Summary: Although single-supply rail-to-rail op amps are widely used, it is often necessary to generate two supply rails from a single (positive) input supply rail to power the rest of the analog signal chain. The current in these parts is generally low, and the positive and negative supplies have relatively well-matched loads. To address this problem, this paper proposes a more optimal approach beyond the common solution, which uses a SEPIC-Cuk converter consisting of a Cuk converter with an unregulated output connected to a SEPIC converter with a regulated output The switch node composition of the device. The two high-efficiency power supplies produced by this combination agree very well under almost all conditions.
Key words: ADI; SEPIC-Cuk Converter; Dual Supply Rails
Although single-supply rail-to-rail op amps are widely used, it is often necessary to generate two supply rails (eg, ±15 V) from a single (positive) input supply rail to power the rest of the analog signal chain. The current of these parts is generally low (eg 10 mA to 500 mA), and the positive and negative power supplies have relatively well-matched loads.
One solution is to use two different DC-DC converters for the positive and negative rails, but this is expensive and unnecessary. Another solution is to use a flyback converter. However, the two power supplies often do not align very well under differential loads, requiring bulky and expensive transformers and being inefficient.
A better solution is to use the SEPIC-Cuk converter shown in Figure 1[1], which consists of the switch node of a Cuk converter with an unregulated output connected to a SEPIC converter with a regulated output. This combination produces two high-efficiency supplies that align very well under almost all conditions, unless the load is 100% mismatched.
At the most basic level, a SEPIC-Cuk converter is a SEPIC converter connected to a Cuk converter, and the two are connected to each other at the switch node. Instead of a single Inductor, the parallel connection of L1 and L2 is used in Figure 1 to make the coupling of the two converters more pronounced. This coupling between converters is possible because the slew rates are equal in magnitude and opposite in sign and the voltage waveform at SN1 is the same for both converters.
Interestingly, the currents of the components in each converter are the same except for Cout (because Cuk produces a negative voltage, so the case of the negative sign needs to be considered), as shown in Figures 2 and 3. With the same load on each output, IL1=IL2, IL3=IL4, IC1=IC2 and IQ3=IQ2.
Therefore, each component should have the same value, which results in a better match between the two output voltages and simplifies small-signal analysis.
This topology can be built with 3 single-winding inductors, 2 coupled inductors, a custom 1:1:1 transformer, or a Coilcraft six-winding Hexapath device (or equivalent). Inductive coupling reduces current ripple in the inductor to 1/3 of its original value[1]greatly reduces the complexity of the small-signal model, and achieves higher bandwidth by eliminating SEPIC and Cuk resonances.
One of the main advantages of this topology is that a single standard current-mode boost controller such as the ADP1621[2]or ADP1613[3]) can realize both positive and negative outputs, and the feedback comes from the SEPIC (positive) output. The small-signal mode of this complex converter looks almost identical to the single-output current-mode SEPIC if the device is selected according to the following principles: (1) the same capacitor is used for each output; (2) the value of C2 should be slightly greater than C1. These capacitors are usually ceramic capacitors, so the difference in DC bias must be considered; (3) coupling capacitors should be used to couple L1 and L2 with L4 and L3 respectively. The same inductor should be used for both the SEPIC and Cuk outputs.
Essentially, the Cuk (negative) output of the SEPIC-Cuk is unregulated, so a change in output current compared to the SEPIC (positive) output will bring some load variation, especially if the load is mismatched. Note that its tracking performance is much better than a similarly configured flyback converter, especially under transient or load mismatch conditions. This is because the coupling between the channels is a direct connection, not through a transformer, which itself has leakage inductance.
When the two power supplies have the same load, at steady state, the more weighted error term is caused by the mismatch between the DC resistance of the inductor and the forward voltage of the Diode. Try to make these errors very large relative to the output voltage. Small.
When the loads are significantly mismatched, the error increases, as shown in Figure 4. Therefore, in some applications it may be necessary to place a small dummy load on one or both channels so that both supplies are within their regulation windows. In general, analog devices such as op amps are not very sensitive to DC changes in their power supply, as long as there is enough headroom.
Figure 5 shows the response to applying a 30 mA transient to the Cuk(-Vout) output of the SEPIC-Cuk converter, maintaining a constant 100 mA load on the SEPIC output. Note that both outputs respond to this transient load.
This is the worst case transient because the Cuk (negative) output is unregulated. It is worth noting that most of the deviation shown by the -Vout rail is actually the DC regulation offset caused by the mismatch between the loads (Iout+, Iout-) applied to the two rails.
The test board shown in Figure 6 is the Excel-based ADIsimPowerTM design tool[4]And build. With this comprehensive design tool, users can quickly and accurately create complete schematics and BOMs for SEPIC-Cuk and many other switching converter topologies.
The measured efficiency is shown in Figure 7.references[5]An improved topology for split rails from a single input voltage is presented in[5].
references
[1] A general unified approach to modelling switching-converter power stages. http://www.ee.bgu.ac.il/~kushnero/temp/guamicuk.pdf.
[2] AD1621 data sheet[Z].http://www.analog.com/static/imported-files/data_sheets/ADP1621.pdf.
[3] ADP1613 data sheet[Z].http://www.analog.com/en/power-management/switching-regulators-integrated-fet-switches/adp1613/products/product.html.
[4] ADIsimPower.http://designtools.analog.com/dtPowerWeb/dtPowerMain.aspx.
[5] AN-1106. http://www.analog.com/static/imported-files/application_notes/AN-1106.pdf.
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