Electronic – Diode identification

Actually, there are two effects that cause switching losses in diodes. On the one hand, some kinds of diodes just need time to "notice" that they are no longer forward biased, and need to stop conducting current. This time is mostly independent of the reverse current flowing, so you can not give a reverse recovery charge that is valid for all circumstances, as the reverse current during that (mostly) fixed time depends on the external current.

For other types of diodes, pushing a reverse current through the diode is actually the way of making it stop to conduct, so the core parameter to turn off this diode is the charge needed to be sent through the diode to turn it off. You can not give an universal time, because the diode will turn off faster if there is a higher current through that diode.

In practice, typical silicon diodes are described sufficiently well using a fixed reverse recovery time (and thus a circuit-dependent reverse recovery charge), while schottky diodes (at least for low forward currents, see Wikipedia on Schottky diodes for details) are described sufficiently well using a fixed reverse recovery charge (and thus a circuit-dependent reverse recovery time). In both cases, if there is a considerable reverse voltage across the diode, you need to consider the junction capacitance additionally to the turn-off action (no matter whether it is mainly time- or charge-based).

The plot of the first diode looks typical for a silicon p-n diode, so you need to guess what current your circuit is able to force through the conducting diode until trr is over to get to reverse recovery charge, while the second diode is a Schottky-barrier diode, and thus the reverse recovery diagram looks suspiciously like capacitor discharge. I am unable to find a good value for the reverse recovery charge from that datasheet, though. The specification of "IR" makes no sense to me, as the diagram seems to exceed "IR = -IF" widely for a short time.

You will struggle to find a datasheet that explicitly states the turnon information. TurnOn switching losses of a diode are typically quite small & unlike the TurnOff characteristics, ONLY affect the diode

The reason for this is the turn-off characteristics have bigger ramifications.

The key information with regards to turn-off are

1. Reverse recovery time \$t_{rr}\$
2. Reverse recovery charge \$Q_{rrm} \$
3. Peak reverse recovery current \$I_{rr}\$
4. Peak rate of fall of recovery current \$dI_{rr}/dt\$

The turn-off of the DIODE characteristic have an immediate knock on effect on the associated commutating device (eg IGBT). The reverse recovery current is additive to the turn-on switching current of the associated device and thus that devices turn-on (the IGBT). Switching 600A through an inductive load can easily have 50A of additional current

This however is turn-off of the DIODE & this information is provided in the datasheet & equally the question was associated with turn-on

Turn-OFF characteristics (for minority carriers) is dominated by die,package, installation: resistance AND inductance. As the current is rising at the \$dI_{rr}/dt\$ rate, an additional voltage is generated on-die. In practice however due to application specific installation stray inductance the \$dI_{rr}/dt\$ will be lower.

To ESTIMATE the turn-OFF switching losses of a power Diode requires three things

1. Reverse Recovery Charge
2. blocking Voltage
3. switching frequency.

Depending on the size of the diode, you may or may not be provided with \$Q_R\$ against \$dI_f/dt\$ graphs.

If you are provided these graphs, the 1st step is to determine the \$Q_R\$ for your operating case: external inductance limiting the \$dI_f/dt\$ and your operating current.

The switching power can then be estimated by

\$P_s = Q_R V_r f_s\$

if you are interested in W.s/pulse (\$J_R\$) This can be estimated via: \$J_R = V_R Q_R\$

As mentioned: TurnON losses are not only tricky (due to the complex nature of interacting strays) but equally is a small contributor