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Posted on 26 September 2019

PowerTrench® MOSFET with Shielded Gate Technology

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Device meets major requirements for synchronous rectification

Higher system efficiency and power density in modern data and telecommunication power systems are core focus since making a small and high efficiency power system means saving space and energy bills in the places.

By Won-suk Choi, Dong-wook Kim and Dong-kook Son, Fairchild Korea Semiconductor, HV PCIA PSS Team Bucheon-si Republic of Korea, Application Engineering

In topology point of view, synchronous rectification that converts the AC voltage from the transformer back to DC, becomes essential building block for the secondary-side of switch-mode power supply in many applications because it offers improved efficiency for these conversion stages with both lower conduction loss and switching losses. In device point of view, the power MOSFET transistor has made a significant evolution with last decade and it has enabled new topologies and high power density in power supply. Major requirements are for synchronous rectification MOSFET on the following:

  • Low RDS(ON)
  • Low QG
  • Low QRR and COSS
  • Less snappy body diode characteristics
  • Low Qgd/Qgs ratio

Fairchild designed a new highly optimized power MOSFET, called PowerTrench® MOSFET with shielded gate technology, for synchronous rectification with deep analysis of power losses in synchronous rectification of server power supplies or telecom rectifiers.

Power Losses Analysis in Synchronous Rectification Conduction loss

The conduction loss of diode rectifier contributes significantly to the overall power loss in a power supply. Power losses in synchronous rectification can be lowered when the product of MOSFET's on resistance and drain current is less than the diode forward voltage drop. Therefore secondary side synchronous rectification is excellent solution to improve system efficiency. Conduction loss can be obtained through below equation 1.

Conduction loss

RDS(ON) can be achieved 1~2mohm in TO-220 standard package depend on voltage rating by using modern medium voltage MOSFETs technology. For high voltage MOSFETs, the resistance of packages has not been a concern. Different from high voltage MOSFET, the package itself contributes a significant portion of the total resistance for medium voltage MOSFETs due to wire bonding, lead and source metal. Total on-resistance of medium voltage MOSFET can be dramatically reduced by using SMD package such as Power56. It can also reduce package inductance that causes undesirable voltage spikes. It enable to use lower RDS(ON) MOSFETs by replace to lower voltage rating MOSFETs.

Gate Driving Loss

Driving losses at gate driver is related to gate charge, QG. In low voltage applications, the driving losses could take up quite big portion of total power losses as low voltage switches have very low conduction losses compared to high voltage switches. During light load conditions, conduction losses are minimal, and the driving losses are even more important. It is well known that the driving losses can be obtained through below equation 2.

driving losses

In synchronous rectification, current flows through MOSFET channel from source to drain during conduction time and flows through body diode during dead time. Since the MOSFET does soft switching at switch turn-on and turn-off transients, dVds/dt is zero. There is no plateau region on gate-source voltage of power MOSFFET for synchronous rectification. Therefore, a resulting gate charge in SR, QSYNC, becomes approximately a value that gate-drain portion of gate charge, QGD subtracted from total gate charge, QG. As shown in Table 1, QSYNC of FDP033N08B is reduced by 28% and 34% compared to FDP032N08 and 75V/3.3mΩ competitor. Fig. 1 shows a calculated loss ratio between driving loss and conduction loss in a 12V synchronous rectification stage with gate driving voltage of 10V and switching frequency is 100kHz. There are two synchronous switches, the gate driving losses of Competitor is over three times higher than the conduction losses at 10% load condition. This graph simply indicates that FDP033N08B can dramatically reduce driving loss at light load condition due to small QSYNC.

Critical Specification Comparison of DUTs

Comparisons of loss ratio [driving loss / conduction loss] according to output load

Body Diode Losses

During the dead time, body diode conduction occurs. Body diode conduction leads to substantial power loss because of high voltage drop across the P-N junction compared to the voltage drop caused by MOSFET channel. This MOSFET loss due to body diode conduction during dead time has a degrading effect on overall efficiency, especially at low voltages and high frequencies. Body diode conduction loss can be calculated equation 3.

Body diode conduction loss

At MOSFET turn-off transient, the Qrr has to be removed and Coss has to be charged up to the transformer voltage of secondary side. The reverse recovery charge, Qrr also produces power losses in the system while turning off the switch. Power losses by the body diode characteristics can be obtained through equation 4.

power losses by the body diode characteristics

The stored charge in output capacitance, Qoss also affects to the capacitive losses. This portion of loss is proportional to the switching frequency and VDS. Therefore, power losses by Coss can be obtained through equation 5.

