Isolated output on cheap boost converter?

[rhodges]

I am looking at cheap boost converters. I am interested in a cheap -20 volt source for LCD contrast. I believe that I read in the power supply chapter of "The Art of Electronics" that a boost converter gives a negative output, so my first thought is that most boost converters will have an isolated output, and it is likely that any random boost converter will be fine for my -20 volts.

I am not worried about risking 75 cents on an ebay purchase, but if my idea is doomed to fail, I might as well learn sooner. Thanks!

[jhpadjustable]

Nope. A boost converter can be arranged upside down (with the switch transistor and flyback diode on the ground side) so that it boosts ground relative to Vin (Vout < ground), but the usual arrangement is to boost Vin relative to ground (Vout > Vin). 75 cent eBay boost modules are likely not the topology you're looking for.

The MC34063 is a general-purpose switching controller around which you can build most basic switching converter topologies. Performance isn't state-of-the-art but the price and availability are very good, and there's a ton of application information out there and a bunch of hobbyist designs.

[paullo]

The MC34063

Is swearing allowed on this forum?

[jhpadjustable]

The MC34063

Is swearing allowed on this forum?

By Jove, you're right, I forgot the suffix!

As I mentioned, the MC34063 is not the greatest performer, but it has a broad range. It's a fair, bare, breadboard-friendly building block for someone to get acquainted with switching converters and controllers, in a variety of basic topologies. Also, it's jellybean cheap in DIP. What more could a beginner (with properly managed expectations) want?

[tkamiya]

Just be careful with those cheap power converters.  I recently used one I bought from A******.  When input voltage dropped significantly due to overload, the board went into oscillation and spit out 45volt or something.  It blew few things downstream.  You might want to load test it before putting it into use.

[Buriedcode]

I've done a few negative supplies for LCD contrast, they rarely require over 15mA for the smaller displays.  As such, you can use a charge pump, tied to a boost converter.  The boost converter steps up a voltage (and provides switching), the charge pump, which can just be two diodes and a couple of caps, creates the negative of this voltage, minus the forward voltage of the two diodes.  It isn't isolated galvanically like a transformer/coupled inductor would be, but as there is a cap in the path, the output can be shorted to ground without damaging any components.  Effectively, create a boost converter and set its output to ~21.4V.  The charge pump will be -21.4 + 2Vf.  Vf = ~0.7, so -21.4 + 1.4 = -20V. 

See: https://e2e.ti.com/blogs_/b/powerhouse/archive/2016/04/01/charge-it-up-with-charge-pumps-part-3#

Alternatively, like above, a boost converter, with a coupled inductor (a transformer).  This can provide an isolated output, but with a 1:1 coupled inductor the output of the boost has to be set at your desired negative voltage, plus its power supply.  eg.  5V in, -20V, the boost converter would have to step up to 20+5 = 25V.   The second winding would be grounded, and provide ~-20V.  Called a flyback converter (a boost converter, with a coupled inductor).

Another way - use an inverting buck-boost converter, although devices specific for this task are not that common.  the MC34063A can be configured like this, its simple, cheap and it works, but its pretty inefficient for low output currents (<50%), and the low switching frequency means you'll need a bigger inductor and caps.  Not huge, just >100uH and >47uF output cap.

Yet another method is using a buck converter to create the negative voltage, there are app notes on this, just google "buck negative voltage".

Finally, dedicated device for negative voltage for LCDs.  Maxim have a few devices, but these are essentially just switchers like above, with other features like digital output adjust.  Plus they are expensive >$8.