We are very familiar with the basic characteristics of diode, such as unidirectional conductivity, conduction when the voltage is greater than a certain value, but with the deepening of the work, we will find that it is not enough to only know these, especially when facing the problems of high-power equipment or EMI. So we need to have a deeper understanding of the diode. Today we will focus on the switching process of the diode (also known as the dynamic characteristics of the diode) and its impact.
The state switching of any switching device is not accomplished overnight. What happens during this switching period is what engineers should pay attention to. Because of the existence of junction capacitance, there will be a transition when the diode switches between the three states of zero bias, forward conduction and reverse cut-off.
Open (Zero Bias Converted to Forward Pass)
The opening of the diode does not mean that the forward voltage drop is greater than a certain voltage (e.g. 0.7V) and then the forward voltage drop is 0.7V.
In fact, it should be when the forward voltage drop increases from zero to an over-voltage VFP before gradually reaching a stable voltage (e.g. 2V), during which the forward current increases continuously. Call this time forward recovery time tfr. As shown in Figure-1 below:
Figure-1: Zero Bias Converted to Normal Pass-through
That is to say, the diode will produce a forward peak voltage at the moment of conduction, which is larger than the steady-state voltage UF. The over-voltage increases with the increase of di/dt.
Turn off (forward to reverse cut-off)
When a reverse voltage is applied to a diode, it does not stop it immediately, but takes a certain amount of time. During this period, there will be voltage and current overshoots. And this is what we often call reverse recovery time trr. The following figure-2:
Figure-2: Forward Pass-through to Reverse Cut-off
At tF, the external voltage of the diode changes.
At t0, the forward current drops to zero, but the diode can not return to the blocking state.
At t1, the reverse current reaches the maximum IRP, and then the reverse overvoltage reaches the maximum URP. The reverse current then decreases rapidly. It can be seen that this curve is similar to the discharge curve of the capacitor. In fact, it also represents the process of the discharge reset of the junction capacitor.
At t2, the current is almost unchanged, and the diode regains its ability to block the reverse voltage.
What's the use of all this?
Diode switch has instantaneous loss. At the moment when the diode is on and off, the multiplier of current and voltage is not zero, so there is switching loss. This is similar to the switching waveform of MOSFET as a hard switch. Therefore, this loss is even worse in high-power and high-frequency (more switching times) situations, which can not be ignored.
Diode switches cause EMI problems instantaneously. It can be seen that the diode will produce overvoltage and peak current at the moment of switching, so high dv/dt and di/dt will inevitably cause EMI problems.
Usually the peak of reverse recovery current is related to conduction in EMI. In addition, the peak current may be accompanied by a period of oscillation, which is related to radiation in EMI, so sometimes we put a magnetic ring on the diode.
So, in order to avoid these problems, we sometimes choose fast recovery diodes or Schottky diodes, which have very short recovery time.
By the way, the forward recovery time is much less than the reverse recovery time, so we usually don't see the parameters of the forward recovery time in the device manual of the diode.