POWER CONVERTER
Continuous Conduction Mode for
High Peak Power Quasi Resonance
Flyback Power Converter
ON Semiconductor CCPG Segment Application
JEAN-PAUL LOUVEL, TV SYSTEM APPLICATION MANAGER, ON SEMICONDUCTOR
W
hen designing a switching power
converter, a design engineer
typically designs the thermal
solution based on the maximum
output power specified by the
product. For example, a 50-W converter will have
to include enough heat sinking capability so that it
can permanently deliver 50 W right upon start-up
at the lowest input line and at the highest ambient
temperature.
Some applications, however, do not absorb a
constant power from the source. For example, a
printer draws power by pulses when the print heads
are active or when the paper is processed. In this
particular case, the converter will not be designed
to thermally handle the peak power excursions but
rather a much lower average value. Today we
typically see ratios of peak to nominal power up to
2:1 in many applications.
Flyback Power Converter
Low power converters (Pout < 70 W) generally use
a flyback topology as this provides a) good crossregulation performance in multi output
applications, b) the ability to work on a wide input
voltage range (90-264 Vac) without Power Factor
Correction (PFC) pre-converter and c) a very low
power standby mode. The transformer provides
isolation from the mains and stores the energy on
the primary side before dumping it on the
secondary side at the switch opening event. This
simple technique limits the surrounding parts with
one single power MOSFET switch and one single
diode for each output voltage. If this above
principle is common to all flyback converters, there
are multiple sub-families that we will now analyze
with their respective pros and cons.
The Fixed Frequency Pulse Width Modulated
(PWM) flyback converter is probably the most
common architecture found today. Driven by an
internal clock, the switching frequency value can be
chosen so that the Electro Magnetic Interference
(EMI) generated by the converter stay away from a
critical frequency range. The regulation loop
defines the conduction time of the power switch and
controls the energy stored/transferred by the
converter. This type of converter can operate in the
Continuous Conduction Mode (CCM) where the
energy stored in the transformer is not fully
transferred to secondary-side capacitors at the end
of the off-time. The transformer remains
magnetized until the next cycle appears. This mode
offers a trapezoidal primary current waveform
(rather than a triangular shape) and provides
higher output power with limited rms currents.
Owing to the CCM operation, the transformer can
be designed with a higher primary inductance to
improve low power / standby efficiency figures.
Another popular version is the Quasi Resonance
(QR) type offering so-called valley switching turnon: the power MOSFET is turned on in the valley
(the minimum) of the drain-source voltage. This
technique provides better EMI performances
compared to a hard-switching version. The
switching losses are reduced however, switching
frequency reduction is mandatory to pass peak
power while maintaining a complete transformer
demagnetization. The higher energy stored by
switching cycle imposed by this lower operating
frequency will require a much larger transformer
size (and cost) with high peak currents in both
primary MOSFET and secondary-side diode.
A third type is the hysteretic converter where the
power transfer at a frozen primary peak current is
ensured by modulating the switching frequency to
provide the requested energy (higher frequency for
more power transferred).
The main subject of this article is the peak power
capability for a QR flyback converter, we will now
analyze in detail the behavior of this type of
POWER CONVERTER
converter, in particular how it copes with peak
power requirements.
Detailed Behavior of QR Flyback
Converters with a High Peak Power
Unlike the traditional PWM converter, this converter
works with a variable switching frequency to turn
the MOSFET on exactly at the point where its drainsource voltage is minimum. It naturally reduces
Electromagnetic Interferences and switching
losses.
To keep valley switching operation, in the low
output power case, the on-time reduction implies a
higher switching frequency which asks again for a
shorter on-time (and so-on) to limit the energy
transferred from the primary to the secondary side,
cycle by cycle, to be below the requested limit. To
limit the switching frequency excursion, new
solutions have been developed where a clamp
controls both the minimum on-time and the
maximum switching frequency. To keep the QR
operation while ensuring minimum drain-source
voltage switching, an innovative valley−lockout
solution has been developed. It can work down to
the 4th valley and toggles to a variable frequency
mode beyond calls VCO mode (Figure 1). It
ensures an excellent light load / standby power
performance as demonstrated by the
NCP1379/1380 from ON Semiconductor.
