AN827 Datasheet, PDF (9/20 Pages) STMicroelectronics – A 500W HIGH POWER FACTOR WITH THE L4981A

AN827 APPLICATION NOTE
Pin 3 OVP (Overvoltage protection input)
The overvoltage protection trip level is determined by the voltage divider across the output bulk capacitor, C6.
The resistor values R11, R21 and R22 are calculated as follows:
R_-----2_----1R_-----+1---1-R_-----2_---2_{-- =} ^V_-----o_---u_---t_-V-+---r--e^∆_---f--^V_----o_---u_--t _{– 1 =} 4_----0_---0_{-5---.-+-1----}4_---7_{-- – 1 =} 9_----0_---9_----k_{-2---+1----k-}9_----0_---9_----k_-
Where ∆V_out = 47V is the maximum overvoltage limit.
The overvoltage limit selection is dependent upon the voltage rating of the output bulk capacitor (450VDC) and
the power mosfet (500BVdss). Care must be taken that the level is not set too low, thus causing false tripping
of the OVP.
Pin 4 IAC (AC current input)
This pin must be connected through resistors R1 and R2 to the rectified line to drive the multiplier with a current
IIAC proportional to the instantaneous line voltage as shown below:
I_{I A C} ( 88 V )
=
-----^V----ⁱ-ⁿ---^p---^k------
R1 + R2
=
8^----0^---6^----k^{-2---+--⋅---88---8-}0^---6^----k^-
=
77 µ A
I_IAC(264V) = ₈----₀---₆---²-_k----^â‹_+---²--₈-⁶---₀-⁴--₆----_k- = 231µA
Thus I_IAC ranges from 77µA to 231µA. The relationship between I_IAC and multiplier output current, Imult, is de-
scribed in section Pin 8 (MULTOUT).
Pin 5 CA-OUT (Current amplifier output)
The current amplifier output delivers its signal to the PWM comparator. An external network defines the suitable
loop gain to process the multiplier output and the inductor current signals. To avoid oscillation problems, the
maximum inductor downslope (Vout/L) must be lower than the oscillator ramp-slope (Vsrp*fsw). The current am-
plifier high frequency gain can be described as follows:
G_ca = ^RR-----¹¹----⁵⁴- + 1 ≤ _V-^V----_os---_ur--^p-_t--_â‹-^â‹--_R-^f--^S--_s--^W-_e---_n-^â‹--_s-^L-_e-- = ⁵----^.-⁰-₄----₀^â‹---₀₈----⁰-_â‹--^k-₀---_.-^â‹-₀--⁰-₃--^.--₃₅----^m---
Where:
V_srp = 5.0V is the oscillator ramp peak-peak voltage
G_ca is the current amplifier gain
f_sw = 80kHz is the switching frequency
R_sense = 0.033Ω is the parallel combination of R30-32
Thus, use R14=R16=2.7k, and R15=36K.
To define the value of the compensation capacitor, C9, it is useful to consider the open loop current gain, defined
by the ratio of the voltage across the sense resistor and the current amplifier output voltage. The crossover fre-
quency is given by the following equation:
f_c = -₂-^f--^s-_â‹--^w-_π-- = -₂₈---⁰-_â‹---^k_π-- = 12.7kHz
To ensure a good phase margin, the zero frequency, fz, should equal approximately f_c/2.
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