1. Introduction #
A lot of industrial machines use a 24V DC power supply. It is used for control panels, communication devices, and edge AI hardware such as industrial cameras and on-site inference boxes.
In actual use, different problems can happen together. These can include reverse polarity, hot-plugging, voltage surges, reverse current, and sudden load changes.
One component can pass its test, but it does not mean the whole power path is safe. The system is reliable when all parts work together properly. One part alone is not enough.
Figure 1 shows how the power path works. The power enters from the fuse or PTC device, then goes through the TVS diode and reverse-polarity protection. Next is the input capacitor, and finally it runs the DC/DC converter and load.
2. Define the Electrical Threats Before Selecting Components #
Before choosing a component, the engineer should first understand the possible threats. Different threats need different protection and components. The table below shows the main threats in a power path.
| Fault | Protection Task | Key Component |
|---|---|---|
| Reverse polarity | Block current from flowing the wrong way | Series diode, Schottky diode, or MOSFET |
| Surge / transient overvoltage | Clamp sudden voltage spikes | TVS diode plus upstream impedance |
| Reverse current | Stop current from flowing backward | Back-to-back MOSFETs or ideal-diode controller |
| Inrush current | Limit the sudden current at power-on | Capacitance, MOSFET SOA, current limiting |
| Inductive kickback | Absorb voltage spikes from coils/motors | Freewheeling diode or clamping diode |
This threat list follows TI’s classification of power-path threats, which covers overcurrent, overvoltage, reverse voltage, reverse current, inrush current, and automotive load dump.
3. Reverse-Polarity Protection: Diode or MOSFET? #
3.1 Diode Solutions
Two common diodes can be used for reverse-polarity protection. They are standard rectifier diodes and Schottky diodes.
These two types differ in forward voltage drop (VF), reverse-voltage rating, leakage current, and thermal loss.
Schottky diodes have lower VF than rectifier diodes, so the conduction loss is lower. But they usually have more leakage current. The leakage current also increases quickly at high temperature.
Standard rectifier diodes have higher VF, but their leakage current is lower. Total thermal loss should not be judged from leakage current alone — it should account for forward voltage drop, leakage current, duty cycle, and thermal resistance together, since a diode with lower leakage can still run hotter overall if its VF and duty cycle are high.
Conduction loss can be estimated with a simple formula: P ≈ I × VF. At high current, this loss becomes large, so the choice of diode has a real effect on system efficiency.
Engineers comparing parts can browse the Schottky Diodes category for low-VF options.
3.2 MOSFET Solutions
A P-channel MOSFET is usually easier to design with than an N-channel MOSFET. The gate can often be controlled with a low-side transistor connected to ground. However, at a 24V input, the gate-source voltage must be checked carefully. The VGS voltage must stay within the MOSFET’s rated limit across the full input range. A gate resistor can help limit switching current, while a gate-source resistor keeps the gate at a known level when it is not being driven. A VGS Zener can also protect the gate from voltage spikes. In some designs, a dedicated gate driver or hot-swap controller may be needed to keep VGS within its safe range at 24V ±10%.
N-channel MOSFETs usually have lower RDS(on), so they can have lower conduction loss. But driving the gate is more difficult. A charge pump is often used because the gate voltage needs to be higher than the input voltage.
The body diode direction of the MOSFET is important. It decides the current path before the MOSFET turns on. So, the MOSFET orientation must be correct for the required protection.
There are some other important things to check. These include the VDS and VGS voltage margin, the RDS(on) value at high temperature, the Safe Operating Area (SOA), and the reverse-current path. RDS(on) also increases when the temperature increases.
Back-to-back MOSFETs are used when current needs to be blocked in both directions. One MOSFET has a body diode that allows current in one direction, so two MOSFETs are used together to block current from both sides.
This article does not suggest any specific part numbers. It focuses on the important parameters to check when selecting a component. Engineers can use these points to compare different options and choose what is best for their design.
Engineers can compare MOSFET options across VDS ratings, VGS limits, RDS(on), package types, and thermal characteristics before selecting a final part. Unikeyic’s MOSFET category provides parametric filters that can help narrow the available options based on these design requirements.
| Solution Type | Conduction Loss | Circuit Complexity | Key Consideration |
|---|---|---|---|
| Schottky Diode | High (P ≈ I × VF) | Very Low | High leakage current at high temperature |
| P-Channel MOSFET | Low (P = I² × RDS(on)) | Low | Direct gate drive needs protection (gate resistor, VGS clamp); higher RDS(on) than N-channel |
| N-Channel MOSFET | Lowest | Medium-High | Lowest RDS(on), requires charge pump / gate driver |
4. TVS Selection Must Match the Entire Voltage Window #
A TVS diode has several key ratings: VRWM (maximum working voltage), VBR (breakdown voltage), VC (clamping voltage), IPP (peak pulse current), and peak pulse power. A common mistake is to treat VBR as the actual clamping voltage. VBR is only the point where the TVS starts to conduct. VC, the clamping voltage, is usually higher than VBR, and VC is the value that downstream components actually see during a pulse.
