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Error amplifier also amplifies the switching frequency

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Remcoremcoremco

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error amplifier

Hi,

I have a question about the error amplifier in a voltage mode controlled buck converter.

I designed a TYPE III erroramp with the correct transfer function to give me a stable loop in combination with the LC plant and the PWM modulator. According to the books, so to say. In simulation everything works fine, exept for one thing: the error amplifier also amplifies the switching frequency quite a lot. So its output is not a more or less DC-like voltage but -in my case- disturbed with a 1MHz signal of a few 100 mV.

The PWM modulator still works, but there is a lot of jittering.

I want to filter the 1MHz but that's not possible. If I do so, I get phase shifts and my overall loop is not stable anymore.

In this link you find a typical transfer of type 3 compensated error amp. You see that also here at 1MHz the erroramp has quite some gain.

http://a330.g.akamai.net/7/330/2540/20090623155729/www.edn.com/contents/images/6666155_fig4.gif

What can I do to get rid of the 1MHz component on the output of the error amp?

Regards,
Remco[/img]

Added after 30 minutes:

Hi,

Here a picture to clarify my problem. The upper one is the ideal situation, the lower one is what I see in simulation.

The 1MHz component on the output cap. is amplified by the error amp. It's quite large and disturbed and because of it, the duty cycle is jittering. I need to filter is to get the situation in the first picture back. But filtering introduces phase shifts that compromises my loop stability of the complete DC DC system.
 

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