AN966 Datasheet, PDF (15/21 Pages) STMicroelectronics – The front-end stage of conventional off-line converters

AN966 APPLICATION NOTE
Pin 7 (GD) is the output of the driver. The pin is able to drive an external MOSFET with 400mA source
and sink capability.
To avoid undesired switch-on of the external MOSFET because of some leakage current when the sup-
ply of the chip is below the UVLO threshold, an internal pull-down circuit holds the pin low. The circuit
guarantees 0.3V maximum on the pin (@ Isink = 10mA), with Vcc > 3V. This allows omitting the "bleeder"
resistor connected between the gate and the source of the external MOSFET used to this purpose.
Pin 8 (Vcc) is the supply of the device. This pin will be externally connected to the start-up circuit (usu-
ally, one resistor connected to the rectified mains) and to the self-supply circuit.
Whatever the configuration of the self-supply system, a capacitor will be connected between this pin and
ground.
To start the L6561, the voltage must exceed the start-up threshold (13V max.). Below this value the de-
vice does not work and consumes less than 90µA from Vcc. This allows the use of high value start-up re-
sistors (in the hundreds kΩ), which reduces power consumption and optimises system efficiency at low
load, especially in wide range mains applications.
When operating, the current consumption (of the device only, not considering the gate drive current)
rises to a value depending on the operating conditions but never exceeding 4.5mA.
The device keeps on working as long as the supply voltage is over the UVLO threshold (10.3V max).
If the Vcc voltage exceeds 18V an internal zener diode, 30 mA rated, will be activated that clamps the
voltage. In that case the power consumption of the device will increase considerably, but there is no
harm as long as the current is below the maximum rating.
PRACTICAL DESIGN EXAMPLE
To fix the main concepts, here below the wide range demonstration board design is described and the
results of the board evaluation are presented.
The target specifications are summarised in table1. To meet them an appropriate selection, especially
as to critical components, is an important step.
Table 1. Wide Range PFC Target Specification.
AC mains RMS voltage
DC output regulated voltage
Rated output power
Minimum switching frequency
Expected efficiency
Full load output voltage ripple
Maximum output overvoltage
V_irms = 85 to 265V
Vo = 400V
P_o = 80W
f_sw(min) = 35kHz
η > 90%
∆Vo ≤ ±10V
∆VOVP = 60V
POWER MOSFET:
Two parameters are useful to select the suitable device: the minimum blocking voltage V(BR)DSS and the
RDSON because of power dissipation.
The device selected is the STP8NM50 (MDMESH) (V(BR)DSS = 500V, RDSON = 0.8Ω @ 25°C, 1.6Ω @
125°C). The estimated power dissipation is 1.6W total. A 40°C/W heat sink is provided to keep die tem-
perature at a safe value.
BOOST DIODE (D1):
The plastic axial diode STTH1L06 (Turbo2, 1A 600V) has been selected. The power dissipation is esti-
mated about 0.24W.
BOOST INDUCTOR (T):
The inductance value (L) is as high as 0.7 mH, which leads to a minimum switching frequency of 35kHz.
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