1. What is a diode?
A diode is a semiconductor element with a PN junction that is unidirectionally conductive.
2. Volt-ampere characteristics of the diode
The voltage at both ends of the diode and the current passing through it are called the volt-ampere characteristics of the diode, and its volt-ampere characteristics are consistent with the volt-ampere characteristics of the PN junction.
2.1 The positive part of the volt-ampere characteristic
When the forward voltage applied to the diode is small, the resistance of the diode is large because the applied voltage is not enough to overcome the blocking effect of the electric field in the PN junction on the carrier movement, so the forward current is almost zero. The voltage corresponding to this part is called the dead zone voltage (also known as threshold voltage or threshold voltage), and the size of the dead zone voltage is related to factors such as diode material and temperature.
When the forward voltage is greater than the dead zone voltage, the diode is turned on. After turn-on, with the increase of forward voltage, the forward current increases sharply, and the relationship between voltage and current is basically an exponential curve.
The figure below shows the forward portion of the simulated diode volt-ampere characteristics. A forward voltage (VT1-VT2) is applied across the diode, and when the voltage is less than 0.7V, the current flowing through the diode is ID=0A. After the voltage is greater than 0.7V, the diode current starts to increase. The forward voltage across the diode after turn-on is called the forward voltage drop. As can be seen from the diagram, this voltage is stable around 700 mV and hardly varies with the amount of current flowing through the diode.
The diode has a negative temperature coefficient, and as the temperature increases, the positive voltage drop of the diode decreases. As shown in the figure below, under the same forward current IF, the higher the ambient temperature around the diode, the smaller the forward voltage drop VF
The figure below shows the results of a temperature simulation of the forward voltage and forward current of the diode. It can be seen that as the temperature increases, the forward voltage of the diode becomes smaller and smaller, and the forward current becomes larger and larger.
2.21 The reverse part of the volt-ampere characteristic
When the diode is reverse-voltage-added, the electric field in the PN junction is strengthened, and only a few carriers pass through the PN junction under the action of the reverse-voltage, forming a very small reverse current. When the reverse voltage increases but does not exceed a certain value, the reverse current is small and basically unchanged, and the reverse current here is usually called the reverse saturation current, and this area in the characteristic curve is called the reverse cut-off zone. The reverse current is formed by a small number of carriers, which increases with increasing temperature, and in practice, the smaller the value, the better. The figure below is a diagram in the spec of a Schottky diode. As the temperature around the diode increases, the reverse current IR increases.
In the figure below, when a reverse voltage is applied to the diode, the reverse leakage current increases as the temperature increases.
When the reverse voltage increases to more than a certain value (the corresponding voltage in the characteristic curve diagram is called the reverse breakdown voltage, and the reverse breakdown voltage of different diodes is different), the reverse current increases sharply, and the diode loses its unidirectional conductivity, which is called reverse breakdown (belonging to electrical breakdown). After the reverse breakdown, the current is very large and the voltage is very high, so the power consumed on the diode is very large, and it is easy to make the PN junction heat up and exceed its dissipated power, resulting in thermal breakdown. Under the premise that the product of the reverse current and the reverse voltage does not exceed the allowable dissipated power of the PN junction, the two electrical breakdown processes are reversible, and the diode can restore its unidirectional conductivity after the reverse voltage is reduced. Otherwise, it will burn out due to overheating, and the breakdown process will be irreversible, and the diode will fail.
3. Diode parameters
- 导通压降VF(Forward voltage)
VF is the voltage drop between the two ends of the diode when the diode is forward conducted, and the greater the current through the diode, the greater the VF; When the diode temperature is higher, the VF is smaller.
- 额定电流IF(Forward continuous current)
Refers to the average current value of the diode converted according to the allowable temperature rise during long-term operation.
- 最大反向峰值电压VRM （Peak reverse voltage）
Maximum Reverse Peak Voltage VRM refers to the maximum reverse voltage that can be added to avoid breakdown. The highest VRM value for Schottky is currently 150V.
- 最大直流反向电压VR（Reverse voltage）
VR is the value when the DC voltage is continuously applied. For DC circuits, the maximum DC reverse voltage is important to determine the allowable and upper limits. The VRM is a peak voltage that is added repeatedly.
- 反向饱和漏电流IR (Reverse leakage)
IR refers to the current flowing through the diode when the reverse voltage is added at both ends of the diode, and the reverse leakage current of the Schottky diode is larger, and the selection of Schottky diode is to choose the diode with smaller IR as much as possible.
- 最大浪涌电流IFSM (Peak forward surge current)
Excess forward current that is allowed to flow through. It is not a normal current, but an instantaneous current, which is quite large. This value drops when the temperature increases. As the power-on period increases, the diode surge capability decreases.
- Reverse recovery time trr
When the operating voltage changes from forward to reverse, the diode operates in an ideal situation where the current energy is instantaneously cut-off. In fact, there is usually a slight delay. The amount that determines the current cut-off delay is the reverse recovery time. Although it directly affects the switching speed of the diode, it does not necessarily mean that this value is small. That is, when the diode is suddenly reversed from on, the time required for the reverse current to decay from a large amount to close to the IR. This indicator is very important when the high-power switch is working in the high-frequency switching state.
- Maximum power dissipation Ptot
When an electric current flows through a diode, it absorbs heat and increases its temperature. In practice, the external heat dissipation also has a great impact on P. Specifically, the voltage applied to the diode is multiplied by the current flowing through it plus the reverse recovery loss.