The Secretary shall fix premium charges for the insurance of mortgages under this subchapter, but such charges shall not be more than 1 per centum per annum of the amount of the principal obligation of the mortgage outstanding at any time, without taking into account delinquent payments or prepayments. In addition to the premium charge, the Secretary is authorized to charge and collect such amounts as he may deem reasonable for the analysis of a proposed project and the appraisal and inspection of the property and improvements. Where the principal obligation of any mortgage accepted for insurance under this subchapter is paid in full prior to the maturity date, the Secretary is authorized to require the payment by the mortgagee of an adjusted premium charge. This charge shall be in such amount as the Secretary determines to be equitable, but not in excess of the aggregate amount of the premium charges that the mortgagee would otherwise have been required to pay if the mortgage had continued to be insured until the maturity date. Where such prepayment occurs, the Secretary is authorized to refund to the mortgagee for the account of the mortgagor all, or such portion as he shall determine to be equitable, of the current unearned premium charges theretofore paid. Premium charges fixed under this section shall be payable by the mortgagee either in cash, or in debentures which are the obligation of the General Insurance Fund at par plus accrued interest, at such times and in such manner as may be prescribed by the Secretary.