AN827 Datasheet, PDF (12/20 Pages) STMicroelectronics – A 500W HIGH POWER FACTOR WITH THE L4981A

AN827 APPLICATION NOTE
Pin 13 Vva-out (Error amplifier output)
To ensure system stability, the compensation network must be designed with sufficient phase margin. Addition-
ally, the system must not regulate the twice mains frequency output ripple voltage in order to avoid line current
distortion. The compensation capacitor, C10, can be calculated as follows:
C10 > ₄-----_â‹---_π-----_â‹---_f--_m----_a---_i--_n---_s----_â‹--₍---_R-¹----₉-----+----_R-----₁----₀---₎----_â‹---_G-----_e---_a- = K_{a R}-----₉-^∆----^V_+----^o-_R--^u---₁^t---₀--
Where:
R9 + R10 are the resistors from the output voltage feedback resistor divider
G_ea is the small signal gain of the error amplifier
∆V_out is the maximum output voltage ripple
Ka = ₆--¹--₀-- for 50Hz and ₇--¹--₂-- for 60Hz mains frequency
C₁₀ > ₆--¹--₀-- ⋅ ₈----₂--⁸-₄----_k- = 162nF , therefore use standard value 220nF
The voltage open loop gain contains two poles at the origin, causing stability problems. This can be avoided by
shifting the error amplifier pole from the origin to near the crossover frequency. This can be accomplished by
placing a resistor, R19, in parallel with the compensation capacitor, C10. The crossover frequency is calculated
as follows:
f_c =


V^----_o^---_u^---_t^---⋅^---∆^----V^---P--_e^---oa---u-⋅^--t-2^----π^-----⋅^--C^-----_o^---_u^---_t^


2^----π^-----⋅^---(^--R^-----9^-----+----1-R^-----1^---0^----)^----⋅^--C^-----1^---0^{--}
=


4^----0^---0^-----⋅^---3^---.^--8^---2^--5---⋅^-0--2^-0---π^-----⋅^---3^---3^---0^----π^----F^{--}


2^----π^-----⋅^---8^---2^----4^----1k----⋅^---2^---2^----0^----n^---F^{-- }
=
11.77 H z
Use R19 = 120k to increase error amplifier dc gain.
Pin 14 VFEED (Error amplifier input)
This pin is the error amplifier inverting input. This pin is connected to the resistor divider connected across the
boosted output voltage to provide regulation. The boosted output voltage is specified at 400VDC. The resistor
divider network is calculated as follows:
R_-----9_-----+-----R_-----1_---0_{-- = -}8_----2_---4_----k_{-- =} ^V_-----o_---u_---t _{– 1 =} 4_----0_---0_{-- – 1}
R 20
10.6k V_ref
5.1
Use R9 = R10 = 412k
Pin 15 P-UVLO (Programmable supply undervoltage threshold)
This pin may be used to modify the turn-on and turn-off power supply thresholds. This circuit does not employ
this feature and the pin is left floating. The typical turn-on threshold is 15.5V and the turn-off threshold is 10V.
Pin 16 SYNC (In/Out synchronization)
This function allows for synchronization in master or slave mode with other circuits in the system. This demo-
board does not use this function and the pin is left floating.
Pin 17 ROSC (Oscillator resistor)
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