# how to calculate the duty ratio of L4981???

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

#### davy_shi

anyone know how to calculate the duty ratio of L4981???
i just want to use this IC for a battery charger, but don't know how to use it,

#### Ouroboros

How exactly do you want to connect the L4981 in a battery charger?

The chip is a boost power-factor-correction circuit, and is normally used to convert 120/230V input mains to 400V dc, while keeping the input current from the mains at close to a power factor of one.

I use the L4981B as the PFC stage in a 300W off-line SMPSU that I have designed here at work. It seems a strange chip to use as part of a battery charger.

#### davy_shi

L4981b for battery charger

i mean the circuit is one phase bridge with two switch, 220v AC input and 36VDC output ,i want to use L4981B to control the two switch in the bridge,

may be i should pose my circuit here

#### davy_shi

any suggestions for my design

well, if i want to design the battery charger,with 220vac input and 36vdc 5a output ,could anyone give me any suggestion please?

#### steveu

The duty cycle output of a PFC chip varies; follows the 120Hz/100Hz full wave rectifier output, so that the input current flows for (most of) the whole AC power cycle, not just on peaks. For a 240VAC input, when the AC input voltage is at 45º, the duty cycle will be about 30%, falling to near zero% at the 90º peak. With lower voltage input, the duty cycle is higher. The PFC circuit dumps a constant ~400VDC through a single diode to the main input storage capacitor. That is then the input for another switcher, the one that produces useable voltages like 12V, 5V etc.