Concatenate the files in the following order:
Root CA Certificate - AddTrustExternalCARoot.crt
Intermediate CA Certificate - COMODORSAAddTrustCA.crt
Intermediate CA Certificate - COMODORSADomainValidationSecureServerCA.crt
Your EssentialSSL Certificate - www_example_com.crt *
(*) Exclude the real certificate "www_example_com.crt" if CA bundle is entered separately.