Low I R Schottky Barrier Diode Series

                                                                                            Mitsuhiro Kakefu
                                                                                            Masaki Ichinose




1. Introduction                                            Fig.1 Cross-sectional structure of SBD chip

     Representative of the recent trends towards small-                 Guard ring       Schottky electrode
                                                                                           (barrier metal)
er size and higher functionality of portable devices and                                                      SiO2
towards higher speed CPUs for computers, electronic
devices are rapidly becoming smaller in size, lighter in
weight and are achieving higher performance, and it is                               Epitaxial layer
essential that their circuit boards and switching power
supplies be made to consume less power, are more
efficient, generate less noise and support higher densi-                              Si substrate
ty packaging. Moreover, in order to suppress the surge
voltage that is applied across a diode during switching
and the noise generated by a steep dv/dt characteristic,
snubber circuits, beads and the like are used, but as a
result the number of components increases, leading to      becomes large, and as a result reverse loss increases,
greater cost. In order to achieve better portability, AC   efficiency decreases and thermal runaway may occur,
adapters for notebook computers are being miniatur-        making it difficult to use this low VF SBD in a small
ized; however, the trend toward higher power con-          power supply packages such as an AC adapter.
sumption results in higher internal temperatures,               The newly developed low IR-SBD is considered to
increasing the severity of the environment in which        be the ideal diode for secondary source rectification in
these semiconductor devices are used. Consequently,        a switching power supply, and is especially well suited
semiconductor devices are strongly required to provide     for rectification in a high temperature environment.
the characteristics of lower loss, improved suppression    Figure 1 shows the cross-sectional structure of the SBD
of thermal runaway, higher maximum operating tem-          chip. The chip design incorporates a guard ring to
perature and lower noise. In particular, an improve-       prevent premature breakdown, and the doping density,
ment in the characteristics of the secondary source        specific resistance and thickness of the epitaxial layer
output rectifying diode, which accounts for nearly 50 %    (n- layer), diffusion depth, and barrier metal that have
of the loss in a switching power supply, is strongly       been optimized to develop a low IR-SBD series that
desired.                                                   provides not only low IR, but also breakdown voltages
                                                           of 40, 60 and 100 V, comparable to the conventional VF.
2. Overview                                                Compared to a conventional SBD having the same
                                                           breakdown voltage, this product achieves an approxi-
    Schottky barrier diodes (SBDs) exhibit the proper-     mate single-digit decrease in IR, a large decrease in
ties of low forward voltage (VF), soft recovery and low    reverse loss, a higher temperature at which thermal
noise, and are widely used in the secondary source         runaway occurs, and a higher maximum operating
rectifying circuits of switching power supplies. Fuji      temperature. Moreover, this new series has a high
Electric has previously developed a product line of        avalanche breakdown voltage and is expected to be
conventional 20 to 100 V SBDs (low VF type) and 120        capable of withstanding the large surge voltage that
to 250 V SBDs [low reverse current (IR) type] as a         occurs when a power supply is turned on. The new
diode series available in a variety of packages and        series is also expected to enable the design of switching
supporting various output voltages and current capaci-     power supply circuits that realize increased efficiency,
ties in order to be applicable to a wide range of power    smaller size and greater flexibility. Table 1 lists the
supply applications. However, when the conventional        absolute maximum ratings and electrical characteris-
low VF type SBD operates at high temperatures, its IR      tics of this low IR-SBD series and Fig. 2 shows external


Low IR Schottky Barrier Diode Series                                                                                 57