This energy transfer looks promising but
unfortunately problems come up as soon as a
higher peak power is needed. To increase the
energy transferred cycle by cycle, the current in the
primary should be larger with corresponding wider
on-time. The conduction time of the secondary
diode also increases forcing the controller to
decrease the operating frequency so that complete
demagnetization can be ensured. While the
frequency is lower, the per-cycle energy should
increase forcing further down the switching
frequency. This “double” effect , necessary to
Fig. 1. The drain-source voltage of a Quasi Resonance flyback
operated with valley switching and valley lock-out (From 1st to
4th valley and VCO mode while energy transferred reduces).
secure a complete transformer demagnetization,
will force the converter to dramatically reduce the
frequency to accept a peak power excursion. The
transformer should therefore be designed to accept
the higher energy that has to be stored cycle by
cycle: an oversized transformer will be needed to
support QR Flyback Converters operating with a
High Peak Power.
If the above behavior creates a problem at high
power, it becomes a natural advantage when the
secondary output is shorted to ground. In presence
of a short circuit, the demagnetization will take a
longer time, forcing the frequency to be very low,
thus reducing the transmitted power. Safety is
greatly improved owing to this operating mode.
The switching frequency will go up as soon as the
secondary output voltage increases again, e.g.
when the short circuit is removed.
A New Solution Featuring Peak
Power Excursion
We want to define a new Power conversion solution
that provides a higher peak power capability
without oversized parts and while keeping all the
advantages of a QR-operated flyback converter.
a) Keep QR for both low EMI and lower switching
losses benefits at nominal/average power
b) Use CCM in high power to avoid frequency
reduction and larger/oversized transformer
c) Keep QR Output short circuit behavior and very
good natural safety performances
Higher Peak Power QR Flyback with
Continuous Conduction Mode
The principle of operation for the Zero Current
Detection section in a NCP1380 controller
appears in figure 2: With auxiliary winding voltage
applied to the ZCD input, the controller is able to
control the end of energy transfer. The added diode
D203 and resistance R206 are used here for Over
Power Compensation function of NCP1379 during
the on time of the MOSFET.
The circuit should be modified to allow CCM
operation when a high peak power demand
occurs. To avoid the complete transformer
demagnetization over a defined power illustrated
by a transfer time larger than a given value, an
additional transistor is added. This is shown as
Q206 in figure 2, whose presence forces the Zero
Current Detection to re-start the controller while
complete demagnetization has not yet occurred.
We should also keep complete demagnetization
POWER CONVERTER
GND).
This new solution allows an increase of ~50% of
the power capability without increasing size and
cost of the overall parts (mainly the transformer)
keeping all the QR nominal load and safety
behavior advantages.
1
R205
1K
Aux. winding
2
F
R206
470k
D203
MMSD4148
NCP1379
IC200
Q206
BC848ALT1
C210
68p
ZCD
CT
FB
BO
CS
VCC
GND
DRV
2
R211
1K
4
F
7
6
3
F
CCM QR Flyback without Frontend
PFC
8
1
5
Fig. 2. Principle of Continuous Conduction Mode Quasi
Resonance Flyback
control with ZCD for average power, starting phase
and output short circuit to Ground to avoid over
stress and over size of multiple parts thanks to the
very good natural QR safety behavior. This is done
by a control of the reflected voltage on the auxiliary
winding which provides a perfect image of
secondary output voltage.
Detail on Added Circuit
Allowing CCM for High Power
Principle: Inhibition of QR ZCD allowing
CCM when Power is over a given limit
• C231 is charged with negative voltage
proportional to supply voltage during the
conduction of the primary switch
• R235 and R254 combined with C231
set the delay time T and transistor Q206
is the switch to turn the circuit ON (simple
timer)
• Over defined time T, Q206 is switched
ON and pulls the ZCD pin 1 to GND (To
restart the next cycle with energy stored in
the transformer). The serial cap C238
guaranties the low voltage level of the IC
input despite Vce-sat of Q206
• As Q206 is supplied by R234
connected directly to the winding, the
CCM is linked to the reflected voltage
from the secondary side during the
conduction of the secondary diode:
CCM cannot be activated if reflected /
secondary output voltage is too low
(starting phase or output shorted to
The power capability is reduced at low mains
supply. Despite shorter off time control by the CCM
timer (Reflected negative voltage proportional to
supply is smaller with lower mains input), the larger
on time (to get the same drain current in the
MOSFET) at lower mains supply has a small impact
on the switching frequency reducing the power
capability. This new solution can also be used
without a frontend PFC to increase the peak power
for applications below the 75 W limits without PFC.
Limitation of the Proposal
There are two limitations to be taken into account
for this solution.
