How to use diodes to reduce parasitic capacitance

The diode plays an important role in the rectifier switch due to its unidirectional conductivity. In the reverse breakdown state, it has a voltage stabilizing effect within a certain current range. Surprisingly, the reverse bias junction capacitance of the diode can effectively reduce the parasitic capacitance of the signal line. This application will be further discussed here.

Last time we shared the knowledge about "How to make good use of the conduction voltage drop of a diode". After that, some users asked to know more about electronic devices. Here we will talk about "How to make good use of diodes to reduce parasitic capacitance".

Diode Parameters—Unidirectional Conductivity

    When it comes to diodes, what everyone is most familiar with is the unidirectional conductivity of the diode, which is reflected in the volt-ampere curve as shown in Figure 1. When the forward bias voltage U=0.5V (silicon tube), the diode begins to conduct, the larger the current, the larger the voltage, and has very low impedance; when the reverse bias voltage is applied, the diode does not conduct, there is a small leakage current within a certain range, and has a large impedance. Its unidirectional conductivity also plays the role of a switch, so it has a wide range of applications in rectification and switching.

Figure 1 Diode volt-ampere characteristic curve
    The diode has a parameter that is not as well known as unidirectional conductivity, but it is also crucial to the impact on circuit design, that is, "junction capacitance".

Diode Parameters—Junction Capacitance

In some high-speed occasions, it is necessary to select a diode with a relatively small junction capacitance; in some occasions, it is necessary to use this junction capacitance to achieve a specific purpose, such as the voltage-controlled oscillator (VCO), which uses the variable capacitance diode to have different capacitance values ​​under different reverse bias voltages, thereby achieving the purpose of voltage control frequency.

Figure 2 Voltage-controlled oscillator application circuit - Example
    In high-speed circuits, as the frequency is getting higher and higher, the influence of parasitic capacitance can no longer be ignored. In the system, these unexpected capacitances come from various aspects, such as the material, thickness, board layer structure, and parallelism of the PCB, which are all parasitic capacitances that affect the PCB board, as well as the parasitic capacitance of the components themselves. The most hateful thing is that these things are also affected by the ambient temperature.
 

Figure 3 Is there no way to deal with the "ringing" caused by parasitic capacitance
    ? Through the unremitting efforts of engineers, it is found that these effects can be reduced through reasonable circuit design. Next, we will discuss how to "use the capacitance characteristics of diodes to reduce parasitic capacitance on high-speed signals."

Diode Magical Use - Reducing Parasitic Capacitance

First, let's get familiar with the capacitance characteristics of the diode: Figure 4 shows the capacitance characteristics of the IN4448HWS diode. Under zero reverse bias, the capacitance is 3pF, and as the reverse bias increases, the junction capacitance decreases.
 

Figure 4 Capacitor characteristics
    On high-speed signal lines, some functions are usually added, which usually bring adverse effects, such as generating large parasitic capacitance, which depends on the specific circuit module. If this capacitance is ignored, it may affect the frequency of the signal. The most unfortunate thing is that even if you notice this capacitance, it seems that you can't do anything because the capacitance generated by the additional functional module is too large. The general attachment function access method is shown in Figure 5:

 
Figure 5 General accessory function access method
    In order to reduce the parasitic capacitance on the signal line, a diode can be added at the access point of the accessory function. This diode must have a relatively small capacitance, and a small signal switch tube is usually selected. If the high current problem is considered, the selection problem needs to be carefully considered.

Figure 6 Forward access method

The forward access method is shown in Figure 6. The diode is connected between the signal line and the additional functional module, which means that the additional functional module is output at a high level when it is enabled. In addition, in order to reduce the parasitic capacitance to a greater extent, the diode is usually operated in a reverse bias state, that is, UL is connected to a low level. When the additional functional module is not working, the diode is at the maximum reverse bias, has a smaller junction capacitance, and the signal line can work at a high frequency, and the system obtains higher performance.

Figure 7 Reverse access method

The reverse access method is shown in Figure 7. Unlike the forward access, the positive pole of the diode is connected to the signal line and UH is connected to a high level.
    Regardless of the forward or reverse access method, the equivalent circuit is shown in Figure 8. We assume that the junction capacitance of the diode is 3pF and the total parasitic capacitance of the accessory functional module is 1uF. If the resistance is large enough, it can be ignored. At this time, the two capacitors are connected in series, similar to the parallel connection of resistors, CT=C1*C2/(C1+C2)≈C1 (C2 is larger). Even if the large capacitor changes greatly, the total capacitance in series is almost equal to the small capacitor, that is, 3pF, which effectively reduces the access capacitance.

Figure 8 Equivalent circuit

The above applications are based on the unidirectional conductivity of the diode and the small junction capacitance. Forward access and reverse access can only be unidirectional, which cannot solve all situations, that is, it can only be used for special functional modules. If the additional functional module needs to be bidirectional, combining Figure 6 and Figure 7 may be a good choice