PowerViews

Conferences and Forums

January 14, 2013

Diode Selection: A critical choice

Doug Bailey
VP Marketing, Power Integrations

Power Channels: Batteries and Portable Power, Communications Power, Packaging and Cooling, Power Components, Switch-Mode Power

Darnell Group interviewed Doug Bailey, VP marketing for Power Integrations, to get his view on the “humble” diode; a part that can be of vital importance in modern electronic systems. Often overlooked as a commodity in low end systems such as chargers, the selection of these devices in higher performance systems is a critical factor that can determine the success or otherwise of the design project.

In power electronic systems, the performance of rectifier diodes is often a key factor in the overall system operation, despite their superficial simplicity. Systems employing power factor correction and applications such as motor drives, DC-AC inverters, bridge converters and DC-DC converters, all place great stress on diodes. When a power electronic system is scanned for temperature rise while operating, the components found to be dissipating most heat will more often than not be the rectifier diodes. Time spent at the design stage in selecting the best combination of diode parameters for a given application will be well-rewarded in terms of increased efficiency and reduced system cost.

Doug explained that the range of diode technologies available has expanded and now includes compound semiconductor materials; novel dopants and lifetime-killer strategies allow designers to take advantage of both the core characteristics and secondary or parasitic dynamic behaviors to optimize their design’s performance. In power switching applications, the behavior of the rectifier diode as it transitions from conducting to reverse biased (i.e. blocking current flow) varies greatly between differing technologies.

Designers of power switching electronics are generally agreed that their least favorite problem they have to deal with is that of EMI. If a prototype design suffers from excessive EMI, the solution often requires the addition of filtering or snubbers. The cost of adding snubbers to a circuit can be significant if in fact they actually work. In addition to the cost, size and weight penalties, there is the loss of efficiency due to energy absorbed and converted into heat by the snubbers. The key to dealing with EMI is to simply avoid generating it in the first place, through an understanding of the root causes and by making the appropriate choices when selecting components.

Frequent culprits for the cause of EMI are fast rectifier diodes producing overshoot and ringing at the output of a DC:DC converter.

If a power diode is quickly reverse biased while it is conducting a high forward current (known as hard switching) a finite amount of time is required to clear it of charge carriers so that it can block the reverse voltage. The stored charge (QRR) is due to either the barrier junction capacitance of Schottky devices or the minority carriers that flow within the cathode and drift region of PN junction based devices. All stored charge must be removed so that the depletion region can become big enough to block the reverse voltage.

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