Two-quadrant controller

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A two-quadrant controller is an electronic circuit that combines two different DC controllers in order to use the advantages of both types.

backgrounds

In principle, the current can only flow in one direction with a DC chopper (in the other direction the diode would be in the blocking position). Combining a deep and a boost converter , you get a two-quadrant, in which not only flow from the source to the consumer, but also in the opposite direction by the consumer can flow back to the source. This effect is used, for example, with the electromotive brake , in which the drive motor acts as a generator during the braking process and feeds the released energy back into the network.

There are two different ways of combining the two DC choppers into a two-quadrant chopper. The differences lie in the direction of flow of the currents:

  • With parallel connection , the drive current flows in one direction and the braking current in the other.
  • when connected in series , the drive and braking currents flow in the same direction.

The graphics shown here differ in the arrangement of the components from those in common textbooks and are based on the graphics from the previous section. In the presentation, emphasis was placed on making the step-down and step-up converters visually visible, not showing the components in optimal cable routing.

Parallel connection

A buck converter is connected in parallel with a boost converter. The boost converter is mirrored horizontally in these two graphics (compared to its representation above) (coil on the left, diode on the right), i.e. its input is connected to the output of the buck converter (left) and vice versa. Since both coils would be next to each other, they can be combined into one coil. The components that are irrelevant in the respective operating mode are shown in light blue .

Dcdc2qS1a.svg

Effect as buck converter (drive)

In drive mode, only switch 1 and the lower diode play a role:

  1. Switch 1 closed ("driving", red line): The circuit acts like a buck converter, the current in the coil increases.
  2. Switch 1 open ("freewheeling", green line): The consumer is supplied from the coil, the current in the coil drops.
  3. Both switches open ("freewheeling", green line): Same effect as case 2, since the switch is parallel to the diode.
  4. Both switches closed: This position is invalid because the voltage source is short-circuited with it ("hot wire").
Dcdc2qS1b.svg

Effect as a step-up converter (braking)

In braking mode, only switch 2 and the upper diode play a role:

  1. Switch 2 open ("feed back", red line): The current flows from the consumer via the coil back into the source.
  2. Switch 2 closed ("freewheeling", green line): The current only flows via switch 2, the current in the coil increases.
  3. Both switches open ("feedback", red line): Same effect as case 1, since the switch is parallel to the diode.
  4. Both switches closed: This position is invalid because the voltage source is short-circuited with it ("hot wire").

Note: In the two operating modes, the current flows through the source and consumer in different directions. It is therefore not possible to “reverse” by reversing the polarity of the source. Driving and braking forces act in opposite directions . This type of circuit is used in electric vehicles such as electric locomotives .

Series connection

A buck converter (left yellow area) is connected in series with a boost converter (right yellow area). With this circuit, too, the two coils can be combined into one (whether the coil is on the left or right of the consumer does not matter).

Effect as buck converter (drive)

Dcdc2qS2a.png

Switch 1 must always be closed in drive mode:

  1. Both switches closed (“driving”, red line): The consumer is supplied via the source, the current in the coil increases.
  2. Switch 1 closed, 2 open ("free running", green line): The consumer is fed from the coil, the current in the coil drops.
  3. Switch 2 closed, 1 open: Both diodes are in the blocking position (no current flow possible).
  4. Both switches open: Both diodes are in the blocking position (no current flow possible).

Effect as a step-up converter (braking)

Dcdc2qS2b.png

In braking mode, at most one switch may be closed:

  1. Both switches open ("feed back", blue line): The current flows from the consumer back to the source.
  2. Switch 2 closed, 1 open ("freewheeling", green line): The current only flows through the coil, the current in the coil increases.
  3. Switch 1 closed, 2 open ("freewheeling", not shown): Same effect as in case 2, only that the current flows through the upper line.
  4. Both switches closed. This position is invalid. There is no electricity.

Note: The current flows through the source in different directions, but through the consumer it always flows in the same direction. This means that the drive and braking forces act in the same direction . This type of circuit comes e.g. B. for use in elevators , when the drive should act upwards when traveling upwards and the brake should also act upwards when traveling downwards.

See also