Performance characteristics of fast recovery diodes

Performance characteristics of fast recovery diodes

Reverse recovery time

The definition of reverse recovery time (tr) is the time interval during which the current transitions from forward to a specified low value through zero. It is an important technical indicator for measuring the performance of high-frequency freewheeling and rectifier devices. IF is the forward current, and IRM is the maximum reverse recovery current. Irr is the reverse recovery current, usually specified as Irr=0.1IRM. When t ≤ t0, the forward current I=IF. When t>t0, due to the sudden change of the forward voltage on the rectifier device to the reverse voltage, the forward current rapidly decreases. At t=t1, I=0。 Then a reverse current IR flows through the rectifier device, and IR gradually increases; At time t=t2, the maximum reverse recovery current IRM value is reached. Afterwards, under the action of forward voltage, the reverse current gradually decreases and reaches the specified value Irr at t=t3. The reverse recovery process from t2 to t3 is similar to the discharge process of capacitors.

Structural characteristics

The internal structure of a fast recovery diode is different from that of a regular diode, as it adds a base region I between P-type and N-type silicon materials to form a P-I-N silicon wafer. Due to the thin base region, the reverse recovery charge is very small, which not only greatly reduces the TRR value, but also reduces the transient forward voltage drop, allowing the diode to withstand high reverse operating voltages. The reverse recovery time of a fast recovery diode is generally several hundred nanoseconds, the forward voltage drop is about 0.6V, the forward current is several amperes to several thousand amperes, and the peak reverse voltage can reach several hundred to several thousand volts. The reverse recovery charge of the ultrafast recovery diode is further reduced, allowing its TRR to be as low as tens of nanoseconds. Fast recovery and ultra fast recovery diodes below 20A are mostly packaged in TO-220. From the perspective of internal structure, it can be divided into two types: single diode and double diode (also known as double diode). There are two fast recovery diodes inside the diode, which can be divided into common cathode pair and common anode pair according to the different connection methods of the two diodes.

Fast recovery diodes with tens of amps are generally packaged in TO-3P metal shells. Pipes with larger capacities (several hundred to several thousand amperes) are packaged in bolt or flat form.

Reverse recovery time

The specified IF is supplied by a DC current source, and the pulse generator applies a pulse signal through a DC blocking capacitor C. The trr value observed using an electronic oscilloscope is the time elapsed from the moment I=0 to the moment IR=Irr. Assuming the reverse recovery charge inside the device is Qrr, the relationship is:

Trr ≈ 2Qrr/IRM (5.3.1)

According to equation (5.3.1), when IRM is constant, the smaller the reverse recovery charge, the shorter the reverse recovery time.

Conventional testing methods

Under amateur conditions, a multimeter can be used to detect the unidirectional conductivity of fast recovery and ultra fast recovery diodes, as well as whether there are open or short circuit faults inside, and to measure the forward conduction voltage drop. If equipped with a megohmmeter, it can also measure the reverse breakdown voltage. Example: Measure a C90-02 ultrafast recovery diode, with its main parameters being: trr=35ns，Id=5A，IFSM=50A，VRM=700V。 Set the 500 type multimeter to the R × 1 position and read the forward resistance as 6.4 Ω, n '=19.5 bars; The reverse resistance is infinite. Further calculate VF=0.03V/grid × 19.5=0.585V. Prove that the pipe is good.

Quick recovery diode precautions

(1) Some single diodes have three pins, with the middle one being an empty pin, which is usually cut off at the factory, but there are also some that are not cut.

(2) If one of the pipes is damaged, it can be used as a single pipe.

(3) When measuring the pressure drop of the positive guide, the R × 1 gear must be used. If using the R × 1k gear, the measured VF value will be significantly lower due to the test current being too small, far below the normal operating current of the diode. In the above example, if R × 1k measurement is selected, the forward resistance is equal to 2.2k Ω, and n '=9 grids. The VF value calculated from this is only 0.27V, much lower than the normal value (0.6V).