CCM should not be used for high output voltage
applications, as this requires a very low trr
secondary diode. CCM is typically limited to low
Added parts for CCM control
1
R253
1K 1/4W
res500d
R235 3K3 1/4W
R254 3K3 1/4W
Q206
BC848ALT1
sot23
D212
MMSD4148
sod123
D213
BAV21
do41d
R234
10k
D220
MMSD4148
sod123
2
D214
MMSD4148
sod123
C238
100n
Aux. winding
C231
1n5 250V
R206
470k
D203
MMSD4148
Ic200
NCP1379
1
ZCD
CT
2
FB
BO
3
CS
VCC
GND
DRV
8
7
C210
68p
R211
1K
6
5
4
Fig. 3. Detailed solution of Continuous Conduction Mode Quasi Resonance Flyback
POWER CONVERTER
T200
SRW4549EM-XXXX
3
PFC_OUT
13
R251
33K 2W
R252
33K 2W
R200
33K 2W
C200
10n 500V
11
NC2
D220
MMSD4148
sod123
R253
1K 1/4W
res500d
R205
1K
R234
10K
D201
MUR180
do41d
D213
BAV21
d041d
R235 3K3 1/4W
Q206
BC848ALT1
sot23
R254 3K3 1/4W
D205
D214
MMSD4148
sod123 C231
1n5 250V
D212
MMSD4148 C238
100n
sod123
R206
D203
470K
SOD123
MMSD4148
1
D206
MMSD4148
1n
220p
R249
4K7
DRV
CS
R209 33K
4
3
R214 10K
8
Q203_G
6
sod123
MMSD4148
R209 100
sot23
1
C216
2
Q204
BC808-25LT1
BO
C218
100p 1KV
Q203_S
3
R220
47k
R221
0.47R 2W
res1000
C225
Y 1nF
F
10u 50v
C236
100n
dip4_3
F
Q203
STP6NK70Z
to220decalD
3K3
PFC-OK
PC102B
SFH817A
7
D209
R218
10
F
9
DRV_Q203
R217 27
VCC4
FB
10K
DRV_QRM
C215
100n
F
F
C217
47u 25V
Hs3
Heat_Sink
1
Q208
BC858ALT1
sot23
R246
1
2
5
GND
C211
F
PWM_VCC
VCC
4
C212
C214
100n
6
CS
C210
68p
470
BP
3
CS
4
R213
sod123
330p
62
sod123
CT
FB
BAV21
do41d
C213
7
2
5
12
Zd200
(15V)
8
ZCD
R211
1K
MMSZ4702T1
BO
Ic200
NCP1379
R207
VCC3
F
F
FB
1
T200-01 1
F
F
Fig. 4. Detailed & complete primary part of NCP1379 CCM QR Flyback SMPS TND401/D ON Semiconductor Green Point
reference design
voltages applications (< 30 Vdc) naturally built
with Schottky diodes (particularly true for 19 V
adaptor’s or printer’s applications).
CCM is difficult to use with high output current
applications built with secondary synchronous
rectification. The secondary Synchronous
Rectification MOS should be switched OFF before
a new cycle to avoid short circuit with this
bidirectional switch. The very high current in the
primary of the transformer will activate the primary
over current limitation and stop the power supply.
Conclusion
This new solution utilizing QR Flyback with high
peak power provides an increased power
capability of ~50% over standard QR Flybacks.
Designing CCM for peak power avoids over sizing
the transformer, MOS and secondary diodes. The
design, now optimized for average power, can be
more compact with improved low power / standby
performance thanks to increases in the
transformer’s inductance while keeping all
advantages of a QR solution for nominal power.
This increased peak power capability for
applications below 75 W (without PFC), allows
reducing both size and cost of today’s QR Flyback
Adaptors used for computers, games stations and
printers. The solution, easily designed with a
minimal number of low cost parts, has been
demonstrated with the ON Semiconductor
NCP1379 / 1380 which also provides valley lockout for improved low power performance.
Combining advantages of both PWM (Higher peak
power capability with CCM) and QR (Reduced EMI
and switching losses, natural safety behaviors for
output short circuit to Ground), the Continuous
Conduction Mode for Quasi Resonance Flyback
Power Converter is quickly becoming the preferred
Flyback solution for future products with very good
natural safety behavior (lower switching frequency
with output short to ground).
More details are available under the link:
http://www.onsemi.com/pub_link/Collateral/TN
D401-D.PDF