Table 1 Absolute maximum ratings and electrical characteristics of low I R SBD Absolute maximum ratings Electrical characteristics Model number Package VFM (V) IRRM VRRM VRSM IO IFSM PRM IF = 0.5 × IO (µA) Rth(j-c) (V) (V) (A) (A) (W) (Tj = 25°C) VR = VRRM (°C / W) YG862C04R TO-220F 45 45 10 125 330 0.61 150 3.50 YA862C04R TO-220 45 45 10 125 330 0.61 150 2.00 TS862C04R T-Pack 45 45 10 125 330 0.61 150 2.00 YG862C06R TO-220F 60 60 10 125 330 0.68 150 3.50 YA862C06R TO-220 60 60 10 125 330 0.68 150 2.00 TS862C06R T-Pack 60 60 10 125 330 0.68 150 2.00 YG862C10R TO-220F 100 100 10 125 330 0.86 150 3.50 YA862C10R TO-220 100 100 10 125 330 0.86 150 2.00 TS862C10R T-Pack 100 100 10 125 330 0.86 150 2.00 YG865C04R TO-220F 45 45 20 145 330 0.63 175 2.50 YA865C04R TO-220 45 45 20 145 330 0.63 175 1.75 TS865C04R T-Pack 45 45 20 145 330 0.63 175 1.75 YG865C06R TO-220F 60 60 20 145 660 0.74 175 2.50 YA865C06R TO-220 60 60 20 145 660 0.74 175 1.75 TS865C06R T-Pack 60 60 20 145 660 0.74 175 1.75 YG865C10R TO-220F 100 100 20 145 660 0.86 175 2.50 YA865C10R TO-220 100 100 20 145 660 0.86 175 1.75 TS865C10R T-Pack 100 100 20 145 660 0.86 175 1.75 YG868C04R TO-220F 45 45 30 160 1,000 0.63 200 2.00 YA868C04R TO-220 45 45 30 160 1,000 0.63 200 1.25 TS868C04R T-Pack 45 45 30 160 1,000 0.63 200 1.25 YG868C06R TO-220F 60 60 30 160 750 0.74 200 2.00 YA868C06R TO-220 60 60 30 160 750 0.74 200 1.25 TS868C06R T-Pack 60 60 30 160 750 0.74 200 1.25 YG868C10R TO-220F 100 100 30 160 750 0.86 200 2.00 YA868C10R TO-220 100 100 30 160 750 0.86 200 1.25 TS868C10R T-Pack 100 100 30 160 750 0.86 200 1.25 Fig.2 External view of the packages 10 4.5 10 4.5 10 4.5 2.7 1.3 1.32 See view from arrow direction P 9.5 1.5 TS868C 15 15 3 YG868C YA868C 10 13.5 13 Arrow direction P 9.5 Model number : YG868C□□R Model number : YA868C□□R Model number : TS868C□□R views of the packages. The current ratings are 10 A, Pack (S) surface mount type. 20 A and 30 A and the product packages are available The newly developed low IR-SBD is described as the TO-220, the TO-220F full-mold type, and the T- below. 58 Vol. 51 No. 2 FUJI ELECTRIC REVIEW

Fig.3 Comparison of forward characteristics 10 10 10 Forward current IF (A) Forward current IF (A) Forward current IF (A) 1 1 1 0.1 0.1 0.1 YG805C04R 100°C YG805C06R 100°C YG805C10R 100°C YG805C04R 25°C YG805C06R 25°C YG805C10R 25°C YG865C04R 100°C YG865C06R 100°C YG865C10R 100°C YG865C04R 25°C YG865C06R 25°C YG865C10R 25°C 0.01 0.01 0.01 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 0.2 0.4 0.6 0.8 1.0 1.2 Forward voltage VF (V) Forward voltage VF (V) Forward voltage VF (V) Fig.4 Comparison of reverse characteristics YG805C04R Tj = 100°C YG805C06R Tj = 100°C YG805C10R Tj = 100°C 103 103 103 Reverse current IR (µA) Reverse current IR (µA) Reverse current IR (µA) YG865C04R YG865C06R 102 YG865C10R 102 102 Tj = 25°C Tj = 25°C YG805C10R Tj = 25°C 101 YG805C04R 101 YG805C06R 101 100 100 100 YG865C04R YG865C06R YG865C10R –1 10 10 20 30 40 10 20 30 40 50 60 0 10 20 30 40 50 60 70 80 90 100 110 Reverse voltage VR (V) Reverse voltage VR (V) Reverse voltage VR (V) when IR is reduced. The newly developed 40 to 100 V 3. Device Characteristics SBD achieves a dramatic decrease in loss at high temperatures through the use of a new barrier metal Figure 3 compares the forward characteristics of as described in chapter 2 and optimized crystal specifi- the low IR-SBD with those of conventional products, cations in order to achieve an approximate 10 % and Fig. 4 compares their reverse characteristics. The increase in VF at rated current compared to a conven- SBD loss is the sum of the forward and reverse losses, tional product, and an IR that is reduced to approxi- and it is desirable that this loss be reduced within the mately 1/10th that of the conventional product. actual operating temperature range. In particular, the reverse loss caused by increased IR at higher tempera- 4. Consideration of the Generated Loss tures must be considered. A tradeoff relation exists between VF and IR, however, and VF typically increases A simulation was performed to calculate the loss Low IR Schottky Barrier Diode Series 59

