AN829 Datasheet, PDF (6/9 Pages) STMicroelectronics – SEMICONDUCTOR KIT FOR

AN829 APPLICATION NOTE
VOLTAGE ERROR AMPLIFIER:
Operating with P_out = 3000W and V_out = 400Vdc, the mean I_out is 7.5A. The corresponding 100Hz capacitor cur-
rent is defined by: I_capa = (P_out / V_out) cos 4!ft So the peak current is: P_out/V_out = 7.5A
Then the output voltage ripple V_r can be evaluated as:
V_r = 7.5A / 2π × 100 × 3500 × E-6 = 6Vpeak
Voltage divider R141/R142 attenuates the output voltage ripple to:
V_ri = 6Vpeak x 5.1/400 = 0.0765Vpeak
Assuming 3% peak voltage ripple at the voltage error amplifier output, ripple can be evaluated as:
Vro = (5.1V - 1.27V) 0.03 = 0.115Vpeak
where the first term is the output error amplifier voltage range.
Then the global voltage error amplifier gain at 100Hz must be:
G_vea = V_ro/V_ri = 0.115/0.0765 = 1.5
as G_vea # 1 / R14 2πf C13
with R14 = 47kΩ then C13=22nF
To maintain voltage loop stability requires the placement of a pole at a unity-gain frequency of about 18Hz (see
#5)
then R13C13 = 1/2π ^. 18
and R13 = 390kΩ
LOAD COMPENSATION:
A voltage applied at pin 6 can be used to perform compensation, for example a load feed forward compensation.
With reference to the multiplier section, this input is equivalent to the voltage error amplifier output. If not used,
it should be connected to the reference voltage (pin 11).
MULTIPLIER circuit has the following response:
Imult = K_mult Iac (V_vea-1.27) (0.8LFF-1.27)/Vrms
where:
K_mult is the multiplier constant (.37) V_vea is the voltage error amplifier output. LFF is the voltage at pin 6.
Iac is the current supplied into pin 4. Vrms is voltage at pin 7.
To keep the maximum multiplier output current below 300µA, R4 should be set to 1MΩ, meaning I_ac rms varies
from 95µA to 270µArms.
The previous equation gives: I_mult # 60µArms with Vin=95Vrms and Pout=1400W, or Vin=195Vrms and
Pout=3000W.
CURRENT AMPLIFIER SECTION:
Using a low R_DSon Power MOSFET as current sense instead of a resistance leads to a reduction of the parasitic
inductance and allows the heat generated to be dissipated in the heatsink. Using the specifications of the
STH80N05, R_sense max is 11m_W at 25°C and 15m_W at 80°C.
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