Switching losses

As switching losses ( english switching losses ) is understood in the electronics , especially in the power electronics and digital technology , those electrical services , performing in a semiconductor switches are implemented during switching and power off on this and thus occur as losses.

Together with the conduction losses - that flow heat dissipation which occurs during the conductive phase of the electronic switch - give the switching losses , the total power loss which occurs in a semiconductor switch.

Cause and occurrence

Electrical power is defined as the product of current and voltage . If a semiconductor switch is in the blocking state, the voltage that drops across it is at a maximum, but the current through the switch is zero due to the blocking state. Accordingly, no power is implemented in the switch itself. If the switch is in the conductive state, the maximum current flowing through the switch is, however, the voltage across the switch is minimal - ideally zero. This means that almost no power is converted even when the switch is on. The power that occurs due to the minimal residual voltage at the semiconductor switch when it is switched on corresponds to the on-state losses.

Between these two power loss minima, i.e. during the time when the semiconductor switch neither completely blocks nor fully conducts, neither the voltage at the switch nor the current through the switch is zero, which means that a considerably higher power is converted in the switch during the switching process.

Since all electronic switching elements, such as bipolar transistors , field effect transistors , diodes or thyristors , have a finite switching time - i.e. they cannot instantly switch from the conductive to the blocking state and vice versa - when switching, the range in which both the voltage and the voltage are passed is inevitable the currents are not zero and power loss is converted. There are many reasons why the switching time of electronic switches is finite. In the case of field effect transistors, for example, the gate capacitance must first be reloaded before the transistor switches. In the case of bipolar transistors and diodes, on the other hand, charge carriers have to be removed when they are switched off , which delays the switch-off process.

Switching losses therefore only occur during the switchover and therefore only play an important role if semiconductor switches are switched frequently per unit of time, i.e. are operated at a high switching frequency . This is especially the case with DC voltage converters , since a high frequency enables small components such as coils and capacitors . In digital technology too, for example in processors , many transistors are switched at a very high frequency, which means that the switching losses are decisive for the total power loss of the system.

Mathematical description

To simplify the mathematical description of the switching losses, it is assumed that the resistance of the transistor changes linearly during switching. In addition, the transmission losses, as shown in the diagram, are not taken into account. All components (except the MOSFET) are assumed to be ideal.

The switch-on losses can be calculated according to the same considerations. The addition of the switch-on losses and the switch-off losses result in the total switching losses of the semiconductor switch.

Switching off a MOSFET on an ohmic load

Circuit of a MOSFET with an ohmic load

Representation of the current, voltage and power ratios when switching off a MOSFET with an ohmic load

The voltage at the transistor during switch-off as a function of time is given by:

${\displaystyle u={U_{+}}\cdot {\dfrac {t-t_{{\text{off}}1}}{t_{{\text{off}}2}-t_{{\text{off}}1}}}}$

The time at which the shutdown begins is denoted by and the time at which the shutdown ends with . ${\displaystyle t_{{\text{off}}1}}$ ${\displaystyle t_{{\text{off}}2}}$

The current through the transistor during switch-off as a function of time can be specified as follows:

${\displaystyle i=I_{\text{Ton}}-{\biggl (}{I_{\text{Ton}}}\cdot {\dfrac {t-t_{{\text{off}}1}}{t_{{\text{off}}2}-t_{{\text{off}}1}}}{\biggr )}}$

The power converted at the transistor during switch-off as a function of time results in:

${\displaystyle p=u\cdot i={U_{+}}\cdot I_{\text{Ton}}\cdot {\dfrac {t-t_{{\text{off}}1}}{t_{{\text{off}}2}-t_{{\text{off}}1}}}\cdot {\bigg (}1-{\dfrac {t-t_{{\text{off}}1}}{t_{{\text{off}}2}-t_{{\text{off}}1}}}{\bigg )}}$

During the entire switch-off process, the following energy is converted in the transistor:

${\displaystyle \int _{t_{{\text{off}}1}}^{t_{{\text{off}}2}}p\,\mathrm {d} t={U_{+}}\cdot I_{\text{Ton}}\cdot \int _{t_{{\text{off}}1}}^{t_{{\text{off}}2}}{\dfrac {t-t_{{\text{off}}1}}{t_{{\text{off}}2}-t_{{\text{off}}1}}}\cdot {\bigg (}1-{\dfrac {t-t_{{\text{off}}1}}{t_{{\text{off}}2}-t_{{\text{off}}1}}}{\bigg )}\,\mathrm {d} t={\dfrac {{U_{+}}\cdot I_{\text{Ton}}}{6}}\cdot ({t_{{\text{off}}2}-t_{{\text{off}}1}})}$

