Standard solutions and devices are compared to a 60 V MOSFET with monolithic Schottky diode as evaluated in SMPS and motor control environments.
BY FILIPPO SCRIMIZZI and FILADELFO FUSILLO, STMicroelectronics, Stradale Primosole 50, Catania, Italy
On synchronous rectification and in bridge configuration, RDSon and Qg are not the only requirements for power MOSFETs. In fact, the dynamic behavior of intrinsic body-drain diode also plays an important role in the overall MOSFET performances. The forward voltage drop (VF,diode) of a body-drain diode impacts the device losses during freewheeling periods (when the device is in off-state and the current flows from source to drain through the intrinsic diode); the reverse recovery charge (Qrr) affects not only the device losses during the reverse recovery process but also the switching behavior, as the voltage spike across the MOSFET increases with Qrr. So, low VFD and Qrr diodes, like Schottky, can improve overall device performance, especially when mounted in bridge topologies or used as synchronous rectifiers—especially at high switching frequency and for long diode conduction times. In this article, we compare standard solutions and devices to a 60 V MOSFET with monolithic Schottky diode as evaluated in SMPS and motor control environments.
Intrinsic MOSFET body-drain diode and Schottky features
In FIGURE 1, the typical symbol for an N-channel Power MOSFET is depicted. The intrinsic body-drain diode is formed by the p-body and n–drift regions and is shown in parallel to the MOSFET channel.
Once a Power MOSFET is selected, the integral body diode is fixed by silicon characteristic and device design. As the intrinsic body diode is paralleled to the device channel, it is important to analyze its static and dynamic behavior, especially in applications where the body diode conducts. So, maximum blocking voltage and forward current have to be considered in reverse and forward bias, while, when the diode turns-off after conducting, it is important to investigate the reverse recovery process (FIGURE 2). When the diode goes from forward to reverse bias, the current doesn’t reduce to zero immediately, as the charge stored during on-state has to be removed. So, at t = t0, the diode commutation process starts, and the current reduces with a constant and slope (-a), fixed only by the external inductances and the supply voltage. The diode is forward biased until t1, while from t1 to t2, the voltage drop across the diode increases, reaching the supply voltage with the maximum reverse current at t=t2. The time interval (t3-t0) is defined as reverse recovery time (trr) while the area between negative current and zero line is the reverse recovery charge (Qrr).The current slope during tB is linked mainly to device design and silicon characteristics.
The classification of soft and snap recovery is based on the softness factor: this parameter can be important in many applications. The higher the softness factor, the softer the recovery. In fact, if tB region is very short, the effect of quick current change with the circuit intrinsic inductances can produce undesired voltage overshoot and ringing. This voltage spike could exceed the device breakdown voltage: moreover, EMI performances worsen. As shown in Fig. 2, during diode recovery, high currents and reverse voltage can produce instantaneous power dissipation, reducing the system efficiency. Moreover, in bridge topologies, the maximum reverse recovery current of a Low Side device adds to the High Side current, increasing its power dissipation up to maximum ratings. In switching applications, like bridge topologies, buck converters, or synchronous rectification, body diodes are used as freewheeling elements. In these cases, reverse recovery charge (Qrr) reduction can help maximize system efficiency and limit possible voltage spike and switching noise at turn-off. One strategy to reach this target to integrate a Schottky diode in the MOSFET structure. A Schottky diode is realized by an electrical contact between a thin film of metal and a semiconductor region. As the current is mainly due to majority carriers, Schottky diode has lower stored charge, and consequently, it can be switched from forward to reverse bias faster than a silicon device. An additional advantage is its lower forward voltage drop (≈0.3 V) than Si diodes, meaning that a Schottky diode has lower losses during the on state.
Embedding the Schottky diode in a 60V power MOSFET is the right device choice when Qrr and VF,diode have to be optimized to enhance the overall system performance. In FIGURE 3, the main electrical parameters of standard and integrated Schottky devices (same BVDSS and die size) are reported.
Benefits of Mono Schottky in a power management environment
In a synchronous buck converter (FIGURE 4), a power MOSFET with integrated Schottky diode can be mounted as a Low Side device (S2) to enhance the overall converter performance.
In fact, Low Side body diode conduction losses (Pdiode,cond) and reverse recovery losses (PQrr) are strictly related to the diode forward voltage drop (VF,diode) and its reverse recovery charge (Qrr):
As shown in (1) and (2), these losses increase with the switching frequency, the converter input voltage, and the output current. Moreover, the dead time, when both FETs are off and the current flows in the Low Side body diode, seriously affects the diode conduction losses: with long dead times, a low diode forward voltage drop helps to minimize its conduction losses, therefore increasing the efficiency. In FIGURE 5, the efficiency in a 60W, 48V – 12V, 250 kHz synchronous buck converter is depicted.
Now, considering isolated power converters’ environment, when the output power increases and the dead time values are high, the right secondary side synchronous rectifier should have not only RDSon as low as possible to reduce conduction losses, but also optimized body diode behavior (in terms of Qrr and VF,diode) in order to reduce diode losses (as reported in (1) and (2)) and to minimize possible voltage spikes during turn-off transient. The 60V standard MOSFET and one with Schottky integrated devices are compared in a 500W digital power supply, formed by two power stages: power factor corrector and an LLC with synchronous rectification. The maximum output current is 42 A, while the switching frequency at full load is 80 kHz, and the dead time is 1μs. The efficiency curves are compared in FIGURE 6.
In both topologies, the 60 V plus Schottky device shows higher efficiency in the entire current range, an improvement in overall system performance.
Switching behavior improvement in bridge topologies
In bridge topologies, reverse recovery process occurs at the end of the freewheeling period of the Low Side device (Q2 in FIGURE 7), before the High Side (Q1 in Fig. 7) starts conducting. The resulting recovery current adds to the High Side current (as previously explained). Together with the extra-current on the High Side device, the Low Side reverse recovery and its commutation from Vds ≈ 0 V to Vdc can produce spurious bouncing on the Low Side gate- source voltage, due to induced charging of Low Side Ciss (input capacitance) via Crss (Miller capacitance).
As a consequence, the induced voltage on Q2 gate could turn-on the device, worsening system robustness and efficiency. A Low Side device, in bridge configuration, should have soft commutation, without dangerous voltage spikes and high frequency ringing across drain and source. This switching behavior can be achieved using power MOSFETs with integrated Schottky diode as Low Side devices. In fact, the lower reverse recovery charge (Qrr) has a direct impact on the overshoot value. In fact, the higher the Qrr, the higher the overshoot. Lower values for Vds overshoot and ringing reduce the spurious voltage bouncing on the Low Side gate, limiting the potential risk for a shoot-through event. Furthermore, soft recovery enhances overall EMI performances, as the switching noise is reduced. In FIGURE 8 are shown the High Side turn-on waveforms for standard and embedded Schottky devices; purple trace (left graph) and green trace (right graph) are Low Side gate-source voltages. The device with Schottky diode shows a strong reduction of Low Side spurious bouncing.
In many applications (synchronous rectification for indus- trial and telecom SMPS, DC-AC inverter, motor drives), choosing the right MOSFET means not only considering RDSon and Qg but also evaluating the static and dynamic behavior of the intrinsic body-drain diode. A 60V “F7” power MOSFET with integrated Schottky diode ensures optimized performances in efficiency and commutation when a soft reverse recovery with low Qrr is required. Furthermore, the low VF,diode value achieves higher efficiency when long freewheeling periods or dead-times are present in the application.
1. “Fundamental of Power Semiconductor Devices”, B.J.Baliga – 2008, Springer Science