AN1467 Datasheet, PDF (1/16 Pages) Microchip Technology – High-Power CC/CV Battery Charger Using an Inverse SEPIC (Zeta) Topology

AN1467
High-Power CC/CV Battery Charger Using an Inverse
SEPIC (Zeta) Topology
Authors: Mihnea Rosu-Hamzescu
Sergiu Oprea
Microchip Technology Inc.
INTRODUCTION
As the number of photovoltaic systems and electric
vehicles increases, so does the demand for intelligent,
high-power and high-efficiency battery chargers. Most
systems on the market today use either lead-acid or
lithium type batteries, requiring constant current/con-
stant voltage charging algorithms.
This application note contains the necessary informa-
tion to build a 100W inverse SEPIC (also called Zeta
converter) battery charger. The novelty consists in driv-
ing this topology synchronously using Microchip com-
ponents, essentially pushing the efficiency over 95% at
8A. The Zeta converter has many advantages, such as
input to output DC insulation, buck-boost capability and
continuous output current, but it is difficult to control.
The control scheme is also interesting, as it uses the
Numerically Controlled Oscillator (NCO) peripheral to
implement a form of fixed on-time, variable frequency
control that allows 15 bits of resolution for the control
system. This opens up quite a few new possibilities in
low-cost software controlled power supplies.
Finally, since this implementation allows the control of
the output voltage and current with a high resolution, it
is quite easy to attach multi-chemistry battery charging
algorithms to the basic output regulation loop, greatly
increasing its usefulness.
The complete implementation of the regulator and
charger library uses only 1k words of program space
and 55 bytes of RAM.
THE ZETA CONVERTER
Considered by many designers as an “exotic” topology,
the ZETA converter (also known as the inverted
SEPIC) offers certain advantages over the classical
SEPIC. This topology has the same buck-boost
functionality as the SEPIC, but the output current is
continuous, providing a clean, low-ripple output voltage
make. This low-noise output converter can be used to
power certain types of loads, such as LEDs, which are
sensitive to the voltage ripple. The ZETA converter
offers the same DC isolation between the input and
output as the SEPIC converter, and can be used in
high-reliability systems.
This topology can also offer high efficiency, especially
if the synchronous rectification is used. The synchro-
nous rectification can be easily implemented here,
because this topology, unlike the SEPIC converter,
uses a low-side rectifier.
The ZETA converter power train is depicted in Figure 1.
FIGURE 1:
Q1
THE ZETA CONVERTER
POWER TRAIN
SW ^C1 SW’
L2
VOUT
Vin
L1
Q2
C2
RL
The two switches, Q1 and Q2, operate out of phase. As
with the SEPIC converter, there are two switching
cycles that are presented in Figure 2.
FIGURE 2:
VIN
SWITCHING CYCLES OF THE ZETA CONVERTER
C1
L2
-+
V_C1
+
-
V_L2
+
L1 V_L1
-
+
V_OUT
C2 RL
-
C1
- _{VC1 +}
-
L1 VL1
+
L2
- _{VL2 +}
+
V_OUT
C2
R_L
-
Cycle 1: Q1 closed, Q2 open
Cycle 2: Q1 open, Q2 closed
 2012 Microchip Technology Inc.
DS01467A-page 1