Buck converter

Circuit diagram of a buck converter. During operation, the power is supplied from a voltage source connected to the left-hand side with the voltage U _E , while a load connected to the right-hand side receives the voltage U _A ; neither are shown here.

The down converter , and buck converter, buck regulator, English step-down converter or buck converter , is in the electronics a form of a switching DC-DC converter . The output voltage U _A is always less than or equal to the magnitude of the input voltage U _E .

Layout and function

The switch S (usually a transistor ) is regularly switched on and off by a controller not shown in the picture; Usually a few hundred to several million switching cycles are carried out per second. As a result, electrical energy is transferred from the voltage source connected on the left to the load connected on the right . The two energy stores coil and capacitor enable the load to be supplied in the phases in which the switch is open. The inductance of coil L keeps the higher input voltage away from the load. The output variable can be set by controlling the switch S on and off times. This control is usually done by a regulator in order to keep the output voltage or current at a desired value.

During the switch-on time T _e , the load current flows through the coil L and through the consumer; the diode  D blocks. During the switch-off phase T _a , the energy stored in the coil is dissipated: The current through the consumer continues to flow, but now through the diode D and out of the capacitor C.

The coil L and the capacitor C form a second-order low-pass filter. The downward conversion is effectively achieved by filtering out the alternating component from the square-wave voltage. How high the remaining DC component is can be set using the duty cycle.

Intermittent and non-intermittent operation

In the continuous mode ( English Continuous Current Mode (CCM) , continuous operation) to listen to the current through the coil during the entire cycle never flow on to; the switch is closed again before the stored magnetic energy is completely dissipated. In contrast, the stands lückende operation ( English Discontinuous Current Mode (DCM) , discontinuous current operation), in which the current drops through the coil periodically during each cycle to zero. The cycle can be divided into a third phase: In addition to the phases of energy storage (with the switch closed) and energy release, which also occur in non-discontinuous operation, the gap phase comes without current through the coil, in which the connected load comes exclusively from the capacitor C is supplied.

Whether there is continuous or intermittent operation depends on inductance , switching frequency , input voltage, output voltage and the flowing output current. Since these parameters can sometimes change quickly, the transition between the two operating modes must generally be taken into account (e.g. prevented) when designing the circuit, especially a controller. The two operating modes differ with regard to the control characteristic, i.e. the dependence of the output voltage on the duty cycle (see below), as well as with regard to the radiated interference .

Regulation / control

Function of the buck converter

There are various methods for regulating the output voltage , of which the pulse width modulation (PWM) in non-discontinuous operation (continuous operation or continuous current mode) is shown as an example below.

With pulse width modulation, there is a fixed switching frequency and period  T . The switch S turns on during the entire period T only for the time T _e < T . The fraction is called the duty cycle . ${\displaystyle d={\frac {T_{e}}{T}}}$

The relationship applies approximately or for ideal components . ${\displaystyle U_{\mathrm {A} }=U_{\mathrm {E} }\cdot d}$

Voltage and current curve

The graphic on the right shows the voltage and current curves of the step-down converter for about one and a half periods; the steady state is shown. The current in the coil always fluctuates around the mean value (red line, load current) and never drops to zero. Let the capacitor C be so large that the output voltage (green line) can be regarded as constant over the observed period of the period. ${\displaystyle i_{L}}$ ${\displaystyle I_{L,av}}$

The coil current is generally:

${\displaystyle i_{L}={\frac {1}{L}}\int _{}^{}u_{L}\mathrm {d} t+I_{0}}$

During the switch-on phase, the magnetic memory (the coil) is charged. The current increases linearly. ${\displaystyle i_{L}}$

The coil voltage is the difference between the input and output voltage during the switch-on time:

${\displaystyle u_{L}=U_{\mathrm {E} }-U_{\mathrm {A} }}$

and is approximately constant, the diode blocks.
During the following switch-off time, the output voltage is applied to the coil:

${\displaystyle u_{L}=-U_{\mathrm {A} }}$

The above integral is now negative and the coil current decreases linearly because the polarity of the coil voltage has now changed. Then the entire process is repeated.

The direct current component of the coil current is also called the bias current and must not saturate the core of the coil, which is why it has an air gap . The direct current component is the constant of integration in the above integral. ${\displaystyle I_{0}}$

The alternating current component in the coil and also at the input is called ripple current .

The graphic clearly shows that non-discontinuous operation cannot be maintained when the load current drops, since the coil current cannot become negative due to the diode.

The voltage at the junction of switch S, diode D and coil L has steep voltage jumps during operation. In intermittent operation there is also a phase in which switch S and diode D block (not conduct) at the same time. As a result, a resonant circuit formed from the coil and the parasitic capacitances of switch S and diode D can be excited to a damped oscillation, which can cause additional interference and also stress the components. This node should therefore be particularly short in the circuit board layout.

