All ways to generate symmetrical rails from a single SMPS

 Introduction

While digital systems are increasingly taking over the world, analog circuits are still used in many places, especially in musical equipment. Most of these devices use symmetrical power rails to simplify internal construction. While such approach is indeed good for devices powered from mains with a transformer, it fails short when supplied from a single DC source like battery. In a current world where many devices are getting smaller and more portable, this design philosophy becomes a limiting factor. While we can't change already created devices, we can create a simple switching mode power supply to generate symmetrical power rails from a single DC source.

Theoretical implementations

There are many ways to generate multiple output rails from a single DC source. Listed in decreasing popularity:

1.  Flyback converter with multiple secondary windings
2. Two independent converters (inverting and buck or boost)
3. Inverting converter based on coupled inductor with rectifier producing positive rail
4. Buck based on coupled inductors producing negative rail

Flyback

The first solution is used mostly in mains powered power supplies as it offers higher efficiency with big voltage ratios and galvanic isolation. It's the topology behind very popular Meanwell power supplies. RT-65 is very popular among eurorack DIYers. The downside is that it stabilizes voltage only on primary output only, others depending on winding ratio. Because of transformer nonidealities (leakage inductance at high loads, parasitic capacitance resonance at light loads) other rails are only so-so stabilized. The biggest downside of flyback converters is their discontinuous output current which coupled with low switching frequencies makes noise filtering very hard. The biggest upside is symmetry of all output signals.

Dual converter configuration

The second one is often used when input voltage is same order of magnitude as output one (5V to 48V for +-12V supply). It's the most "canonical" solution, employed in many audio power supplies. Most of them are designed as step-down (Vin > 12V) and while they look symmetrical they aren't. I compiled the biggest differences in table:

Buck	inverting
continuous output current	discontinuous output current
inductor current slightly higher than output current	inductor current a few times higher than output current
lower voltage swing -> lower ripple current	Bigger voltage swing -> bigger ripple current
higher efficiency -> higher power rating	lower efficiency -> lower power rating

Which is the main reason why many switching power supplies (for eg. tiptop uZeus, 4ms RP25, konstant lab uZDROJ) have asymmetrical maximum load on +-12V rails.

The alternative using step-up converter (Vin < 12V) for positive rail, while more symmetrical, inherits most downsides of inverting topology with little benefit. Additionally because of lower input voltage higher currents are required further complicating the design and reducing efficiency.

The "new" ways

If one looks carefully at buck, boost and inverting converters it's easy to realize that they're all essentially the same thing with just different pin names. Buck converter connected "in reverse" is a boost converter, but with grounded output the previous ground becomes output of inverting converter. This diagram should help visualize that:

all standard topologies compared

Now if another winding to the inductor gets added and rectified with strategically placed diode symmetrical output can be obtained. This idea isn't mine, it's not new either. Modified inverting and modified buck are described by TI application note and Maxim application note, respectively. They're rarely seen in practice though and I think it's because they aren't thought at any university. They share many similarities:

• they use just a single IC and coupled inductor
• they regulate just a single parameter and depend on inductor's tight coupling
• the available max power is shared in a complex way between both outputs 

modified topologies for bipolar output

You could say that that it reintroduces issues of flyback converter and you'd be correct. The difference  is that here there are only 2 windings, in 1:1 ratio, that don't require high voltage isolation. Because of that they're tightly wound together, achieving much higher coupling ratios. While typical flyback converters have 98-99% coupling, coupled inductors have 99.5-99.8% coupling, greatly improving regulation of secondary output.

The main difference between the two is that modified inverting is very symmetric, trying to optimize both rails at the cost of lower efficiency and output power for the same elements. Modified buck on the other hand treats positive rail preferentially, stabilizing just it and offering higher maximum output current.

The natural question is why there's no modified boost converted that'd generate symmetrical rails. The answer is simple - both inverting and buck converter have part of the cycle when inductor voltage is equal to Vout, but boost doesn't.

Practical implementation

To prove the usefulness of modified topologies I created TAC - tactical eurorack power supply. Because of very asymmetrical power consumption of modular the design is based on modified buck converter. The design uses 3A, forced PWM buck converter (TPS62933F), running at 1.2MHz together with 22uH, 45mΩ inductor (SRF1260-220M), along a bunch of passive components to set operating point and filter output ripple.

schematic of built converter

Because of cooling issues and poor component selection the output power was limited to around 2.3A at +12V and 1.0A at -12V. Negative output voltage changed with load applied to either rail (as expected) and was regulated best when it consumed exactly all power in lower switching cycle. That implied optimal load ratio of 4:1 for Vin=15V and 3:2 for Vin=20V. Unloaded -12V output jumped up to -14V, due to ringing of inductor, but adding just 2mA load for LEDs brought it down to -12.6V.