Fig.5 Junction temperature vs. estimated loss (60 V/20 A) Table 2 Ambient temperature when beginning thermal runaway at LCD-TV 24 V output power supply Condition : installation cooling fin (30°C /W) 8 YG805C06R Forward side 23-inch LCD-TV 30-inch LCD-TV 7 YG805C06R Flyback side power supply power supply Model number Estimated loss WO (W) YG865C06R Forward side (+24 Vout /3.5 A) (+24 Vout /5.0 A) 6 YG865C06R Flyback side 5 Forward Flyback Forward Flyback New device: YG865C06R 4 Conventional device : Approx. Approx. Approx. Approx. Conventional device: YG805C06R YG805C06R 74°C 84°C 72°C 77°C 3 New device : Approx. Approx. Approx. Approx. 2 YG865C06R 98°C 108°C 97°C 100°C 1 0 20 40 60 80 100 120 140 160 180 increases, and IR becomes more noticeable as it Junction temperature Tj (°C) increases at higher temperatures. As a result, a vicious cycle ensues in which the increase in IR leads to an increase in loss, which generates heat in the Fig.6 Thermal runaway data (TS868C04R, TS808C04R) element, leading to an increase in IR, etc. In some cases, this phenomenon ultimately leads to thermal damage (thermal runaway) of the element. Figure 6 45 shows thermal runaway data of the ambient tempera- 40 1 ture vs. reverse voltage for a 40 V/30 A product. For DC 35 2 the sake of comparison, a conventional SBD is also Reverse voltage VR (V) New device: TS868C04R 30 DC shown. Compared to the conventional product, it can Conventional device: be seen that the allowable operating temperature 25 TS808C04R range has been expanded due to the lower IR. Table 2 20 shows the estimate thermal runaway temperatures for 15 a 60 V/ 20 A product in 24 V output power supplies 10 (Vdc = 380 V, I = 3.5 A or 5 A) for 23-inch and 30-inch 5 LCD TVs, which approximate actually installation 0 conditions. Compared to the conventional product, the 0 50 100 150 200 thermal runaway ambient temperature is estimated to Ambient temperature Ta (°C) be 32 % higher (98°C) at the forward side and 28 % higher at the flyback side (108°C) in the case of the 23- inch LCD, and 34 % higher (97°C) at the forward side generated in the case of a 24 V power supply and 29 % higher at the flyback side (100°C) in the case (Vdc = 380 V, I = 5 A) for a liquid crystal display (LCD) of the 30-inch LCD. With a high maximum allowable TV. Figure 5 shows the relationship between junction operating temperature, these new devices are well temperature (Tj) and estimated loss (Wo) for a 60 V/ suited for high temperature applications. 20 A product. For the sake of comparison, a conven- tional SBD is also shown. In the region of low Tj, the 6. Conclusion conventional product has less loss, but because IR has a large effect on loss at high temperatures, the low IR An overview of the low IR-SBD and its application device achieves less loss than the conventional device to secondary source rectification applications in switch- at high temperatures, and at Tj = 150°C, the low IR ing power supplies have been presented. product achieves approximately 76 % less loss than the In response to the anticipated future requests for conventional product, and its application to higher power supplies that are smaller in size, generate less efficiency power supplies is anticipated. loss and have higher efficiency, Fuji Electric intends to further improve SBD characteristics and to develop a 5. Consideration of the Thermal Runaway Tem- product line of small package products. Fuji will perature continue to make additional improvements in order to develop high quality products and enrich this product The temperature of an element rises as its loss series. 60 Vol. 51 No. 2 FUJI ELECTRIC REVIEW