If you rearrange the equations according to the duty cycle d , you get the control characteristic:

${\displaystyle d={\frac {U_{A}}{U_{E}}}}$.

The output voltage increases when the switch-on time increases (with the same period duration ). ${\displaystyle T_{e}}$ ${\displaystyle T}$

Current account

If the losses of the circuit are not taken into account, the following power equation results:

${\displaystyle P_{\mathrm {E} }=U_{\mathrm {E} }\cdot I_{\mathrm {E} }=P_{\mathrm {A} }=U_{\mathrm {A} }\cdot I_{\mathrm {A} }}$

The real buck converter has its main losses in the following components:

• Coil - it has ohmic losses due to its winding resistance as well as magnetic losses in the core material.
• Switching transistor - it has a voltage drop when switched on as well as switching losses (it switches in a finite time).
• Freewheeling diode - it has a typical forward voltage of 0.4–1 V and switching losses .

To reduce the losses in the diode, a controlled MOSFET can be used in its place . A synchronous rectification is then obtained , the coil current and the output current can now become negative - the direction of energy flow can be reversed.

properties

Power section of a three-phase switching regulator for supplying power to the processor on a PC main board

The power balance results in u. a. that the output current of a buck converter is always higher than its mean input current. For a short time, however, a current flows at the input that is even slightly higher than the average output current. This means that a back-up capacitor with a particularly low equivalent series resistance ( low ESR ) is required on the input side, particularly in the case of step-down converters with a large difference between the input and output voltage, in order to avoid additional external power losses and disturbances in the supply voltage.

I.a. This problem led to the development of multi-phase step-down converters: They consist of several parallel, time-shifted step-down converters of lower power, which are usually controlled with a single control circuit.

The output voltage of the step-down converter is always less than the input voltage, i.e. d is always less than 1. The circuit must be adapted exactly to the load (not shown in the circuit) or the semiconductor switch - usually a transistor , IGBT or MOSFET  - must can be controlled via a control circuit in order to regulate the current flow through the load or the voltage at the load via the pulse-pause ratio.

In the case of multi-phase step-down converters, the current balance between the individual phases must also be maintained. Usually a smoothing capacitor is connected in parallel to the load for voltage stabilization on the output side .

If the step-down converter is used to drive motors, the coil L and the smoothing capacitor can u. It may also be omitted because the motor winding usually already provides sufficient inductance. However, the increased losses in the motor and the possible interference emissions must be taken into account.

Applications

In contrast to series regulators , step-down converters have lower losses if they generate output voltages that are significantly lower than the input voltage. In contrast to series regulators, their mean input current is less than the output current.

Applications include

• generating lower voltages (12 V, 5 V) from 24 V ( trucks , industrial power supplies);
• Provision of the processor supply voltage (1.2… 3.5 V) in notebooks ;
• Chargers for rechargeable batteries ;
• Operation of semiconductor lasers ;
• Current control on stepper motors and speed control on DC motors ;
• Operation or control of Peltier elements for heating or cooling;
• Driving LED flashlights and LED bicycle headlights;
• as a special design in class D amplifiers of audio output stages.

There is the realization of down-converters integrated monolithic circuits ( English integrated circuit, IC ) , which contain a part or all of the semiconductor devices that are required to regulate a constant output voltage under varying load.

For small outputs, hybrid circuits are also offered, which also contain the coil.

It should be noted that the output voltage - depending on the load current - has a certain ripple ( triangle ripple) due to the switching cycles , which can be significantly reduced by an LC element or a subsequent low-drop linear regulator .

Synchronous converter

Synchronous buck converter (without control logic)

If, in the circuit diagram above, the diode D is replaced by a further switch S ₂ , together with the control logic necessary for the correct timing, this becomes the synchronous converter . The name is derived from the necessary, timely correct control of the switches, which takes place as with synchronous rectifiers . The synchronous converter is by exchanging input and output of a boost converter . The topology is, so to speak, the generalization of the down and up converter. With one and the same circuit, the direction of the energy flow can be reversed - depending on the duty cycle and the ratio of the voltages on both sides.

Web links

Commons : Buck converters  - collection of images, videos and audio files

• Heinz Schmidt-Walter: "Down converter" in: Lecture script "Switching power supplies". (PDF) Darmstadt University of Applied Sciences
• Jörg Rehrmann: The new Internet power supply and converter manual.
• Interactive applet (1) to understand the buck converter

Individual evidence

1. ^ Robert W. Erickson: DC-DC Power Converters. (PDF) p. 2 , accessed on July 11, 2017 . 
2. ↑ https://www.researchgate.net/publication/275349755_Magnetics_Design_Tool_for_Power_Applications Esguerra, Mauricio: Magnetics Design Tool for Power Applications. in researchgate , April 2015, accessed on March 1, 2020