Working principle and application of fast recovery diode

For diodes , When the voltage applied at both ends changes from positive to negative , The response time is usually very short , On the contrary, when it changes from reverse to forward, the time is relatively long , This is the reverse recovery time . When diodes are used in high-frequency applications , The reverse recovery time is required to be short , At this point, we need a diode with fast reverse recovery , We call this kind of diode fast recovery diode .

How does fast recovery diode achieve fast recovery ？？

One 、 Physical structure
The internal structure of fast recovery diode is similar to that of ordinary PN Junction diodes are different , It belongs to PIN Junction diode , That is to say P Silicon material and N A base region is added in the middle of the silicon material I, constitute PIN Silicon . because PD The main active region of is the barrier region , Therefore, broadening the barrier region can improve the sensitivity .PIN Junction fast recovery diode is actually artificially PN The barrier width of the junction is expanded ,PIN The effective area of junction fast recovery diode is mainly the area with electric field I Type layer （ Barrier region ）, Then the effective region for generating photogenerated carriers increases , The influence of diffusion has weakened , And the junction capacitance is also greatly reduced , Therefore, the sensitivity and response speed of its optical detection have been greatly improved .

Two 、 Electrical performance

In the figure IF Is the forward current ,IRM Is the maximum reverse recovery current ,Irr Is the reverse recovery current , It is usually stipulated that Irr=0.1IRM. When t≤t0 when , Forward current I=IF. When t＞t0 when , Because the forward voltage on the rectifier suddenly becomes the reverse voltage , therefore , The forward current decreases rapidly , stay t=t1 moment ,I=0. Then the reverse current on the rectifier IR Gradually increase ; stay t=t2 The maximum reverse recovery current is reached at any time IRM value . After that, it is affected by the forward voltage , The reverse current gradually decreases , And in t=t3 Reach the specified value at any time Irr. from t2 To t3 The reverse recovery process of is similar to the capacitor discharge process . from t1 To t3 The time interval of is the reverse recovery time trr.
The main feature of fast recovery diode is its reverse recovery time （trr） In hundreds of nanoseconds （ns） following , Ultrafast recovery diodes can even reach tens of nanoseconds .

3、 ... and 、 Application in bootstrap circuit
Fast recovery diode （ abbreviation FRD） It has good switching characteristics 、 Semiconductor diode with short reverse recovery time , Mainly used in switching power supply 、PWM Pulse width modulator 、 In electronic circuits such as frequency converters , As a high-frequency rectifier diode 、 Freewheeling diode or damping diode use .

The voltage at both ends of the bootstrap capacitor of the system is zero at the moment of power on , If IPM Normal startup is required , Drive circuit VCC You need normal power supply , There is no voltage during initialization , stay IPM Before work , The bootstrap capacitor needs to be charged , Sufficient number of pulses by controlling the drive signal , Precise control IGBT Opening , Raise the voltage at both ends of the capacitor to the target voltage .
The specific working process is ：
a、 The bootstrap capacitor needs to be charged at the moment of power on , Lower bridge arm IGBT Turn on and pull the output voltage of the corresponding phase down to the ground , The power supply passes through the bootstrap resistor 、 Bootstrap diode charges the capacitor .
b、 When on the bridge IGBT When opening , The output voltage rises to the bus voltage again . The voltage at both ends of the capacitor cannot change suddenly , The voltage at both ends remains at the supply voltage level , At the same time IGBT Drive supply voltage . Bootstrap diode reverse cut-off , Effectively isolate the weak current power supply from the bus voltage , Avoid strong current leading into weak current breakdown circuit devices , The above is half a cycle , Follow up cycle after cycle .
Through to IPM The working principle and working process of the bootstrap circuit at the moment of initial power on are analyzed and found , Before the circuit starts to work, the system initializes , Xiaqiao IGBT During the charging process of bootstrap capacitor, the second straw bears the minimum voltage , The diode will not have the possibility of overvoltage failure , Get on the bridge IGBT During the opening process, the diode can effectively isolate strong and weak current , Both ends bear the maximum voltage , except IP TongWai is the device with the highest voltage impulse frequency for this circuit , If the reverse voltage withstand of the device is low due to various factors, it is very easy to cause the device to have insufficient reverse voltage withstand and Breakdown Failure .