PDE solution positivity constraint

I have a 2d PDE and NDSolve returns a solution that takes negative values but I want the function to be positive

     f[x,y]>=0

everywhere. How do I include this simple constraint?

DreamProxies - Cheapest USA Elite Private Proxies 100 Cheap Private Proxies 200 Cheap Private Proxies 400 Cheap Private Proxies 1000 Cheap Private Proxies 2000 Cheap Private Proxies 5000 Cheap Private Proxies ExtraProxies.com - Buy Cheap Private Proxies Buy 50 Private Proxies Buy 100 Private Proxies Buy 200 Private Proxies Buy 500 Private Proxies Buy 1000 Private Proxies Buy 2000 Private Proxies ProxiesLive.com Proxies-free.com New Proxy Lists Every Day Proxies123.com Buy Cheap Private Proxies