Make a negative -24V from positive 24V - Page 1
Forum section: electronics / beginners

author: dalevova
author_rank: Newbie
timestamp: February 26, 2018, 12:03:51 pm
content: Hi community,I need -24V ~100mA for PT6315 VFD display driver.I have +5V power supply and boost converter to +24V (from +5V).Can't find a simple schematic to convert +24V (or more if needed) to -24V.Please help to noob.

author: Rbastler
author_rank: Frequent Contributor
timestamp: February 26, 2018, 12:37:44 pm
content: The ICL7660 might do the trick. But I have no idea how much current you can draw from it.Sent from my A0001 using Tapatalk

author: danadak
author_rank: Super Contributor
timestamp: February 26, 2018, 01:07:58 pm
content: Something lkike this -https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwi6yODt0sPZAhXjYt8KHVKvDYMQFgg-MAA&url=http%3A%2F%2Fwww.ti.com%2Flit%2Fan%2Fslyt516%2Fslyt516.pdf&usg=AOvVaw3YuY7qq7w6UveVi8nz5DRVhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwi6yODt0sPZAhXjYt8KHVKvDYMQFghKMAE&url=http%3A%2F%2Fwww.ti.com%2Flit%2Fan%2Fsnva022e%2Fsnva022e.pdf&usg=AOvVaw1dVkNASb7NMqngq2w_41FbAlso consider doing this off you 5V supply, should yield better efficiency.Regards, Dana.

author: madires
author_rank: Super Contributor
timestamp: February 26, 2018, 01:35:13 pm
content: You could buy a 3W DC-DC converter module with isolated output and connect the positive output to ground. Or see the LM2575's datasheet for an example of an inverting converter.

author: Buriedcode
author_rank: Super Contributor
timestamp: February 26, 2018, 02:11:25 pm
content: I think charge pumps would struggle with that current. Sure you can get inverting charge pumps for 200mA+ but these are generally for lower voltages (5V max). The ICL7660 has a max input voltage of 12V, and even when cascading them, they have a high output resistance.Using a two diode charge pump on the switching node of the boost converter could get you Vout - 2*Vd (so about -23.4V for schottky diodes) but would require a large fly cap and I can't see it producing >50mA.Ultimately, you're looking at either an off-the-shelf module or you could use a coupled inductor with your boost converter. 1:1 inductors are fairly common, and you would need to set the boost to 29V to get -24 from the secondary (a boost converters output is the inductors voltage + Vin = 24 + 5V). If you can find coupled inductors with higher ratios it would make life easier, but of course, these tend to be much harder to find, and if you can find a source of such inductors then you could probably find miniature inductors with split secondaries - that way you can just use your boost converter and generate +/-24V from it.Does this need to be regulated? A buck converter can be modified to be inverting, and some have the facility to allow regulation of the negative rail. Just google "negative voltage buck".

author: janoc
author_rank: Super Contributor
timestamp: February 26, 2018, 02:22:06 pm
content: You can easily make a simple flyback supply like this:http://www.kerrywong.com/2013/06/05/vfd-filament-driver-using-555/That one gives about 2.2V AC for the filament but nothing prevents you from winding more turns for the secondary to have a higher voltage and to add an rectifier. The large advantage is that the supply is floating (don't tie the center tap of the secondary to 5V in that case!), so you can tie either end to the system ground to get you either positive or negative voltage, as needed. You can even wind several secondaries because you will likely want the offset filament drive too to prevent ghosting on the VFD.I have built this circuit for testing an old VFD and it works just fine. It is not super well regulated but it rarely matters for this type of application.