Buck converter

The down-converter, and buck converter, english to step down converter, buck converter english or buck regulator is in electronic form of a DC-DC converter according to the method of pulse width modulation. The output voltage UA is always less than the magnitude of the input voltage UE.

  • 2.1 Current account

Design and function

The magnitude of the output voltage can be adjusted by controlled switching on and off of the switch S (usually a transistor).

The switch S switches over the entire period T only for the period of. The following equation applies:

With d the duty cycle.

: Duty

During the on time of the load current flowing through the inductance L and by the consumer, the diode D is blocked.

: off-

During the deactivation phase, the energy stored in the inductance is reduced: the current through the load will continue to flow, but now through the diode D and from the capacitor C.

: Period.

Regulation / control

For the regulation of the output voltage there are various methods, of which the following are the pulse width modulation ( PWM) in the continuous current mode (continuous or non- continuous mode ) is shown as an example: The current in the inductor swings always around the mean ( red dashed line ) and never drops to zero.

Discontinous operation

In discontinuous mode ( DCM german, discontinuous mode ) the inequality, within the time so no output current flows. Whether continuous or continuous mode is present depends on inductance, switching frequency and output current.

Voltage and current waveform

The adjacent graph shows the voltage and current waveforms of the buck converter are shown, it is shown the steady state.

During the activation phase of the magnetic memory is being charged ( the inductance ). The current rises evenly:

The coil voltage () is approximately constant, the diode blocks. In the subsequent switch-off phase, the output voltage across the inductor is on. The output current continuously decreases as the polarity of the coil voltage has now changed. Then the entire process is repeated.

If you compare the equations for the duty cycle d to, we obtain the control characteristic:

Thus, the output voltage rises when the switch is greater ( for the same period ).

The creation of the control characteristic can be reproduced in the interactive Java applet.

Current account

Disregarded the losses of the circuit results in the following power equation:

The real down-converter has its significant losses in the following components:

  • Coil - it has resistive losses through its winding resistance and magnetic losses in the core material.
  • Switching transistor - it has a voltage drop in the ON state and switching losses ( he turns in a finite time ).
  • Free-wheeling diode - it has a typical forward voltage of 0.4-1 V and switching losses. To reduce these losses, you can instead use a controlled MOSFET. This is called synchronous rectification.

Properties

From the current account arises, inter alia, that the output current of a buck converter is always greater than its average input current. However, in each case for a short time at the input of a current to flow which is even higher than the average output current. As a result that especially in down-converters with a large difference between input and output voltage on the input side, a back-up capacitor with a particularly low equivalent series resistance ( ESR Sheet low ) is required to prevent additional external power losses and disturbances in the supply voltage.

Among other things, this problem led to the development of multi-phase buck converter: these consist of several parallel, time delay controlled buck converters of lower power, which are usually controlled by a single control circuit.

The output voltage of the buck converter is always less than the input voltage, that is, d is always less than 1, the circuit must be accurately adjusted load or semiconductor switch ( not shown in the circuit ) to the - usually a transistor, IGBT or MOSFET - must are controlled by a control circuit to control on the pulse-pause ratio of the current flow through the load or the voltage across the load.

In multi-phase buck converters in addition, the current balance between the individual phases must be maintained. Most still a smoothing capacitor is connected in parallel with the load to the output side voltage stabilization.

Is the down-converter used for driving motors, the inductance L and the smoothing capacitor can also may be omitted since the winding of the motor usually is already a sufficient inductance. Attention will be paid, however, if necessary, the increased losses in the engine and the noise radiation may occur.

Applications

In contrast to linear regulators down converter capable of producing low-loss output voltages that are lower than the input voltage. Their mean input current is therefore less than the output current.

  • Generating lower voltages (12 V, 5 V) from 24 V (trucks, industrial power supplies)
  • Providing the processor supply voltage ( 1.2 ... 3.5 V) in the notebook
  • Chargers for batteries
  • Operation of semiconductor lasers,
  • Current control of stepper motors and speed control of DC motors
  • Operation / control of Peltier elements for heating / cooling
  • Bustle of LED flashlights and LED bicycle headlights
  • As a special shape in class - D amplifiers audio amplifiers.

There is the realization of monolithic integrated circuits buck converters (English: integrated circuit, IC), containing a portion or all semiconductor devices that are required to regulate a constant output voltage with varying load.

For small loads, and hybrid circuits are available, in addition even contain the coil.

Generalization

If in the above circuit diagram of the diode D is replaced by another switch S2, together with the necessary time for the correct activation control logic, it is the synchronous converter. The name is derived from the necessary, correct time control of the switch, which is similar to synchronous rectifiers. The synchronous converter can then be converted by swapping input and output directly in a boost converter and is therefore in the topology of the generalization of the downward and upward converter are

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