Choosing from multiple diode technologies for Boost CCM PFC and DC-DC converters.
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 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.
The range of diode technologies available has expanded and now includes compound semiconductor materials, novel dopants and lifetime-killer strategies allowing designers to take advantage of both the core characteristics and secondary or parasitic dynamic behaviors to optimize their designs 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. This can be illustrated by reference to two common applications: a power factor correction circuit operating in continuous conduction mode (Figure 1) and a DC-DC converter (Figure 2).
The boost converter in Figure 1 switches the rectified AC mains input to produce a high voltage DC output. The PFC control circuit adjusts the switching to ensure that the AC mains current input waveform closely follows the input voltage waveform. In this way current is drawn from the mains at a power factor of close to 1.
The PFC control circuit turns the boost switch (MOSFET) on and off at a switching frequency of typically 60 kHz to 100 kHz. The duration of the on-time is controlled based on the output voltage, the current through the switch and the phase angle of the AC input voltage. When the switch turns off, the stored energy in the inductor discharges through the diode as ID_FORWARD and charges up the output capacitor COUT.
Boost converters that deliver more than 250W are usually designed to operate in the continuous conduction mode. The MOSFET switch turns on while the diode is still conducting a high forward current. When the reverse bias is established across the diode it ceases conducting and isolates the output capacitor COUT.
For a PN junction diode to turn off, sufficient charge must be removed from the barrier junction capacitance so that the depletion region can become big enough to block the reverse voltage. The charge that must be removed is known as the reverse recovery charge QRR. Removing QRR requires a finite amount of time (the reverse recovery time tRR) and during this period a large reverse current IRR can be drawn through the diode. The reverse recovery current flows through the MOSFET in Figure 1, adding to the inductor current IL. Consequently, the MOSFET must be sized to accommodate the combined peak current. The reverse current IRR wastes energy and generates EMI noise that may require dampening. Similar issues arise in a DC-DC converter.
In Figure 2, the two output rectifier diodes both require snubbers to control the oscillation generated by the sharp turn-off of the ultra fast diodes. Even so, the overshoot peaks at 332 V.
In both applications the diode technology selected can have a big impact on the switching currents and overshoot. When selecting a diode and MOSFET switch, the designer must consider both the amplitude of IRR and the period for which it flows (tRR) to assess the peak currents and energies involved.
Broadly speaking, the system designer has five main rectifier diode technologies to choose from: bulk silicon (Si) technologies, standard diodes, Schottky barrier diodes, ultra fast diodes and merged PIN-Schottky diodes. Merged PIN-Schottky diodes, also known as the Qspeed family(1), are advanced Si technology diodes produced by Power Integrations Inc. (PI). Conventional Schottky diodes vary in their performance based on the materials used to form the Schottky junction. Silicon carbide (SiC) Schottky barrier diodes have become more popular in recent years, particularly for high-voltage applications.
Table 1 lists these diode technologies together with some key parameters for comparison.
Standard PN junction diodes, although they have a high voltage capability and low VF, must be discounted immediately because of their long tRR. Schottky barrier diodes act more like ideal switches than standard PN junction devices. With low VF and tRR, Schottky barrier diodes make an ideal choice. Unfortunately, technology limits Si Schottky barrier diodes to low voltage applications.
Si ultra fast diodes are made by doping the PN junction with platinum. The VF and trr are inferior when compared with Si Schottky barrier diodes and this impacts the efficiency of a design. On the plus side, Si ultra fast diodes are available up to 1000V VBR. On the down side, the platinum doping in Si ultra fast diodes causes a high peak reverse recovery (IRR) current with an abrupt or 'snappy' cutoff. This creates high peak currents through the MOSFET switch (Figure 1) and high EMI, both of which must be dealt with.
Qspeed (merged PIN-Schottky) diodes contain a hybrid structure that combines the characteristics of PN and Schottky barrier diodes. The VF is similar to that of ultra fast diodes. The tRR is as fast as Schottky barrier diodes and the maximum VBR is 600V. These characteristics enable Qspeed diodes to be use in 230VAC mains applications with efficiency levels higher than that of the ultra fast diodes. Other key benefits are that Qspeed diodes do not exhibit either the high peak IRR or snappy cutoff of ultra fast diodes.
