Schottky Diode Questions - RC Racing Talk

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29 June

Hello, we wish you a wonderful day. In this essay, we first pose the following query: what determines the diode's maximum operating frequency? To address this question, we need to differentiate between junction capacitance and reverse recovery time, as they are two distinct concepts. The charging and discharging times of the junction capacitance do not coincide with the reverse recovery time. Why is that? Let’s examine these facts.

1. Simple Basics

1. Junction Capacitance

The diode exhibits parasitic capacitance, primarily represented as the junction capacitance in a simple diode model.

Figure 1

2. Reverse Recovery Time

When the voltage abruptly reverses, the diode current in practical applications does not instantly drop to zero. Instead, a relatively substantial reverse current lingers, which must be reduced to 0.1 times its maximum value. This duration is known as the reverse recovery time.

Figure 2

2. Facts

Why is the reverse recovery time never equal to the charge and discharge times of the junction capacitance?

The junction capacitance and reverse recovery time (Trr) characteristics for diodes are typically provided by the manufacturer. Now, let's compare the characteristics of four distinct diode types: Schottky diodes, ultra-fast recovery diodes, rapid recovery diodes, and standard diodes.

To enhance the credibility of our results, we ensure that the four manufacturers of these diodes possess the same withstand voltage, packaging, and maximum working current. All have a maximum reverse withstand voltage of 100V, the same SMA package, and a maximum operating current of 1A. The selected manufacturer is DIODE Semiconductor.

The models are as follows:

Schottky diode: B-13-F

Ultra-fast recovery diode: US1B-13-F

Fast recovery diode: RS1B-13-F

Ordinary diode: S1B-13-F

Here are screenshots of these diode parameters:

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Figure 3

The gathered parameters are as follows:

Figure 4

As demonstrated, the Schottky diode possesses the highest junction capacitance among the given types. However, the Schottky diodes do not operate at the highest frequency, so why?

Although the reverse recovery time for Schottky diodes isn't detailed in the standard documentation, we know it is the shortest.

In strict terms, the operation of the Schottky diode differs from that of a PN junction diode in that a reverse recovery time does not exist. To summarize, due to parasitic capacitance, a maximum operating frequency is established.

We note that various types of diodes rank from highest to lowest operating frequency as follows:

Figure 5

We now understand that the Schottky junction capacitance, measuring 80 pF, is the highest. The reverse recovery times of the remaining three diodes, with capacitances ranging from 10 pF to 20 pF, vary significantly.

We can also estimate the time required for the junction capacitance to fully charge, assuming that the reverse recovery time acts as the charging duration of the junction capacitance.

For instance, take the fast recovery diode RS1B-13F, which has a junction capacitance of 15 pF. Its reverse recovery current is depicted in the figure below (obtained from the specification). Since the typical reverse current is around 0.5 A, we can easily conclude that it will take about 1.5 ns to charge a 15 pF from 0V to -50V, which is significantly less time than the actual reverse recovery time of 150 ns.

Figure 6

Therefore, it is clear that the duration of the reverse recovery time isn't determined by the PN junction capacitance.

3. Answers to Questions

Returning to the original question, what factor determines the diode's maximum operating frequency?

In essence, the answer is straightforward: Firstly, a high operating frequency cannot be achieved if the junction capacitance is excessively large. Since the capacitor's impedance decreases with frequency, the signal bypasses the capacitor, and the diode's reverse cut-off capability is lost.

Secondly, if the reverse recovery time is overly lengthy, a high operating frequency is unattainable. This is because voltage transitions occur more rapidly at elevated frequencies. If the voltage changes again following the application of reverse bias, the reverse current does not return, thus impeding the diode's function as a reverse cut-off.

Consequently, the maximum operating frequency of a diode is contingent upon both junction capacitance and reverse recovery time. The degree of influence varies based on the diode type.

For Schottky diodes, the operating frequency is mainly dictated by the junction capacitance since the reverse recovery time is relatively swift.

In contrast, PN junction diodes see their maximum operating frequency determined by the reverse recovery time, which has a much greater impact than the junction capacitance, typically measured in tens of pF.

Ultimately, we recognize that Schottky diodes enable operation at higher frequencies and boast the fastest switching speed when compared to PN junction diodes.

In conclusion, the reverse recovery time does not equate to the charging and discharging time of the junction capacitance.

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