Bootstrapping (electronics)

Bootstrapping [' butstræp - ] ( from English- bootstrap, dt, Bootstrap ') denotes an electric circuit in which a potential change in one part of the circuit and abruptly becomes effective in another. In this case, the effect is exploited, that capacitors at low currents its voltage change little. Unplug it were a potential change on the one hand with the other ( bootstrap effect ). Which is used for example, in amplifier circuits in which the output voltage is fed back through a capacitor to the input, known as bootstrap circuits. This feedback causes a drastic increase of the input resistance of the amplifier.

Of operation and application

Two circuit points are connected to each other via a sufficiently large capacity, and in any one of the potential changes very rapidly, the potential at the other point to the same extent changes - if only low currents are flowing. This behavior has the consequence that this compound acts as a short circuit for the rapidly changing part of the signal, since the influence of such currents is slower. This fact is utilized in the AC coupling in amplifiers. With such a bootstrap circuit, the output of an amplifier - usually the emitter or source nodes of a transistor - a device connected at its input via a coupling capacitor.

In the analog signal processing, these ( co) is coupling used to a significant increase of the input resistance. A further application of the bootstrap effect is the starting of a NMOS transistor in the high branch of a bridge circuit. Receipt by the voltage of the capacitor can be realized even voltages which are higher than the supply voltage.

Example

Using the example of the half-bridge circuit in the adjacent circuit, the operation will be explained as an example. The two n- channel MOSFET, the MOSFET is referred to below as the low-side FET and the upper MOSFET as the high-side FET, to be alternately conducting, the potential at point B changes from 0 to Uin. n -channel MOSFETs are generally then low when the potential at the gate to the threshold voltage UGS, on, typically 6 to 10 V, is more positive than at the source terminal. The low-side FET is at a sufficiently high input voltage is no problem to put alternately the gate at 0 V and to values ​​of about 10 V to achieve the threshold voltage.

But in order to drive the high-side FET gate potentials are higher than the threshold voltage Uin necessary. This is achieved in the bootstrap circuit with a diode D and a capacitor C in combination with a specific gate driver. The upper gate driver is connected to its reference potential to the center point B. Initialization of the bootstrap circuit, referred to as precharge, the capacitor C is charged to the input voltage by the low-side FET, the lower is switched on, a certain minimum time. When the lower FET is turned off, the inductive load by a possibly still present output current flows for a short time by a non-illustrated in the circuit diagram freewheeling diode. In any case, the potential at point B remains low, until the upper FET becomes conductive. Until then, the gate driver takes the current to the recharging of the gate capacitance of Uin, with even senior diode. These locks as soon as the potential increases in B. When the upper FET is fully turned on, B ( = supply of the driver ) is close Uin and the upper terminal of the capacitor at a potential which is approximately equal to twice the input voltage - both the diode and the driver must be designed.

Since the capacitor C is capable of storing only a finite amount of charge, and discharges through the upper driver, the charging process must be periodically repeated: The capacitor C is charged in the half-period during which the lower FET is conducting, to the input voltage. In the second half period, the capacitor supplies the gate driver and the gate terminal of the high-side FET, which turns on this. The bootstrap circuit is not suitable when the upper FET to remain on for an extended time. Typically is in control circuits for influencing the average potential at the point B, the driving of the bootstrap circuit of pulse width modulation (PWM).

The coil L serves as an energy store in order to produce a constant output voltage Uout in this circuit example. Furthermore, the gate driver includes not shown here called level shifter, which lift the control signal ( PWM) internally to the reference potential of the node B.

Alternatively, the bootstrap circuit is the use of an independent charge pump from the FET driver to supply the high-side FET and the driver. In this design, then the permanent switch on the upper FET is possible. Can continue to be used on the upper side instead of an n -channel FET with a usually somewhat poorer operation data and higher costs associated p-channel MOSFET which is driven by a negative voltage with respect to its source terminal.