Electronic Design

Low Stored Charge Separates Diode From The Pack

I n an idealized diode, no reverse current flows from cathode to anode when the device is reverse-biased. However, with real-world diodes, large amounts of stored charge can flow from the cathode—back through the anode— before the diode enters its blocking state. That stored charge is QRR, and it causes the reverse recovery current (IRR) that flows as the diode transitions from forward to reverse bias.

For example, a conventional 600-V diode can experience forward currents upward of 8 A. But as it commutates to the blocking state, narrow pulses of reverse current—as high as 6 A—can shoot back through the anode.

Circuit designers try to prevent this diode reverse current from flowing into other parts of the circuit. In boost power- factor-correction (PFC) converters, diode IRR flows into the power MOSFETs. As a result, junction temperature rises and efficiency of the PFC circuit reduces from 0.5 to 2%. Replacing the traditional diode with a low-QRR PFC diode can boost power-supply efficiency, eliminate snubber circuitry, reduce MOSFET temperature, and, in some cases, allow the use of lower-cost or fewer MOSFETs.

Some fast silicon diodes have what is known as a snappy or abrupt recovery characteristic. Not so with the newer low-QRR diodes from Qspeed Semiconductor. The company’s first launches into the high-voltage, offline world have a lower reverse recovery current and a softer, more benign recovery characteristic than other silicon-based rectifiers.

AHEAD OF THE CURVE
The waveforms illustrated in the figure compare Qspeed diode characteristics with other silicon and siliconcarbide (SiC) diodes. In diode reverse recovery, the current waveform reaches its peak reverse value and then starts to decay back (up) to the zero line.

These effects become more pronounced as PFC switching frequency surpasses 65 kHz and the MOSFETs switch on and off faster. The voltage spikes that result from snappy recovery cause noise, which can interfere with the PFC control IC, causing instability and lower efficiency.

As can be seen in the curves, the absolute amount of recovery current for the Qspeed diode is lower than that of the other silicon rectifiers. This causes less current stress on the switching power MOSFETs, which in turn reduces the amount of power they dissipate. This lowers the temperature of the heatsink and other nearby components.

High-frequency EMI is another common side effect of high-frequency power processing. Boost rectifier recovery current is typically a major source of both conducted and radiated EMI. Even with an adequate conducted emissions filter, the radiated portion can make it very difficult to meet regulatory agency requirements. EMI chamber testing has demonstrated that Qspeed’s Q-series rectifiers reduce conducted and radiated EMI on the level of SiC rectifiers as well as about 10 dB/µV more than other silicon rectifiers.

A further benefit of low QRR is that designers can increase the power density of their PFC circuits. Q-series rectifiers can operate at much higher frequencies than the typical 65 kHz that’s used in most PFC circuits. Testing showed efficient operation at up to 225 kHz and 1000 A/µs di/dt.

By doubling the PFC frequency from 65 kHz to 120 kHz, the designer can often shrink down the PFC choke about twofold. If PFC switching frequency increases to 180 kHz, the PFC choke shrinks accordingly. In particular, the ability to operate at higher switching frequencies makes these diodes attractive in power supplies above 500 W.

Qspeed employs proprietary device design techniques and processes to produce its low-QRR silicon diodes. Among the company’s most recent products are the LQA16T300 and LQA20T300C. The LQA16T300 is rated at 300 V/16 A and typically exhibits about 44 nC of stored charge at a junction temperature of 125°C. The LQA20T300C is a dual, common-cathode diode rated at 300 V/20 A (10 A per diode). Its typical stored charge (at 125°C) is only 38 nC.

The diodes are available in throughhole TO-220AC and TO-220AB packages, respectively.

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