Power converter efficiency calculation

The losses in the magnetic, inductive and capacitive components can be controlled and minimized to realize a high conversion efficiency. One of the major causes of efficiency loss in switch-mode circuits are the output diodes. If the output current is 1 A and the forward voltage drop across the diode is 0. FETs consume more power when switching than in a steady on or off state.

This is because their internal gate capacitance must be charged and discharged to switch the output. Peak gate currents of 2 A or more are not unusual. Factoring in parasitics The level of energy efficiency among switch-mode converter topologies varies.

One reason is that the components they use are non ideal. Textbook descriptions of converter topologies assume ideal components and ignore the parasitic effects. It is, however, a fact of life that inductors have capacitive and resistive elements and vice versa. The choices of components used in a switching supply therefore have a large influence on its performance.

Critical components, such as switching and rectifying elements, magnetic components and filter capacitors, all affect both the switching frequency and also the overall efficiency of the converter. In particular, semiconductor switches have many non-ideal properties.

FETs place high peak current demands on the driving circuit, especially the current needed to charge and discharge the parasitic Miller capacitance between gate and drain. Diodes have a parallel equivalent capacitance that slows their switching speed and, of course, the internal forward voltage drop. All these effects depend on frequency, so an inductor can behave as a capacitor at high frequencies, just as a capacitor can behave as an inductor.

Transformers have similar issues. The disadvantage of using a transformer is that the energy transfer from primary winding to secondary winding involves additional losses. Parasitic elements Parasitic effects in transformers include interwinding coupling capacitances for both the primary and secondary windings, a magnetizing inductance of the core, and leakage inductances for both the primary and secondary.

These transformer parasitic effects strongly influence the converter performance. Coupling capacitance causes common-mode EMC problems. Core saturation caused by magnetizing inductance limits the transformer current. Leakage inductances are especially troublesome, reducing efficiency and generating radiated EMI. Leakage inductances are also responsible for the voltage spikes that arise whenever the current changes rapidly in the windings.

Such overvoltages stress the primary switch and secondary diodes, so they must either be sized to withstand the peak voltage or fitted with a parallel snubber network to dissipate the energy in the spikes. However, the energy in the spikes and the power that the snubber has to absorb constitute an energy loss that diminishes the efficiency of the converter. The energy in the spikes and the power that the snubber must absorb can be calculated according to:.

A snubber cannot eliminate the power loss caused by the spikes. The power that would otherwise be dissipated in the switch or rectifier diode is now dissipated by snubber network resistors instead. Besides the spikes caused by the parasitic leakage inductance, any coupled reactive system will also exhibit resonant frequencies. Most transformer-based designs try to either reduce these parasitic elements to a minimum or choose operating frequencies where resonance is not an issue.

However, a quasi-resonant or resonant converter design deliberately encourages resonance by increasing the winding inductance or by adding additional inductors because controlling this resonance can facilitate an efficient converter design. As has been mentioned before, one big source of efficiency loss in any converter is the power dissipation in the output diodes.

Low forward-voltage-drop Schottky diodes can sometimes serve as an alternative for low-power converters, but they are expensive when sized to cope with higher currents. Even so, the forward drop is around mV, so the power loss can still be significant. A big leap forward in efficiency improvement has been the development of synchronous rectification. In a typical circuit with diode rectification, one diode acts as a rectifier and another is a freewheeling diode. Both diodes are alternately loaded with approximately the same current.

The losses from the forward voltage drop in the diodes is just the voltage drop times the diode current. With a typical forward voltage of 0.

The power dissipated in the diode would be 5 W, so the diode would probably have to be heat-sink-mounted to have any useful operating temperature range. Fortunately, FETs can be used as rectifying elements by switching them on during the forward part of the cycle and turning them off during the reverse part of the cycle. Their advantage as fast switches with low on-resistance makes them suitable as rectifiers. The disadvantage of FETs is that they must be actively driven, so there are additional timing and drive circuits required.

More precisely, the efficien What does 1U, 2U or 3U mean? Many rack-mounted power systems are specified as being 1U, 2U, 3U, etc. What does this mean? For electronic equipment racks e. Class 2 or Class II power supplies? What type of LED driver or power supply do I need? Linear vs. Switch-mode Power Supplies. The Power Guy blog focuses on modern switch-mode power supplies and converters. However, to provide the newbie newcomer with some backgro Search This Blog.

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