Two rules must both be true for safe design. First, the maximum normal input voltage must stay below VRWM. Second, the worst-case VC must stay below the absolute maximum voltage rating of the downstream MOSFET or DC/DC converter.
Other factors can also affect how well a TVS diode protects a circuit. The type and length of the voltage surge can change how much stress the TVS diode has to handle. High temperature can also reduce the amount of surge power the TVS diode can safely handle. The resistance in the power source and wires can affect how much current flows through the TVS during a surge.
Choosing the right TVS diode is also important. A unidirectional TVS placed before the reverse-polarity protection can become forward-biased when the input polarity is reversed. What happens next depends on the fuse or PTC, current limiting, and the power supply. The current may be limited, or the fuse may blow. It does not always mean a direct short circuit. A bidirectional TVS does not become forward-biased when the input polarity is reversed. This means the TVS itself does not create a low-resistance path for the current. However, it does not replace reverse-polarity protection. You still need a diode or MOSFET to block reverse current from reaching the rest of the circuit.
Finally, the TVS diode should be placed close to the power input and protected circuit. Keeping the PCB traces short helps reduce extra voltage caused by the traces during a fast surge.
A TVS device only protects against short transients. Under sustained overvoltage, a TVS cannot replace a disconnect switch or a current-limiting device. If overvoltage lasts too long, the TVS can overheat and fail.
A full range of TVS Diodes & ESD Suppressors is available for matching against the voltage window defined above.
For a deeper step-by-step method, see Toshiba’s TVS/ESD diode selection guide. This article builds on this method by adding coordinated analysis of the MOSFET and capacitor stages, rather than considering the TVS device alone.
5. Capacitors: Transient Support Can Also Create Inrush #
MLCCs (multi-layer ceramic capacitors) give good high-frequency bypassing. But their effective capacitance drops under DC bias, and the actual value depends on operating voltage, dielectric type, and package size.
Aluminum electrolytic capacitors are used to store a large amount of energy. When choosing a capacitor, check its ESR, ripple-current rating, operating temperature, and service life. Their life can become shorter when they are used at high temperature.
Large input capacitance can create high inrush current at power-on. This extra stress affects the MOSFET’s SOA and the connectors. It can also form a ringing circuit together with cable inductance.
The best approach is a combination of MLCCs and electrolytic capacitors together. It should not be treated as a simple either-or choice.
Suitable parts can be sourced from the Ceramic Capacitors category for the high-frequency stage, and from Aluminum Electrolytic Capacitors for bulk energy storage.
6. Worked Example: A 24V Industrial Input #
To demonstrate the step-by-step design process, let’s consider a typical 24V industrial system with the following specs:
- Input Voltage: 24V DC ± 10% (21.6V to 26.4V)
- Nominal Load Current: 3A (72W)
- Surge Protection Level: IEC 61000-4-5 (1kV / 42Ω surge waveform)
- Downstream Max Voltage Rating: 40V (DC/DC converter input limit)
- Operating Ambient Temperature: -40°C to +85°C
The surge level above uses an application-specific 1kV test condition based on IEC 61000-4-5. The exact IEC severity level depends on the applicable product standard, the port under test, and the coupling mode — in common reference tables, 1kV typically corresponds to Level 2, while Level 3 is usually 2kV, so no specific level is claimed here. The 42Ω source impedance used in this article is the test configuration assumed for this example, not a universal impedance for all 24V power-supply ports; the actual impedance must be confirmed against the applicable product standard and port type. The test can be applied line-to-line or line-to-earth using the proper coupling/decoupling network (CDN), with the standard 1.2/50µs open-circuit voltage waveform and 8/20µs short-circuit current waveform. The CDN and coupling method should match the port being tested, and this is what makes the test condition representative of a real 24V power-supply input.
With these values set, the design work follows six steps, using the solution-comparison table from Section 3 to weigh conduction loss against circuit complexity for this specific 24V input:
Step | What To Do | | 1 | Set the protection goals and the voltage window | | 2 | Compare Schottky diode loss vs MOSFET loss | | 3 | Check MOSFET voltage rating, RDS(on), VGS, and SOA | | 4 | Pick a TVS device and check worst-case clamping voltage (VC) | | 5 | Set the MLCC and electrolytic capacitor values | | 6 | Test surge, hot-plug, reverse polarity, load step, and temperature rise |
6.1 Example Calculations
The calculations below use some example component values. These values are used only to explain the design and are not a recommendation for a specific part. In a real design, use the actual specifications of the selected components.
