If you are working for a top 500, very likely you have already signed some docs to surrender the rights to any work related inventions to your emplyer (except that you will be named as inventor)
But if you can prove that you conceived the invention well before your employment and what you have done at work was just perfecting the invention. The invention might belong to you. Or you can reach a deal with your employer.
Before your employer files US application you can file China application (if you believe you are the rightful owner) because China uses first-to-file principle whreas US applies first-to-invent principle. But very likely your employer will sue you on whatever grounds.
After your employer has filed US application, it can use patent treaty to claim one year priority to prevent anyone from acquiring foreign patents.
