You can encode I frames only with Adobe Media Encoder. Proceed as follows.
1-Select File>Export>Adobe Media Encoder
2-Select MPEG-2 as format and "PAL MPEG-2 Generic" as Preset
3-Go to the Video tab and select CBR as Bitrate Encoding; set the bitrate to the value you want
4-In the GOP setting set the M frames to 1 and the N Frames to 1 (If I recall correctly M is the number of frames between I frames while N is the number of frames between P frames)
5-Click OK, select a file name and location and click Save.
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