Diodes In Parallel

If the load current is greater than the current rating of a single diode, then two or more diodes can be connected in parallel (see Figure 1) to achieve a higher forward current rating. Diodes connected in parallel do not share the current equally due to different forward bias characteristics. The diode with the lowest forward voltage drop will try to carry a larger current and can overheat. Figure 2 shows the V-I characteristics of two diodes. If these two diodes are connected in parallel at a given voltage, a different current flows in each diode. The total current flow is the sum of I_D1 and I_D2. The total current rating of the pair is not the sum of the maximum current rating for each but is a value that can be just larger than the rating of one diode alone.

Parallel diodes can be forced to share current by connecting a very small resistor in series with each diode. In Figure 3, the current-sharing resistor R establishes values of I_D1 and I_D2 that are nearly equal. Although current sharing is very effective, the power loss in the resistor is very high. Furthermore, it causes an increase in voltage across the combination. Unless using a parallel arrangement is absolutely necessary, it is better to use one device with an adequate current rating.

The value of the current-sharing resistor can be obtained as follows.

V = V_D1 + I_D1 x R = V_D2 + I_D2 x R

Solving for R,

R = (V_D2 – V_D1) / (I_D1 – I_D2)

The power dissipated in R is

PR = I²_D1 x R + I²_D2 x R

The voltage across the diode combination is

V = V_D2 + I_D1 R = V_D2 + I_D2 R

Characteristics

When diodes are connected in parallel, they share the same voltage across their terminals but carry individual currents. Here are the characteristics and considerations when diodes are connected in parallel:

1. Voltage Sharing

   Diodes in parallel have the same voltage across their anode and cathode terminals. This voltage is determined by the external circuit or power supply connected to them.

2. Independent Currents

   Each diode in parallel carries its own current based on its forward voltage drop (typically around 0.6 to 0.7 volts for silicon diodes) and the external circuit connected to it.

3. Current Imbalance

   Due to manufacturing variations and slight differences in forward voltage drop, diodes connected in parallel may not share the current equally. Some diodes may conduct more current than others, leading to imbalance.

4. Reverse Bias Protection

   When diodes are connected in parallel, they provide reverse bias protection to each other. If one diode experiences a reverse voltage, the other diodes will block this reverse voltage and protect the reverse-biased diode from breakdown.

5. Current Sharing Resistors

   To improve current sharing and balance among diodes in parallel, you can use current-sharing resistors in series with each diode. These resistors help equalize the current flow through the diodes.

6. Thermal Considerations

   Diodes generate heat when they conduct current. When diodes are in parallel, heat dissipation becomes important. It's essential to ensure that each diode can handle the heat generated and that they don't overheat.

7. Redundancy

   Parallel diodes can be used for redundancy in critical applications. If one diode fails, the others can continue to conduct, ensuring the circuit's functionality.

8. Lower Voltage Drop

   When diodes are connected in parallel, the overall voltage drop across the combination is lower than that of a single diode. This can be advantageous when you want to minimize voltage loss.

In summary, diodes in parallel share the same voltage but carry individual currents. They offer some protection against reverse bias conditions and can be used for redundancy. However, it's important to consider current imbalance, thermal issues, and the need for current-sharing resistors when using diodes in parallel to ensure proper operation and reliability in your circuit.

Advantages and Disadvantages of connecting diodes in parallel

Advantages

1. Redundancy: One of the primary advantages of connecting diodes in parallel is redundancy. If one diode fails or becomes faulty, the others can continue to conduct current, ensuring the circuit's reliability and functionality.
2. Current Sharing: When appropriately designed with current-sharing resistors, diodes in parallel can share the current load more evenly. This helps prevent the overloading of individual diodes and ensures balanced operation.
3. Lower Voltage Drop: When diodes are connected in parallel, the overall voltage drop across the combination is lower than that of a single diode. This can be beneficial when you want to minimize voltage loss in a circuit.
4. Higher Current Handling: Parallel diodes can collectively handle a higher current compared to a single diode, making them suitable for applications requiring increased current-carrying capacity.
5. Improved Heat Dissipation: Distributing the current load across multiple diodes can improve heat dissipation, reducing the risk of overheating and increasing the overall reliability of the diodes.

Disadvantages:

1. Current Imbalance: Without proper current-sharing resistors or careful selection of diodes, there can be a current imbalance among the diodes in parallel. Some diodes may conduct more current than others, potentially leading to early failure or reduced efficiency.
2. Complex Circuitry: Adding current-sharing resistors and ensuring proper current balance can make the circuit more complex, requiring careful design and calculation.
3. Increased Component Count: Using diodes in parallel increases the number of components in the circuit, which can be a drawback in terms of cost, space, and complexity.
4. Voltage Tolerance: When diodes are connected in parallel, they share the same voltage across their terminals. It's crucial to ensure that all diodes can withstand this voltage without breaking down.
5. Thermal Management: Parallel diodes can generate heat, especially when carrying high currents. Proper thermal management is necessary to prevent overheating and ensure long-term reliability.
6. Reverse Bias Protection: While diodes in parallel can provide reverse bias protection, they may not completely eliminate the risk of reverse voltage damage, especially if the reverse voltage exceeds the breakdown voltage of the diodes.

In summary, connecting diodes in parallel offers redundancy improved current handling, and lower voltage drop, but it also requires careful consideration of current balance, voltage tolerance, and thermal management. The decision to use diodes in parallel should be based on the specific requirements and constraints of the application, taking into account both their advantages and disadvantages.

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