Alternating DC

Do you mean a metal oxide varistor (or MOV)?

http://en.wikipedia.org/wiki/Varistor

A small capacitor may do the job also without distorting the pulse too much (but I don't think this is what you want).

What load you driving?

A common method is "snubbing" which is usually using diodes that pass the spikes back from the output to the source and common rails or a RC circuit to attenuate the spikes.

The diodes sort of recycle the spikes back into the supply side of the system and a RC snubber absorbs them in a way. Its common to see both used together though.

Large voltage spikes are almost always caused by the the inductance of the circuit being driven insisting that the current cannot change that fast. Depending on the speed of the turn on and off of the switches, this inductance can be just the nanohenries of inductance from cabling. All snubber networks of one type or another provide an alternate path besides the breakdown characteristic of the transistors. I prefer a diode that seems to be permanently reverse biased to the initial current flow. But when turning off the inductor the current instead now flows in the same direction through the inductor but now through the diode. An example of this is the diode found across a relay's coil. Counter-intuitively the speed that the current stops flowing through the inductor is inversely proportional to the impedance of the path. So placing a resistor in series with the diode will speed up magnetic discharge but then produce a larger voltage spike.

Depends on if you want to clamp them or eliminate their production. A low value series resistance can attenuate their production if it doesn't interfere with circuit operation. In effect, this lowers the "Q" of the inductor so that it doesn't "ring" due to the large amount of high frequency energy that results from the fast rise or fall time. No matter what you do you ultimately must dissipate energy as heat. Even diodes will incur losses due to switching times so you must use fast diodes to minimize this but ultimately the energy has to go somewhere. MOV's are ineffective since they are too slow for such an application and do not very precise nor sharp "knee" (zener threshold).

I think it is "Filtering". one/ two capacitors in between inductor shall filter our out negative. Please correct if I am wrong!

This is a very bad idea. All you do is create resonant circuit and/or consume massive power depending on where you put them.

The trade term used to communicate what you have described is "clipping voltage." Another term, "clamping voltage" uses a capacitor to keep the voltage level close to the positive peaks between pulses thus creating a relatively constant voltage level which is good enough to drive simple DC circuits.

In the diagram below the output of the AC source is clipped by a Diode. The direction of the apex of the black triangle that represents the diode is important. If it points the opposite way then the positive peaks would be clipped off instead of the negative peaks.

This diagram shows a clamped voltage derived from an AC source. The blue lines represent the voltage dissipating from the capacitor. From the perspective of the load (Vo) the voltage level appears to be constant.

Diagrams coutesy of http://people.seas.harvard.edu/~jones/es154/lectures/lecture_2/diode_circuits/diode_appl.html

The post has nothing to do with this. It is not a question about a power supply. It is about ringing on the edges of a square wave coming from a PWM (pulse width modulation) circuit.

Correct term is Square-Wave; Alternating-DC

Some points need clarification:

1.) (+) & (-) pulses refferred to 0V & are above & below the 0V base

or

2.) base is 1/2 the level of pulses

Voltage level ?

Your question is ambiguous needs drawing of your output waveform.

In any case a single diode in series [in proper direction will work 7 a Choke-Input filter is recommended.

A bridge rectifier can be used in case of 1.) above

The question is clear enough if you are versed in the type of circuit he refers to. Refer to my first post and re-read his original.