PowerShell Script to backup WSP from Central Admin's solution gallery

For a specific solution backup

$solution = Get-SPSolution $packageName -ErrorAction SilentlyContinue
# Get the File name from Solution file.
$filename = $solution.SolutionFile.Name
# script to get the location where the script is running -- folder location.
$BaseFolder = (Get-Location -PSProvider FileSystem).ProviderPath
# Save the file to Disk
$solution.SolutionFile.SaveAs("$BackupFolder\$filename")

To backup all the solutions from central admin' solution gallery

$BaseFolder = (Get-Location -PSProvider FileSystem).ProviderPath
$SPSolutions = get-spsolution
foreach($solution in $spsolutions)
{
$filename = $solution.SolutionFile.Name
$solution.SolutionFile.SaveAs("$BackupFolder\$filename")
}