Converter In-Rush Thermistors etc, UPDATE -4

BFL13 wrote:
So everybody, need a hand here with this page 4/12 stuff. I can't figure out what is the "in rush current" and "the allowed in-rush current" and the "desired current limit" which??? is the difference between them??? So the thermistor only has to stop that much of it???

We have that rule of thumb using 1/3 of the in rush current, but is what info you get from knowing the diode bridge spec the same thing as the in- rush you would see on a scope that you take 1/3 of?

I just want to know (I think!) the desired limit amps to use with that 170v that keeps getting waved around so I can get minimum R :(

EDIT: reading back in the thread I see where there is the in-rush current and then the steady current. the steady current is supposed to be 13a.

I don't understand the relationship with the 25 to 50a spec of the diode bridge to the 13a steady current. If 13a is it then why do I need R to handle 25 to 50? or do I? :(

A capacitor acts initially as a short circuit when its fully discharged and as an open when fully charged.

When power is first applied, it is possible that the peak transformer secondary supply voltage could be applied to the diode bridge while the capacitor is acting as a short.

During this condition the surge current would be very high and would only be limited by the transformer winding resistance, diode bulk resistance, and capacitor ESR.

The thermister is inserted as a series element to the diode bridge in an effort to reduce the surge current during turn on.

The thermister cold resistance is typically much larger than the other circuit element resistance values so it is used to simplify the calculations.

So a 170V peak voltage divided by the thermister cold value determines the maximum initial surge current that can be created by the capacitor when its uncharged.

Once the capacitor reaches its steady state charge, the current flowing in/out is much smaller so a lower series resistance is needed - the reason a thermister was designed.

The thermister you originally specified has a 5 ohm resistance when cold and then drops to 0.03 ohm when hot - this allows it to limit the charge current during turn-on and maximize load current during steady state.

So to answer your question, the bridge is rated at 25 amps maximum for steady state, which leaves a fairly conservative margin of extra current capacity since actual steady state is 13 amps.

The surge current information of 50 amps was pulled off the data sheet which shows the bridge can handle about 50 amps of current for 100 cycles and still survive.

The transient current condition typically exists for 8-10 cycles so 50 amps is probably reasonable.

However, many designers do use maximum bridge steady state current values when sizing the thermister.

EDIT: spelling/grammer