At the high end of the voltage scale are silicon carbide Schottky barrier diodes. These diodes are both fast and exhibit the same low reverse recover current as Qspeed diodes. With the availability of SiC transistors, silicon carbide technology can be used to make very high efficiency power converters for applications such as photovoltaic inverters. There is a price to be paid however. The manufacturing costs of silicon carbide make SiC diodes substantially more expensive than comparable silicon products.
Figure 3 illustrates the behavior of differing diode technologies during reverse recovery. The red line shows the characteristic of a standard fast diode. The area of the curve encompassing the negative current phase is equal to the reverse recovery charge QRR. This energy is wasted each time the diode turns off. Therefore, for a high efficiency design, the QRR must be as low as possible. The purple trace is the characteristic of a platinum doped ultra fast diode. The QRR is obviously much lower, 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 is for a Qspeed diode. In this case the peak current is half that of the ultra fast diode and there is no oscillation.
The soft turn-off characteristic translates into performance and efficiency advantages for both the PFC and DC-DC converter examples.
In the PFC application shown in Figure 4, substuting a 600V Qspeed diode enabled the MOSFET size to be reduced from 2A to 12A with a 10°C reduction in MOSFET temperature. The lower heat energy dissipated translated into a 2.5 percent increase in efficiency.
Stability over temperature
In PN-junction based power diodes, QRR, IRR and tRR vary with junction temperature. Thermal interference slows down minority carrier recombination as junction temperature increases(2). Therefore, QRR, IRR and tRR will all increase as the junction temperature rises. Figure 5 shows the dependence of QRR on junction temperature, for the same three diodes shown in Figure 3. Qspeed diodes can be designed to have a low, positive temperature coefficient. The QRR, IRR and tRR of these diodes does not increase significantly over the normal operating junction temperature range.
The consistent QRR, IRR and tRR over temperature simplify the designer's task in keeping power supply efficiency and EMI within specifications, at worst-case operating conditions and eliminate the risk of thermal runaway.
Raising efficiency in output rectification
In the DC-DC converter example (Figure 2), substituting 300V Qspeed diodes eliminates the need for snubbers and increased efficiency by 2.2 percent.
In Figure 6, 300V Qspeed diodes were used in place of ultra fast diodes and the snubbers were removed. No other circuit changes were made. Without snubbers, the peak voltage on overshoot still reduced from 332V to 289V.
Further advantage can be taken of the low QRR of Qspeed diodes. The reduced switching losses enable a circuit to be designed to run at higher switching frequencies. By doubling the switching frequency the inductance required of chokes and transformers is approximately halved. Significant savings in the size and cost of magnetics may thus be achieved.
Making a choice
The analysis has shown there is not a simple choice to diode selection for power switching applications. For low voltages the silicon Schottky barrier diode is the clear choice. For high voltages in the 1000 - 1200V region silicon carbide offers significant technical advantages that may be sufficient to offset the high cost.
At 600V, switching frequency and current become guiding factors. For switching frequencies under 100KHz where low VF and higher thermal performance are required, SiC diodes are the better match. Also, if the required current rating is very high (>30A), the SiC diode is recommended due to its thermal robustness. For applications with switching frequencies equal to or greater than 100KHz and current rating up to 20A, the lower switching losses of Qspeed diodes provide some efficiency advantage.
For 600V and 300V output rectification applications, when measured against Si ultra fast diodes, there may be circumstances that require low VF in which case ultra fast may be the correct choice, However, the 'soft' reverse recover characteristic, lower QRR, and low peak IRR of Qspeed point to lower switching losses, lower EMI, and lower peak reverse voltage across the diodes, making Qpeed the clear winner.
1. Qspeed Family of Advanced Diodes. Power Integrations Inc. www.powerint.com
2. Application Note AN-301. Qspeed Family Reverse Recovery Charge, Current and Time. January 2011. Power Integrations Inc. www.powerint.com
PI-A164 New diode technologies enable power supply designers to optimize topology choice and benefit from performance enhancements