Can Si rectifiers compete with SiC diodes?

Silicon can reduce cost and/or improve equipment performance

BY JOHN JOVALUSKY
Qspeed Semiconductor
Santa Clara, CA
http://www.qspeed.com

Power semiconductors and reactive components (capacitors and inductors) are found inside most of today’s electrically powered equipment. Their normal operation produces two unwanted side effects on the alternating current (ac) power lines that supply them with electricity.

First, these devices can cause low power factor. Second, they can distort the line current and cause it to become electrically noisy or shift it out of phase with the line voltage.

Power factor is the ratio of the actual power used to the apparent power drawn from the ac line. Significant capacitance or inductance within the equipment can cause the apparent power to be greater than the used power, resulting in a low power factor.

The lower the power factor, the more energy will be lost along the ac line that supplies the equipment. If power semiconductors in the equipment are switched on and off at high frequencies, that switching can distort the ac line current and make it noisy. This is especially true in switching power supplies.

Fig. 1. The easiest and most cost-effective way to achieve power factor correction is to use a boost-converter stage, which produces an output voltage that is higher than its input voltage.

International standards such as IEC 61000-3-2 specify acceptable levels of line current distortion and power factor for various types of electrically powered equipment. The easiest and most cost-effective method of achieving power factor correction (PFC) uses a boost converter stage (see Fig. 1 ), which produces an output voltage that is higher than its input voltage.

Boost-diode performance

For equipment that utilizes 300 W or more, boost converters operating in the continuous conduction mode (also known as CCM) are normally used. Of the boost converter’s two power semiconductorsa MOSFET and a diodethe diode has the more demanding role, since its reverse recovery affects the MOSFET’s performance.

In continuous-conduction-mode operation, every time the control IC turns on the MOSFET, the diode is conducting a high forward current. Because the boost diode is being quickly reverse biased while it is fully forward biased, and since silicon diodes require a finite amount of time to turn off, the reverse-recovery current (IRR ) that flows back through the diode as it turns off can be quite large (see the red trace in Fig. 2 ).

Fig. 2. Shown above are the reverse-recovery waveforms of four common boost diodes (400 V, 5 A, 200 A/µs, 125C).

That reverse current flows through the MOSFET, increasing its operating temperature. Specialized silicon diodes have been designed that have very short reverse recovery times (tRR ), but their IRR reduction is typically not that substantial and they often have abrupt turn-off characteristics (see the black trace in Fig. 2 ).

Low QRR and high softness factor

Schottky diodes behave more like an ideal switch than do p-n junction devices. The two most important performance benchmarks of a Schottky are its low reverse-recovery charge (QRR ) and its recovery softness factor.

Both benchmarks are important in boost converters. Low QRR results in low IRR as the diode turns off. A high softness factor reduces the amount of EMI noise that turn-off generates, the size of the voltage spike that develops on the anode of the device, and the likelihood that its commutation will interfere with the PFC control IC.

The limitations of Schottky diodes

Schottky diodes could improve the performance of PFC boost converters significantly, but silicon Schottky diodes have a reverse-voltage limit of about 250 V. Because boost diodes must withstand 500 to 600 V, engineers turned to devices made from silicon carbide (SiC), since that compound can have higher voltage ratings. However, due to the cost of SiC devices (three to five times that of comparable silicon parts), few applications can afford to use them.

Better silicon diodes have been introduced over the past few years, but none of them have performed as well as SiC Schottky devices. However, a new family of silicon rectifiers was recently developed that has SiC Schottky-like reverse recovery performance (see the green trace in Fig. 2 ).

The QRR that must be removed from a p-n junction silicon diode before it becomes reverse biased determines the amount of IRR that can be pulled back through it as it turns off. QRR is mainly determined by the duration or the lifetime of minority charge carriers near the p-n junction.

Schottky diodes have no minority carriers since they only consist of a metal contact to n-type material. The small IRR that occurs when a Schottky diode is reverse-biased comes from discharging the capacitance of the metal contact to the diode body.

Silicon diode designers have a number of techniques for controlling minority carrier lifetimes in their devices, but have been unable to match the low QRR of SiC diodes until now. As can be seen by the green trace in Fig. 2 , the newest silicon devices—the Q-Series, from Qspeed Semiconductor—have an IRR almost as low as that of SiC Schottky devices (the blue trace in Fig. 2 ).

Schottky diodes have no minority carriers since they only consist of a metal contact to n-type material.

Softness is a measure of how quickly the diode’s IRR drops or falls back to zero, once it has reached its peak negative value. Silicon diodes that have been designed to recover quickly often use a minority-carrier-lifetime control technique that causes a very abrupt decrease of the IRR (see the black waveform in Fig. 2 ). That snappy turn-off generates significant EMI noise and a large voltage spike on the anode of the diode.

Elaborate snubbing circuits are required to offset those undesirable effects when using snappy-type diodes. A high softness factor means that the diode’s IRR drops back to zero at a rate of change (di /dt ) that is equal to or less than the rate at which it increased to its peak negative value. When a diode turns off softly, it generates less EMI noise, develops a smaller voltage spike on the anode of the diode, and will be less likely to interfere with control IC operation.

Since silicon rectifiers that rival SiC Schottky diodes are now commercially available, engineers should re-evaluate their PFC boost converters to see if they can reduce the cost and/or improve the performance of their designs by taking advantage of the SiC-like performance of these new silicon devices. ■

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