Power Management Circuits

Introduction

Power management circuits bridge batteries and loads, handling voltage conversion, current distribution, and safety protection. This section covers the design principles and selection of BMS, DC-DC converters, LDO regulators, and protection circuits.

BMS (Battery Management System)

A BMS is an essential component for multi-cell lithium battery packs, responsible for monitoring each cell's state and providing protection.

BMS Core Functions

graph TD
    BMS[BMS Battery Management System] --> CV[Voltage Monitoring<br/>Cell Voltage]
    BMS --> CT[Temperature Monitoring]
    BMS --> CC[Current Monitoring]
    BMS --> BAL[Cell Balancing]
    BMS --> PROT[Protection Functions]
    BMS --> COM[Communication Interface<br/>SMBus/CAN]

    PROT --> OVP[Overvoltage Protection OVP]
    PROT --> UVP[Undervoltage Protection UVP]
    PROT --> OCP[Overcurrent Protection OCP]
    PROT --> OTP[Over-Temperature Protection OTP]
    PROT --> SCP[Short Circuit Protection SCP]

    BAL --> PB[Passive Balancing]
    BAL --> AB[Active Balancing]

Protection Thresholds

Protection Function	Typical Threshold (Li-ion)	Action
Overvoltage (OVP)	4.25–4.30V/cell	Disconnect charging
Undervoltage (UVP)	2.5–3.0V/cell	Disconnect discharge
Overcurrent (OCP)	110–150% of setpoint	Disconnect output
Short circuit (SCP)	Several times OCP threshold	Immediate disconnect
Over-temperature (OTP)	60–70°C	Disconnect charge/discharge

Cell Balancing

Passive Balancing: Dissipates excess energy from high-voltage cells through resistors

Principle: $P_{dissipated} = \frac{(V_{cell} - V_{target})^2}{R_{balance}}$
Pros: Simple, low cost
Cons: Energy wasted as heat

Active Balancing: Transfers energy from high-voltage cells to low-voltage cells

Methods: Inductive coupling, capacitor switching, transformer
Pros: High efficiency (>90%)
Cons: Complex circuitry, higher cost

Common BMS Chips/Modules

Model	Cells	Features	Suitable Scenario
HX-2S-01	2S	Low-cost module, 7.4V	Small robots
HX-3S-01	3S	11.1V, common RC battery	Medium robots
BQ76920 (TI)	3–5S	Integrated AFE, I2C communication	Professional grade
BQ76930 (TI)	6–10S	High cell count support	Large battery packs
BQ76940 (TI)	9–15S	High-voltage packs	EVs / large robots

DC-DC Converters

DC-DC converters transform one DC voltage to another, forming the core of a robot power system.

Buck (Step-Down) Converter

Reduces high voltage to low voltage, with efficiency typically 85–95%.

Operating Principle:

A high-frequency switch (MOSFET) chops the input voltage, and an LC filter produces a stable lower output voltage.

\[V_{out} = D \times V_{in}\]

Where $D$ is the duty cycle, $0 < D < 1$.

Common Buck Modules/Chips:

Model	Input Range	Output	Max Current	Features
LM2596	4.5–40V	1.2–37V adjustable	3A	Classic module, ~$2
MP1584	4.5–28V	0.8–20V adjustable	3A	Compact, efficient
LM2596HV	4.5–60V	1.2–57V adjustable	3A	High voltage input
TPS5430	5.5–36V	Adjustable	3A	TI industrial grade
XL4015	8–36V	1.25–32V adjustable	5A	High current
LTC3780	5–32V	1–30V	10A	Buck-boost

Typical Applications: 24V battery → 12V (motors), 24V → 5V (Jetson/RPi)

Boost (Step-Up) Converter

Raises low voltage to high voltage.

\[V_{out} = \frac{V_{in}}{1 - D}\]

Common Boost Modules/Chips:

Model	Input Range	Output Range	Max Current	Features
XL6009	3–32V	5–35V	4A	Boost module
MT3608	2–24V	5–28V	2A	Micro boost
TPS61088	2.7–12V	4.5–12.6V	10A	TI high efficiency

Typical Applications: 3.7V Li-cell → 5V (USB power), 5V → 12V (small applications)

Buck-Boost Converter

Used when input voltage may be higher or lower than output voltage.

SEPIC topology: Output same polarity as input
Inverting Buck-Boost: Output has negative polarity
Four-switch Buck-Boost: Highest efficiency but complex

Typical chips: LTC3780, TPS63000 series.

LDO (Low Dropout Regulator)

An LDO linearly regulates input voltage to a stable output voltage via a pass transistor.

