Power Entry and Protection
Power entry is where the outside world meets the board. It must handle normal supply current, wrong polarity, cable transients, ESD, surge, inrush, and user mistakes without damaging the product or creating unsafe conditions.
Learning Objectives
By the end of this lesson, you should be able to choose input protection blocks, estimate fuse and diode losses, place TVS devices, control inrush, and review the power-entry path for safety and serviceability.
Typical Power Entry Chain
The exact order depends on product requirements, surge level, hot-plug behavior, and safety standards.
Fuse or PTC
A fuse protects wiring and reduces fire risk. It does not protect every semiconductor instantly. Select it based on normal current, startup current, ambient temperature, interrupt rating, and downstream fault energy.
[
P_{FUSE}=I^2R
]
PTCs reset, but their resistance and trip behavior vary strongly with temperature.
Reverse-Polarity Protection
Common methods:
- series diode: simple but wastes voltage and heat;
- Schottky diode: lower drop but still dissipates power;
- P-channel MOSFET or ideal-diode controller: efficient but more complex.
Diode power loss:
[
P_D=V_F I
]
At (V_F=0.4\text{ V}) and (I=1.5\text{ A}), loss is (0.6\text{ W}), which may require thermal copper.
TVS and Surge Clamping
A TVS diode clamps fast transients. Place it close to the connector with a short return path. Check working voltage, breakdown voltage, clamp voltage, peak pulse power, capacitance, and package surge rating.
The protected circuit must tolerate the TVS clamp voltage, not just the nominal input voltage.
Inrush and Hot Plug
Large input capacitors draw high current when plugged in:
[
I=C\frac{dV}{dt}
]
Use series resistance, soft-start regulators, hot-swap controllers, NTCs, or active inrush control when connector stress or supply collapse is a risk.
Practical Review Checklist
- Connector voltage, current, polarity, and keying are clear.
- Fuse rating matches cable and load risk.
- Reverse protection power loss is acceptable.
- TVS is close to the connector and returns to the intended path.
- Input capacitors and hot-plug inrush are checked.
- Creepage, clearance, and isolation needs are satisfied.
Common Mistakes
- Placing TVS devices far from the connector.
- Choosing a fuse only by normal operating current.
- Ignoring diode heat in reverse-protection paths.
- Assuming a TVS clamp voltage equals the supply voltage.
- Letting inrush reset the upstream supply.
Summary
Power entry protection manages abuse before it reaches sensitive circuits. Use fusing, polarity protection, transient clamping, filtering, and inrush control based on the real external environment.
Further Reading
- Littelfuse, "TVS Diode Application Notes."
- Texas Instruments, "Reverse Current and Reverse Polarity Protection."
- IEC 61000-4-x immunity test family overview.