The QRF power converter is a popular choice in low-power offline applications in the 150-W-or-less output power range for many reasons

BY MICHAEL O’LOUGHLIN
Systems
Application and Design Engineer, High-Voltage PWM Controller Product Group, Texas Instruments
www.ti.com

The quasi-resonant
flyback (QRF) power converter is a popular choice in low-power offline
applications in the 150-W-or-less output power range for many reasons. The QRF
converter has a low component count, which translates into a low-cost bill of
materials. The topology can hit system efficiencies as high as 90% to 91% when
synchronous rectifiers are used on the converter’s output.

To manage heat
and power dissipation in the traditional QRF, engineers have limited the
maximum switching frequency (fsw)
of their designs below 100 kHz. The only problem with this is that consumers
would like their adapters even smaller than they are today. To accomplish this,
it requires being able to run the offline flyback converter at switching
frequencies at least two to three times greater than the QRF is presently used
to reduce magnetic size. In this article, we discuss how using an active clamp
flyback (ACF) converter for these applications can run faster and cooler than a
traditional QRF, enabling the reduction of magnetic size in offline flyback
applications and making the overall power supply smaller.

QRF switching losses increase with frequency
The QRF converter
is a very efficient power converter because of its ability to valley switch,
which has lower switching losses compared to traditional hard-switched flyback
converters. However, the QRF does have switching losses (Q1) that
increase with switching frequency, especially at high input voltages.

Equation 1 expresses
the QRF switch-node (VSW)
switching losses (PSW(QRF)),
where NP/NS
is the flyback converter’s transformer turns ratio (T1, Fig 1).
Variable CSW
is the flyback converter’s switch-node (VSW) capacitance. From this equation,
it can be observed that PSW(QRF)
will increase fSW.

PS0818_TI_Fig-1

Fig. 1: Offline flyback converter showing an RCD, diode, and
active clamp.

Passive clamp losses increase with switching frequency

Another problem
with the traditional QRF converter is that it traditionally uses a diode clamp
or resistor capacitor diode (RCD) clamp to provide a current path to
de-energize the transformer’s (T1) primary leakage inductance (LLK) and
protect the converter’s main switch (Q1) from electrical overstress and damage. Unfortunately,
these passive clamps are not free, and they dissipate power (PCLAMP) that increases
with switching frequency (fSW).

See how Equation
2 calculates the flyback passive clamp power dissipation (PCLAMP). In
this equation, VCLAMP
is the voltage across the clamp when Q1 is off, and IP is the transformer’s peak
primary current. Similar to the switch-node losses, the clamp losses also
increase with fSW.

PS0818_TI_Equation-2

The PCLAMP and PSW(QRF) losses
in QRF with a passive clamp are detrimental to high-frequency designs. If you
try to reduce the power converter’s magnetic size by running the design at a higher
switching frequency, you will have to increase your heat sink size for Q1, hurting
the design’s power density. The increased losses will hurt system efficiency and
require the use of higher power-rated and more expensive components for the
main switch and passive clamp. This discourages most designers from designing
QRF converters over 100 kHz.

ACF recycles clamp energy
The active clamp in
Fig. 1 consists of a clamp switch (QC) and a
clamp capacitor (CC)
that replaces an RCD or diode clamp. This gives a place for the discharge and
storage of transformer T1’s
leakage inductance (LLK)
and thus protects Q1
from electrical overstress. Because QC allows bidirectional clamp current (IC), the
leakage energy can be returned to the output through the flyback converter’s
transformer primary-to-secondary turns ratio (NP/NS) every switching cycle,
making the active clamp near lossless and a better choice for higher-frequency
designs in the 200-kHz range or higher.

ACF can achieve zero-voltage switching
There are pulse-width
modulator (PWM) controllers on the market that are designed for use in active
clamp power converters that have two drive outputs for QC and Q1. The power supply designer
can then take advantage of bidirectional current by delaying the turn-off of
the clamp switch. This delay allows the primary magnetizing inductance (LPM) to charge
in the reverse direction and develop a negative peak primary current (–IP) large
enough to resonate with the switch-node capacitance (CSW) to achieve zero-voltage
switching (ZVS).

The delay does
require adjusting the turn-off delay of switch Q1, allowing the power converter time
to achieve ZVS. It will require some work in the lab, but removing the
switching losses is well worth the effort.

To achieve ZVS,
the energy in LPM
needs to equal the energy in CSW.
This relationship is shown in Equation 3:

PS0818_TI_Equation-3

Calculating the
amount of negative current that is needed to achieve ZVS can be derived by
algebraically solving for I
P from Equation 3,
which yields Equation 4:

PS0818_TI_Equation-4

To make the
design process easier, Texas Instruments developed the UCC28780 active flyback
controller. The UCC28780 monitors the switch node and will adjust the Q
C turn-off
delay and Q
1
turn-on delay to achieve ZVS. The controller continuously monitors the switch
node and will adjust the delays within a few switching cycles to ensure that the
design still achieves ZVS if the line voltage and/or the power converter load
should change. This should reduce the active clamp flyback converter’s design
cycle time, reducing development costs even further.

Fig. 2
shows a graphical comparison between QRF and ACF switch-node switching and
transformer primary current. There are notes in this waveform describing why
ACF is more efficient than the traditional QRF flyback.

PS0818_TI_Fig-2

Fig. 2: ACF ZVS and active clamp vs. QRF and passive clamp.

Summary

When
designed correctly, the ACF controller will not only recycle transformer
leakage energy to remove switch-node clamping losses but can also achieve ZVS, removing
primary switching losses. This enables designers to push the flyback power
converter’s switching frequencies from 100 kHz to 200 kHz or more without
adding additional heat sinking, which then allows them to reduce the design’s
magnetic size and increase power density when compared to a QRF using passive
clamps.