How to do worst case calculation of Diode in reverse polarity protection?

 

Reverse polarity protection is a crucial design consideration in many electronic circuits to prevent damage when the power supply is connected with reverse polarity. Diodes are commonly used for this purpose due to their ability to block current flow in one direction. In this guide, we'll walk through the steps to perform a worst-case calculation for a diode used in reverse polarity protection.

1. Identify the Maximum Input Voltage

The first step is to determine the highest voltage that could be applied to the circuit in a reverse polarity condition. This includes considering the power supply’s nominal voltage and any possible over-voltage conditions.

For example, if your system operates on a 12V battery, the maximum reverse voltage could be around 12V, but you should also consider potential voltage spikes that could occur due to various factors like load dumps in automotive applications.

2. Choose a Suitable Diode

Select a diode that can withstand the maximum reverse voltage. The diode’s reverse voltage rating (also known as Peak Inverse Voltage, PIV, or Reverse Stand-Off Voltage) should be higher than the maximum input voltage.

• Schottky Diodes: Known for their low forward voltage drop and fast switching speed, but typically have lower reverse voltage ratings.
• Silicon Diodes: Have higher reverse voltage ratings but come with a higher forward voltage drop.

For instance, if the maximum reverse voltage is 15V, choose a diode with a reverse voltage rating of at least 20V to provide a safety margin.

3. Check Reverse Leakage Current

Every diode has a small reverse leakage current when a reverse voltage is applied. This current should be low enough to not affect the circuit’s operation. Check the diode's datasheet for the reverse leakage current at the specified reverse voltage.

For example, if the datasheet specifies a reverse leakage current of 10μA at 25V, ensure that this current is acceptable for your design, considering it will cause minor power dissipation.

4. Thermal Considerations

Calculate the power dissipation due to the reverse leakage current and ensure the diode can handle the thermal load. The power dissipated (P) can be calculated using the formula:

$P = V_{R} \times I_{R}$

Where:

• $V_{R}$ is the reverse voltage.
• $I_{R}$ is the reverse leakage current.

For example, if the reverse voltage is 12V and the reverse leakage current is 10μA, the power dissipation is:

$P = 12V \times 10\muA = 0.12mW$

While this seems small, it’s important to consider higher leakage currents at elevated temperatures. Diode datasheets often provide leakage current values at different temperatures.

5. Consider Transient Conditions

Transient voltage spikes can occur due to switching events, inductive loads, or external disturbances. These spikes can exceed the normal operating voltage. Use transient voltage suppression (TVS) diodes or snubber circuits to protect against these spikes.

For example, if you expect voltage spikes up to 30V, choose a diode with a reverse voltage rating significantly higher, such as 40V, to handle these transients without breaking down.

6. Verify the Diode’s Reverse Recovery Time

In some applications, especially those involving high-frequency switching, the diode’s reverse recovery time can be critical. This is the time it takes for the diode to switch from conducting in the forward direction to blocking in the reverse direction. Ensure that the chosen diode has a reverse recovery time suitable for your application to avoid potential issues like reverse current flow.

Practical Example

Let’s consider a practical example where you need to protect a 12V automotive circuit from reverse polarity.

• Maximum Input Voltage: 14V (considering typical automotive voltage).
• Selected Diode: 1N5819 Schottky diode with a reverse voltage rating of 40V and a reverse leakage current of 1mA at 25V.
• Reverse Leakage Current at 14V: Assume it’s proportionally less than at 25V, let’s approximate it to be 0.5mA.
• Power Dissipation:

$P = 14V \times 0.5mA = 7mW$

• Transient Voltage Spikes: Design for spikes up to 40V.

Thus, the 1N5819 diode can comfortably handle the maximum reverse voltage, the reverse leakage current is acceptable, and the power dissipation is minimal, ensuring the diode does not overheat under normal operation.

Conclusion

Performing a worst-case calculation for a diode used in reverse polarity protection involves understanding the maximum input voltage, selecting an appropriate diode with a suitable reverse voltage rating, checking the reverse leakage current, considering thermal dissipation, accounting for transient conditions, and verifying the reverse recovery time. By carefully considering these factors, you can ensure reliable protection for your electronic circuits against reverse polarity connections.

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