Making The Most Of Discretes

Thomas Bluhm looks at improved technologies for bipolar transistors and Schottky diodes.

Advances in design and improved packaging has made it possible to develop optimised discrete semiconductor devices with low saturation voltage and Schottky rectifiers with very low forward voltage that meet heat generation, efficiency, space, and cost requirements.

The collector power dissipation PC = VCEsat x IC is a major contributor to losses in bipolar transistors. Since the collector current IC is predefined by the application, the device manufacturer has only the option to reduce the losses in the transistor by reducing the collector-emitter saturation voltage VCEsat. With low VCEsat transistors, this is essentially achieved by using mesh-emitter technology.

With the mesh-emitter design, the emitter series resistance is reduced by spreading the emitter region over a much larger area, and by contacting it from the base as a "mesh." This results in an evenly driven base, providing a more efficient use of the active emitter area on the die and thus a significantly lower collector-emitter saturation voltage (Fig. 1).

Enlarging the die area within the limits further reduces the occurring losses. The development of new leadframes and the use of 6-pin packages (e.g. SOT457) also allow a better heat dissipation (Fig. 2).

BISS transistors in SOT23 package are comparable to medium power transistors at lower cost. Since the overall costs of a transistor are largely influenced by the costs of its package, a transistor in a SOT23 costs less than a transistor in a SOT223.

With conventional transistor designs, the die size that is often required for the necessary collector current limits further miniaturization. For example, transistors with collector currents > 0.5A are not feasible in a SOT23 package using the traditional design. If, on the other hand, mesh-emitter technology is used, transistors in this package can already provide collector currents of more than 2A. Therefore, a mesh-emitter transistor (SOT23) can replace a much larger transistor in a SOT223 package at comparable or sometimes even better characteristics.

Table 1 compares the BISS transistors PBSS4350T and PBSS4320T with a medium-power transistor BDP31.

Figure 3 shows a detailed curve of the collector-emitter saturation voltage of the three transistors. The saturation voltage of the mesh-emitter transistors at a collector current of 1A is about 40 to 50% lower than with a conventional transistor, although it is driven only with 50mA (IC / IB = 20) instead of 100 mA (IC / IB = 10). A SOT23 transistor requires less than 20% board space than a SOT223 type.

The advantages of higher efficiency, reduced temperature increase, and available output voltage are demonstrated in the example of a simple load switch (low side switch). The supply voltage VCC is 3.3 V, and the load current is VLoad = IC = 2A.

The conventional medium power transistor BDP31 in a SOT223 and the mesh-emitter transistor PBSS4320T in a SOT23 are compared. The transistors are characterized by the values given in Table 1.

The temperature increase T is calculated from the total power dissipation Ptot and the thermal resistance Rth:

T is calculated as 128K for the transistor BDP31 and as 109K for the BISS transistor PBSS4320T, if they are mounted on a 1 cm2 collector pad.

It is important for a number of applications that the available output voltage VLoad closely match the supply voltage. VLoad is calculated as the difference between the supply voltage VCC and the collector-emitter saturation voltage VCEsat at only 2.6V for the BDP31, compared to about 3.1V for the mesh-emitter transistor PBSS4320T.

The efficiency n results from the ratio of the load power PLoad and the supply voltage PSupply:

While only a circuit efficiency of 79% can be achieved using the standard transistor BDP31, this increases to 94% for the mesh-emitter transistor PBSS4320T.

To summarise, the efficiency can be significantly improved, the available load's voltage is much closer to the supply rail, and the temperature rise is lower if a SOT23 BISS transistor is used instead of a standard medium power transistor in the much larger SOT223.

With diodes, the forward power dissipation PF = IF x VF is a major contributor to the overall loss. Since the diode current (IF) is predetermined by the application, the diode manufacturer can only reduce the power dissipation by reducing the forward voltage drop (VF). For Schottky rectifiers, the forward voltage VF depends on the barrier level of the metal used and of the active area.

Reducing the forward voltage VF by enlarging the active area conflicts with the requirement of miniaturisation and increases the circuit losses due to the increased diode capacitance (CD).

