1. Schottky diode Schottky diode, also known as Schottky barrier diode (SBD for short), is a low-power, ultra-high-speed semiconductor device. The most notable feature is that the reverse recovery time is extremely short (can be as small as a few nanoseconds), and the forward voltage drop is only about 0.4V. It is mostly used as high-frequency, low-voltage, high-current rectifier diodes, freewheeling diodes, and protection diodes. It is also useful as rectifier diodes and small-signal detector diodes in microwave communication circuits. It is more common in communication power supplies, frequency converters, etc.
Schottky diode parameter description:
(1) VF forward voltage drop Forward Voltage Drop.
(2) Maximum Forward Voltage Drop of VFM.
(3) VBR reverse breakdown voltage Breakdown Voltage.
(4) The reverse RMS input voltage that VRMS can withstand.
(5) VRRM peak repeated reverse voltage Peak Repetitive Reverse Voltage.
(6) VRSM Non Repetitive Peak Reverse Voltage (halfwave, single phase, 60 Hz) Non-repetitive peak reverse voltage (half wave, single phase, 60Hz) (7) VRWM Working Peak Reverse Voltage
.
(8) VDC Maximum DC Blocking Voltage.
(9) Trr Reverse Recovery Time
(10) IF(AV) Forward Current
(11) IFSM Maximum Forward Surge Current
(12) IR Reverse Current Reverse Current
(13) TA Ambient Temperature or Free Air Temperature Ambient Temperature
(14) TJ Operating Junction Temperature
(15) TSTG Storage Temperature Storage Temperature Range
(16) TC Case Temperature
Disadvantages of Schottky
The biggest disadvantage of Schottky diodes is their low reverse bias voltage and large reverse leakage current. For example, Schottky diodes made of silicon and metal have a rated reverse bias voltage. The maximum withstand voltage is only 50V, and the reverse leakage current value has a positive temperature characteristic, which tends to increase rapidly as the temperature rises. In practical design, it is necessary to pay attention to the hidden danger of thermal runaway. In order to avoid the above-mentioned problems, the reverse bias voltage of Schottky diodes in actual use will be much smaller than its rated value. However, Schottky diode technology has also improved, and its reverse bias voltage rating can reach a maximum of 200V. That is to say, Schottky can reach up to 200V.
2. The main parameters of semiconductor diodes
(1). The reverse saturation leakage current IR
refers to the current flowing through the diode when the reverse voltage is applied to both ends of the diode. The current is related to the semiconductor material and temperature. At room temperature, the IR of the silicon tube is at the nanoampere (10-9A) level, and the IR of the germanium tube is at the microampere (10-6A) level.
(2). Rated rectification current IF
Rated rectification current refers to the average current value converted according to the allowable temperature rise when the diode is in long-term operation. At present, the IF value of high-power rectifier diodes can reach 1000A. The forward current of the diode is actually divided into the rated rectification current IF, the repeatable peak current IFRM, and the non-repeatable peak current IFSM. IFSM is the maximum surge current that the diode can withstand.
(3).Maximum average rectified current IO
In the half-wave rectifier circuit, the maximum value of the average rectified current flowing through the load resistor refers to the rectified DC current, which is a very important value in design. It is usually smaller than IF in the specification.
(4). The maximum surge current
is the excessive forward current that is allowed to flow. It is not a normal current, but an instantaneous current. This value is quite large. (5). Even if the
maximum reverse peak voltage VRM has no reverse current, as long as the reverse voltage is continuously increased, the diode will be damaged sooner or later. The reverse voltage that can be added is not an instantaneous voltage, but repeated.
Forward and reverse voltage. Since the rectifier is supplied with AC voltage, its maximum value is an important factor specified. The maximum reverse peak voltage VRM refers to the maximum reverse voltage that can be added to avoid breakdown. At present the highest VRM value can reach several thousand volts.
(6). Maximum reverse DC voltage VR
The above-mentioned maximum reverse peak voltage is the peak voltage that is repeatedly added. VR is the value when DC voltage is continuously applied. It is used in DC circuits. The maximum DC reverse voltage is for determining the allowable value and The limit value is very important.
