Why use components for derating? Because if the component's operating conditions do not exceed the specifications provided by the supplier, it can ensure full-life operation. Derating improves product reliability by reducing stress on components, thereby extending their lifespan and minimizing failure risks.
Derating rules are typically based on worst-case scenarios. Components that operate under such extreme conditions are a key factor in determining whether the expected lifetime of the product meets its rated life or falls short.
The worst-case condition refers to the situation where a component experiences maximum stress during operation. This usually involves a combination of environmental parameters like temperature, voltage, switching frequency, and load. The limits for these stresses are generally specified in the component’s datasheet.
A good design should evaluate reliability by assessing the risk associated with component performance under worst-case conditions. Risk assessment helps identify potential failure causes, the likelihood of failure, and the severity of consequences.
To develop a derating specification, a failure risk assessment is conducted under worst-case conditions. An accelerated testing model is often used to simulate this scenario. If the assessment is based on normal usage time, the market might already have been impacted before results are available.
The accuracy of the model significantly affects the risk assessment outcome. To ensure model accuracy, collaboration with academic institutions is often required. In our case, we will briefly analyze the qualitative aspects.
Accelerated testing typically follows the Arrhenius equation:
$$ A = \exp\left(\frac{E_a}{k} \left( \frac{1}{T_{\text{ref}}} - \frac{1}{T} \right) \right) $$
Where:
- $ A $: Acceleration factor
- $ E_a $: Activation energy
- $ k $: Boltzmann constant (8.63 × 10â»âµ eV/K)
- $ T $: Absolute temperature
- $ T_{\text{ref}} $: Reference temperature
If an acceleration factor is known for a specific derating condition, the following formula can estimate life under different conditions:
$$ t = T_{\text{ref}} \times A^{\frac{T - T_{\text{ref}}}{10}} $$
Where:
- $ T $: Temperature in degrees Celsius
- $ T_{\text{ref}} $: Reference derating temperature in degrees Celsius
- $ T_{\text{ref}} $: Reference service life in KHrs (thousand hours)
- $ t $: Service life in KHrs
- $ A $: Acceleration factor per 10°C
For example, if a component has a 30KHrs life at 90°C and an acceleration factor of about 2 per 10°C, what temperature would result in a 20KHrs life?
$$ \text{Temperature} = 90^\circ C - \frac{10^\circ C \times \log_2\left(\frac{30}{20}\right)}{1} $$
This calculation helps determine the impact of temperature on component life.
Integrated circuits are complex and often confidential, so their reliability is typically inferred based on junction temperature. Common derating guidelines include:
1. Maximum working voltage ≤ 80% of rated voltage.
2. Maximum output current ≤ 75% of rated current.
3. Junction temperature ≤ 85°C or ≤ 80% of rated junction temperature.
Diodes come in various types, each with unique requirements. General guidelines include:
- Long-term reverse voltage < 70–90% of VRRM.
- Forward average current < 70–90% of rated value.
- Junction temperature varies depending on type (e.g., signal diodes up to 150°C).
Power MOSFETs require careful derating:
- VGS ≤ 85% of VGSmax.
- ID_peak ≤ 80% of ID_M.
- VDS ≤ 80–90% of rated voltage.
- dV/dt ≤ 50–90% of rated value.
- Junction temperature ≤ 85°C or ≤ 80% of Tjmax.
Transistors also need derating:
- All voltages ≤ 85% of rated values.
- Power loss ≤ 70–90% of rated value.
- IC must be derated by 30% within RBSOA and FBSOA.
- Junction temperature ≤ 85–125°C.
Electrolytic capacitors are critical in power supplies. Key guidelines include:
- Vdc + Vripple ≤ 90% of rated voltage.
- Ripple current ≤ 70–90% of supplier rating.
- Initial capacitance with 20% margin.
- Life cycle affected by temperature (life halves every 10°C).
- Shell temperature limited by design life.
Ceramic capacitors:
- Operating voltage ≤ 60–90% of rated voltage.
- Surface temperature ≤ 105°C.
- Self-temperature rise ≤ 15°C or as specified.
Film capacitors:
- Avoid polystyrene due to poor heat resistance.
- Surface temperature ≤ 85°C.
- Polyester self-temperature rise ≤ 8°C.
- Polypropylene self-temperature rise ≤ 5°C.
- Life depends on voltage and pulse slope.
Resistors:
- Fixed linear resistors (carbon, metal, etc.) and nonlinear (NTC, PTC) have different derating needs.
- Reliability depends on temperature from ambient and self-heating.
- Power and voltage limit resistor use.
- Derating rules vary by type and application.
Magnetic components:
- Voltage between wires must stay within limits.
- Wire gauge conversion formula: $ d = 25.4 \times 0.005 \times 92^{(36 - AWG)/39} $.
- Enamel wire lifespan reduces by 2.5x per 10°C increase.
- Core derating: Bmax ≤ 80% of Bsatt, TCORE ≤ 70% of Tcurie – 10°C.
Metal Oxide Varistors (MOV):
- Tcase ≤ 85°C under any conditions.
- Recommended ratings based on AC input (e.g., 150Vrms for 120V AC).
Printed Circuit Boards (PCBs):
- Material and surface temperature limits vary (e.g., FR4 up to 125°C).
- Use vias for heat dissipation, but limit via current to 2A.
- Spacing rules follow UL935 standards.
Fuses:
- Derating balances protection and fuse life.
- At 25°C, derate by 25%. For every 1°C increase, slow-blow fuses derate 0.5%, fast-blow 0.1%.
Optocouplers:
- Max voltage ≤ 70–90% of rated voltage.
- Max current ≤ 25–90% of rated current.
- Current transfer ratio with 20% margin.
- Junction temperature ≤ 85–100°C.
These guidelines help engineers ensure reliable, long-lasting designs while managing costs and performance trade-offs.
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