Cracking the diode compromise
The days of analog power systems are long gone, giving way to modern power systems that invariably apply switching topologies in their operation. Today’s systems, such as power factor correction, motor drives, DC-AC inverters, bridge converters and DC-DC converters are all called upon to operate at high levels of efficiency, generate low EMI, be physically small, light in weight and low cost to manufacture. These requirements all point to high-frequency switching, the higher frequency the better, provided that EMI regulations can be met. Successfully meeting requirements often relies on the characteristics of what may be regarded as a relatively simple component, the rectifier diode.
Whenever an inductor is switched in a power circuit, a diode is generally present to carry the freewheel current through the inductor during each cycle. When a power electronic system is scanned for temperature rise while operating, the components found to be dissipating most heat will often be the rectifier diodes, so attention given to careful selection of diodes at the design stage will be well rewarded. A range of technologies and architectures are now applied in the manufacture of diodes, attempting to produce the ideal diode for each application. A number of diode technologies are identified in Table 1.
The first four technologies employ bulk silicon and the fifth uses a compound semiconductor material to achieve breakdown voltages higher than achievable with silicon. The simplest is the silicon standard diode which consists of P-type and N-type silicon forming a single junction at the interface. If reverse bias is applied while the diode is conducting a high forward current, a finite amount of time (tRR) is required for minority carriers to be removed from the junction and a depletion region established. Because the tRR of standard diodes is in the range 1-2 ms they are limited to only low-frequency applications.
A Schottky diode is produced by replacing the P-type material with a metal contact. Current flow is only carried by electrons so there are no minority carriers. When reverse bias is applied, electrons are attracted to the opposite pole, creating a depletion region to block current. Because there are no minority carriers and the depletion region is very thin, the Schottky diode reverse recovery is much faster than with a standard silicon diode. The penalty is that the breakdown voltage for silicon Schottky is limited to around 200V and leakage can be significant. Higher breakdown voltages can be achieved by using compounds such as silicon carbide instead of silicon, but at the high material cost, limits the use of silicon carbide only to the most demanding applications.
The P intrinsic N (PIN), often known as ultra-fast, diode uses a region of lightly doped N-type silicon between the normally doped P and N regions. The lightly doped N region is often doped with platinum to create recombination centers that reduce minority carrier lifetime. When the diode is reverse biased, holes and electrons are attracted to the recombination centers in the drift region where they recombine. This creates a much faster reverse recovery than with a standard diode.
The larger junction thickness results in a much higher forward voltage to carry the same amount of current when compared with a Schottky diode.
As can be seen from Table 1, the Schottky diode has lower VF and faster tRR than ultra-fast diodes but is limited to 200V VBR. The ultra-fast diode can be used above 200V but efficiency is compromised.
There is another disadvantage of ultra-fast diodes in that the platinum doping causes high peak reverse recovery currents (IRR) with an abrupt or snappy cutoff. This tends to generate EMI, often requiring the use of energy sapping snubbers to contain it.
The ideal solution would be to combine the best characteristics of Schottky and ultra-fast diodes. Power Integrations Inc. (PI) produced the novel Qspeed diodes, merging the PIN-Schottky range of diodes thus realizing this ideal solution1 . The Qspeed diodes are manufactured using three-dimensional diffusion and deposition techniques more commonly found in the world of memories and microprocessors. Figure 1 provides a simplified diagram of the structure.
Figure 1. Qspeed diode structure.
Tubular trenches of P-type silicon are deposited into lightly doped N-type material. The side walls of the trenches are coated in insulating material so conduction can occur only at the bottom of the trenches where there is a junction between the P- and lightly doped N-type silicon. This creates a PIN diode. A metal contact is then deposited across the tops of the trenches to create a Schottky junction with the N- material in between the P wells. So QSpeed diodes contain both Schottky and PIN diodes within them.
Figure 2. Forward conduction.
When forward biased the Schottky junctions conduct immediately and electrons flow between the P-type trenches. At higher current levels, a voltage drop builds up in the channels and the PIN junctions at the base of the trenches start to conduct, providing an additional forward current path. The combination of junction types provides fast early turn-on and high current density capability.
When the diode is reverse biased the Schottky diode elements stop conducting almost immediately, followed by the PIN junctions. Once reverse recovery is complete, the depletion layers formed around the base of the trenches build up and overlap. These create a continuous barrier pinching off the channels through to the Schottky diodes.
Figure 3. Reverse bias. Depletion regions overlap.
Thus, the reverse breakdown voltage and leakage current is determined by the strong depletion layers, not the Schottky junctions. This enables the QSpeed diode to exhibit the fast turn on characteristics of the Schottky diode together with the high reverse-voltage capability (up to 600V) and low leakage characteristics of the PIN diode.
The combination of Schottky and PIN diodes provides another very important benefit. It overcomes the snappy switching of platinum doped ultra-fast diodes and produces very soft turnoff characteristics as shown in Figure 4.
Red = Standard fast diode
Purple = Platinum-doped, ultra-fast diode
Green = Qspeed diode
The area of the curve encompassing the negative current phase for each device type is equal to the reverse recovery charge QRR. This energy is wasted each time the diode turns off. Also the reverse current passes through any driving elements, meaning they have to be rated to accommodate the peak current. Therefore, for high efficiency the QRR should be as low as possible. The trace for the platinum-doped, ultra-fast diode shows a much lower QRR, but the peak reverse current is still high and the turn off is very abrupt. The abrupt or ‘snappy’ turn off causes oscillation, hence EMI. The green trace for a Qspeed diode reaches a peak reverse current half that of the ultra-fast diode and there is little oscillation.
Figure 5. Effect of different trench depths.
By altering the depth of the trenches in Qspeed diodes the conduction voltage of the PIN diodes can be adjusted, giving different tradeoffs between conduction losses and soft recovery. This tradeoff has been exploited by PI in the development of three Qspeed families, providing optimized performance for different applications. The Q-Series has highest softness, the X-Series lowest conduction losses and the H-Series lowest switching losses.
Qspeed diodes are the result of applying VLSI design and production techniques to a power device. They offer greater efficiency than Schottky diodes at any frequency above 50 kHz without exhibiting the leakage problematic in Schottky diodes. Qspeed diodes are faster and more efficient than ultra-fast diodes at any frequency and the soft switching of Qspeed diodes eliminates the need for snubbers in many cases. In fact, Qspeed diodes challenge the performance of silicon carbide diodes at 600V at a small fraction of the cost. The power electronics designer can choose the ideal diode for every application without need to compromise.
1. Qspeed diodes. Power Integrations Inc. https://www.powerint.com/en/products/qspeed-family