Is it OK to connect multiple diodes with the same part number in parallel?
It is not recommended to connect two diodes in parallel. Every diode has a slightly different forward voltage; even diodes with the same part number are not perfectly matched.
It is not recommended to connect diodes in parallel.
Every diode has slightly different VF characteristics. Even diodes with the same part number are not perfectly matched. If diodes are connected in parallel, a large amount of current will flow to the side with the lower VF, possibly causing deterioration or destruction.
In high-current designs in which the current ratings of diodes can be a concern, use diodes with a higher current rating instead of connecting multiple diodes in parallel.
I1 = I2
If there is no variation in the diode characteristics, you can expect the current to flow equally through the two diodes.
I1 < I2
If there is variation in the diode characteristics, more current will flow through the diode on the lower VF side. As a result, the junction temperature also rises on the lower side, further reducing VF. This causes more current to flow.
Suppose, for example, that a given diode has the following characteristics. The minimum forward voltage (VF) is not specified in this standard. The forward voltage is 0.92 V typical and 1.20 V maximum at an IF of 100 mA. Fig. 3 shows a typical IF–VF curve and its copy shifted according to the maximum value.
Suppose that a diode with typical VF and a diode with maximum VF are connected in parallel. Fig. 3 indicates that when VI = 0.8 V, a forward current of 22 mA flows through the diode with typical VF while a forward current of 400 μA flows through the diode with maximum VF. When two diodes are connected in parallel, most current flows through the one with a lower VF unless measures are taken. In this case, the diode with a lower VF dissipates more power, causing its junction temperature to increase more than the one with a higher VF.
The electrical characteristics of Si diodes are dependent on temperature as indicated by Fig. 4 (IF–VF curves given at different temperatures). VF generally decreases as temperature increases. Therefore, positive feedback is formed with more current flowing through the diode with a lower VF. This condition might lead to thermal runaway. In the worst-case scenario, the junction temperature could exceed the maximum rated temperature, causing device destruction.
Therefore, use a diode with a high current rating instead of connecting multiple diodes in parallel. When you need to connect two diodes in parallel, it might be possible to prevent thermal runaway by connecting current-limiting resistors (R1 and R2) in series with each diode as shown Fig. 5. In this case, select resistors, taking variations in resistance into consideration. Large-value resistors help absorb variations in the electrical characteristics of diodes.
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