TI explains why a buck-boost charger is the right choice for USB power delivery

By Will Zhou, product marketing engineer, Texas
Instruments, Battery Management Systems (BMS) group,
www.ti.com

The
USB Type-C and USB Power Delivery (PD) specifications were first released
in 2014. Since then, electronic devices equipped with USB Type-C ports supporting
USB PD have become common. USB Type-C ports are engineered to support high-power
capabilities with the USB PD standard, facilitating more flexible and faster
charging for devices like laptops, cellphones, portable speakers, and power
banks.

The
USB Type-C port has flip ability and four VBUS power pins (versus one in the traditional
USB Type-A and Micro-B). Compared to the previous 5-V/0.1-A to 1.5-A VBUS, USB PD
offers a flexible range of power profiles from 5 V/0.1 A up to 20 V/5 A,
allowing faster charging. The newest USB PD 3.0 standard added a programmable power
supply (PPS) feature, with the VBUS supporting a 20-mV-or-smaller tuning step
to meet the requirements of fast charging schemes like flash chargers or
switched-capacitor chargers. These chargers significantly increase charging current
and efficiency but also need constant VBUS adjustment to keep up with the
changes in battery voltage.

With
the ability to support a wide-range of power profiles and its backward-compatibility,
USB Type-C/USB PD becomes a more universal charging solution, thus significantly
reducing the number of different types of cables and connectors. You can use a USB
Type-C/USB PD wall adapter or power bank to charge every compatible electronic
device with the same cable.

Fig. 1: The pinout of the USB Type-C connector.

Buck-boost chargers for USB PD
The
application’s power level will usually determine the battery-cell configuration
and the USB PD power profile. Here are some typical applications across various
Li-ion battery-cell configurations and power levels:

  • Single-cell
    (1S) configuration:
    Mostly used in small electronics like
    smartphones, sports cameras, and small wireless speakers. Most of these
    electronics have a 5-V/3-A power profile or less; the exceptions are high-end
    smartphones, in which higher power and fast charging schemes are often touted
    as product features.
  • Two-cell-in-series
    (2S) configuration:
    These applications often support higher power
    profiles and are usually employed by applications such as two-in-one tablets,
    USB PD power banks, and electronic point-of-sale devices.
  • Three-cell-
    and four-cell-in-series (3S–4S) configurations:

    These batteries have higher voltages and are used in bigger devices like laptops,
    high-power smart speakers, and consumer drones. The common power profiles in 3S
    to 4S applications may get to 45 W (15 V/3 A or 20 V/2.25 A) and above.

It
can be a useful feature for a multi-cell charger to support single-cell configuration
as well. This flexibility allows the reuse of designs with minimal changes
across products that use different battery configurations.

A
buck-boost charger provides the buck, buck-boost, and boost working modes. There
are two reasons why these working modes are needed in PD. The first reason has
to do with the USB PD’s wide input voltage range. For example, let’s say that you
want to design a battery-powered smart speaker with USB Type-C/USB PD. To make
it easy to support the powerful audio amplifiers, you choose a 3S battery. You
want it to be compatible with a small 5-V/2-A to 3-A travel adapter, a bigger
laptop adapter with 20-V/2.25-A USB PD capability, and something in between. In
the 3S configuration, the nominal battery voltage is about 11.1 V. With the 5-V
travel adapter, the charger needs to boost the input voltage up to the battery;
with the 20-V/2.25-A adapter, however, the charger needs to buck it down.

The
second reason is that the battery voltage varies with different states of
charge (SoC). A typical 3S battery voltage can vary from 9 mV when it is empty (0%
SoC) to 12.6 V when it is full (100% SoC). If an adapter provides 12 V after
power negotiation, it will be in buck mode to charge the 3S battery in a low state
of charge and in buck-boost and boost mode as the battery continues to charge (the
charger is in buck-boost mode when the input voltage is on a similar level to
the battery voltage). Fig. 2 shows an
example of the different working modes of a buck-boost charger IC with a 3S
battery with different VBUS USB PD profiles and battery states of charge.

TI-USB-PD-figure2

Fig. 2: Working modes of a buck-boost charger,
with various input voltages and states of charge in charging and source modes.

You
may sometimes need to use a USB charging port as a source device to supply
power to external devices with the USB source mode (or On-The-Go mode in the past).
To support USB PD as a source device, you will also need a buck-boost charger.
Say that you want to add power sourcing capability to the smart speaker that you
are designing. To supply 5 V as a source on VBUS, the speaker needs to step the
voltage down from the 3S battery; to supply 20 V, the VBUS needs to boost up from
the 3S battery. The buck-boost charger can manage both sinking and sourcing power
with the same set of external field-effect transistors (FETs), making the
design more feature-abundant without adding cost. To support the PPS feature in
source mode in a power-bank design, for example, the VBUS needs to yield voltages
in incremental steps of less than or equal to 20 mV.

