at the gate of the MOSFET to configure 12V at the MC34063 output and measures the voltage one more time.There are two types of the scripts - .blr scripts where you can see end edit the source code and .ipr compiled and encrypted scripts where you can't examine how the script works (at least not in an easy way). Each script consist of 3 parts - the first part defines the user interface (similar to winforms), the second part defines the event handlers and the third part is compiled and uploaded to the AT91SAM7S microcontroller on the main board. Unfortunately I don't have fulltest script in .blr format, I have only encrypted version but I can upload some .blr script if you are interested to see how the device works.iProg is an excellent device, I bought it because I must read and reprogram the locked MC68HC908AZ60 processor from the Column Integrated Module of Opel Vectra but now that I have it I am using it with everything else because it can program "everything". It can replace every Atmel, PIC and EEPROM programmer I was using before I got it + it can do many more things. For example, it can be used as a vehicle emulator for testing car instrument tables - the device emulates the traffic on the CAN BUS and instrument table then works like it is connected to the car.I tested it with some old car computers and it works great. I tried to find the PIN code from some car ECUs, tried to clear the crash data from some car airbag modules etc. and everything worked well.But to read the MC68HC908AZ60 the 10V voltage must work well besides 3.3 and 5V because "high voltage" signal is used to enable monitor mode on that processor.There are many adapters ready to be used with the device and the scripts to be used with those adapters but the device could be used as the µC development environment too and by making your own adapter and the script you can make anything - even multichannel sound generator :-)I am not sure if I could replace the MC34063 with some other controller. Maybe if there is one with the same pinout.BTW, seems as the word "sizable" I marked with red is breaking the quote parser.psI couldn't find the way to type the message outside of the quotation when using full editor. The only way I could type outside of the quotation was by using Toggle View button to disable the WYSIWYG editor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 06, 2022, 11:04:24 pm
content: Yeah, WYSIWYG doesn't work very well... the blockquote clashes with the (traditional?) quoted text besides.  isn't bbmarkup so it shouldn't affect anything... but who knows?Tim

author: chupocro
author_rank: Contributor
timestamp: June 07, 2022, 06:54:49 pm
content: Quote from: T3sl4co1l onTodayat 01:04:24 am isn't bbmarkup so it shouldn't affect anything... but who knows?TimNewlines are weird too, sometimes I have to enter two newlines for the empty line and sometimes one newline is enough :-/ You can see in my last reply - "sizeable" caused the rest of the quote wasn't indented. The same is happening now, I can already see that in the preview.BTW, you asked what is the input voltage under the load - the input voltage when everything is off is 4.87V which is normal because there is SS54 Schottky diode in series with the USB power line. When I enable 12V without load the input voltage drops slightly to 4.80V but when I connect 100 Ohm load the input voltage still remains 4.80V meaning 100 Ohm load doesn't stress the USB port.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2022, 08:24:44 pm
content: Ah, good. Because input drooping would've been another possibility. Well, maybe it still does on the leading edge, but probably also not much.Tim

author: KubaSO
author_rank: Contributor
timestamp: October 14, 2023, 12:34:20 am
content: Quote from: T3sl4co1l on June 06, 2022, 05:37:16 amThat NPN Darlington emitter follower switch doesn't help. For low-voltage operation, I use it very roughly like this:The dropout at low loads (say 0.1A) is 0.5V give or take. Basic idea is to saturate the switch. The boost circuit has to be tweaked for efficiency, last time I did it I added an inductor so the booster was switching as well.