ST AN829 Application note
AN829
APPLICATION NOTE
SEMICONDUCTOR KIT FOR
POWER FACTOR CORRECTOR
by J.M. Borgeous
This paper present a new line of P.F.C. dedicated products. Both silicon and packaging have been optimized to reduce system cost, including filtering. The products shown here are offered as a kit for power factor correction.
BOOST CIRCUIT OPTIMIZATION PARAMETERS
Among the available topologies, a boost circuit operating in continuous current mode is the only topology which enables the RFI noise across the input capacitor to be limited, and consequently a lower cost filter is required. Also, the boost inductor stores only a part of the transferred energy, because the mains still supplies energy during the demagnetization phase of the boost inductor - so the magnetic part required is smaller than needed with any other topology. Therefore the boost topology leads to the cheapest solution. Figure 1 shows the general topology of a boost PFC. Its optimization requires careful adjustment of the following parameters:
–the value of the input capacitor Ci
–the current ripple in the boost inductor Lb
–the parasitic capacitances of the boost inductor and power semiconductors, including those associated with the heatsink
–the operating frequency and also the frequency modulation technique.
Figure 1. Semiconductor Kit for PFC Schematic.
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PROTECTION |
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D95IN252A
November 2003 |
1/9 |
AN829 APPLICATION NOTE
SEMI-CONDUCTOR KIT
The semiconductor kit consists of an L4981 controller, an STE36N50-DK power module and an STH80N05 power sense (see figure 1).
L4981A/B controller:
The L4981 operates with an input voltage range of 85V to 270V and uses average current mode PWM control, providing feed forward line and load compensation. Two versions are available: version (A) provides synchronization with the down stream converter, whereas version (B) provides linear frequency modulation, spreading the RFI noise spectrum.
Both versions incorporate overvoltage and overcurrent protection, soft start and under voltage lockout with programmable threshold.
Other features include an on chip voltage reference (2%) which is externally available, a typical starting current of only 0.5mA and separate grounds for the power and signal stages.
L4981 use an optimum current control method. It is an average current control using feed forward line regulation and variable or fixed switching frequency.
The oscillator simultaneously turns on the power switch and starts the ramp of the PWM current control. The average inductor current is compared with the current reference by means of the current error amplifier. It operates as an integrator, allowing the circuit to accurately follow the current reference generated by the multiplier. This current reference is obtained by sinewave modulating the error voltage of the voltage control loop.
A feed forward compensation of the mains voltage has been added to the multiplier in order to keep constant the voltage control loop bandwidth whatever the mains fluctuation. A fourth multiplier input allows external compensation to be applied to the current modulation.
The oscillator can operate at constant or modulated switching frequency. In applications where modulated frequency is used, the RFI noise spectrum can be spread adjusting the depth of modulation by means of an external resistor. Then the maximum inductor current occurs at the minimum operating frequency.
STE30NA50-DK Power Module:
Built in an isolated ISOTOPTM package, which can be mounted directly on a PCB, this module integrates a low RDS(ON) Power MOSFET and a TURBOSWITCHTM Diode. Putting these two components in a single isolated package with very low parasitic inductance and capacitance reduces the component count, and significantly reduces transient overvoltages, and EMI and RFI.
As a result, the design safety margin can be relaxed and the voltage rating of the power MOSFET can be just 500V(br)DSS, meaning also that the RDS(on) of the MOSFET can be lower - in this case it is 0.14 Ohm. Both the current and avalanche handling capabilities of the power MOSFET section are specified at 100°C junction temperature, allowing for maximum utilization of the device. The MOSFET is a low gate-charge type and so its drive requirements are compatible with the 2A peak current capability of the L4981 controller.
The integrated TURBOSWITCHTM freewheeling diode is an ultra-fast, soft recovery device using planar epitaxial technology, and is a part of the STTA series. Its low trr (30ns) keeps the MOSFET switching losses to a minimum. Other ratings are 600VRRM and a maximum VF of 1.5V at the rated average forward current (IFav = 20A).
STH80N05 Power sense:
Using a high density low voltage Power MOSFET for current sensing has many advantages:
– low resistance, typically 10m2
2/9
AN829 APPLICATION NOTE
–Can be mounted on the same heatsink as the ISOTOP
–intrinsic diode for controller input protection
–very low parasitic inductance improving signal/noise ratio.
However, the peak current limitation will be affected by the MOSFET thermal characteristic.
DESIGN RULE EXAMPLE
Taking the following operating conditions (see figure 2):
Vin rms = 230Vac +10% -15% (f=50Hz)
= 16Arms Pout = 3000W Vout = 400Vdc
or
Vin rms = 120Vac +20% -20% (f=60Hz)
Iin rms = 15Arms
Pout = 1400W
Vout = 400Vdc
Figure 2. 1500W/3000W PFC Schematic.
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D95IN253A |
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