Using fast recovery MOSFETs for synchronous rectification

Energy efficiency regulations and voluntary standards are driving the adoption of resonant converters and increasing the design-in base of synchronously rectified output stages. Both applications benefit from the use of fast recovery MOSFETs, used here to describe a MOSFET having a fast recovery body diode. At first glance, the benefit of using such MOSFETs is restricted to applications where the body diode is hard switched, such as in hard switching inverter topologies. However, the use of MOSFETs with normal recovery diodes without special compromise results in high failure rates in resonant converter applications. Further, the combination of excellent body diode characteristics and high dv/dt characteristics of fast recovery MOSFETs make them very suitable for synchronous rectification applications. This article reviews why fast recovery MOSFETs are beneficial, and indeed sometimes necessary in such applications. RESONANT CONVERTER SWITCHING

Figure 1 shows the MOSFET and body diode conduction in a zero voltage switching half-bridge resonant converter. In such converters, the current waveform lags the voltage. Assume that Q1 is on and that Q2 is off. Q1 conducts the resonant current. After a certain time, determined by the control loop, Q1 is turned off. The resonant current now flows through the body diode of Q2. At some time during the negative current period, Q2 is turned on, reducing the conduction losses. Q2 is turned on while the voltage across it is the forward voltage of the body diode, which is much less than the bus voltage. This is effectively zero voltage switching. The direction of the resonant current in Q2 reverses. After a certain time, again determined by the control loop, Q2 is turned off. As a high current is flowing, and as the voltage swing on Q2 is approximately equal the bus voltage, the turn off sequence is not low loss. (Reference 1 gives a detailed introduction to resonant converters for more information.)

There is no apparent forced turn off of the body diodes in this application. The body diode of Q2 is switched on as soon as Q1 is switched off. When the current direction in Q2 is reversed, the body diode of Q2 is apparently switched off naturally, resulting in no reverse recovery losses. Based on this analysis, it would make no sense to use a fast recovery MOSFET for either Q1 or Q2. First, they are more expensive based on the additional processing steps required to produce them. Second, the RDS(ON) is generally higher. For example the FQPF5N50CF 500V fast recovery MOSFET has a room temperature maximum RDS(ON) of 1.55Ù vs. 1.4Ù for the FQPF5N50C standard version. The exception is the 600V SuperFET family, which shows no or little worsening of the R_DS(ON) between the fast and normal recovery versions.

Problems with the reliability of phase-shift full-bridge converters were investigated and reported in the late 1990s. This investigation was started because of unexplainably high failure rates for the power supplies using these topologies. MOSFETs with normal recovery diodes were used in the application. A similar analysis applies to standard zero voltage switching resonant converters. This shows that the body diode characteristics are indeed important.

Returning to the previous explanation of the resonant converter switching cycle, it was simply assumed that all of the current flowing through Q2 during the negative current phase will flow through the channel. In practice, the body diode will conduct a portion of this current. When the current direction reverses, the diode undergoes a normal reverse recovery process, which can take some time. Under certain unfavourable conditions, the body diode will still have a small charge when high voltage is applied across the MOSFET. At heavy loads, there is a high initial body charge which is rapidly removed in the channel. At light loads, the initial body charge is lower, but is takes much longer to clear. In some cases, this small charge is sufficient to induce destructive secondary breakdown in the MOSFET, caused by a parasitic bipolar transistor in the MOSFET structure. As fast recovery MOSFETs have a much lower stored charge, and clear this lower charge much faster than a MOSFET with a normal body diode, the extent of this potential problem is far less.

One method to remove the effect of the body diodes is to isolate the MOSFET body diode with so-called steering diodes. This adds extra component cost and space, and will increase conduction losses, working somewhat against the motivation to use resonant converters in the first place. This method is used in resonant converter applications today where sufficiently fast recovery MOSFETs are not yet available. Another method is to use an anti-saturation clamp between the gate and the drain of the MOSFET. This increases losses at lighter loads, but reduces the stored charge in the body diode. The clamp adds extra cost and space, and is quite difficult to dimension over temperature. For systems with low dv/dt, a third method is to switch on the MOSFET before the body diode conducts. This is possible with phase-shift full-bridge architectures but less useful for half-bridge and full-bridge resonant topologies due to the risk of shoot through.

The best solution is to use fast recovery MOSFETs. The 600V SuperFET technology shows no or little worsening of RDS(ON) between the normal and the fast recovery versions. Fast recovery versions are available in a range of sizes and packages from 11A to 47A. These devices have the best-in-class RDS(ON) for fast recovery MOSFETs. As a result, using these devices in resonant converters helps to improve the overall system efficiency while maintaining robustness.

BODY DIODE DV/DT RATING

Synchronous rectification applications benefit greatly from the use of MOSFETs with fast recovery body diodes. In most applications, the diode undergoes forced commutation, resulting in losses in both the diode and the switch which is commutating the diode. So fast recovery MOSFETs offer an advantage over normal MOSFETs for such applications, as the diode reverse recovery characteristics are better.

One more subtle advantage is the improvement of the body diode dv/dt rating of fast recovery MOSFETs over normal body diode MOSFETs. In standard MOSFET applications – for example, driving a clamped inductive load – the current will build up first, then the drain to source voltage will drop when the device is turned on, and the reverse procedure will happen when the device is turned off. So the MOSFET is only subjected to dv/dt, or changes in voltage, when the channel is conducting. In synchronous rectification applications and in some resonant ones, the MOSFET is subjected to high dv/dt when the channel is off. For synchronous rectification applications in particular, it is important to ensure that the diode dv/dt rating is not exceeded by the driving circuits, as this could induce destructive secondary breakdown.

Fast recovery MOSFETs generally have better diode dv/dt ratings than their slower counterparts. For example the FQP44N10F 100V FRFET fast recovery MOSFET has a rating of 15V/ns compared with 6V/ns for the FQP44N10 standard planar MOSFET.

For 600V resonant applications, the improvement is more notable. The FCB20N60F FRFET SuperFET has a diode dv/dt rating of 50V/ns, whereas the standard FCB20N60 has a rating of 4.5V/ns. Both devices have the same room temperature RDS(ON) rating of 190mÙ.

References

• R. W. Erickson and D. Maksimovic, “Fundamentals of Power Electronics,” Second Edition, Springer, 2001, Chapter 19, ISBN 0-7923-7270-0.

• L. Saro, K. Dierberger and R. Redl, “High-Voltage MOSFET Behavior in Soft-Switching Converters: Analysis and Reliability Improvements,” Proc. INTELEC 1998, pp. 30-40,


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Figure 1