TOPICS DISCUSSED:

1. NEW 150V POWER SCHOTTKY FAMILY

2. IS THE LOWEST FORWARD VOLTAGE DROP OF REAL SHOTTKY DIODES ALWAYS THE BEST CHOICE?

3.WHY DO WE NEED ESD PROTECTION?

NEW 150V POWER SCHOTTKY FAMILY

As a result of computer, telecom and consumer applications, the Switch Mode Power Supply (SMPS) is becoming more widespread nowadays. The constant increase in services and performance, which offers these applications, tends to move conversion systems towards higher output power. In addition to these developments dictated by the market, SMPS manufacturers are in competition, their battlefield being the criteria of power density, efficiency, reliability and cost, with this last factor being very critical.

Today, SMPS designers of 12-24V output have practically the choice between a 100V Schottky or a 200V bipolar diode. The availability of an intermediate voltage has therefore become necessary to gain in design optimization.

This is why a new family of 150V POWER SCHOTTKY diodes, intended for 12V and more secondary rectification, in applications such as desktops, file servers or adaptors for notebook PCs.

When we have the choice between 150V Schottky and 200V PN Diode, the 150V Schottky is the best choice for the safety of the component and the environment, the limitation of parasitic effects and for the efficiency of the converter. In addition to the low Vf, the 150V Schottky has a better4 switching behavior, due to its essentially capacitive recovery (less sensibility to the temperature). We have the advantage of a soft recovery diode in terms of EMC and the Schottky is preferable to a fast recovery diode in terms of losses. The 150V Schottky diode is the better choice versus the 200V bipolar as for EMC and losses at turn-off are concerned. Experimental measurements confirm this.

With the arrival of the EN6100-3-2 standard and the introduction of the PFC, whatever the input voltage is, there will be a continuous voltage on the primary. This will lead to a reduction of the transformation ratio, and at the same time, the reverse voltage of the diode. Consequently, a lower breakdown voltage diode will be needed in the future to replace a 200V PN diode used today. Also, the tendency is for the output power of adaptors to increase. This involves an increase in the output voltages. The voltage requirements of the diode in this case will be higher than 100V and a 150V diode is likely to be the appropriate component.

IS THE LOWEST FORWARD VOLTAGE DROP OF REAL SCHOTTKY DIODES ALWAYS THE BEST CHOICE?

According to the thermionic emission model, pure Schottky barriers exhibit a forward voltage drop, which decreases linearly as the barrier height diminishes; whereas the reverse current increases exponentially as the barrier height decreases. Consequently, there exists an optimum barrier height, which can minimize the sum of forward and reverse power dissipation for a particular application. However, discussions with the users of Schottky diodes reveal that they do not search for the minimum of forward and reverse power dissipation but always for the minimum forward voltage drop. Values of reverse current are very rarely asked for. Once must know how the Schottky diode is being applied in order to objectively select the most appropriate part.

LOW VOLTAGE LEVEL APPLICATIONS
In high power applications with low circuit voltages and using Schottky diodes with blocking voltages less than 25V, the forward power losses of the diodes still dominate in the balance of power losses. The prevailing applications are switched mode power supplies (SMPS). Here it is argued that a 4 mV decrease of the forward voltage drop results in a reduction of the forward power losses of approximately 1%. Therefore, the components created for this application have low barrier heights (less than 0.74 eV) and highly doped, thin epitaxial drift layers. This results in a device with a low forward voltage drop and high but still acceptable reverse current.

MEDIUM AND HIGH VOLTAGE LEVEL APPLICATIONS
On the other hand, reverse power losses in high power applications using medium or high voltage Schottky types are comparable or can be even higher than the forward power losses. Nevertheless, most users don’t ask for low reverse currents but again for only a low forward voltage drop.

DIODE WITH AN IDEAL DYNAMIC BEHAVIOR
In addition to forward and the reverse power losses, there is apparently a third quality, but one that is difficult to quantify. Nevertheless it has an impact on forward voltage drop as shown by experience. This quality is exhibited by the dynamic properties and switching loss of the real Schottky diode. Due to their short appearance in the range of only a few nanoseconds, they can only be measured with costly test equipment and moreover, the slight differences in their dependencies cannot be made evident.

With regard to the dynamic properties, the Schottky diodes is generally considered as an ideal diode with a junction capacitance connected in parallel. Concerning the switching characteristics, the ideal diode is a pure majority-carrier component (only electronics in the n-region). After the zero crossing of the current from the forward state to the reverse state, the ideal diode fully ignores the previous conductive state and blocks reverse voltage immediately after the current crosses zero.

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