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How does overvoltage protection work with the aid of avalanche diodes?

created by Jürgen Gerber, Product Group Manager Discrete Semiconductors at Rutronik Elektronische Bauelemente GmbH and Jochen Krieger, Senior Manager Application and Product Engineering - Diodes Division - TVS & ESD Protection Diodes & EMI Filter, Vishay |   Rutronik

Overvoltage can always occur when an IGBT is switched on and off at high speed in a high-performance application. For instance, when switching off the load current circuit, the collector-emitter voltage goes up abruptly, reaching very high peaks. Overvoltage caused by switching events can severely damage or even destroy switching transistors.

How do avalanche diodes help to prevent overvoltage?

A common overvoltage protection method is "active clamping". In this case, an avalanche diode is used as the direct feedback. If switching off results in an inductive load overvoltage peak, it is conducted by the avalanche diode to the IGBT gate and the IGBT is switched back on.

The diagram shows the basic principle: While the voltage is rising the diode is blocked (A). In the moment when one free electron in the depletion region triggers an avalanche, the voltage drops suddenly down below the breakdown voltage level of 30V so that the avalanche immediately breaks down (B). Only sometimes is it possible to keep the avalanche current stable for a short period of time before it restarts and the voltage rises again (C). The breakdown delay (D), i.e. the time between two breakdown events, cannot be forecast.

Avalanche diodes with improved noise performance are recommended for active clamping overvoltage protection. As they enable:

  • faster breakdown at fast-rising reverse voltages
  • more stable breakdown voltage at low currents (below ~ 1mA), and thus
  • longevity of the other components, e.g. IGBTs or Mosfets, resulting in
  • cost savings for applications such as frequency converters or motor controllers, as the components need to be replaced less often.

How does avalanche diode noise occur?

The perpetual switching on and off of the avalanche causes the noise of the avalanche diode, i.e. the constant generation of voltage peaks and their sudden breakdown (see diagram). There are two preconditions to trigger an avalanche breakdown:

  1. The presence of a sufficient breakdown voltage to generate a critical electric field strength for impact ionization.
  2. The presence of free electrons as they form the leakage current.

For example, a leakage current of 1.6pA = 1.6 x 10-12A is equal to an electron flow rate through the barrier layer of 107 electrons per second. This means that statistically they can only trigger an avalanche every 100ns. However, since not every electron triggers an avalanche, the time will be much longer in reality. The probability of triggering an avalanche breakdown is thus proportional to the leakage current. In other words: The higher the leakage current, the higher the probability of triggering an avalanche breakdown or the shorter the breakdown delay time (in the diagram: D).

Between two impact leakage current electrons, the reverse voltage at the diode can rise considerably above the breakdown voltage level. It is only when the next impact electron triggers an avalanche that the voltage at the diode suddenly breaks down to the breakdown voltage level.

If the applied voltage source delivers sufficient current, e.g. 1mA, the avalanche breakdown can keep itself running through continuous impact ionization, resulting in a stable avalanche current.

However, if the source current is too low, e.g. 100μA, the diode is discharged by the abrupt drop in avalanche voltage below the breakdown voltage level, so that the avalanche breakdown immediately stops again. A certain amount of time is now needed to charge the diode and line capacitance with the low source current up to the required voltage level before the next electron can trigger a new avalanche. This perpetual switching on and off of the avalanche causes the typical noise of the avalanche diode breakdown.

The difference the diode noise performance makes is also visible in the diagram: It shows the breakdown voltage range of two Z-diodes (Zener diodes) with a breakdown voltage of 30V measured at a reverse current (IR) of 100μA. One was generated with standard technology using a very low leakage current, the other with "Low Noise Technology". The Zener diode with "Low Noise Technology" has a significantly more robust voltage characteristic than the other diode that can only retain a constant avalanche current for a short period of time (C).

Z-diodes with "Low Noise Technology" are available from Vishay. The new generation of Z-diodes in the SMF, BZD27, BZG 03, BZG04, BZG05, PLZ and VTVS series clearly increase the probability of triggering an avalanche breakdown due to their moderately increased leakage current (IR~ 10nA) and thus reduced noise. This offers the user a more stable breakdown voltage at low currents (below ~ 1mA) and a faster breakdown of fast-rising reverse voltages.

Further influencing factors for diode noise

The leakage current rises exponentially with the temperature, i.e. the noise decreases as the temperature rises. Light can also release free electrons in the depletion region of the diode, thus reducing the noise level. This means: The darker and colder the ambient conditions, the greater the noise level.


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