Separation and enrichment of platelets in blood plasma play a major role in monitoring the human health condition and transfusion purposes. Although centrifugation is frequently employed to achieve platelet separation, recent advancements in the field of cell separation place microfluidic platforms in advantageous positions over the conventional ones. In this work, we report the development of a PDMS-based passive microfluidics device to separate and enrich platelets in blood plasma. The devices were tested with whole blood in a practically relevant flow rate range of 0.2-0.6 mL/min. One of our proposed devices, comprising gradual constriction-expansion, is able to separate and enrich platelets with 8.7-fold enrichment factor, 25.5% purity, and with minimum activation at a flow rate of 0.4 mL/min using whole human blood. The amount of enrichment and purity is found to be sensitive to the ratio of RBCs and WBCs to platelets in the incoming sample. The ability of the microdevice to handle whole blood at a high flow rate, and enrich platelets in plasma with minimal activation, makes the microdevice unique. These results also demonstrate the strong effect of the RBCs and WBCs to platelet ratio on the amount of enrichment and purity that can be obtained from passive microdevices.