Generating +/- 14V from 3V supply

[ez24]

ha ha
The first time I have been right on this forum 

[new299]

ha ha
The first time I have been right on this forum 

Ah well, at least I made somebody happy. Looks like the LT1054 might be suitable...

[flynwill]

I recently built such a thing with the LT1372, It probably falls short of 100 ma with 3V input (It'll easily get there with 4.5-5 V input).  I just used an additional LC trap to further suppress the switching ripple.

[new299]

As an update on this I build a board around the LM3224 and LT1054 and it appears to be working well. Wrote up some notes here:

http://41j.com/blog/2015/08/13v-psu-board-bring-up/

[Beninot]

Hi, I don't know if this may help you, but the way I found to generate +/- voltage with simple variables voltage regulators like the lm317, is to get a lm 317 to generate a voltage that is half the voltage of your supply and then, you can use it as a "virtual ground", the voltage between it and your positive will be +1/2 your input and for the negative, it will be -1/2 your input

Hope it can help you!   

[new299]

Hi, I don't know if this may help you, but the way I found to generate +/- voltage with simple variables voltage regulators like the lm317, is to get a lm 317 to generate a voltage that is half the voltage of your supply and then, you can use it as a "virtual ground", the voltage between it and your positive will be +1/2 your input and for the negative, it will be -1/2 your input

Hope it can help you!   

That's an interesting idea thanks! In my case I also needed to boost from 3v. I guess I could boost from 3v to 28v and then split it was the lm317. It could be an option perhaps.

[fivefish]

Can you post scope shots of your +/- DC supply? How clean are the outputs? Ripple, etc.

[edavid]

Hi, I don't know if this may help you, but the way I found to generate +/- voltage with simple variables voltage regulators like the lm317, is to get a lm 317 to generate a voltage that is half the voltage of your supply and then, you can use it as a "virtual ground", the voltage between it and your positive will be +1/2 your input and for the negative, it will be -1/2 your input

This is a bad idea, unless you can guarantee that the upper part of the circuit always passes less current than the lower part.  The LM317 can't sink current, so if the upper part of the circuit passes too much current, the "virtual ground" will be pulled up.

[Zero999]

Where are you getting the 3V from?

You want an output power of 3W so at an efficiency of 80% that's an input current of 1.25A.

[new299]

Can you post scope shots of your +/- DC supply? How clean are the outputs? Ripple, etc.

It's very noisy, so I guess I need to work on that. The transients seem to dominate, they're about 50mV! What can I do to reduce this? Any suggestions out there?

[fivefish]

Can you post scope shots of your +/- DC supply? How clean are the outputs? Ripple, etc.

It's very noisy, so I guess I need to work on that. The transients seem to dominate, they're about 50mV! What can I do to reduce this? Any suggestions out there?

Can you post it anyway. I know there'll be noise. I'm really curious and thinking of doing a similar thing.  So even with the linear regulator after the DC converter, it still has some transients? 

Have you tried a pi filter at the outputs?

[new299]

Can you post scope shots of your +/- DC supply? How clean are the outputs? Ripple, etc.

It's very noisy, so I guess I need to work on that. The transients seem to dominate, they're about 50mV! What can I do to reduce this? Any suggestions out there?

Can you post it anyway. I know there'll be noise. I'm really curious and thinking of doing a similar thing.  So even with the linear regulator after the DC converter, it still has some transients? 

Have you tried a pi filter at the outputs?

Attached. I'm not 100% confident in my test setup (I've read I should really shield the board?) but it seems like there's so much noise it's not worth trying that yet. It doesn't feel like the regulators do a not from probing around.

[Beninot]

Hi, I don't know if this may help you, but the way I found to generate +/- voltage with simple variables voltage regulators like the lm317, is to get a lm 317 to generate a voltage that is half the voltage of your supply and then, you can use it as a "virtual ground", the voltage between it and your positive will be +1/2 your input and for the negative, it will be -1/2 your input

This is a bad idea, unless you can guarantee that the upper part of the circuit always passes less current than the lower part.  The LM317 can't sink current, so if the upper part of the circuit passes too much current, the "virtual ground" will be pulled up.