LDO vs. Switching Regulator

Feature	LDO	DC-DC (Switching)
Efficiency	$\eta = \frac{V_{out}}{V_{in}}$	85–95%
Noise	Very low (uV level)	Higher (mV-level ripple)
Cost	Low	Medium-high
Size	Small	Medium
Heat dissipation	$P_{loss} = (V_{in}-V_{out}) \times I_{out}$	Low
Suitable for	Low dropout, small current	Large dropout, large current

Common LDOs

Model	Output Voltage	Max Current	Dropout	Package
AMS1117-3.3	3.3V	1A	1.0V	SOT-223
AMS1117-5.0	5.0V	1A	1.0V	SOT-223
AP2112K-3.3	3.3V	600 mA	250 mV	SOT-23-5
MCP1700-3302	3.3V	250 mA	178 mV	SOT-23
TLV1117-33	3.3V	800 mA	1.1V	SOT-223

When to Choose LDO

5V → 3.3V (low dropout, efficiency $\frac{3.3}{5} = 66\%$ is acceptable, current <1A)
Low noise requirements (analog sensors, ADC reference voltage)
Space-constrained, cost-sensitive

When to Choose DC-DC

24V → 5V (large dropout, LDO efficiency only $\frac{5}{24} = 21\%$, severe heat)
High-current loads (>1A)
Boost needed

Hot-Swap Circuit

Allows safe insertion and removal of power supplies or batteries while the system is running.

Key Challenges

Inrush current: Charging current surge to output capacitors
Arcing: Arc discharge at connector contacts
Voltage droop: Brief power interruption during switching

Hot-Swap Controllers

Model	Voltage Range	Features
LTC4352	2.9–18V	Ideal diode controller
TPS2490	9–80V	Wide voltage hot-swap
LM5069	9–80V	Programmable current limit

Design Essentials

V_IN ---[eFuse/Hot-Swap IC]---+---[DC-DC]--- V_OUT
                              |
                          C_bulk (large buffer capacitor)
                              |
                             GND

Soft start: Controls MOSFET gate ramp rate to limit inrush current
Current limiting: Sets maximum current, $I_{limit} = \frac{V_{sense}}{R_{sense}}$
Large buffer capacitor: Maintains power during source switching

eFuse (Electronic Fuse)

An eFuse uses MOSFETs to replace traditional fuses, providing resettable overcurrent protection.

Advantages

Resettable: Automatically recovers after overcurrent, no replacement needed
Programmable: Trip current set by external resistor
Fast response: Microsecond-level cutoff, far faster than traditional fuses

Common eFuse Chips

Model	Voltage Range	Current Range	Features
TPS2596 (TI)	2.7–19V	0.1–5.2A	SOT-23, compact
NIS5135 (ON)	2.5–23V	0.2–5A	Adjustable current limit
STEF01 (ST)	4–48V	0.2–6A	Wide voltage

Typical Application

Providing independent eFuse protection per subsystem for fault isolation:

graph LR
    BUS[24V Bus] --> EF1[eFuse 1<br/>5A]
    BUS --> EF2[eFuse 2<br/>3A]
    BUS --> EF3[eFuse 3<br/>2A]
    BUS --> EF4[eFuse 4<br/>1A]

    EF1 --> M[Motor Drivers]
    EF2 --> J[Jetson Orin]
    EF3 --> L[LiDAR]
    EF4 --> S[Sensors/MCU]

Power PCB Design Guidelines

Layout Principles

Input caps close to IC: Minimize parasitic inductance
Short, wide traces: Keep high-current paths as short and wide as possible
Copper pour area: Copper pour under power MOSFETs and inductors for heat dissipation
Separate signal and power: Analog signals away from switching nodes
Grounding strategy: Star grounding or ground plane partitioning

Trace Current Capacity Estimate (1 oz copper, outer layer)

Trace Width	10°C Rise	20°C Rise
0.5 mm	0.7A	1.0A
1.0 mm	1.2A	1.7A
2.0 mm	2.0A	2.8A
5.0 mm	4.0A	5.5A

For high current (>5A), use multi-layer stacking or external wires.

References

Texas Instruments: DC-DC Converter Design Guide
Analog Devices: LT Journal of Analog Innovation
"Switching Power Supply Design" (Pressman)
EEVBlog: Power Supply Design Tutorials

Feature	LDO	DC-DC (Switching)
Efficiency	\(\eta = \frac{V_{out}}{V_{in}}\)	85–95%
Noise	Very low (uV level)	Higher (mV-level ripple)
Cost	Low	Medium-high
Size	Small	Medium
Heat dissipation	\(P_{loss} = (V_{in}-V_{out}) \times I_{out}\)	Low
Suitable for	Low dropout, small current	Large dropout, large current