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.