Sorry I'm a beginer, this is the way I figured out to do this, but do you know a voltage regulator witch works at 12v and can sink current?

[edavid]

Hi, I don't know if this may help you, but the way I found to generate +/- voltage with simple variables voltage regulators like the lm317, is to get a lm 317 to generate a voltage that is half the voltage of your supply and then, you can use it as a "virtual ground", the voltage between it and your positive will be +1/2 your input and for the negative, it will be -1/2 your input

This is a bad idea, unless you can guarantee that the upper part of the circuit always passes less current than the lower part.  The LM317 can't sink current, so if the upper part of the circuit passes too much current, the "virtual ground" will be pulled up.

Sorry I'm a beginer, this is the way I figured out to do this, but do you know a voltage regulator witch works at 12v and can sink current?

TLE2426 is good for low power circuits, otherwise it makes more sense to use a conventional split supply.

[new299]

Well, my current guess is that the transients are due to poor layout? (current layout attached) In particular that the feedback resistors are very close to the inductor (as shown in the attached). I kind of assume EMF from the inductor effects the feedback network and results in the spikes I'm seeing?

So I plan to revise the layout and see if that helps. But any suggestions would be very welcome.

[MagicSmoker]

It's very noisy, so I guess I need to work on that. The transients seem to dominate, they're about 50mV! What can I do to reduce this? Any suggestions out there?

Ahem...

You have a 3V supply - that means your converter needs to use bipolar junction transistors (BJTs).
You want low noise - that suggests a converter topology that features sinusoidal currents and/or voltages

The simplest circuit that meets both of the above conditions is a current-fed Royer. This is a push-pull converter with a choke input and a resonating capacitor in parallel with the primary. Not really a "Beginners" type of thing to design mainly because of the transformer but there aren't too many other choices that will work at 3V.

[fivefish]

Magic, are you referring to the type of DC-DC converters that require the use of small transformers with primary and secondary windings?  The secondary is then DC rectified and filtered?

[SaabFAN]

Well, my current guess is that the transients are due to poor layout? (current layout attached) In particular that the feedback resistors are very close to the inductor (as shown in the attached). I kind of assume EMF from the inductor effects the feedback network and results in the spikes I'm seeing?

So I plan to revise the layout and see if that helps. But any suggestions would be very welcome.

Have you tried to add a LC-Filter to the output?

[RoadRunner]

i suggest LT3463 - Dual Micropower DC/DC Converters do very good job. i have used them for e-ink bias circuit.

http://www.linear.com/product/LT3463

[rs20]

I would not chain Dc-Dc converters, i tried, too much noise. The downstream converter adds to its output noise from upstream converter.

Citation needed? Sorry to dig up this old post, but how can this be? Put simplistically, the switching noise from the first regulator has to make it through through at least two stages of smoothing, so even if the two sources of noise are added, I would think the first source of noise is heavily attenuated. I grant there could be subtleties, like perhaps the control loop of the second converter being upset by noise from the first... but at the end of the day, the second converter is slamming the output of the second inductor from 5V down to 0V up and down again continually, how can adding a ~0.1V of ripple to that 5V be of any relevance?

[fivefish]

Citation needed? Sorry to dig up this old post, but how can this be? Put simplistically, the switching noise from the first regulator has to make it through through at least two stages of smoothing, so even if the two sources of noise are added, I would think the first source of noise is heavily attenuated.

I've seen it.

Example: 12VDC switching wall wart (typical china stuff you can get from eBay), powering a 12V to <whatever> DC-DC switching converter circuit.  Scope the output of the 2nd converter and you'll see switching spikes caused by/originating from the 12VDC wall wart.  Unplug the wall wart, the switching spikes disappear, and for a few seconds you have "cleaner" power until the wall wart capacitor charge runs out.

[rs20]

In particular that the feedback resistors are very close to the inductor (as shown in the attached). I kind of assume EMF from the inductor effects the feedback network and results in the spikes I'm seeing?

