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2. Selection of the switches

Lecture's plan

Introduction

  1. Considered structures
  2. The selection of the switches
  3. Commutations
  4. Possible configurations of the switches' states
  • At the level of the switches of the arm j: in order to allow the current ij to flow, it is necessary that at least one of the two switches Kj and K¢j to be in the ON state
  • In order to avoid the short-circuit of the source U, the two switches cannot be in the ON state simultaneously.

The states of the two switches of each arm must therefore be complementary, one being ON and the other OFF (figure 4)

Figure 4a

Figure 4b

When the Kj switch is in the ON state, it is passed by the current ij. As this current is an alternating current, it is sometimes positive, sometimes negative: the switch Kj must therefore be current reversible.

When the Kj switch is OFF, the switch K¢j is ON: the voltage at the terminals of Kj is therefore equal to U. This voltage is a continuous voltage, therefore always positive.

The Kj switch is a switch which has 3 running sections (3 segments) obtained by connecting in anti-parallel a transistor and a diode (figure 5).

Figure 5a

Figure 5b

  • the section 1 corresponds to the blocking state of the transistor and of the diode.
  • the section 2 corresponds to the conduction state of the transistor.
  • the section 3 corresponds to the conduction state of the diode.

When the transistor is in conduction state (section 2), the diode is submitted to a very weak negative voltage equal to the voltage drop (VCE)sat of the transistor in the passing state.

When the diode is in conduction state (section 3), the transistor is submitted to a very weak negative voltage equal to the voltage drop VDON of the diode in the passing state: when we are in the section 3 we can command the ON state of the transistor in order to allow an automatic passage to the section 2, if the current ij stops being negative and becomes positive.

The switch K¢j presents the same characteristics that the switch Kj has; it is therefore also obtained by connecting in anti-parallel a diode and a transistor.

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Last update: 2005, September, 30 | Translation: Sergiu Ivanov