Generate Auxiliary Voltages at Low Cost

Jan 1, 2008 12:00 PM
By John Betten, Application Engineer, Texas Instruments, Dallas

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Frequently, the lowest possible cost is the fundamental challenge that engineers encounter when designing power converters. When multiple output voltages are required, it is often tempting to provide one switching regulator for each output voltage. While this approach provides excellent output-voltage regulation, it certainly does nothing to help achieve the top design priority — lowest possible cost.

Fortunately, circuits such as coupled inductors and charge pumps can be easily implemented in traditional switching converters to provide additional output-voltage rails at minimal cost. Using the proper circuit configuration, it is often possible to achieve good voltage regulation along with high efficiency.

Coupled Inductors

Fig. 1 shows three different ways to generate an auxiliary output from a coupled inductor. In all three configurations, current flows in the auxiliary winding only during the on time of the synchronous FET. During this period of the switching cycle, the V_OUT1 output voltage plus the drain-to-source voltage (V_GS) drop of the synchronous FET are impressed across the inductor's primary winding. The FET's voltage drop generally is quite small, usually less than 0.1 V, compared to around 0.5 V if a diode rectifier is used.

The auxiliary output voltage for an inductor with a 1:1 winding ratio is equal to the primary-winding voltage, less the forward-voltage drop of the secondary-side diode, or V_OUT2 = V_OUT1 + V_FET - V_D. The actual V_OUT2 output voltage obtained is dependent on the load currents in both outputs since the voltage drops of V_FET and V_D are current dependent. Additionally, good coupling between the windings is necessary to reduce leakage-inductance effects at higher operating frequencies. Lower switching frequencies and light loading provide the best voltage regulation for V_OUT2.

As shown in Fig. 1, the ground reference of the auxiliary winding can be connected to any point. In circuit A, the V_OUT2 ground can be connected to a separate, isolated ground (as shown) or to the V_OUT1 ground if isolation is not required. In circuit B, the V_OUT2 ground floats on top of V_OUT1, making the output of V_OUT2 approximately twice that of V_OUT1. In circuit C, the diode's cathode is grounded, making V_OUT2 a negative output voltage.

Fig. 2 shows an example of an isolated coupled inductor design that uses a linear regulator (U2) to provide a low-current, low-noise, well-regulated output voltage. Standard off-the-shelf coupled inductors are typically bifilar wound and have 1:1 turn ratios. Low-cost custom magnetics also can be easily designed and quickly obtained, providing an avenue for generating unusual turn ratios or multiple output voltages.

Fig. 3 shows an example of a multiple-winding coupled inductor that provides matching positive and negative auxiliary output voltages (V_OUT2 and V_OUT3). Various voltage configurations can be implemented by using the circuit grounding arrangements shown in Fig. 1.

While diode rectification on the secondary side of the coupled inductor is easy to implement, the forward-voltage drop of the diode can introduce a large output-voltage variation over load current and temperature. Fig. 4 shows a circuit that uses secondary-side synchronous rectification to reduce the voltage variation and FET losses.

When low-resistance FETs are used, FET voltage drops are minimal and the output-voltage regulation for V_OUT2 improves. Under certain loading conditions, the voltage drops of FETs Q1 and Q2 will perfectly cancel each other, resulting in an output voltage for V_OUT2 that is equal to V_OUT1 times the turns ratio of the coupled inductor.

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