Power losses

Voltage Spikes effect

For a practical alternative, snubbers could be used to manage the voltage spikes within the maximum drain-source voltage ratings. Additional power losses are inevitable in this case. In addition, the power losses due to the snubbers are not negligible at light load. Besides the circuit board parameters, device characteristics also affect to the voltage spike level. In synchronous rectification, a major device-related parameter is a softness of the body diode during reverse recovery. Basically, reverse recovery characteristics of diodes are determined by device design. Figure 2 shows measured peak drain-source voltage levels due to body diode characteristics comparing Fairchild’s FDP033N08B, FDP032N08 and 75V/3.3mOhm competitor at ID=50A, VDD=40V and di/dt of 400A/μs. The peak drainsource voltage of FDP033N08B during body diode reverse recovery is lower to 10.1V and 12.8V than that of FDP32N08 and 75V/3.3mOhm competitor thanks to its soft body diode characteristics.

Voltage Spikes Comparisons under Reverse Recovery behavior

The parasitic inductance can strongly influence MOSFET switching characteristics, usually causing increased switching losses and deviations from the expected performance. Parasitic inductance arising from both component packaging and circuit layout is a reality of any circuit. The length of the lead takes quite a bit of the source inductance of the package. Industry standard through-hole type TO-220 package has 7nH of typical lead inductance, but typical lead inductance of PQFN56 SMD package is only 1nH. Other important parasitic components are layout parasitic inductance and capacitance. In circuit board layout, 1cm of trace pitch has an inductance of 6-10nH. These parasitic inductances directly affect to body diode reverse recovery characteristics and peak voltage spikes. The body diode recovery charge on datasheet is sum of COSS displacement current, the recovered minority carrier current, and the reactive currents arising from common source inductance of test circuit. Figure 3 shows the simulation waveforms of body diode reverse recovery according to various common source inductances. It is clear that higher inductance could cause larger Qrr and higher peak voltage. The peak voltage is reduced from 59.2V to 55.6V by using 1nH source inductance of Power56 SMD package. Minimizing common source inductance is therefore critical to system efficiency.

Body diode reverse recovery waveforms comparisons according to source inductance

Medium Voltage MOSFETs Technologies

Several new technologies have been developed to improve the RDS(ON) × QG FOM. The trench gate structure can dramatically reduce the channel resistance (Rchannel) and JFET resistance (RJFET) that are the major contributors to on-resistance of medium voltage MOSFETs (BVDSS < 200V). With compelling advantage of the trench structure in the ability to reduce RDS(ON) by providing the shortest possible current path (vertical) from drain to source, it is possible to increase cell density without any JFET pinch-off effect. Figure 4 shows the Fairchild PowerTrench® MOSFET family. The conventional trench gate structure of Figure 4(a) enables lower on-resistance by increasing the channel width to length ratio. The another concept that was originally developed for high-voltage devices, but now being used for low-voltage devices as well, is the use of charge balance or Super-Junction device structures. With the use of charge balance approach, one can obtain two dimensional charge coupling in the drift region. The latest middle voltage power MOSFETs at Fairchild employ this shielded-gate structure, where the shield electrode is connected to the source as shown in Figure 4(b). The shield electrode, along with the thicker oxide between electrode and drift region,

Vertical Structure of Fairchild PowerTrench® MOSFET Family

provides charge balance for drift region. This enables the use of higher doping in the drift region, resulting in reduced drift resistance. The specific resistance of new medium voltage power MOSFET has been significantly improved compared to the previous generation while improving on the already superior switching characteristic. Apart from RDS(ON) and QG, other parameter, such as like the body diode reverse characteristics, internal gate resistance and the output charge of the MOSFET (QOSS) are now becoming more relevant in synchronous rectification. The importance of these loss components rises at higher switching frequencies and higher output currents. Fairchild new medium voltage MOSFETs are now being optimized to improve the diode characteristics as well as the output capacitance. The latest PowerTrench® MOSFETs, FDP033N08B, employ shieldedgate structure that provides charge balance.

Efficiency comparisons in 600W synchronous rectification

Conclusion

As light load efficiency is getting important, the gate driving losses and the snubber losses become serious factors. Consequently, low QSYNC and soft body diode are critical characteristics for better synchronous rectification efficiencies. However, the RDS(on) is still key parameter of the application. Figure 5 shows the system efficiency comparison for the three devices from the Table 1 in 600W phaseshifted full bridge converter with synchronous rectification. The total system efficiency with FDP033N08B, the latest shielded-gate trench MOSFET is 95.36% at light load condition and 95.34% at full load condition. It is 0.1% and 0.19% higher than that of FDP032N08, conventional trench gate MOSFET and 75V/3.3mOhm best competitor due to lower driving loss and turn-off switching loss at 10% load condition. From the efficiency results as shown in Fig. 5, it is clearly seen that FDP033N08B, the latest shielded-gate trench MOSFET shows significant loss reduction under both full load and light load conditions because of its optimum design. The Fairchild latest shielded gate trench power MOSFET, which combines a smaller QSYNC and soft reverse recovery intrinsic body diode performance with fast switching could substantially improve the efficiency of synchronous rectification.

E-mail: wonsuk.choi@fairchildsemi.com

 

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