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Power Tip 60: Three simple split-rail power supply topologies

Power Tip 60: Three simple split-rail power supply topologies

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By eeNews Europe



Figure 1 presents a simple method, which involves a charge-pump integrated with a boost converter. The boost output is a regulated positive voltage while the charge pump supplies the negative voltage. Referring to Figure 1, when MOSFET Q18 turns off, capacitor C86 charges through D18 to a voltage equal to the positive output voltage plus a diode drop. When Q18 is turned on, C86 discharges through D17 into output capacitor C87. D15 and D16 add a diode drop to the charge on C86 to offset the voltage drops of D17 and D18 in the charge pump. Removing D15 from the circuit results in the -12 volts, being a diode drop less than the +12 volts. This circuit needs to have equal or greater loading on the positive output, or the ripple voltage becomes excessive on the negative output. For instance, if the positive output is unloaded, the power supply switching stops and the negative output capacitor voltage decays until the next switching cycle.


Click on image to enlarge.

Figure 1: A charge-pump added to a boost creates a negative output when VO>VIN.

Figure 2 presents an alternative approach using a buck-boost converter with a coupled inductor. This circuit is useful for a wide range of input and output voltages and is not constrained to just a boost relationship. This circuit uses an integrated-FET buck controller in a buck-boost power stage. The controller is referenced to the negative output voltage, but starts through the output diode D2. As current in the power inductor’s primary winding builds, the negative voltage is driven lower. In this circuit, the sum of both the positive and negative output voltages is regulated. This improves regulation of the outputs, compared to only regulating one. If you only regulate one output it will be tightly controlled, and you may have a +/-10% variation on the other. In this case, regulating the sum improves the regulation of each to +/-5%. The controller’s return is connected to the negative output, which has advantages and disadvantages. It eliminates a differential amplifier that would be needed, if the return were connected to ground. However, it requires level shifting signals like power good, enable and clocks. The other choice that you have to make with this circuit is whether to design it so that it always operates with continuous inductor current. For continuous operation, D2 and perhaps D1 are often replaced with MOSFETs, which allows the current to reverse in them during the 1 – D off time. If D1 is not replaced with a FET and the turns-ratio of the inductor is 1:1, the positive output will be about a diode drop less than the negative. Continuous conduction provides better efficiency and cross regulation, but at the expense of complexity and cost.


Click on image to enlarge.

Figure 2: Buck-boost produces two outputs with no restriction on VIN to VO with a coupled inductor.

Flybuck

Figure 3 presents a simple isolated design of a split- rail power supply called a flybuck. In this case, there is a primary regulated 12 volt output and secondary referenced +/Ð15 volt outputs. The circuit comprises a synchronous buck regulator with coupled windings. Synchronous operation is necessary to guarantee regulation in the case of 12V no-load operation with loading on the secondaries. With synchronous operation, the current is allowed to reverse in the primary to eliminate charge build up on the output capacitor and avoid peak detecting. Regulation is maintained on the primary side while secondary side regulation uses the 1 – D portion of the switching period. During this time the inductor’s primary voltage is clamped to 12V and the secondary output voltages are determined by the turns-ratio. This circuit can achieve +/-10% secondary regulation over a wide range of loads.


Click on image to enlarge.
Figure 3: Flybuck with coupled inductor provides one non-isolated and two isolated outputs.

Table 1 presents a summary of the topology selection criteria. Many times the charge pump, which is the least expensive approach, is appropriate. However, if good regulation over wide-load current ranges is needed, the other two approaches should be considered. Since the flybuck is basically a coupled inductor buck, the ratio of the input voltage to the main output voltage is always greater than 1. Adding an additional winding to this topology allows different ratios and also allows for isolation. Finally, the buck-boost provides the most flexibility between input and output voltages.


Click on image to enlarge.

Table 1: VIN/VO and isolation requirement help make topology choice.

Please join us for the next Power Tip where we will look at using boost converters for high conversion ratios.

For more information about this and other power solutions, visit: www.ti.com/power-ca and TI’s Power House blog: www.ti.com/powerhouse-ca.

Robert Kollman is a senior applications manager and distinguished member of technical staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University and an MSEE from Southern Methodist University.

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