Ensayo Sobre Convertidor Dc Dc

Power Management

Texas Instruments Incorporated

Designing DC/DC converters based on ZETA topology
By Jeff Falin
Senior Applications Engineer

Introduction
Similar to the SEPIC DC/DC converter topology, the ZETA converter topology provides a positive output voltage from an input voltage that varies above and below the output voltage. The ZETA converter also needs two inductors and aseries capacitor, sometimes called a flying capacitor. Unlike the SEPIC converter, which is configured with a standard boost converter, the ZETA converter is configured from a buck controller that drives a high-side PMOS FET. The ZETA converter is another option for regulating an unregulated input-power supply, like a low-cost wall wart. To minimize board space, a coupled inductor can be used. Thisarticle explains how to design a ZETA converter running in continuous-conduction mode (CCM) with a coupled inductor.

Figure 1. Simple circuit diagram of ZETA converter
CC Q1 C IN L1a D1 COUT L1b VOUT

VIN

Basic operation
Figure 1 shows a simple circuit diagram of a ZETA converter, consisting of an input capacitor, CIN; an output capacitor, COUT; coupled inductors L1a and L1b; an ACcoupling capacitor, CC; a power PMOS FET, Q1; and a diode, D1. Figure 2 shows the ZETA converter operating in CCM when Q1 is on and when Q1 is off. To understand the voltages at the various circuit nodes, it is important to analyze the circuit at DC when both switches are off and not switching. Capacitor CC will be in parallel with COUT, so CC is charged to the output voltage, VOUT, duringsteady-state CCM. Figure 2 shows the voltages across L1a and L1b during CCM operation. When Q1 is off, the voltage across L1b must be VOUT since it is in parallel with COUT. Since COUT is charged to VOUT, the voltage across Q1 when Q1 is off is VIN + VOUT; therefore the voltage across L1a is –VOUT relative to the drain of Q1. When Q1 is on, capacitor CC, charged to VOUT, is connected in series with L1b; sothe voltage across L1b is +VIN, and diode D1 sees VIN + VOUT.

Figure 2. ZETA converter during CCM operation
Q1 is On

IL1a VIN + C IN VIN – L1a

– VOUT + CC

+ VIN – L1b VOUT IL1b COUT GND

(a) When Q1 is on
Q1 is Off C IN VIN – VOUT + GND L1a

– VOUT + CC

– VOUT + L1b VOUT

IL1a

IL1b

COUT

(b) When Q1 is off

16 High-Performance Analog Products www.ti.com/aaj 2Q 2010Analog Applications Journal

Texas Instruments Incorporated

Power Management

The currents flowing through various circuit components are shown in Figure 3. When Q1 is on, energy from the input supply is being stored in L1a, L1b, and CC. L1b also provides IOUT. When Q1 turns off, L1a’s current continues to flow from current provided by CC, and L1b again provides IOUT.

Figure 3. ZETAconverter’s component currents during CCM

TS VIN + VOUT VQ1 D × TS (1 – D) × TS I Q1(Peak) I IN + lOUT

Duty cycle
Assuming 100% efficiency, the duty cycle, D, for a ZETA converter operating in CCM is given by D= VOUT . VIN + VOUT (1)
l D1 I IN + lOUT l Q1

This can be rewritten as D I V = IN = OUT . 1 − D IOUT VIN Dmax occurs at VIN(min), and Dmin occurs at VIN(max). (2)

I IN l* C C –IOUT I IN

Selecting passive components
One of the first steps in designing any PWM switching regulator is to decide how much inductor ripple current, ∆IL(PP), to allow. Too much increases EMI, while too little may result in unstable PWM operation. A rule of thumb is to assign a value for K between 0.2 and 0.4 of the average input current. A desired ripple current can be calculated as follows:Desired ∆I L(PP) = K × IIN
l L1a

I L1b *Measured flowing into CC from Q1’s drain.

I OUT

(3) D . 1− D In an ideal, tightly coupled inductor, with each inductor having the same number of windings on a single core, the coupling forces the ripple current to be split equally between the two coupled inductors. In a real coupled inductor, the inductors do not have equal inductance and the...