Homomorphic encryption is the type of encryption scheme which does not require decryption for encrypted data in order to do arithmetic computations on this encrypted data. Homomorphic encryption methods have been developing using several approaches. CKKS is the HE scheme which supports an approximate addition and multiplication of encrypted messages, together with a new rescaling procedure for managing the magnitude of plaintext. After CKKS is revealed, it is widely used in privacy preserving in data analyses. However, as the computations are performed, the ciphertext modulus in CKKS decreases and eventually, more computations cannot be done and therefore CKKS needs bootstrapping procedures. By far, the Residue Number System (RNS) variants of CKKS and new approximation methods for CKKS and RNS-CKKS are proposed. In this paper, the new approximation methods for CKKS are analyzed and advantages and disadvantages of latest variants are discussed. The aim of this work to compile various papers about bootstrapping of HEAAN and give brief survey of those papers.