 |
Qspeed Semi's 600V Rectifiers have been specifically designed to provide the lowest possible reverse recovery current (Qrr) and the softest recovery of any silicon diode. This reduction in Qrr offers many benefits to circuit designers:
- Lower MOSFET temperatures
- Significantly lower EMI
- Higher energy conversion efficiency
- Elimination of snubber components
- Higher frequency, higher-density operation
Click here for Qspeed Rectifier Product Listing |
Reverse Recovery Explained
In a perfect diode, current would flow forward from anode to cathode and then at a time decided by other components in the circuit, the diode would stop conducting and then go into a blocking state where it could support the full rated voltage. In the idealized diode there would be no reverse current flowing from cathode to anode.
That was the ideal diode -- which of course does not yet exist.
In a real diode, say any typical ultrafast diode, there is a large component of charge that must flow out of of the anode before the diode can go into its blocking state. This is known as reverse current or Qrr.
In the example below, the traditional 600V diode sees 8 Amps of forward current, and then when it attempts to commutate to a the blocking state we see up to 6 Amps of reverse current shooting back out the anode.
Circuit designers have spent years building "work-arounds" to this problem. Their goal was to prevent this diode reverse current from flowing into other parts of the circuit.
In boost power factor control (PFC) circuits, this charge Qrr will flow into the Power MOSFETs, causing their junction temperature to rise significantly. This can typically reduce the overall efficiency of the PFC circuit by .5 to 2%.
By replacing traditional ultrafast diodes with a Qspeed PFC rectifiers, the designer can boost overall efficiency of the power supply, eliminate snubber circuitry, reduce MOSFET temperatures, and in some cases use lower-cost or fewer MOSFETs.
The Benefits of Soft Recovery
Qspeed rectifiers are intentionally designed to have a soft recovery. Other diodes have what is known as a snappy or abrupt recovery.
Qspeed's first launch into the high-voltage offline world, our new
Q-Series rectifiers have a lower reverse recovery current and a softer, more benign recovery characteristic than any other silicon-based rectifier in the world.
The unique recovery characteristic of Qspeed's rectifiers result in several benefits. Our customers have thus far confirmed that Qspeed products help them to:
- Boost energy conversion efficiency
- Consume less electrical energy
- Run their system cooler and generate less heat
- Generate less electromagnetic interference (EMI)
- Use fewer components
- Save space
- Reduce BOM costs
- Improve system reliability
How can a single component make so many improvements to power supply circuits that have been designed for decades?
Looking at the traces below you can see that when the current waveform reaches its peak reverse value it starts to decay up to the zero line. Qspeed diodes follow a smooth, tapered path where as snappy diodes are abrupt and cause ringing and voltage overshoot.
These effects become more pronounced as you increase PFC frequency beyond 65kHz and have to switch the MOSFETs faster.
The resulting voltage spikes of snappy diodes can run up to 800V and may destroy the MOSFET.
Noise from snappy diodes has also been found to trip the PFC controller (causing instability and lower efficiency).
Notice in the traces above that the absolute amount of recovery current is lower than that of other Silicon rectifiers as well. This results in less current stress on the switching power MOSFETs, which in turn reduces the amount of power being dissipated, which in turn lowers the temperature of the heatsink and of the other components in close proximity to it.
Improving EMI with Q-Series Rectifiers
High-frequency EMI, another common problem in PFC circuits, is emitted by the recovering boost rectifier. Even with an input filter, this internally-generated EMI is radiated externally. EMI chamber testing has conclusively demonstrated that Qspeed's Q-series rectifiers reduce externally radiated EMI by 5dB relative to Silicon Carbide rectifiers, and by substantially more than 10dB versus other Silicon rectifiers.
We believe the softer recovery is ultimately more reliable, more efficient, and eliminates many of the expensive, mitigative measures that designers must take when using high-voltage rectifiers.
Improving Power Density with Q-Series Rectifiers
An further benefit of having such low Qrr is that designers can now increase the power density of their PFC circuits. Q-Series rectifiers were designed to operate at much higher frequencies than the typical 65kHz used in PFC today. Q-Series Rectifiers have been tested successfully up to 225kHz and di/dt of 1000A/us.
By doubling the PFC frequency from 65kHz to ~120kHz the designer should be able to reduce the size of the PFC choke by nearly 2x.
Taking that a step further, the designer could operate PFC at 180 kHz, and the PFC choke should shrink in size by a factor of 3X over the 65kHz design.
This ability to operate well at higher frequencies makes Qspeed's Q-Series rectifiers a real advantage for power supplies > 500 W. |
