MC34063 as simple voltage inverter? - Page 1
Forum section: electronics / beginners

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: November 12, 2015, 04:56:34 pm
content: This question is a follow up for aprevious thread, where I came to the conclusion that the MC34063 might be just what I need.Reading itsdatasheet, I found this circuit (page 9, Fig. 7):Based on the above, my questions are:- If I understood correctly, this circuit will invert any input voltage (Vout= - Vin), is that correct?- I can go either way, using a PNP or NPN transistor, but are there advantages of one over another?- What are proper transistors to be used?- The resistor on the left, between pins 6 and 7, is Rsc, and should be around 0.22 Ohms. How to I get that value?- I figure the inductor can be around 100uH, based on other schematics in the datasheed, and the Schottky diode is a 1N5819. What about the 2 resistors that are connected to the base of the transistor? What are suitable values for them?Thanks in advance and sorry if these questions are too basic.

author: go0fy42
author_rank: Newbie
timestamp: November 12, 2015, 11:03:20 pm
content: No, it will not simply invert the input voltage, but you will have to go through the whole design process for the inverting converter (9.2.1.1 in the datasheet). Dave actually has a (rather old...) video on using the MC34063, check it out:. Inverting here only means, that the output voltage will have the opposite polarity of the input. The magnitude is still up to your design.You will have to set a design output voltage, which you want to get and then do the calculations from that. This will also give you the inductor values (Lmin from the design procedure). Check Dave's video, which takes you through the necessary steps.Also I would at least in the beginning try to stay away from the "extended current" schematics, as it can be tricky enough to get the chip to do what you want without them. The external transistors do not necessarily simplify things.While the datasheet says "easy to use" this will not magically work by throwing a few components around the chip - it is more involved than that.Also the output from the MC34063 will be quite noisy (i.e. it will have noticable ripple voltage), so you may want to think about how "smooth" your output needs to be. Some serious output caps and potentially a linear negative regulator to smooth the output may be needed, depending on what you need.As to the .22 ohms resistor: simply parallel five 1ohm resistors, which should give you .2ohm doesn't look nice, but does the job. .22ohms is also available as a standard value, but is something not everyone has lying around...regards,go0fyedit: unf-ed the URL

author: LukeW
author_rank: Frequent Contributor
timestamp: November 13, 2015, 11:54:54 am
content: The 34063 is not the most high frequency, efficient, modern device on the market - but I guess it is common and cheap.There are a few "calculator" websites around for the 34063 - you may find they're helpful with some of the maths.http://www.nomad.ee/micros/mc34063a/

author: coppice
author_rank: Super Contributor
timestamp: November 13, 2015, 12:29:20 pm
content: Quote from: LukeW on November 13, 2015, 11:54:54 amThe chip is cheap, but the BOM isn't. Overall you can achieve considerably better pricing with something that needs less ferrite.

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: November 13, 2015, 02:32:06 pm
content: I can't say I am not disappointed with the info you guys provided. Not that it is your fault, but that the whole idea I had went down the drain. I was really hopping I could just "mirror" my PSU's regulated output as a negative voltage.I'll continue to find a way to add negative voltage output to my PSU project. In the meantime, thank you all for your valuable feedback.Quote from: LukeW on November 13, 2015, 11:54:54 amI got mine from an old automotive USB charger.

author: macboy
author_rank: Super Contributor
timestamp: November 13, 2015, 03:18:06 pm
content: Quote from: AlxDroidDev on November 13, 2015, 02:32:06 pmHow about a little detail on your PSU project. It is a classic linear design with a transformer, rectifier, and capacitors followed by some kind of linear regulator? If so, then you can probably create a negative (unregulated) rail and build a second regulator for the negative side. If you want it to track the positive side (same magnitude but negative) with a common zero-volt terminal then that is usually easily accomplished.As usual, you need to tell use what you want to accomplish, not how you want to do it.