How a buck-boost charger works
Looking
at the buck-boost charger topology in Fig.
3
, there are four switching FETs that implement the buck-boost function
(not to be confused with buck-boost mode, wherein the input voltage is on a
level similar to the system or battery voltage). In buck mode, the Q1 and Q2 FETs
keep switching to step the adapter voltage down; Q3 stays off and Q4 stays on.
In boost mode, Q1 stays on, Q2 stays off, and Q3 and Q4 switch to boost up the
voltage.

In
buck-boost mode, when the input and output voltages are on similar levels, Q1,
Q2, Q3, and Q4 will need to switch once in a cycle traditionally. The same
operation principle holds true in USB PD source mode as well. Texas Instruments
(TI) has an innovative technology that allows only two FETs switching once per
cycle in the buck-boost mode. This technology greatly improves power efficiency
and reduces heat dissipation. The pass-through mode (PTM) can further improve
the efficiency by keeping Q1/Q4 always
on, and Q2/Q3 always off.

In
PTM, the system voltage is equal to the adapter input voltage. The input power passes through Q1, the
inductor, and Q4 directly to the system with negligible switching loss and no
inductor core loss. PTM can be used for high-current flash charge and for low-current
light-load operations to improve efficiency.

TI-USB-PD-figure3

Fig. 3: Application diagram of the TI BQ25713 buck-boost
charger.

USB Type-C/USB PD charging system and design
considerations
In
a USB Type-C/USB PD charging system, apart from the buck-boost charger, other
functional blocks may include an interface protection chip, a PD controller, a
gauge and a protector for the battery, and a general controlling function for
power path coordination and LED operation. The general control can be either
implemented by a standalone microcontroller (MCU) or integrated into the PD
controller. The USB PD controller detects plug-in events, identifies device roles,
and negotiates power levels. An I2C or SMBus interface configures the
charging parameters.

The
gauge and battery protector are usually inside the battery pack. The gauge
measures the battery’s state of charge and can communicate with the MCU to
adjust the charging parameters in real time. The battery protector implements cell
balancing, voltage, current, and temperature protections and is sometimes
integrated into the gauge. Fig. 4 shows
the main blocks of a typical battery system.

TI-USB-PD-figure4

Fig. 4: Block diagram of a battery system with a USB
Type-C port.

You
will need to consider charging performance and features to make your systems
less heat-dissipative and more cost-effective and compatible with universal
power sources. To obtain a cool charging experience, the power efficiency needs
to be high in various working modes (buck, boost, and buck-boost) with
different loads.

In
applications in which the system needs to know parameters like input and
battery currents/voltages to take action, an integrated analog-to-digital
converter (ADC) makes measurements readily available. Because different USB
adapters can have different power profiles and cables can have different
resistances, it’s important to know the source’s maximum power capability to
avoid crashing it. TI’s input current optimization (ICO) feature enables systems
to capture the maximum power of a source automatically upon insertion. This
feature allows the system to safely draw maximum power from various adapters, even
an unknown adapter with large impedance and a long cable. The power path
feature is preferred to instantly power on the system with a deeply discharged
battery upon the insertion of the adapter.

Conclusion
The
market demand for USB Type-C/USB PD will only continue to rise due to the
technology’s ease of use and wide range of power profiles. To meet USB
Type-C/USB PD charging system design needs, buck-boost chargers are needed to
step up and step down the input voltages to charge the battery and supply for
the system. Be sure to look for a charger that operates at high efficiency in
different modes and integrates functions like ICO, ADC, and power path in your
next design to achieve fast and cool charging and an optimized system solution
cost.

TI’s
USB Type-C/USB PD solutions include:

  • BQ25713
    and
    BQ25703A
    1S–4S
    buck-boost chargers
  • The BQ25871
    battery
    switch charger and BQ25970
    switched-capacitor
    battery charger for flash charging on the sink side
  • The TPS65987D
    USB
    Type-C and USB PD controller
  • The TPD6S300A
    USB
    Type-C short-to-VBUS protection IC

For
more USB Type-C and USB PD products, check out TI’s overview
page
.

Will Zhou is a product marketing engineer in the
Texas Instruments Battery Management Systems (BMS) group. Will has bachelor’s
and master’s degrees in electrical engineering from Nankai University and the University
of Florida. You can reach Will at
willzhou@ti.com.