Bootstrapping (electrical engineering)
Bootstrapping [ 'butstræp- ] (from English bootstrap , dt.' Boot straps ') describes an electrical circuit in which a change in potential in one part of the circuit suddenly becomes effective in another. This makes use of the effect that capacitors change their voltage only slightly at low currents. They drag a change in potential on one side to the other ( bootstrap effect ). This is used, for example, in amplifier circuits in which the output voltage is fed back to the input with the aid of a capacitor, so-called bootstrap circuits. This feedback causes a drastic increase in the input resistance of the amplifier.
Functionality and application
If two circuit points are connected to one another via a sufficiently large capacitance and the potential changes very quickly at one of the points, the potential at the other point changes to the same extent - if only low currents are flowing. This behavior has the consequence that this connection acts like a short circuit for the part of the signal that changes very quickly, since the influence of the equalizing currents is slower. This fact is also used in the AC coupling in amplifiers. In such a bootstrap circuit, the output of an amplifier - usually the emitter or source node of a transistor - is connected to a component at its input via a coupling capacitor .
In analog signal processing, this (co-) coupling is used to significantly increase the input resistance. Another application of the bootstrap effect is to start an NMOS transistor in the high branch of a bridge circuit. Due to the voltage retention of the capacitor, voltages can even be realized that are above the supply voltage.
example
Using the example of the half-bridge circuit in the adjacent circuit, the mode of operation will be explained as an example. The two n-channel MOSFETs , the lower MOSFET is also known as the low-side FET and the upper MOSFET as the high-side FET , should alternately become conductive, then the potential at point B changes between 0 and U in . N-channel MOSFETs generally have a low resistance when the potential at the gate is more positive by the threshold voltage U GS, on , typically 6 to 10 V, than at the source connection. With the low-side FET , if the input voltage is sufficiently high, it is not a problem to alternately set the gate to 0 V and to values around 10 V to reach the threshold voltage.
However, in order to be able to control the high-side FET , gate potentials higher than U in by the threshold voltage are necessary. In the bootstrap circuit, this is achieved by a diode D and a capacitor C in combination with a special gate driver . The upper gate driver is connected to the center point B with its reference potential . To initialize the bootstrap circuit, also referred to as precharge , the capacitor C is charged to the input voltage by the lower low-side FET being switched on for a certain minimum time. As soon as the lower FET is switched off, any output current that may still be present through the inductive load flows briefly through a free-wheeling diode not shown in the circuit diagram. In any case, the potential at point B remains low until the top FET becomes conductive. Until then, the gate driver takes the current to recharge the gate capacitance from U in while the diode is still conducting. This blocks as soon as the potential at B rises. When the upper FET is fully switched on, B is close to U in and the upper connection of the capacitor (= supply of the driver) is at a potential which corresponds approximately to double the input voltage - both the diode and the driver must be designed for this.
Since the capacitor C can only store a finite amount of charge and is discharged via the upper driver, this charging process must be repeated periodically: The capacitor C is charged to the input voltage in the half-cycle while the lower FET is conductive. In the second half cycle, the capacitor supplies the gate driver and the gate connection of the high-side FET , which turns it on. The bootstrap circuit is therefore not suitable if the upper FET is to remain switched on for a long time. Typically, in control loops to influence the mean potential at point B, the bootstrap circuit is controlled with pulse width modulation (PWM).
The coil L serves as an energy store in order to generate a constant output voltage U out in this circuit example. Furthermore, the gate driver includes so-called level shifters , not shown here , which raise the control signal ( PWM ) internally to the reference potential of node B.
As an alternative to the bootstrap circuit, a charge pump that is independent of the FET driver can be used to supply the high-side FET and the driver. In this design, the upper FET can also be permanently switched on. Furthermore, instead of an n-channel FET, a p-channel MOSFET, which is usually associated with somewhat poorer operating data and higher costs, can be used on the upper side, which is controlled by a negative voltage with respect to its source connection.
literature
- Ulrich Tietze, Christoph Schenk, Eberhard Gamm: Semiconductor circuit technology . 12th edition. Springer-Verlag, Berlin 2002, ISBN 3-540-42849-6 .
- Günther Koß, Wolfgang Reinhold, Friedrich Hoppe: Text and exercise book electronics: Analog and digital electronics. With examples and tasks and solutions . 3. Edition. Hanser Fachbuchverlag, 2005, ISBN 3-446-40016-8 .