In some applications especially those involving high current may need more than one diode in parallel. For DIY projects like for personal consumption or school projects, this could be acceptable. However, in talking with professional product development wherein the quality and reliability are the focus, parallelling diodes is highly discouraged. This article will discuss the 2 main reasons why it is not advisable to connect diodes in parallel.
1. Diode with Same Part Number Slightly Differs in Forward Voltage Drop
Even the diodes have the same part number, they will still differ in the forward voltage. Diode datasheets always specify the forward voltage in terms of range.
Examples:
RFN20TF6SC9 – Rohm Semiconductor
The forward voltage of this diode is specified in typical and maximum values; 1.25V and 1.55V respectively.
STTH25M06 – STMicroelectronics
The forward voltage of this diode is specified also in typical and maximum values; 2.5V and 3.4V respectively.
Based on the two examples above, the forward voltage is a range. This means that even the same part number diode will still differ in the forward voltage which is within the specified range.
A diode with a lower forward voltage will allow more forward current. This will make the other diode to dissipate more power than the other diode. In stress perspective, this is not good. This is not only up to this event. The worst-case scenario is discussed item number 2 below.
(By the way, both Rohm Semiconductor and STMicroelectronics are two of the best diode manufacturers in the market today. I have first hand experience in using their parts in the past and the quality and reliability is very good.)
2. Diode has Negative Temperature Coefficient
Diodes has negative temperature coefficient. This means that a rise in temperature will correspond to a decrease on a certain parameter and in this case the forward voltage.
Examples:
RFN20TF6SC9 – Rohm Semiconductor
The graph below indicates four curves with respect to specific junction temperatures. As you can see, the higher the temperature, the lower the forward voltage.
STTH25M06 – STMicroelectronics
As you can see from the table below, the forward voltage at 25’C is higher than the value at 150’C.
As discussed above, diodes will slightly differ in the forward voltage. The diode with the lower forward voltage will share more current. When this happens, the junction temperature of that diode will increase. When the junction temperature increases, the more the forward voltage will decrease. This means that the more the current that diode will allow. This will end up to thermal runaway and destroy the diode.
Should you have other explanations why it is not advisable to connect diodes in parallel, right it in the comments section and let us discuss.