AN829 datasheet(6/9 Pages) STMICROELECTRONICS

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AN829 APPLICATION NOTE

6/9

VOLTAGE ERROR AMPLIFIER:

Operating with Pout = 3000W and Vout = 400Vdc, the mean Iout is 7.5A. The corresponding 100Hz capacitor cur-

rent is defined by: Icapa = (Pout / Vout) cos 4!ft So the peak current is: Pout/Vout = 7.5A

Then the output voltage ripple Vr can be evaluated as:

Vr = 7.5A / 2π × 100 × 3500 × E-6 = 6Vpeak

Voltage divider R141/R142 attenuates the output voltage ripple to:

Vri = 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:

Gvea = Vro/Vri = 0.115/0.0765 = 1.5

as Gvea # 1 / R14 2πf C13

with R14 = 47k

Ω 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 = 1/2

π . 18

and R13 = 390k

Ω

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 = Kmult Iac (Vvea-1.27) (0.8LFF-1.27)/Vrms

where:

Kmult is the multiplier constant (.37) Vvea 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 300

µA, R4 should be set to 1MΩ, meaning Iac rms varies

from 95

µA to 270µArms.

The previous equation gives: Imult # 60µArms with Vin=95Vrms and Pout=1400W, or Vin=195Vrms and

Pout=3000W.

CURRENT AMPLIFIER SECTION:

Using a low RDSon 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, Rsense max is 11mW at 25°C and 15mW at 80°C.

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