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 C
– the current ripple in the boost inductor L
– the parasitic capacitances of the boost inductor and power semiconductors, including those associ-
ated with the heatsink
– the operating frequency and also the frequency modulation techniqu e.
i
b
Figure 1. Sem ico nd uc tor Kit for PFC Sc he m ati c .
Lb
C1
STH80N05
89 10
ONE CHIP
L4981A/B
CONTROLLER
November 2003
any
compensation
7
6
13
4
1/KY
X
+-
14VREF
+
STE30NA50-DK
Lp3 Lp4
Lp1
Lp2
Lp5
FREQ. MODUL.(B)
SYNCHRON. (A)
5
+
-
FULL
PROTECTION
+
A
+
20
1
D95IN252A
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AN829 APPLICATION NO TE
SEMI-CONDUCTOR KIT
The semiconductor kit consist s of an L4981 controller, an S TE36N50-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 PW M 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 t he mult ipli er i n order t o 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 os cillator 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 m ounted directly on a PCB, this m odule integrates a low
R
DS(ON)
Power MOSFET and a TU RB O SWI TC H
TM
Diode. Putting these tw o components in a s i ngle i solat ed
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
, meaning also that the R
(br)DSS
of the MOSFET can be l ower - in this case it is 0.14 Ohm. Both the
DS(on)
current and avalanche handling capabilities o f the po w er M O SF ET sec tio n a re s pe cif ied 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 TURBOSWITCH
TM
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 600V
and a maximum VF of 1.5V at the rated average forward current (I
RRM
Fav
= 20A).
STH80N05 Power sense:
Using a high density low voltage Power MOSFET for current sensing has many advantages:
– low resistance, typically 10m2
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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):
V
= 230Vac +10% -15% (f=50Hz)
in rms
I
= 16Arms
in rm s
P
= 3000W
out
V
= 400Vdc
out
or
V
= 120Vac +20% -20% (f=60Hz)
in rms
I
= 15Arms
in rm s
P
= 1400W
out
V
= 400Vdc
out
Figure 2. 1500W/3000W PFC Schematic.
J1
1
F1
D1
C1
1
J2
R16
+-
R71 R72
C72C71
R20
Dz
STH80N05
R4
IAC
VRMS
R73
S/FM
R2
IPK
2
4
7
16
Lb
+
-
R8 R9
MOUT
8
LFF VREF
R19
D2
Q1
VCC
19
611
TURBOSWITCH DIODE
ISOTOP
STE30NA50-DK
C19
R
CAOUT
20
GF
R
GN
D
GF
GDRV
C
P
R
Z
ISENSE
95
C
Z
L4981
C
C
SGNDPGND
1
10
14
13
3
1217 18
SSCCOSCROSC
C18R17 C12
O10
to
O14
1
1
VFEED
VAOUT
VP
J7
R141
R31
C
O2
to
C
O7
R142
R32
J8
R14
R13
C13
D95IN253A
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