Thursday, April 15, 2010

Trr in Multi-Junction High Voltage Diodes and Switching Circuits

When selecting a high voltage diode for your application, one important characteristic to examine is Reverse Recovery Time, or Trr – especially in fast switching applications such as Switch Mode Power Supplies (SMPS).

When a diode is blocking, a reverse voltage exists between the anode and cathode terminals. Ideally, zero current flows. In this state, the cathode is more positive than the anode. The diode acts like an open circuit. In reality, there is a small current referred to as reverse current, Ir, usually in the nano-amp range.

When a diode is conducting, i.e. acting like a short circuit, current passes through the diode from anode to cathode. The anode is more positive than the cathode. This current is called Forward Current, Io. Io generates a relatively small Forward Voltage drop, Vf.

Sample Diode Data Sheet

Reverse Recovery Time, Trr, is the time it takes the diode to transition between blocking mode and conducting mode.

Trr is especially important in the case of multi-junction diodes used in fast switching applications such as Switched Mode Power Supplies. To create diodes capable of withstanding greater than 1,000V reverse voltage, die are stacked in series and then passivated, thus creating a high voltage diode.

For a SMPS, and other switching applications, running at 50kHz, the slowest Trr recommended for a diode is 70ns. Diode Trr is actually a measure of the reverse recovery time of the fastest die in the stack.

Under most circumstances, stacking die in order to achieve a higher blocking voltage, Vrwm, is not a problem. This technique is common. However, a potential area of concern is widely varying reverse recovery times within the discrete diode. For instance, a problem can arise when individual junction times vary by several orders of magnitude, and operating frequencies approach 50 kHz, typical in switching applications. In this situation, one would expect a diode to fail shortly after turn-on. Oft times, die Trr can be confirmed to range between nano-seconds and micro-seconds.

The solution to the problem of widely varying die Trr is to construct a diode using die cut from the same wafer lot. During the manufacturing process, VMI takes great care to do just that. Our standard manufacturing process greatly reduces concerns over potentially mismatched reverse recovery times.

Every application is different, and many factors can impact how well a component performs. Things to consider include operating conditions, operating frequency, wave-shape, and circuit topology. Trr requirements should be evaluated on a case-by-case basis.

With that said, it is generally not necessary to match die Trr in stacked diodes as long as the die come from the same wafer lot.

With VMI diodes, mismatched die Trr is nothing to worry about.

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