Fairchild FQP7N60 The use of qfets in a flyback converter Datasheet

July, 2000
AN9008
The Use of QFETs in a Flyback Converter
By Il Soo Yang
Introduction
Power supply designers face many challenges in designing more efficient and cost-effective power
supplies. Efficiency is a major consideration in designing switching power supplies. Many factors in
the design process such as the input filter capacitance, transformer core geometry and construction, output rectifier, and switching device etc., affect the efficiency of switching power supplies.
Among the losses all components generate, switching device losses occupy about 30%. Hence,
selecting MOSFETs with optimum efficiency and high reliability is very crucial in power supply
design. This application note compares the key characteristics, power losses, and efficiency of the
new QFET and a conventional MOSFET in a 60 watt flyback converter operated at 180 to 265
VAC.
QFET Characteristics
Almost all the power supplies used in TVs, VCRs, PCs, fax machines, and other home appliances
rely on a switching circuit to convert the AC wall power to DC power or DC to AC. Thus, they are
referred to as switched mode power supplies. To obtain high efficiency, it is crucial for designers to
select switching MOSFETs to give very low losses in the circuits. MOSFETs must exhibit low conduction and switching losses with safety qualifications. Fairchild Semiconductor, in extending its
commitment to develop high quality MOSFETs, now offers new high efficiency QFETs for switched
mode power supply applications.
A power QFET, rated at 600V and used in a 60 watt flyback converter, features a gate charge rating which is 45 percent lower than existing devices for improved switching and drive efficiency. Figure 1 compares the new QFET FQP7N60 with its conventional MOSFET counterpart. By using
unified singular well stripe technology, the Miller capacitance of the new QFET is reduced by about
40 percent.
Rev D, July 2000
1
:
QFET
FQ P7N 60
Figure 1: Gate Charge Improvement
Balanced with gate charge improvement, the on-resistance [Rds(on)] goes down by about 20 percent with respect to previous devices versus drain current. Figure 2 shows the improvement of onresistance in a QFET compared with a MOSFET.
Conventional MOSFET
QFET (FQP7N60)
On-Resistance : Rds
(on)
[ Ω]
1.10
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
1
2
3
4
5
6
7
Drain Current : Id [A]
Figure 2: On-resistance Improvement vs. Drain Current
The combined improvement of gate charge and on-resistance in this 60 watt flyback converter
leads to a more efficient system because of the reduced turn-off conduction loss. It is worth
emphasizing that QFETs offer designers significant improvements in terms of lower overall system
cost due to lower gate driver requirements, a smaller heat sink, and narrower PCB. Table 1 illustrates the features which are useful in flyback converters and other applications.
Table 1: Qg and Rds(on) Improvements
Voltage Rating
Device
On-resistance
Gate Charge
Package
600V
Conventional Part
1.2 Ω
65 nC
TO-220
600V
FQP7N60(QFET)
1.0 Ω
38 nC
TO-220
Rev D, July 2000
2
Performance in a Flyback Converter
Figure 3 shows the design of a commercially available 60 watt flyback converter with two outputs
(+160V, +15V), operating at a switching frequency of 80kHz and an input voltage of 220VAC. This
type of switching power supply is used for applications, such as monitors, TVs, and miscellaneous
instruments, requiring multiple output voltages . This discontinuous mode flyback converter, using
a KA3882 current mode controller, features good voltage tracking with the use of pulse by pulse
current sensing on the primary side, and an isolated secondary feedback loop. The PWM IC
KA3882 directly drives the power MOSFET.
As the power MOSFET sequentially turns on and off, energy is stored in the transformer core during the on time, and is then transferred to the output capacitor during the off time. When the power
MOSFET turns off, the energy stored in the leakage inductance causes a voltage spike across the
drain-to-source terminal of the power MOSFET, which amounts to at least twice the input voltage
(Vin + nVo + leakage inductance voltage1). Most applications need clamp circuits to restrict this
voltage spike from exceeding the BVdss rating of a MOSFET. A power MOSFET must have high
voltage capability with lower on-resistance and smaller gate charge for higher efficiency.
D1
T1
1
D6
5
1
2
1
2
1
8
5
R3
2
Vout2
R2
2
D2
2
2
2
2
4
1
2
C4
D7
2
1
1
1
2 1
C5
2
1
1
1
1
1
C2
C1
8
160V 0.3A
R18
2
C6
4
C3
Vout1
C12
R5
1
L1
3
2
1
1
R1
Fuse
1
1
2
1
1
2
1
2
2
5V 0.8A
2
1
2
2
R19
1
2
3 2
1
R14
Vin=220VAC
2
1
1
R4
C13
1
2
1
2
1
R20
2
R16
1
2
1
3
1
1
2
1
C14
1
1
2
2
C9
U3
C10
2
2
7
1
8
2
1
2
U1
R8
R17
2
R10
1 1
1
2
R7
1
D3
U2
R6
2
2
2
2
2
1
KA3882
1
1
2
C7
R9
QFET(FQP7N60)
R11
1
2
2
1
Q1
1
6
3
or Conventional
MOSFET
2
4
5
D4
3
1
2
1
1
R11
2
1
1
C8
2
C11
D5
R14
R13
2
2
1
2
Figure 3: Flyback Converter Circuit Diagram
1. 'n' indicates a turns ratio of the transformer windings. The voltage of Vin + nVo + leakage inductance voltage of the transformer appears at
the primary side.