(7). The maximum operating frequency fM
Due to the junction capacitance of the PN junction, when the operating frequency exceeds a certain value, its unidirectional conductivity will deteriorate. The fM value of the point contact diode is higher, above 100MHz; the fM of the rectifier diode is lower, generally not higher than a few kilohertz. Excessive junction capacitance in high-frequency lines results in very low capacitive reactance, which may bypass the diode.
The size of the junction capacitance and the reverse recovery time jointly affect the maximum operating frequency of the diode.
(8). Reverse recovery time Trr (what is the relationship between Trr and fM?)
When the working voltage changes from forward voltage to reverse voltage, the ideal situation of diode operation is that the current can be cut off instantaneously. In fact, it generally takes a little time to delay, and this time can be measured by the reverse recovery time Trr, that is, when the diode suddenly reverses from conduction, the reverse current decays from a large amount to close to IR (reverse saturation leakage current ) time required. Although it directly affects the switching speed of the diode, it does not necessarily mean that this value is small. If the Trr of the high-power switching tube works in the high-frequency switching state, the switching speed will be affected, and the tube may be overheated and burned.
The key factor affecting the reverse recovery time is the number of minority carriers when the PN junction is made. When the number of minority carriers is less, the reverse recovery time is shorter.
(9). The maximum power P
The maximum power P is the voltage applied to both ends of the diode multiplied by the current flowing. When the product of the voltage applied across the diode and the current flowing through it is greater than P, the tube will burn out due to overheating. This limit parameter is particularly important for Zener diodes and variable resistance diodes.
3. Selection of diodes
1. Select according to main parameters
(1) Rated forward operating current
The rated forward operating current refers to the maximum forward current value that is allowed to pass through the diode when it works continuously for a long time.
(2) Maximum surge current
The maximum surge current is the excessive forward current that is allowed to flow. It is not a normal current, but an instantaneous current. Its value is usually about 20 times the rated forward operating current.
(3) Maximum reverse working voltage
When the reverse working voltage applied to both ends of the diode reaches a certain value, the tube will break down and lose its unidirectional conductivity. In order to ensure the safety of use, the maximum reverse working electric value is stipulated. For example, the reverse withstand voltage of lN4001 diode is 50V, and the reverse withstand voltage of lN4007 is 1000V.
(4) Reverse current
The reverse current refers to the reverse current flowing through the diode under the action of the specified temperature and the highest reverse voltage. The smaller the reverse current, the better the unidirectional conductivity of the tube.
The reverse current is closely related to the temperature, about every 10°C increase in temperature, the reverse current doubles.
Silicon diodes have better stability at high temperatures than germanium diodes.
(5) Reverse recovery time
When changing from forward voltage to reverse voltage, the current cannot be cut off instantaneously, and it needs to be delayed for a little time. This time is the reverse recovery time. It directly affects the switching speed of the diode.
(6) Maximum power
The maximum power is the voltage applied across the diode multiplied by the current flowing. This limit parameter is special for Zener diodes and the like.
(7) Frequency characteristics
Due to the existence of junction capacitance, when the frequency is high to a certain extent, the capacitive reactance is so small that the PN junction is short-circuited. As a result, the diode loses its unidirectional conductivity and cannot work. The larger the area of the PN junction, the larger the junction capacitance, and the less able it is to work at high frequencies.
4. Selection of different diodes
(1) Detection diode
The detection diode can generally be a point-contact germanium diode. When selecting, the detection diode with high operating frequency, small reverse current, and sufficiently large forward current should be selected according to the specific requirements of the circuit.
(2) Rectifier diodes
Rectifier diodes are generally planar silicon diodes, which are used in various power rectification circuits. When selecting a rectifier diode, parameters such as its maximum rectification current, maximum reverse operating current, cut-off frequency, and reverse recovery time should be considered. The rectifier diodes used in ordinary series regulated power supply circuits do not have high requirements for the reverse recovery time of the cut-off frequency, as long as the rectifier diodes with the maximum rectification current and maximum reverse operating current meet the requirements are selected according to the requirements of the circuit.
(3) Zener diodes
Zener diodes are generally used in regulated power supplies as reference voltage sources or in overvoltage protection circuits as protection diodes. The selected Zener diode should meet the requirements of the main parameters in the application circuit. The stable voltage value of the Zener diode should be the same as the reference voltage value of the application circuit, and the maximum stable current of the Zener diode should be about 50% higher than the maximum load current of the application circuit.
