What Vishay’s New Power Stage Teaches Us About Reverse RecoveryJuly 20, 2020 by Steve Arar
Modern power devices rely on small on-state resistance and fast reverse recovery. What's behind the power stages driving these two key characteristics?
The efficiency and power density of today’s power systems hinge on two significant features of modern power devices: small on-state resistance and fast reverse-recovery.
Vishay Intertechnology recently released a compact MOSFET half-bridge power stage suitable for applications that require a high power density. In this article, we’ll look at some of the important features of this device, the SiZF300DT. We’ll also briefly discuss the reverse recovery phenomenon and see how a power stage such as the SiZF300DT improves its performance in this regard.
Synchronous Buck Converter
First, let’s briefly examine the reverse recovery phenomenon and see how a power stage such as the SiZF300DT addresses this issue. Consider the synchronous buck converter shown below, which is one of the main use cases of the SiZF300DT:
Conduction losses of a MOSFET. Image used courtesy of Texas Instruments
In this circuit, the two MOSFETs should not turn on at the same time to avoid a short from input to ground. When the high-side MOSFET (Q1) turns on, current flows through the inductor and the load capacitor. The on-time of Q1 determines the voltage that appears at the output. If Q1 is on 10% of the switching time, the output voltage will be about 10% of the input voltage.
When Q1 turns off, Q2 should turn on to provide a path for the inductor current which, according to basic physics laws, cannot change instantaneously. The ideal gate voltage of Q1, Q2, and the output current are shown below:
Synchronous buck converter waveforms. Image used courtesy of ON Semiconductor
Freewheeling Through the MOSFET Body Diode
In practice, Q2 should turn on some time after Q1 turns off to ensure that the two transistors are not on simultaneously. For this small percentage of the switching time that both Q1 and Q2 are off, the freewheel path can be provided by the intrinsic body-drain diode of Q2 that is explicitly shown in the above buck converter schematic.
Reverse Recovery of a Diode
At the end of the freewheeling period of Q2 and just after Q1 turns on, the body-drain diode needs to go from forward bias to reverse bias condition. When Q1 turns on and the diode is reverse biased, we expect the diode to exhibit a high-impedance to ground.
However, in the process of going from forward bias to reverse bias, a diode can conduct current in the reverse direction for a short period of time. This is referred to as the diode reverse recovery and is illustrated in the following figure.
Depiction of diode reverse recovery. Image used courtesy of Ichiro Omura et. al
Since the diode is capable of conducting current in the reverse direction when Q1 is on, there will be a short from input to ground at the end of the freewheeling period. The longer the diode reverse recovery time, the more the power loss we’ll have.
Addressing the Issue With an Integrated Schottky Diode
Unlike a body diode, a Schottky diode does not exhibit a reverse recovery time. That’s why adding a Schottky diode in parallel with the low-side MOSFET can reduce the power losses associated with the reverse recovery current significantly.
Vishay uses the term SkyFET to refer to a MOSFET with an integrated Schottky diode. The company claims that its SkyFET MOSFETs can reduce the power loss associated with reverse recovery by about 40% as compared to a traditional trench MOSFET.
According to ST Microelectronics, an added Schottky diode can also improve EMI performance and reduce the risk of having a shoot-through event. Besides, a Schottky diode has a lower forward voltage drop compared to a body diode. This can reduce the diode conduction loss (IV loss) that is associated with the freewheeling period.
Vishay's New MOSFET Half-Bridge Power Stage
The new device is a 30 V n-channel MOSFET half-bridge power stage. As depicted below, it includes a high side TrenchFET, a low side SkyFET MOSFET as well as an integrated Schottky diode.
The SiZF300DT. Image used courtesy of Vishay
The high-side MOSFET of the SiZF300DT exhibits a maximum on-resistance of 4.5 mΩ at 10 V. The low-side MOSFET features an on-state resistance of 1.84 mΩ at 10 V.
The device is offered in a compact 3.3 mm x 3.3 mm package—which, according to Vishay, is 65% smaller than other products with similar on-resistance. This makes the SiZF300DT suitable for synchronous buck converters employed in space-constrained applications such as graphic and accelerator cards, computers, servers, and telecom and networking equipment.
Vishay also claims the new device can deliver up to 11% higher output current per phase as compared to other products with similar footprint area. Typical gate charge for the high-side and low-side MOSFETs of the device are 6.9 nC and 19.4 nC, respectively. As discussed in a company app note, the gate charge values can be used to estimate the switching behavior of a power MOSFET.
The SiZF300DT incorporates a large PGND pad that enhances the thermal transfer and simplifies the PCB layout.
Placing a Schottky diode in parallel with a MOSFET can enhance a power system's efficiency by two different mechanisms. And, unlike a body diode, a Schottky diode does not exhibit a reverse recovery time. Besides, a Schottky diode has a lower forward voltage drop and exhibits a lower conduction loss during the freewheeling period.
Featured image used courtesy of Vishay.