The Social Security Administration has released guidance regarding when back payments of SSI (Supplemental Security Income) benefits must be paid in small installments, in larger installments, or in one lump sum. When you are approved for SSI disability benefits, if the backpay owed to you is more than three times the maximum monthly benefit ($735 in 2017), you will be paid in three installment payments, at six month intervals. (If your state pays an SSI supplement, that will be included in deciding whether the backpay you are owed is more than three times the monthly benefit amount.)
There are some exceptions to the rule limiting the amount of the first two payments, which Social Security has clarified. The first and second installment payments can be increased if you need funds to purchase a home, pay for necessary medical needs, or pay off debts related to necessities (housing, food, or clothing). Medical necessities can include a car (if it's necessary to get to doctors' appointments or medical treatment), a mobile phone (if it's needed to contact medical offices), or a computer (if used for Social Security's website or online services). Housing debts can include past-due rent, mortgage payments, property insurance, taxes, and utilities (gas, electric, water, sewer, and garbage).
For more information on SSI backpay, see Nolo's article on disability back payments.