(4) Switching diodes
Switching diodes are mainly used in household appliances such as tape recorders, TV sets, and DVD players, and electronic equipment such as switching circuits, detection circuits, and high-frequency pulse rectification circuits.
For the medium-speed switching circuit and the detection circuit, 2AK series ordinary switching diodes can be selected. The high-speed switching circuit can choose RLS series, 1SS series, 1N series, 2CK series high-speed switching diodes.
The specific model of the switching diode should be selected according to the main parameters of the application circuit (such as forward current, maximum reverse voltage, reverse recovery time, etc.).
(5) When selecting a varactor diode, you should focus on whether its operating frequency, maximum reverse operating voltage, maximum forward current, and zero-bias junction capacitance and other parameters meet the requirements of the application circuit . , High Q value, small reverse leakage current varactor diode.
5. TVS diode selection
(1) Minimum breakdown voltage VBR and breakdown current IR. VBR is the minimum breakdown voltage of TVS. At 25°C, TVS will not produce avalanche below this voltage. When the TVS flows through the specified 1mA current (IR), the voltage applied to the two poles of the TVS is its minimum breakdown voltage V BR . According to the degree of dispersion between VBR of TVS and the standard value, VBR can be divided into two types: 5% and 10%. For 5% VBR, VWM = 0.85VBR; for 10% VBR, VWM = 0.81VBR. In order to meet the IEC61000-4-2 international standard, TVS diodes must be able to handle ESD impacts of a minimum of 8kV (contact) and 15kV (air). Some semiconductor manufacturers use higher impact resistance standards for their products. For some portable device applications with special requirements, designers can select components according to their needs.
(2) Maximum reverse leakage current ID and rated reverse cut-off voltage VWM. VWM is the voltage that the diode can withstand in normal state, this voltage should be greater than or equal to the normal working voltage of the protected circuit, otherwise the diode will cut off the circuit voltage continuously; but it needs to be as close as possible to the normal working voltage of the protected circuit, so It will not make the entire circuit face the threat of overvoltage before the TVS works. When the rated reverse cut-off voltage VWM is applied between the two poles of the TVS, it is in a reverse cut-off state, and the current flowing through it should be less than or equal to its maximum reverse leakage current ID.
(3) Maximum clamping voltage VC and maximum peak pulse current I PP . When the pulse peak current IPP with a duration of 20ms flows through TVS, the maximum peak voltage that appears at its two ends is VC. VC and IPP reflect the surge suppression capability of TVS. The ratio of VC to VBR is called the clamping factor, generally between 1.2 and 1.4. VC is the voltage provided by the diode in the cut-off state, that is, the voltage passing through TVS in the ESD impact state. It cannot be greater than the withstand limit voltage of the protected circuit, otherwise the components are in danger of being damaged.
(4) Pppm rated pulse power, which is based on the maximum cut-off voltage and the peak pulse current at this time. For handheld devices, generally speaking, a TVS of 500W is enough. The maximum peak pulse power consumption PM is the maximum peak value of pulse power consumption that TVS can bear. Under a specific maximum clamping voltage, the greater the power consumption PM is, the greater its ability to withstand surge current. Under a specific power consumption PM, the lower the clamping voltage VC is, the greater its ability to withstand surge current. In addition, the peak pulse power consumption is also related to the pulse shape, duration and ambient temperature. Moreover, the transient pulse that TVS can withstand is non-repetitive, and the pulse repetition frequency (the ratio of duration to intermittent time) specified by the component is 0.01%. If repetitive pulses appear in the circuit, the accumulation of pulse power should be considered, which may damage the TVS.
(5) Capacitance C. Capacitance C is determined by the TVS avalanche junction cross-section, which is measured at a specific frequency of 1MHz. The size of C is proportional to the current bearing capacity of the TVS tube, if C is too large, the signal will be attenuated. Therefore, C is an important parameter for selecting TVS for the data interface circuit. Capacitors For loops with higher data/signal frequencies, the capacitors of diodes will interfere more with the circuit, causing noise or attenuating signal strength. Therefore, it is necessary to determine the capacitor range of the selected component according to the characteristics of the loop. Generally, capacitors should be selected as small as possible for high-frequency circuits (such as SAC (500W, 50pF, ±10%), LCE (1.5KW, 100pF), low-capacitor TVS), and capacitors for circuits that do not require high capacitors can be selected higher than 40pF.