The spikes you are seeing are happening at over 1 MHz -- so the spikes you are seeing are surely just switching noise? There's no way that the feedback resistor and DC-DC controller are responding so fast that they are creating the spikes you see. In any case, the little spikes you see are less than 20 nanoseconds long, so simple passive filtering as suggested by SaabFAN is surely the cure you need. But, why you'd want to carefully smooth out the voltage input to a switching converter is beyond me. Save all that trouble for after the converters, even if the spikes make it through the converters, they can't be any worse. Might as well deal with the noise from your supply, and the noise from your switching supplies, all at once -- with passive filtering and maybe linear regulation.

Citation needed? Sorry to dig up this old post, but how can this be? Put simplistically, the switching noise from the first regulator has to make it through through at least two stages of smoothing, so even if the two sources of noise are added, I would think the first source of noise is heavily attenuated.

I've seen it.

Example: 12VDC switching wall wart (typical china stuff you can get from eBay), powering a 12V to <whatever> DC-DC switching converter circuit.  Scope the output of the 2nd converter and you'll see switching spikes caused by/originating from the 12VDC wall wart.  Unplug the wall wart, the switching spikes disappear, and for a few seconds you have "cleaner" power until the wall wart capacitor charge runs out.

How did the amplitudes of the two sources of noise compare? That's weird. That implies the duty cycle of the second converter isn't paying much attention to the output. What was the topology of the second converter? It's quite clear that a Constant-on-time controller cannot exhibit this behaviour, nor a hysteretic controller.

The bottom line is that the second DC-DC converter hacks up whatever input voltage it gets into a massively rippley square wave. How input spikes survive the smoothing that removes all the squareness is hard to comprehend. If you're seeing poor layout allowing coupling from the input directly to the output, or common-mode noise, those are different issues entirely.

[new299]

It's very noisy, so I guess I need to work on that. The transients seem to dominate, they're about 50mV! What can I do to reduce this? Any suggestions out there?

Ahem...

You have a 3V supply - that means your converter needs to use bipolar junction transistors (BJTs).
You want low noise - that suggests a converter topology that features sinusoidal currents and/or voltages

The simplest circuit that meets both of the above conditions is a current-fed Royer. This is a push-pull converter with a choke input and a resonating capacitor in parallel with the primary. Not really a "Beginners" type of thing to design mainly because of the transformer but there aren't too many other choices that will work at 3V.

Thanks, I'd like to see how far I can push the boost based design at the moment. It's helping me understand the limitations of boost converters anyway I think.

[new299]

The spikes you are seeing are happening at over 1 MHz -- so the spikes you are seeing are surely just switching noise? There's no way that the feedback resistor and DC-DC controller are responding so fast that they are creating the spikes you see. In any case, the little spikes you see are less than 20 nanoseconds long, so simple passive filtering as suggested by SaabFAN is surely the cure you need. But, why you'd want to carefully smooth out the voltage input to a switching converter is beyond me. Save all that trouble for after the converters, even if the spikes make it through the converters, they can't be any worse. Might as well deal with the noise from your supply, and the noise from your switching supplies, all at once -- with passive filtering and maybe linear regulation.

I'll tried to add LC filtering and it's not seemed to help. I'll try fiddling some more.

One thing I'm confused about... If I disconnect the boost converter output from the linear regulator at it's output, I still see the transient noise on the regulator output. Is this then likely EMI, noise on the supply (I tried adding LC filtering here too though), or is it a measurement artifact?

[MagicSmoker]

Magic, are you referring to the type of DC-DC converters that require the use of small transformers with primary and secondary windings?  The secondary is then DC rectified and filtered?

Yes. More specifically, the original Royer is a self-oscillating voltage-fed push-pull converter that delivers a nearly-pure DC output voltage even before any filtering. The biggest downside of the original Royer is its low efficiency (due to the high peak current in the transistors when the transformer saturates each half cycle), but this can be fixed by simply inserting an inductor in series with the primary center-tap (and flyback diodes across each BJT), which turns it into a current-fed self-oscillating push-pull converter. Then for the ultimate in noise reduction you can place a resonating capacitor across the primary which changes the voltage waveform from a square wave into a sine wave.

The resonant current-fed push-pull was very popular for CCFL backlights in laptops (which have since been rendered obsolete by white LEDs) so a lot of literature will be aimed at that application, but the principles are the same whether the output is in volts or kilovolts.