A power path controller allows your device to simultaneously charge a battery and power the system from an external source — seamlessly switching to battery when external power is removed. Without power path management, the battery experiences continuous charge-discharge stress that drastically reduces its lifespan. This guide covers power path topologies, IC selection, and practical DIY circuits for Indian makers building battery-powered products.
What Is a Power Path Controller?
A power path controller is a circuit that intelligently manages power flow between three nodes: the external power input (USB, solar, adapter), the battery, and the system load. It decides which source powers the system and how much current goes to battery charging, all while preventing reverse current flow.
Every smartphone, laptop, and tablet uses power path management. When you plug in a charger, your phone runs from the charger while the battery charges separately. When you unplug, the battery takes over instantly with no brownout. This is power path management in action.
Why You Need Power Path Management
Without power path management, the common approach is to charge the battery and run the load from the battery simultaneously. Problems:
- Battery stress: Continuous charging and discharging simultaneously (called “pass-through”) causes localised heating inside the cell, accelerating degradation
- Charge termination failure: The charger IC sees load current as charge current, preventing proper charge termination
- Brownout on unplug: When external power is removed, there is a brief voltage drop as the system switches from VBUS to battery — potentially resetting microcontrollers
- Inaccurate fuel gauging: Current flowing in and out simultaneously confuses battery fuel gauge algorithms
Common Power Path Topologies
1. Ideal diode OR-ing: The simplest approach. Two ideal diode controllers (MOSFET-based) select the higher voltage source. External power wins when present; battery takes over when external drops. Limitation: battery always at system voltage, no voltage regulation.
2. Load switch with priority: External power connects directly to the load via a load switch. Battery connects through a second load switch that activates only when external power is absent. Battery charges through a separate charger path.
3. Integrated power path (BQ24074-style): A single IC manages input current limiting, battery charging, and system output. The IC prioritises system power from external input, diverts excess to battery charging, and seamlessly transitions to battery when input is removed.
Popular Power Path ICs
| IC | Type | Max Current | Features | Cost (India) |
|---|---|---|---|---|
| BQ24074 | Integrated charger+path | 1.5A | USB/solar input, status outputs | ₹150-300 |
| LTC4412 | Ideal diode controller | Load-dependent | Very low quiescent (12uA) | ₹200-400 |
| MAX17710 | Energy harvesting PMIC | 200mA | Boost+charger+path for solar/piezo | ₹300-500 |
| TPS2113 | Power mux | 3A | Automatic source switching | ₹100-200 |
Building a Simple Power Path Circuit
For hobbyists who want power path without a specialised IC, here is a MOSFET-based approach:
Simple Power Path with P-MOSFET:
External 5V ──→ Schottky diode ──→ System Load
↑
Battery ──→ P-MOSFET (Si2301) ──────→ System Load
|
Gate ←── Voltage divider from External 5V
When external power present:
- Schottky diode passes 5V to system (minus 0.3V drop)
- MOSFET gate is pulled high → MOSFET OFF
- Battery isolated from system
- TP4056 charges battery separately
When external power removed:
- No voltage on MOSFET gate → MOSFET ON
- Battery powers system through MOSFET (0.05V drop)
- Seamless transition (~1ms)This circuit is simple but lacks input current limiting and dynamic power sharing. For production devices, use BQ24074 or similar integrated IC.
1A micro USB charger with protection for single Li-ion cells.
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Frequently Asked Questions
Can I use a TP4056 module with power path?
The TP4056 module does not include power path management. It charges the battery while your load draws from the battery — the classic “pass-through” problem. For a simple solution, add the MOSFET power path circuit described above between the TP4056’s battery output and your system load.
What is the voltage drop across a power path circuit?
Schottky diode path: 0.2-0.4V drop. MOSFET-based ideal diode: 0.02-0.1V drop. Integrated ICs (BQ24074): 0.1-0.3V internal MOSFET drop. For 3.3V systems, keep the total path drop under 0.2V to maintain regulation.
Does power path work with solar panels?
Yes. Solar input replaces USB as the external source. The power path controller prioritises solar power for the system and diverts excess to battery charging. When solar drops (night or clouds), the battery takes over. The BQ24074 is specifically designed for solar+USB dual input with power path.
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