How do I backup my mssql databases with powershell

param(
    $serverName,
    $backupDirectory,
    $daysToStoreBackups
)

[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SqlServer.SMO") | Out-Null
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SqlServer.SmoExtended") | Out-Null
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SqlServer.ConnectionInfo") | Out-Null
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SqlServer.SmoEnum") | Out-Null

$server = New-Object ("Microsoft.SqlServer.Management.Smo.Server") $serverName
$dbs = $server.Databases
foreach ($database in $dbs | where { $_.IsSystemObject -eq $False })
{
    if($db.Name -ne "tempdb") #We don't want to backup the tempdb database
     {
    $dbName = $database.Name

    $timestamp = Get-Date -format yyyy-MM-dd-HHmmss
    $targetPath = $backupDirectory + "\" + $dbName + "_" + $timestamp + ".bak"

    $smoBackup = New-Object ("Microsoft.SqlServer.Management.Smo.Backup")
    $smoBackup.Action = "Database"
    $smoBackup.BackupSetDescription = "Full Backup of " + $dbName
    $smoBackup.BackupSetName = $dbName + " Backup"
    $smoBackup.Database = $dbName
    $smoBackup.MediaDescription = "Disk"
    $smoBackup.Devices.AddDevice($targetPath, "File")
    $smoBackup.SqlBackup($server)
    }
    #"backed up $dbName ($serverName) to $targetPath"
}

Get-ChildItem "$backupDirectory\*.bak" |? { $_.lastwritetime -le (Get-Date).AddDays(-$daysToStoreBackups)} |% {Remove-Item $_ -force }
"removed all previous backups older than $daysToStoreBackups days"