AN827 Datasheet, PDF (2/20 Pages) STMicroelectronics – A 500W HIGH POWER FACTOR WITH THE L4981A

AN827 APPLICATION NOTE
POWER COMPONENTS SELECTION
The power component values and types are derived and selected in the next section. Please refer to Figure 2,
500 Watt Demoboard Schematic.
Input Diode Bridge
The input diode bridge, D1, can be a standard slow-recovery type. The selection criteria include the maximum
peak reverse breakdown voltage, maximum forward average current, maximum surge current and thermal con-
siderations.
Maximum peak reverse voltage:
V_prv = V_irmsmax ⋅ 2 ⋅ 1.2(safety m arg in) = 264V ⋅ 2 ⋅ 1.2 = 448V
Therefore use a 600V rated diode.
Maximum forward average current:
I_rmsmax = _V-----_r-_m--^P---_s-^O-_m---^U--_i-_n-^T---_â‹---_n-- = ₈----₈-⁵---⁰-_â‹---₀₀---_.--₉- = 6.31A
I_fave = ^I--^r--^m-----^s---^m---_π-^a---^x----^â‹-------²- = -⁶---^.-³----¹--_π---^â‹-------²-- = 2.84A
The thermal considerations require the I_fave rating to be significantly higher than the value calculated. The part
chosen has a I_fave of 25A. Additionally, a small heatsink is required to keep the case temperature within speci-
fication.
Maximum surge current:
There is a significant inrush current at start-up due to the large value bulk capacitor, C6, at the output. There is
minimal impedance from the mains to this capacitor, thus at the peak of the input voltage waveform a large in-
rush current exists. This inrush current can be significantly reduced by some means of current limiting such as
an NTC or triac/resistor combination. The input bridge diode’s maximum surge current rating must not be ex-
ceeded. This demoboard has a low cost and simple NTC for current inrush limiting. The efficiency can be im-
proved by using the triac/resistor scheme, however the cost and complexity increases.
Input Fuse
The input fuse, F1, must open during severe current overloads without tripping during the transient inrush cur-
rent condition or during normal operation. The fuse must have a current rating above the maximum continuous
current (6.3Arms) that occurs at the low line voltage (88V). The fuse chosen for this demoboard has a continu-
ous current rating of 10A/250VAC.
Input Filter Capacitor
The input filter capacitor, C3, is placed across the diode bridge output. This capacitor must smooth the high fre-
quency ripple and must sustain the maximum instantaneous input voltage. In a typical application an EMI filter
will be placed between the mains and the PFC circuit. This demoboard does not have the EMI filter except for
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