I designed cascaded converters and I have switching spikes at the output c6. spikes and zoom on each spike is attached. how to eliminate it? I used snubber circuit

 

What's it for? Its only purpose seems to be heating up R4.

 

I designed cascaded converters and I have switching spikes at the output c6. spikes and zoom on each spike is attached. how to eliminate it? I used snubber circuit

You could have a poor quality inductor with large leakage inductance or poor PCB layout such as thin tracks or wires in switching paths giving large parasitic inductance. Pehaps consider sharing your PCB layout.

 

What you have is two cascaded switching mode power supplies. And the circuits appear to be the same as when they were in that other thread. The operation of a switching power supply is actually quite complex because of the quite lare currents being switched very rapidly. Thus every interconnection that has current flowing is subject to the influence of the actual and effective impedance of that connection. That means that every current loop needs to be recognized and understood so tghat the PCB layout will avoid causing problems like spikes and noise.
Several semiconductor producers have excellent design resources available and I suggest visiting them and learning what they can provide.

 

Also bear in mind that a lot of the spikes you'll see when probing switch mode circuits are actually a probing issue. How are you grounding your scope probes and where are you placing the probes?

 

1. Have you tried adding ferrite on the output. That may provide some help.
2. Did you guess the RC values for the snubber or selected them measuring the circuits parasitic capacitance and inductance?

 

Doing something about them after the fact is unlikely to produce satisfactory results. You need to eliminate them by design from the get go. I have not seen a single engineer get this right the first time - welcome to the club.

 

Read what Bob Pease and JIm Williams had to say about switching mode power supply design. And if those two masters told us that the designs are complex and tricky I do believe them. Aside from the actual circuit, the PCB layout is also tricky.
Papabravo iscertainly correct.

 

What is the highest voltage of spike