If the transistor is now switched with the switching frequency f , the turn-off losses result in:

${\displaystyle P_{\text{S,off}}={\dfrac {{U_{+}}\cdot I_{\text{Ton}}}{6}}\cdot ({t_{{\text{off}}2}-t_{{\text{off}}1}})\cdot f}$

Switching off a MOSFET on an inductive load

Circuit of a MOSFET with inductive load and free-wheeling diode

Representation of the current, voltage and power ratios when switching off a MOSFET with an inductive load

The voltage at the transistor during switch-off as a function of time is given by:

${\displaystyle u={U_{+}}\cdot {\dfrac {t-t_{{\text{off}}1}}{t_{{\text{off}}2}-t_{{\text{off}}1}}}}$

Since the current through an inductance cannot change suddenly due to Lenz's rule , it continues to flow through the transistor at almost the same level during the switch-off process until the voltage at the transistor assumes supply voltage potential. From this point the freewheeling diode begins to conduct and the current commutates from the transistor to the diode. For this consideration it is assumed that the inductance and thus the energy stored in the initial current is very large in relation to the energy that is converted during the switching process. Thus the current impressed in the inductance hardly changes and can be assumed to be constant in simplified terms.

${\displaystyle i=I_{Ton}}$

The power converted at the transistor during switch-off as a function of time results in:

${\displaystyle p=u\cdot i={U_{+}}\cdot I_{Ton}\cdot {\dfrac {t-t_{{\text{off}}1}}{t_{{\text{off}}2}-t_{{\text{off}}1}}}}$

During the entire switch-off process, the following energy is converted in the transistor:

${\displaystyle \int _{t_{{\text{off}}1}}^{t_{{\text{off}}2}}p\,\mathrm {d} t={\dfrac {{U_{+}}\cdot I_{Ton}}{t_{{\text{off}}2}-t_{{\text{off}}1}}}\int _{t_{{\text{off}}1}}^{t_{{\text{off}}2}}{t-t_{{\text{off}}1}}\,\mathrm {d} t={\dfrac {{U_{+}}\cdot I_{Ton}}{2}}\cdot ({t_{{\text{off}}2}-t_{{\text{off}}1}})}$

If the transistor is now switched with the switching frequency f , the turn-off losses result in:

${\displaystyle P_{\text{S,off}}={\dfrac {{U_{+}}\cdot I_{Ton}}{2}}\cdot ({t_{{\text{off}}2}-t_{{\text{off}}1}})\cdot f}$

Reduction of switching losses

In power electronic circuits, the switching losses can be reduced primarily by reducing the switching frequency. In practice, the switching frequency should not be chosen higher than necessary. Basically, the switching frequency should be selected so high that the switching losses caused by it are not higher than the transmission losses.

Another way of minimizing switching losses is to minimize switching time. Depending on the semiconductor switch used, this can be done by higher control currents and corresponding power drivers . As a result, the switching time can be significantly influenced and reduced, especially with MOSFETs. However, a short switching time and a correspondingly rapid change in current result in significantly poorer EMC properties , which is why in practice higher switching losses are deliberately accepted in some cases.

Furthermore, various relief networks can be used that significantly improve the current and voltage ratios during the switching process and thus reduce the switching power loss. In the simplest case, a capacitor can be connected in parallel to the transistor, which takes over the load current when it is switched off, whereby the transistor can be switched off without current - and thus without power loss. In order not to have to accept higher switch-on losses when the transistor is switched on again, much more complex circuits are necessary that discharge the capacitor before switching on (ringing processes).

In the case of digital circuits, lower cross currents are achieved by reducing the operating voltage, which in turn cause lower power losses during switching.

See also

• Snubber

literature

• Ulrich Schlienz: Switching power supplies and their periphery 3rd edition, Vieweg & Sohn Verlag | GWV Fachverlage GmbH, Wiesbaden, 2007, ISBN 978-3-8348-0239-2
• Franz Zach: Power Electronics: A Manual. 2 volumes, 4th edition, Springer-Verlag, Vienna, 2010, ISBN 978-3-211-89213-8

Web links

• Lecture notes on the subject of power electronics with a description of relief networks, University of Bremen. (PDF; 1.6 MB)