Hello,
i have a H-bridge voltage inverter that takes a 50V battery and creates a pwm sinus with about 15A, at the end there is a filter inductor between 1-10mH. My question is now, can the output filter inductance be too big? The scenario i have in mind is, when the Mosfet closes and the inductor tries to keep the current flowing and the drain voltage of the Mosfet should go down and exceed the rated drain-source voltage which destroys the Mosfet.
could this become a problem?
Hello there,
In a DC to DC converter the inductor helps filter the pulsing that comes from the transistor stage. It works with the filter capacitance, and the two form a filter that helps smooth out the DC. The inductance can not be too small or it will allow peak currents that are too high and that causes an RMS current into the cap that can be too high for the rating of the cap which in turn can blow out the cap. Thus the choice of inductor plays with the choice of cap to some degree.
Because the inductor works in conjunction with the output cap and the ESR of the inductor is always small, that means there is also a natural frequency of oscillation that will occur and that act to prevent perfect control of the output level in a normal feedback arrangement. The inductor energy is proportional to the inductance L/2*i^2 so the larger the inductor the more energy it stores, and this wants to dissipate even in the event that the control circuit wants to reduce the output due to changing environmental conditions. This means the speed of response is reduce with larger inductor values. The tradeoff is to get decent response as well as decent filtering action.
An AC converter however is usually of low frequency like below 100Hz, which means speed of control usually isnt as much of a concern. However, as you noted (although not exactly how it works), when the transistor turns on it has to take the full inductor current at the time it turns on (not quite as much about voltage). That's not really too much of a problem though because the transistor has been doing that all along, since the converter was first turned on. The problem comes in if we run into a condition where the output load current increases beyond the normal load current, and then when the transistor turns on it may have to deal with a rising current, and there is no way to turn it off because the inductor keeps pumping out current until the energy is dissipated. This can be more than normal, but there is no way for the converter to stop this because the control circuit does not do that. This means that the bigger the inductor the longer it will take the converter to respond to an overload. Depending on how long this takes the load may be damaged.
So the trade off is filtering action vs speed of response. The larger the inductor the better the filtering and the less RMS current the output cap has to put up with, but the speed of response is increased with larger inductance.