MSC60028 Output Inductor & Current Sense Resistor

[japlytic]

I happened to find that the MSC60028 was the only PFC chip available from my supplier.

Two problems I encountered when calculating for an active PFC for a 400W and a 500W power supply were:

• The current sensing resistor (2.2 ohms) in Figure 5 of the MSC60028 datasheet needs to be a value that gives a peak voltage of 0.3V across it under minimum load conditions (Page 9 of datasheet).

• The values of Poutmax, fPWM, and L on Page 9 are a bit confusing, because the units of measure have not been specified e.g. A, kHz, mH.

[PeteS in CA]

Re: MSC60028 Output Inductor & Current Sense Resistor

Got a datasheet linky?

You can't get a TI (Unitrode) UC3854?

[japlytic]

Re: MSC60028 Output Inductor & Current Sense Resistor

http://www.alldatasheet.com/datashee.../MSC60028.html - MSC60028 datasheet.

I can't get a UC3854 from my supplier.

[japlytic]

Re: MSC60028 Output Inductor & Current Sense Resistor

Found the calculations for the boost inductor on the UC3854 (had units of measure ) were about the same as the MCS60028.

Boost Inductor
Ip=Peak inductor current in A
Poutmax=Maximum output wattage

Vout=Output DC voltage
Vinmin=Minimum DC voltage after bridge rectifier (multiply AC input voltage by 1.414 to get DC voltage)

fPWM=PWM frequency in Hz.
iL=Ripple Current in A (0.2 * Ip)

Capacitor Selection
Poutmax=Maximum output wattage
T=Holdup time in mS
V2outmax=Maximum output voltage threshold
V2outmin=Minimum output voltage threshold
C=Capacitor in uF.

Current Sense Resistor (not shown on datasheet)
Ip=Peak inductor current in A
iL=Ripple Current in A (0.2 * Ip)
Vrs=Normal Current Sensing Voltage
Vrs(pk)=Peak Current Sensing Voltage
0.3V under minimum load conditions

I still need help with the current sensing resistor calculation to obtain 0.3V across it when the supply has a minimum load.