| Calculation | Assumptions & Formula | Result |
|---|---|---|
| Schottky diode loss | P = I × VF, assuming VF = 0.5V at 3A | 3A × 0.5V = 1.5W |
| MOSFET conduction loss (hot) | P = I² × RDS(on,hot), assuming RDS(on,hot) ≈ 30mΩ after derating to 85°C ambient plus self-heating | (3A)² × 0.03Ω ≈ 0.27W |
| MOSFET voltage margin | Compare worst-case VC (≈38V) against BOTH the MOSFET VDS rating (60V assumed part) AND the article’s stated downstream DC/DC converter absolute maximum input rating (40V) | MOSFET: 60V − 38V = 22V margin (≈63% of rated VDS, within an 80% derating guideline). DC/DC converter: 40V − 38V = only 2V margin, which cannot reliably cover TVS tolerance, temperature drift, and PCB parasitic overshoot → this margin is insufficient and requires further verification (e.g. a lower-VC TVS or a converter with higher input rating), not a pass/fail conclusion of “safe.” |
| TVS peak pulse current (IPP) | IPP ≈ (VOC − VC) / ZS, assuming VOC = 1000V (surge open-circuit voltage), VC ≈ 38V (clamping voltage, not VBR), and ZS = 42Ω source impedance | (1000V − 38V) / 42Ω ≈ 22.9A |
| TVS peak pulse power (PPP) | PPP ≈ VC × IPP, using VC ≈ 38V | 38V × 22.9A ≈ 870W → select a TVS rated ≥ 1kW peak pulse power for margin |
| Effective MLCC capacitance | Nominal 22µF X7R rated part, assuming ~60% derating at 24V DC bias | 22µF × 0.4 ≈ 8.8µF effective at 24V |
| Capacitor inrush current | I = C × dV/dt, assuming 100µF total input capacitance charging to 24V over ~1ms at hot-plug | 100µF × (24V / 1ms) = 2.4A — check against MOSFET SOA |
| Thermal design (MOSFET junction rise) | Tj = Ta + P × RθJA, assuming RθJA ≈ 50°C/W and Ta = 85°C (max ambient) | 85°C + (0.27W × 50°C/W) ≈ 98.5°C, below a typical 150°C Tj(max) rating | Suggested validation checklist:
- Surge test at the rated pulse level
- Hot-plug test at minimum and maximum input voltage
- Reverse-polarity test at full load
- Load-step response test
- Temperature-rise test at maximum ambient temperature
7. Common Integration Mistakes #
- Choosing components based only on the nominal voltage instead of the full voltage range
- Ignoring RDS(on) at high operating temperature
- Using VBR in place of VC when checking downstream protection
- Ignoring MLCC capacitance derating under DC bias
- Adding more capacitance without checking the resulting inrush current
- Placing the TVS device too far from the connector
- Choosing alternative parts based only on package size and rated values without checking their actual operating conditions
Frequently Asked Questions #
What is the difference between VBR and VC in a TVS diode?
VBR is the breakdown voltage. It is the voltage at which the TVS starts conducting. VC is the clamping voltage, which is the voltage that reaches the parts after the TVS during a real pulse. VC is normally higher than VBR, so it is important when checking the protection.
Can a TVS diode replace a fuse?
No. A TVS diode only clamps short voltage spikes. It cannot limit current or stay safe under a long overvoltage or overcurrent event. A fuse or PTC device is still needed for that protection.
Which is better for reverse-polarity protection, a diode or a MOSFET?
A diode is simple to use, but it can get hot at high current because of power loss. A MOSFET has lower conduction loss and can give better efficiency. However, it needs proper gate drive and correct body-diode direction. The best choice depends on the current, board space, and required efficiency.
Why does MLCC capacitance drop under DC bias?
This happens because of the ceramic dielectric material inside the MLCC. As DC voltage rises, the effective capacitance falls. This effect is stronger in small package sizes and high-K dielectrics, so the real capacitance at operating voltage is often lower than the printed value.
What causes inrush current in a DC power supply?
When the power is turned on, the input capacitors need to charge. This can cause inrush current. Larger capacitors can cause more inrush current. If the current is too high, it can put stress on the MOSFET, connectors, and input protection parts.
How do you protect a 24V DC industrial input from surges and reverse polarity?
A power path needs different components, not just one part. A fuse or PTC is used for overcurrent protection. A TVS diode is used for surge protection, and a diode or MOSFET is used for reverse-polarity protection. MLCC and electrolytic capacitors help keep the power stable.
Conclusion #
For a reliable power path, MOSFETs, diodes, TVS devices, and capacitors should be selected together. They work as one protection system, so they should not be treated as separate parts. Unikeyic Electronics supplies electronic components for industrial control, automotive electronics, robotics, medical electronics, IoT, communications, and new energy applications.
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