Hi
It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load.
Hi
It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load.
Another advantage in case of RL load is that the freewheeling diode cuts out negative part of the AC voltage not to appear across the load terminals. Thus it prevents decreasing the value of the rectified DC voltage.
In other words, in case of RL without freewheeling diode voltage wave across the load has a positive and a negative portion, the negative portion increases as load inductance increases (due to the current delay). This reduces the (Average) DC voltage across the load. While when there is a freewheeling diode in parallel to the load, the negative portion of the AC voltage does not appear across the load, preventing the rectified voltage decrease.
Generally, a full wave rectifier does not need any extra freewheeling diode.
A Free Wheel Diode (FWD) at the output of a full wave rectifier will:
1) Increase DC voltage for a given firing angle due to the elimination of negative portions of the instantaneous dc waveform in a SCR phase controlled converter,
2) Will reduce the generated ripple voltage on the DC side of a SCR phase controlled converter due to same reason as in above, reducing the filtering requirements.
3) Will improve the input PF in an SCR phase controlled converter due to ending the input current waveform earlier by permitting internal free-wheeling.
However, the above will happen only if (a) the DC side has sufficient inductance either as filter or as part of load, (b) the firing angle is more than 30 deg for 3-phase, only after which the negative portion appears. Note that there will be no benefit in a diode rectifier since the FWD is always reverse-biased.
Also note that use of the FWD prevents operation of the SCR converter in inversion mode.
In general, it is a good practice to put free wheeling diodes (with short reverse recovery time) across a semiconductor switch, even if the latter has integral diode, in order to provide the additional protection. However, the latest high performance switches appear to suggest that it is no longer necessary. But I will check the data sheet carefully. I personally always put an anti-parallel diode for my development projects.
Several colleagues explain why free wheeling diodes should be used. I can only add that Many power switches have anti-parallel diode for the use of free wheeling diodes. All the best, Okan
The question asked for the advantage of a free wheel diode at the output of a full wave rectifier, but why are so many people talking about using antiparallel diodes across semiconductor switches or reduction of arcing? It is not the same.
The question is not clear within the context of today's available technology. The answer one gets depends upon the question asked and the way it is posed. There is no one answer fits all.
"Full wave" rectifiers may utilise both self commutated (diodes/ thyristor's) or forced commutated (TRANSISTOR/GTO / IGBT...). They may also be UNI or BI- directional in terms of current and power flow. The former are uni-directional whereas the latter may be bi-directional in terms of current and power flow. They may be also be controlled or uncontrolled.
As already noted above by Hossein , flywheel diodes are not normally required, as with self-commutated devices any reverse voltage will simply maintain conduction of the load current through the device until it passes through a natural current zero (here the rectifier output voltage may go negative). In the case of force commutated devices the device anti-parallel diodes will serve the same function (Rectifier output cannot go negative). As Sujit notes if one wishes to avoid any reduction in output voltage during the self-commutating period a freewheeling diode may be used on the output, in which case it acts much like a forced commutating device or, and this is important, where the output supplies electrolytic capacitors which require a unidrectional voltage.
In general fly back diodes are mostly commonly used with forced commutated devices such as PWM controlled series regulators or DC/DC converters to maintain continuous load current and prevent series device failure.
My suggestion is that when asking a question, apply your mind and think a little before doing so.
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