It should also be considered that the reverse current (IR) will increase when the forward voltage decreases. For the development of its so-called MEGA (Maximum Efficiency General Application) Schottky rectifiers, Philips therefore chose the barrier so that either the forward voltage is minimised or the reverse current is minimised at a still low forward-voltage level.

Furthermore, the die size area could be reduced to mount these rectifiers in advanced, very small low-signal packages (e.g. SOD323F).

To further reduce the forward voltage, the thickness of the silicon die was reduced, and the ratio of die area and leadframe area was optimised.

Today, Schottky rectifiers in large packages such as SMA, SMB, and SOD123 still dominate the market for currents between 0.5A and 3A. However, these are unreasonably bulky for applications such as point-of-load DC/DC converters. Now, the MEGA technology enables the development of rectifiers in smaller packages (SOD323F) with forward currents of 0.5 A to 2 A.

Table 2 compares the key characteristics of the widely used diodes SS12 or SS14, respectively, to the new MEGA Schottky rectifiers PMEG1020EJ and PMEG2010EJ. The smaller SOD323F rectifiers either have a forward voltage similar to that of the SMA diode (PMEG2010EJ) or is significantly reduced (PMEG1020EJ).

Though a comparison of the reverse currents is only possible to a limited degree because these are published for different reverse voltages, it still provides an impression of the order of magnitude. The characteristics in Figure 4 show the typical forward behaviour. The forward voltage VF of the PMEG2010EJ rectifier is similar to those of the SS12 and SS14 rectifiers. If an application requires an even lower forward-voltage drop, the MEGA Schottky rectifier PMEG1020EJ provides it, but at the expense of a higher reverse current. This makes it attractive for applications with a relatively high duty cycle.

Some applications, such as battery chargers, require diodes with a reverse current that is as low as possible in order to prevent the battery from discharging via the charger when not connected to power. At the same time, losses in charging mode should be as low as possible.

During the development of the PMEG6010AED rectifier, care was therefore taken to minimise the reverse current at a low forward voltage. Although this diode is mounted in the smaller SOT457 package, the forward voltage and the reverse current are equivalent to those of the SS12 and SS14 diodes in the SMA package (Table 2).

Figure 5 shows a view of the SOD323F mounting area, which is only 20% of the SMA package area.

The simple circuit example of a reverse-voltage protection diode (blocking diode) in a battery-powered device compares a MEGA Schottky rectifier (PMEG1020EJ and PMEG- 2010EJ) to a standard Schottky diode of the SS12 or SS14 type with respect to temperature increase, voltage drop, and efficiency. It is assumed that the battery voltage in this example is 3V, and the device's current consumption is 1A.

The internal temperature rise DT is calculated by multiplying the forward power dissipation PF by the thermal resistance Rth j-s. The resulting values are about 17K for the SS12, 19K for the PMEG1020EJ and 30K for the PMEG2010EJ.

Although the PMEG1020EJ diode is mounted in a much smaller package, its temperature increase is about the same compared to that of the much larger SMA diode, thanks to the lower forward voltage. The relatively high temperature increase of the PMEG2010EJ is caused by the higher forward voltage combined with the higher thermal resistance of the much smaller package, but is still admissible.

To use as much of the full battery voltage as possible, the voltage drop across the reverse-polarity protection diode should be minimised. When the PMEG1020EJ rectifier is used, 2.65V are available to supply the circuits, while about 2.5V are available when using the SS12 or PMEG2010 rectifiers. The efficiency, which is calculated from the ratio of the operating voltage VB and the battery voltage VBatt, is 88 % for the PMEG1020EJ diode, versus about 82% for the SS12 and PMEG2010EJ rectifiers.

The MEGA Schottky rectifier diode meets or even exceeds the characteristics of much larger rectifiers. Its lower losses create a more efficient device, and the smaller package generates a more space-efficient design.

Product cost and required board space could significantly be reduced for bipolar transistors and Schottky rectifiers by applying innovative technologies. Furthermore, a host of applications can derives several benefits from an improved efficiency by opting for low VCEsat (BISS) transistors in addition to low VF (MEGA) Schottky rectifiers.

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