Three.js, Camera rotation around a point - camera

Question

I'm trying to make a camera rotation and zoom around a point.
I don't want to use THREE.TrackBallControls because as i see, it only modify the fov when zooming, it is not what i want.
I'm using this simple equation : http://en.wikipedia.org/wiki/Sphere#Equations_in. To compute a point on the sphere, this point will be the position of the camera.
In three.js
var point = new THREE.Vector3(
radius * Math.cos(theta) * Math.sin(phi),
radius * Math.sin(theta) * Math.sin(phi),
radius * Math.cos(phi)
);
point.add(center);
To manipulate it, i use :
left mouse, for rotation : x coord === theta and y coord === phi
middle mouse, for zooming : zoom in/out === radius
It doesn't work well, especially for 'phi' rotation, maybe because it's define between 0 and PI.
Maybe there is a better solution using Quaternion.Slerp ?
edit :
My camera :
aspectRatio = container.offsetWidth / container.offsetHeight;
camera = new THREE.OrthographicCamera( -aspectRatio * viewSize / 2, aspectRatio * viewSize / 2, viewSize / 2, -viewSize / 2, -10, 10 );
camera.position.x = 0.004;
camera.position.y = 0.004;
camera.position.z = 0.004;
camera.lookAt(new THREE.Vector3(0, 0, 0))
scene.add(camera);
My OrbitControls :
controls = new THREE.OrbitControls ( camera );
controls.rotateSpeed = 1.0;
controls.zoomSpeed = 1.2;
controls.keyPanSpeed = 0.8;
controls.noZoom = false;
controls.noPan = false;
when using left mouse (rotation) the camera.position change.
when using middle mouse (zoom) the camera.position is still the same.
check made in the render function :
function render() {
// update camera controls
controls.update();
console.log(camera.position);
// actually render the scene
renderer.render( scene, camera );
}