MC34063A – A love hate relationship with a boost converter

*Note – I haven’t posted in a while because Volunteer is in the middle of a development cycle. I’m hoping to get back into posting about various projects and observations, but I’m really rushing my production schedule at the moment.

Lately We’ve been doing a lot cost efficiency rework to some of our designs. A project that we’re working on is getting very close to completion, and now we’re looking at ways to bring costs down before giving the thumbs up to an engineering prototype. So suffice to say, if you need different voltage outputs from a single source, and you want to do it cheaply the MC34063A might show up on your radar. It did for us, and honestly we’ve got mixed feelings about it. The MC34063A has formed such a “the Good the Bad and the Ugly” feeling in me that I wanted to write about it.

The Good

Let’s start with the GOOD. Well, for one it has a wide input range from about 0-40V. It can also output 300V if you push it. It can even output negative voltages with some minor component rearrangement. The datasheet itself doesn’t describe voltages greater than 40V, but an application note released by OnSemi does: https://www.onsemi.com/pub/Collateral/AN920-D.PDF .

And if you don’t feel like reading an Application note a more readable interpretation of this note can be found at this blog post: https://www.changpuak.ch/electronics/High_Voltage_Power_Supply_MC34063.php

If you’ve never used this IC before, Dave Jones from EEV blog released an excellent video on using this IC over here: https://www.youtube.com/watch?v=qGp82xhybs4 . This video is quite excellent, but if you want to have an output above 40V than you’re going to need to read the above application note, because the math for the datasheet kind of falls apart at that point.

Also part of the GOOD about this IC is that it’s been around forever and there are plenty of reference designs available online. The patent for the chip has been opened to the public at this point, and now there are many manufacturers of it which has made it very inexpensive. Kind of like the 555 timer IC, the MC34063A just a really useful and generic component.

The BAD

Now for the BAD. Since the MC34063A is no longer patented literally anyone can manufacture it. In fact, you can get them for about 3-4 cents on AliExpress right now and you’ve got sooooo many options to choose from. Logically you may assume that since this IC is very well understood that they all should be roughly equivalent. Unfortunately not. I’ve only personally ordered these ICs from a single supplier, but from everything I’ve been reading online forums about them makes me believe that these ICs can vary wildly from manufacturer to manufacturer.

There are also online calculators that help you design for this IC, but unfortunately by the time I actually go to build these circuits, I inevitably end up modifying component values to get my desired outputs. Given the differences in all of the design tools, manufacturers, and calculators I wouldn’t just blindly design a circuit and send it out for manufacture. And honestly, I wouldn’t think the BAD would be so bad if it weren’t for the UGLY.

The Ugly

There’s simply no internal thermal protection for this IC. What exactly does this mean? Well, if your MC34063A somehow goes into thermal runaway this IC will short to ground. This can result in a catastrophic failure as Matthew Millman found out over at his blog here: http://tech.mattmillman.com/mc34063-a-tough-lesson/ . He discovered that a failure in one of his tantalum capacitors caused his MC34063A to short. This in turn caused his tantalum caps to detonate, and his PCB to be fuse and subsequently burn. This literally caused a fire… I mean, I gotta repeat that…. it caused a fire. Everybody wants to produce a product as inexpensively as possible, but I guarantee a recall would wipe out any cost savings you may have received.

I personally have killed about 8 of these ICs now just in experimenting with them at higher voltages. It’s amazing how simply changing the resistor values used in determining voltage output (R1 and R2 in the datasheet) can throw this thing into self destruct mode. If I didn’t have current limiting on my power supply I’m pretty sure I would have killed more of these things.

Conclusion

So what does this mean? Should you forever shun this chip? I don’t think so. There’s actually a good chance that something you currently own has the MC34063A. You might even be using that device right now to charge your phone, or power something else. But if you decide to use it in your designs you probably should be aware of the risks involved with it. So let’s go over some things you can do to make sure that your devices don’t blow up.

  1. You could potentially find another chip which is similar to (and I’m pretty sure compatible with) the MC34063A like the NCP3063 made by OnSemi. The NCP3063 has internal thermal shutdown protection and cycle-by-cycle current limit. These features do make the replacement cost about 20-30 times what you’d find on AliExpress, but they also are REALLY REALLY important.
  2. You should always put a fuse near this IC to prevent it drawing too much current. In Matt Millman’s article he described that he did actually add fuses to his circuit, but that when his MC34063A shorted that it still provided a few Ohms of resistance which, in turn, failed to blow the fuses he selected. He’s suggesting a 0.5A fuse right before the MC34063A and this seems like good advice. In my personal experimentation I’ve noticed there to be roughly 8 times higher than the current draw at startup. If your MC34063A is providing 20mA then a 500mA gives you plenty of head room so that you’re not destroying your fuse every time you power your device.
  3. You should also test your circuit by shorting it to ground to make sure your protection works the way you expected. A few cents for a sacrificial fuse can save you a massive recall. In my particular application I also designed my IC to only provide a small amount of current anyway, so even if my circuit shorts my MC34063A doesn’t destruct. Clearly this might not be possible for circuits that need to provide more current.
  4. Don’t use tantalum capacitors with this IC. If you absolutely need them, just pick another boost/buck IC. Tantalums short when they internally form dendrites. This is a well known failure of tantalum capacitors and given a long enough operational lifetime forming dendrites is inevitable. If you absolutely have to use tantalums, typically you should choose components that have 2 times or more the max voltage rating to offset this dendrite/shorting risk.
  5. You could try to limit current to your MC34063A. I looked into this a bit… and honestly since I’d be adding more components and engineering to my project it only made me want to just use the NCP3063 instead. It’d save room and probably cost the same amount when everything is said and done. I haven’t personally tried this, but it reasonably makes sense.
  6. You can still use these ICs for learning and experimentation. Yes these ICs are old and janky, but they’re probably one of the harder boost/buck ICs to learn about and design with. Many concepts used in building power supplies became pretty obvious while using this IC. It will certainly make you appreciate how easy more modern boost/buck ICs are in comparison. It did for me. It also opened my eyes to why power supply design is a bit of a specialty in the electronics community.

So yeah. I guess if you’re making generic car chargers or something and exporting them to another country through an easily collapsible company then the MC34063A probably makes a lot of sense. 3 cents for a component this versitile is amazing. But for Volunteer, I think we’re switching to another switcher. Internal thermal shutdown protection is something that we can’t do without.