Diode Reverse Recovery Current
Even if a fast-recovery or an ultra-fast recovery diode is used for free-wheeling
operation, the reverse-recovery characteristic of the diode imposes some constraints.
The circuit that can be used to test the reverse recovery characteristic of
a diode is shown in Fig. 1. An applet, named the first applet, simulates the
reverse recovery property of the diode and is displayed below.
The operation of the test circuit is explained at first. The time constant
due to load inductance and load resistance should be several times the time
corresponding to the switching frequency. Then the current through the remains
more or less steady and its value is given by:
When the MOSFET is ON, the load current flows through the transistor. When
it is turned off, the current flows through the snubber circuit initially
and then the free-wheeling diode, marked as DUT (device under test). When
the MOSFET is switched ON again, the source tends to reverse the current through
the diode. The current through the diode falls rapidly and reverses before
it becomes zero. The reverse recovery has been simulated using the model of
the diode shown in Fig. 1.
The reverse-recovery transient process depends on the diode itself and it
also depends on the junction temperature of the diode, the forward current
prior to being reverse biased, the rate of fall forward current and the source
voltage that applies to the reverse bias to the diode. As the value of any
one of these parameters rises with the other parameters remaining unchanged,
the reverse recovery transient process becomes worse, reflected by an increase
in the peak reverse current. The reverse recovery turn-off period starts once
the diode current becomes negative and lasts till the reverse diode current
increases in ampltidue first and then decays to about 10% of its reverse peak
current. It represents the time that has to elapse before the diode recovers
ability to block reverse voltage.
The manufacture specifies the reverse recovery transient period and the maximum
reverse current for a set defined by its forward current, the rate of fall
of forward current and the junction temperature. For many diodes, the maximum
rate of fall forward current is specified to be 100 A/ms.
This means the test inductance L should set be such that
If reverse recovery transient is to be minimized, it would be preferable
to select L such that
where the rated current of diode is Irated,diode and trr
is its reverse recovery transient period.
The applet simulates the reverse recovery transient. It is an approximation
of the behaviour of the diode. No accuracy for this model is claimed, but
it does show the dependence of recovery transient on the parameters mentioned.
The pull-down choice menu contains rated current of the diode, its turn-off
period, its turn-on period, load current, source voltage, test inductance
in nH, test resistance in Ohms and the junction temperature as its items.
To change the value of a menu-item, pull down the menu, highlight the item,
change its value in the adjacent text-field and then click on Set Value
button. When the Run/More button is clicked, the program
displays the diode current and the voltage across the diode for a period corresponding
to 12 times the turn-off period. If the diode had not turned off by then,
click again on the Run/More button, and the process that occurs
for the next 12 times the turn-off period would be displayed. Whenever the
rate of fall of forward current increases due to either lower test inductance
or higher source voltage, the total period required for the diode to turn-off
decreases, but there would a significant increase in the peak reverse current.
click here to open the applet
APPLET FOR REVERSE RECOVERY TRANSIENT IN A DIODE
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