Rev D, July 2000
3
Table 2: Power Supply Specifications
1. Operating mode
:
Flyback Discontinuous Mode
2. Input voltage (Vin)
:
180 VAC to 265 VAC (50Hz/60Hz)
3. Switching frequency (fsw)
:
80 kHz
4. Output voltage (Vout)
:
A. 160V ± 5% 0.3A
B. 15V ± 5% 0.8A
5. Efficiency (η)
:
75%
This flyback converter was tested in rated conditions of 220VAC input voltage, 80kHz switching
frequency, and 60W output power.
Figure 4 shows the waveforms for rated operating conditions using the QFET(FQP7N60) as the
switching device. The QFET is driven by a gate-source voltage of 15V and the voltage spike
across the drain-source terminal is adequately clamped to about 500V by an additional clamp circuit during off-time.
Vgs(5V.div)
Id(1A/div)
Vds(250V/div)
Figure 4: Operating Waveforms at Rated Conditions (Vin=220VAC, Pout=60W)
Rev D, July 2000
4
Figure 5 compares the waveforms of a conventional MOSFET with the new QFET(FQP7N60) at
turn-off without the additional clamped circuit (R5, C6, and D2), and the high conduction diode (D4)
for gate discharging (refer to Figure 3).
Conventional
MOSFET
Vds(100V/div)
Id(0.5A/div)
QFET
/div
Figure 5: Turn-off Improvement at Rated Conditions (Vin=220VAC, Pout=60W)
Note that the switching time of the QFET is faster than that of the conventional MOSFET because
of the reduction of gate charge by at least 45 percent. Figure 6 shows the difference in turn-off loss
between both MOSFETs without clamped circuits and the conduction diode, D4. The turn-on loss
in the crossover losses is very small and can be negligible. The turn-off loss period is due to the
finite switching time of the MOSFET which is directly related to the gate charge.
Conventional
MOSFET
QFET
/div
Figure 6: Turn-off Loss Improvement
Rev D, July 2000
5
0.90
Efficiency [η]
0.85
0.80
0.75
QFET (FQP7N60)
Conventional MOSFET
0.70
40
60
80
100
120
140
Frequency [kHz]
Figure 7: Efficiency vs. Frequency (40~140 kHz, @ Vin=220VAC, Pout=60W)
The turn-off loss area of QFET(FQP7N60) is half that of of the MOSFET.
During turn on and off, there is a short period when there is a significant overlap of voltage and current across the MOSFET. Figure 5 shows that the QFET(FQP7N60) has a shorter overlap period
than the conventional MOSFET, resulting in a lower loss (Figure 6). In Figure 7 the efficiencies of the
converter are calculated without D4 (high conduction diode, refer to Figure 3) operating at rated conditions of 220 VAC input voltage and 60 watt output as a function of frequency. As shown in Figure 7,
the QFET (FQP7N60) design is more efficient than its conventional MOSFET counterpart. The
advantage of QFET design is more pronounced as the switching frequency of the power supply
increases. These waveforms clearly demonstrate that faster switching translates into lower switching loss and much better efficiency.
Summary
To ensure high efficiency and reliable performance of the flyback converter, or any other converter,
the designer must ensure that the MOSFET operates effectively with lower on-resistance and gate
charge in the system. In this application note, that QFET(FQP7N60) flyback design demonstrates
higher efficiency than the previous MOSFET design because of the improvement of on-resistance
and gate charge. The other series of Fairchild’s QFETs with high voltage ratings (600, 800, and
900 V) allow designers to improve the performance of a switching mode power supply by a significant reduction in gate charge and on-resistance.
Rev D, July 2000
6
Appendix:
A. The printed circuit board layout
Rev D, July 2000
7
B. Parts List
Designator
Value
Designator
Value
Designator
Value
C1, C2, C3, C4
0.0047 µF
R1
NTC
R19
1.9 kΩ(1/4W)
C5
220 µF
R2
220 kΩ(1W)
R20
500 Ω (variable)
C6
0.0022 µF
R3
220 kΩ(1W)
L1
BSF2125
C7
0.0033 µF
R4
220 kΩ(1W)
T1
Transformer
C8
0.0022 µF
R5
68 kΩ(1W)
U1
KA3882
C9
100 µF
R6
12 kΩ(1/4W)
U2
PC817
(Photocoupler)
C10
10 nF
R7
2.7 kΩ(1/4W)
U3
KA431
C11
560 pF
R8
100 kΩ(1/4W)
Q1
FQP7N60
C12
33µF
R9
100 kΩ(1/4W)
C13
1000 µF
R10
9 kΩ(1/4W)
C14
10 nF
R11
50 kΩ(1/4W)
D1
Bridge Diode
R12
1 kΩ(1/4W)
D2
1N4937
R13
100 kΩ(1/4W)
D3
1N4937
R14
0.5 kΩ(1W)
D4
1N4148
R15
5 kΩ(1/4W)
D5
1N4744
R16
1 kΩ(1/4W)
D6
FR304
R17
33 kΩ(1/4W)
D7
UF5404
R18
120 kΩ(1/4W)
Rev D, July 2000
8
TRADEMARKS
The following are registered and unregistered trademarks Fairchild Semiconductor owns or is authorized to use and is not
intended to be an exhaustive list of all such trademarks.
HiSeC™
ISOPLANAR™
MICROWIRE™
POP™
PowerTrench®
QFET™
QS™
Quiet Series™
SuperSOT™-3
SuperSOT™-6
ACEx™
Bottomless™
CoolFET™
CROSSVOLT™
E2CMOS™
FACT™
FACT Quiet Series™
FAST®
FASTr™
GTO™
SuperSOT™-8
SyncFET™
TinyLogic™
UHC™
VCX™
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PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY
ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT
CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.
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properly used in accordance with instructions for use provided
in the labeling, can be reasonably expected to result in
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