Project Overview and Review
1:32
Analyzing the schematic.
Looking for "Device description, MOSFET type, key electrical characteristics including RDS(on), VGS threshold, drain current, package" in AO3400A_ds.pdf
Looking for "Device description, diode type, forward voltage, reverse voltage, average forward current, package" in B5817W-B5819W(SOD-123).pdf
Running design review.
Project overview
How the present circuit is connected
Functional interpretation
Intended behavior
Dependencies
Functional upgrades
This project is a small power-path / load-switch module intended to switch or isolate a load between a battery input and a 5 V rail. The project description says it is:
“A simple load switch module to switch between Battery and 5V power”
At a high level, it is trying to provide a controlled output at 5V_LOAD_SW using the battery input at VBAT and/or the 5 V input at +5V.
What is currently in the design
Core components
| Component | Role in the design | Key details |
|---|---|---|
| Q1 | Main switching element | Q1, 30 V N-channel MOSFET, SOT-23, up to 5.7 A continuous at TA = 25 C, RDS(on) as low as 18 mOhm typ at VGS = 10 V |
| D1 | Protection / steering element | D1, Schottky barrier rectifier, 40 V reverse voltage, 1.0 A average forward current, 0.60 V max forward drop at 1.0 A |
| R1 | Bias / default-state resistor | Generic resistor, 100 kOhm, footprint 0402 |
| +5V | External 5 V input terminal | Power input |
| VBAT | External battery input terminal | Battery input |
| 5V_LOAD_SW | Switched output terminal | Output to the load |
Nets visible in the schematic
The design intent looks like this:
- Q1 is used as the main electronic switch
- R1 pulls the MOSFET gate to ground so the switch has a defined default state
- D1 provides a Schottky path / clamp / steering function between the MOSFET source-side node and the gate node
That combination is commonly used in simple power-path circuits to:
- prevent unintended backfeed
- clamp or shape the MOSFET gate-source voltage
- establish a predictable turn-on / turn-off behavior
Based on the visible netlist, the project appears to be incomplete or only partially wired:
- +5V is present as a terminal and power portal, but it is not shown connected into the active switching netlist
- VBAT is present as a terminal and power portal, but it is also not shown connected into the active switching netlist
- 5V_LOAD_SW exists as an output terminal, but it is not shown connected to
Net 3 - Q1 drain is currently alone on
Net 3
So the project purpose is clear, but the actual implemented schematic is not yet fully realizing that purpose.
Design choices and why they make sense
1. Using an N-channel MOSFET
- it has very low
RDS(on) - it is available in a compact SOT-23
- it supports relatively high current for a small part
An N-channel MOSFET is efficient, but it is more topology-sensitive than a simple diode solution:
- gate drive and source/drain orientation matter
- if used on the high side, it may need a gate voltage above the source to turn on strongly
- if used in an auto-switching path, the exact bias network becomes critical
- low forward drop compared with standard silicon diodes
- fast recovery behavior
- simple passive steering/protection behavior
Even a Schottky still dissipates power:
- at higher current, the forward drop causes efficiency loss
- thermal rise can become important near the 1.0 A average current rating
R1 at 100 kOhm suggests a low-power bias strategy:
- keeps the gate from floating
- minimizes standby current
- gives the switch a defined default state
A larger resistor means:
- lower wasted current
- but slower gate discharge and less noise immunity than a smaller value
Technologies used
This module depends on:
- the external power sources actually being wired to +5V and VBAT
- the output load being wired to 5V_LOAD_SW
- correct source/drain/gate orientation for Q1
- a complete net connection strategy, which is not yet visible in the current schematic snapshot
The module can be efficient if most current flows through the MOSFET path rather than through a diode drop. That is the main advantage of using Q1.
Potential limits:
- any path through D1 incurs a voltage drop
- MOSFET conduction losses depend on actual
VGS - poor gate drive can make the MOSFET behave worse than expected
- Q1 is electrically capable of several amps in principle, per datasheet
- D1 is only a 1.0 A average forward current device
- in practice, PCB copper, thermal environment, and topology will dominate safe operating current
The 100 kOhm pull-down in R1 means startup behavior may be relatively gentle, but:
- the gate node may be more susceptible to leakage/noise
- switching edges may be slower than with a stronger pull resistor
This is the biggest issue in the current project state.
The visible implementation does not yet show:
- how +5V enters the switching path
- how VBAT enters the switching path
- how 5V_LOAD_SW is driven from Q1
So today, the design is best understood as a partially built power-switch concept, not yet a complete module implementation.
2. MOSFET topology correctness
With a single N-MOSFET, correctness depends heavily on:
- source/drain orientation
- body-diode direction
- expected input/output voltage relationship
- whether the MOSFET is intended as low-side, high-side, or ideal-diode-like element
If that is wrong, the circuit may:
- fail to switch properly
- leak current backward
- never fully enhance the MOSFET
Right now there is no comparator, supervisor, or controller IC. That means any automatic source selection must rely purely on passive behavior and transistor physics.
That is simple and cheap, but less robust.
Real-world applications
If completed correctly, this kind of module could be useful for:
- battery-powered gadgets with external 5 V input
- USB-powered devices that need battery isolation
- simple backup-power switchover
- small embedded boards needing basic source selection
- low-cost load disconnect or reverse-current blocking
Priority improvements
| Improvement | Why it helps |
|---|---|
| Connect +5V, VBAT, and 5V_LOAD_SW explicitly into the switching network | Makes the schematic functionally complete |
| Label the active nets clearly | Easier review and less ambiguity |
| Verify Q1 source/drain orientation against intended power flow | Prevents reverse conduction mistakes |
| Define expected load current | Confirms whether D1 and PCB copper are sufficient |
| Add input/output bulk decoupling if the module will feed dynamic loads | Improves transient behavior |
If you want a more production-ready version, future enhancements could include:
- a dedicated ideal-diode / power-mux controller
- undervoltage lockout for the battery path
- reverse-polarity protection on VBAT
- overcurrent or short-circuit protection
- soft-start behavior
- status indication showing which source is active
The present concept may not scale cleanly because:
- D1 is limited to 1.0 A
- SOT-23 thermal limits become important
- trace resistance and heating increase
For higher-current versions, a larger MOSFET and a more robust controller topology would be better.
For smarter power management
If the goal is seamless battery / 5 V prioritization, a future version could add:
- source priority logic
- battery charging integration
- load sharing
- true ideal-diode OR-ing
This project is best described as a compact battery/5 V load-switch module concept built around:
The intent is sound, but the current implementation appears incomplete, because the external terminals and output are not yet fully connected into the active netlist. Once those connections are finished and the MOSFET topology is verified, it could become a simple, compact power-path building block.
If you want, I can next give you:
- a plain-English signal-flow walkthrough, or
- a design review of what is missing / likely wrong in the current schematic.
Review missing connections
Inspect full schematic
Check MOSFET topology
Look for missing caps
30 days
Ready.
+5V
D1
VBAT
VBAT
Q1
Not Recommended for New Designs
+5V
5V_LOAD_SW
R1
Resistance
100kΩ
Reviews
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Battery Load Switch Module Bottom Layer
A simple load switch module to switch between Battery and 5V power
Properties
Properties describe core aspects of the project.
Pricing & Availability
Distributor | Qty 1 |
|---|---|
Digi-Key | $0.09–$0.46 |
LCSC | $0.06–$0.11 |
Mouser | $0.10 |
Controls
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