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: November 13, 2015, 06:42:43 pm
content: Quote from: macboy on November 13, 2015, 03:18:06 pmMy intention is to repurpose a notebook PSU (19.5V, 4.74A, HP Part no. 519330-004) as the basis of my own PSU. After the notebook PSU I'll add a voltage regulator (I haven't decided yet if I'll use a LM338T or a LM2679T-ADJ... I see pros and cons in both).I'll also add a Atmega328 MCU that will measure and display the output voltage (through a voltage divider) and output current (through anACS712-5A). The Atmega328 will also display output power (in W) and will shutdown the PSU if the output current exceeds 4.5A).The MCU will be powered by aMAX608connected to a 9V battery. When turned on, the MCU will close a relay, which in turn turns on the notebook PSU's mains. The fact that the relay is controlled by the MCU allows me to shut it down if the measured current exceeds 4.5A.This is, basically, what I have in mind. Bells and whistles include adding a bluetooth adapter to the project so I can monitor voltage/current/power on the phone with an Android app I can create, and also: open/close the relay, control the brightness of the display, and whatever else I can control electronically with the MCU. I've done something similar on a project that was featured onHack-a-Day. I also planed a few LEDs for status, fuses, cooling fan, reset button, a button that connects a 50R/10W resistor on the output (so I can test the measurements or discharge the notebook PSU).The display is a PCD8544 that I got from an old Nokia 6120 phone.I am trying to do this is an enclosure that is quite compact, because mu current "bench PSU" is a repurposed ATX PSU (pic below). Although it's been serving me weel, it is too bulky and ugly. On the pic below, the top left part of the PSU is a variable output controlled by an LM317T.

author: MagicSmoker
author_rank: Super Contributor
timestamp: November 14, 2015, 01:00:11 pm
content: Quote from: AlxDroidDev on November 12, 2015, 04:56:34 pmThe MC34063 is a fine choice as long as you don't need more than a few hundred mA of output current. Yes, most datasheets say the *switch* can handle 1.5A peak, but that isn't a realist value once temperature rise is considered and output current is much lower than switch current in the buck-boost (inverting) converter. Points in its favor are that it uses a fixed frequency hysteretic control so no frequency compensation is required, it is very cheap and more or less ubiquitously available anywhere in the world. Now, as to your specific questions above:1. Correct - this is one of the 3 basic dc-dc converter topologies, the buck-boost (aka the non-isolated flyback).2. It is rather difficult to properly drive bipolar transistors (aka BJTs) in switchmode converters so I would avoid any application examples with an external switch. Note that in a buck-boost converter where Vout is roughly the same magnitude as Vin, switch current will be twice the output current.3. See #2.4. Rsc sets the switch current limit. The value is determined by Ohm's Law (Rsc = Vtrip/Isc), where Vtrip in the MC34063 is nominally 0.3V (but can be anything from 0.25V to 0.35V), and Isc is the peak switch current. For example, if you wanted to limit switch current to 300mA you would choose a 1R resistor. Note that spread on Vtrip, however, and choose an Rsc value that allows for the worst case tolerance if you want to get the most output current possible. That said, I wouldn't go lower than 0.51R for Rsc (giving a switch current limit range of ~500mA to ~700mA [rounded to nearest 100]).5. The MC34063 is very forgiving of the inductor value w/r/t switching frequency and switch current limit, but don't assume that "one size fits all". In fact, the inductance required is inversely proportional to the peak-to-peak switch current and switching frequency, and in all cases it will need to support some DC current and have a closed magnetic pathway (a toroid [aka - ring] core made of powdered iron would be a safe, if not ideal, choice). I'm not going to run the equations for you but I will suggest that you set the switching frequency to around 50kHz, the peak switch current to no more than ~600mA and the peak to peak variation in switch current to around 40% of the peak current. That will result in an inductance value somewhere in the range of 500uH to 1mH. Lower values of inductance will have a higher peak switch current and faster transient response time; higher values will have less output noise and ripple.As for the diode, note that it must withstand the sum of the input and output voltage in a buck-boost converter, plus a little margin for ringing/overshoot due to stray inductance. An additional caveat is that Schottky diodes such as the 1N5819 start leaking current in rather dramatic fashion as the applied voltage approaches their max rating (but are otherwise excellent choices for switchmode converters). The 1N6819 is rated for 40V so with a 19.5V input I would limit output to no more than -12V in a buck-boost; -15V would really be pushing things and would almost certainly require an RC damping network across the diode.Finally, the STMicroelectronics datasheet for this IC is particularly good as it has many application examples including PC board layouts.