Texas Instruments | Boosting LLC Resonant Converter Efficiency by Using the UCC24624 | Application notes | Texas Instruments Boosting LLC Resonant Converter Efficiency by Using the UCC24624 Application notes

Texas Instruments Boosting LLC Resonant Converter Efficiency by Using the UCC24624 Application notes
____________________________________________________
Boosting LLC Resonant Converter Efficiency by Using
UCC24624 Synchronous Rectifier Controller
The LLC converter is a popular topology for PC,
server, and TV power supplies, due to its simplicity
and high efficiency. The resonant operation achieves
soft switching for the entire load range, which makes it
a good candidate for high frequency and high power
density designs. Besides, LLC converter uses the
capacitive filter, eliminating the need of an output filter
inductor. The capacitive filter also allows the converter
to use lower voltage rating rectifiers and reduce the
system cost. Furthermore, the secondary side rectifiers
can achieve zero current switching with much less
reverse recovery loss. With all the benefits provided by
the LLC topology, to further increase its efficiency, the
loss on the output rectifiers should be reduced.
For the same design, if the rectifier diodes are
replaced with MOSFETs, with proper control, the
conduction loss can be calculated using Equation 2,
approximating the secondary side current shape as
sinusoidal shown in Figure 3. With a 4-mΩ RDSon, each
rectifier loss can be reduced to 0.247 W, which
translates to total 0.4% efficiency loss.
Q1
Lr
Iout
Vout
Vin
Cr
Q2
S1
Synchronous Rectifier for LLC Resonant Converter
When using the diode rectifiers, as shown in Figure 1
the entire output current flows through the output
diodes. For low voltage or high output current
applications, there is a significant efficiency loss and
thermal stress in these diode rectifiers.
S2
Figure 2. LLC Converter with SR
2
è
2
254 = 4&5KJ × +NIO
= 4&5KJ × @ × +KQP A
4
IS1
(2)
IS2
Q1
Iout
Lr
Iout
Vout
Vin
Cr
Figure 3. LLC Secondary Side Current
Q2
D1
D2
Design Challenges for LLC SR Control
Figure 1. LLC Converter with Diode Rectifier
The loss on each rectifier diode can be estimated
based on Equation 1, if the diode is modeled by a
fixed forward voltage drop VF. Using this calculation,
for a 12-V, 10-A output design, with 0.5-V forward
voltage drop, each diode generates 2.5 W of loss,
which translates to about 4% total efficiency loss.
2&EK@A = 8( ×
+KQP
2
(1)
Using the Synchronous Rectifier (SR) as shown in
Figure 2, the voltage drop on the MOSFET could be
much lower than a typical diode forward voltage.
SLUA922 – December 2018
Submit Documentation Feedback
The SRs can be controlled by monitoring its drain-tosource voltage (VDS). Before the SR turns on, the
current flows through its body diode. The body diode
forward voltage drop can be used to trigger the SR
turning on. After the SR is turned on, its on-state
resistance becomes a current sensing resistor, the VDS
can be used to sense the current to turn off the SR
before the current reverses. Even though the control
method is quite straightforward, there are several
design challenges associated with the LLC resonant
converter SR control.
SR Turn off Timing: The biggest challenge of LLC SR
control is to turn off the SR at the right timing. Different
than the Flyback converters, LLC SRs generally carry
much higher current and have higher di/dt. As shown
in Figure 4 , the sensed voltage VSENSE is used for SR
control. It includes the RDSon drop (VSR) and the offset
Boosting LLC Resonant Converter Efficiency by Using UCC24624
Synchronous Rectifier Controller
Copyright © 2018, Texas Instruments Incorporated
1
www.ti.com
voltage on the package inductances (LD, LS) induced
by the di/dt. With high di/dt and package inductances,
this offset voltage could be substantial and the SR is
often turned off too early, which results in a long bodydiode conduction time and large conduction loss.
packages, such as TO-220, UCC24624 allows the
designer to further increase its turn-off threshold by
using an external offset resistor from VSS pin to SR
MOSFET source pin. This makes the controller less
sensitive to the MOSFET package selections.
VSENSE
Q1
Lr
-
VSR
LD
Iout
+
Vout
Vin
LS
Cr
SR
Q2
ISR
Figure 4. Voltage Sensed by SR Controller
Burst Mode Operation: Another challenge associated
with SRs used in LLC converters is the burst mode
operation. During the burst mode, both of the primary
side switches are turned off. The switch node
capacitor resonates with the LLC transformer
magnetizing inductor. This low frequency parasitic
oscillation can potentially cause the SR to falsely turn
on and make the output deliver energy to the primary
side, which results in more conduction loss.
Low Standby Power: Even though the SRs save the
conduction loss, they add extra loss to the system due
to the control circuitry and the gate driver loss. This
extra loss is insignificant at a heavier load, due to the
large conduction loss savings. However, at no load
condition, it is more efficient to disable the SR
controller by putting it into a standby mode and use
the SR body diode for the rectification.
Reliability Concern: Due to the capacitive filter, if both
SRs turn on at the same time, the output is shorted
through the transformer and catastrophic failure is
expected. It is critical to prevent both SRs from
conducting at the same time, even considering the
false triggering caused by circuit noises.
UCC24624 Dual SR Controller for LLC Converters
To achieve better LLC resonant converter efficiency,
UCC24624 dual SR controller is introduced to work
together with LLC controllers such as the UCC25360
series. UCC24624 implements VDS sensing for the SR
control, together with varies features addressing the
LLC SR control challenges, making it an ideal solution
for high efficiency LLC designs.
To address the SR early turn off challenge,
UCC24624 implements proportional gate drive,
together with an adjustable +10.5-mV turn-off
threshold. The proportional gate drive reduces the SR
gate voltage during the current falling edge. The
reduced gate drive voltage increases the SR MOSFET
RDSon, resulting in higher voltage drop across the SR.
This increased voltage drop overwhelms the offset
voltage induced by the package inductance. Together
with the positive turn-off threshold, UCC24624 keeps
the body diode conduction time to a minimum. To
operate better with higher parasitic inductance
2
UCC24624
S1
VG1
VG2
PGND
VDD
REG
VD2
VD1
VSS
S2
(Optional)
Figure 5. UCC24624 Dual SR Controller for LLC
Converter
To improve the burst mode operation, besides the
traditional off-time blanking, adaptive turn-on delay
time is adopted in UCC24624. During normal
operation, the turn-on delay is kept short to minimize
the body diode conduction time and improve the
efficiency. During burst mode operation, the SR
operation changes from complimentary fashion to no
switching. UCC24624 uses this as the indicator to
detect the LLC has entered burst mode operation. It
increases its turn-on delay time to help reject the
parasitic oscillation. The turn-on delay is also
increased at light load conditions to provide extra
noise rejection. This adaptive turn-on delay time helps
to maintain noise rejection without sacrificing efficiency
performance.
UCC24624 also has a built-in automatic standby
mode detection circuit, without using external
components. For the LLC converters at the no load,
the converter operates in burst mode to regulate the
output voltage. The LLC SR conduction time in each
switching cycle could still be long, while the average
switching frequency of the converter is quite low.
UCC24624 detects the light load condition based on
the converter average switching frequency. It allows
the controller to enter standby mode at no load and
helps to achieve low standby power.
To enhance the reliability, and prevent both SRs
turning on at the same time, interlock logic is applied
to both channels of the SR controls. During one
channel SR conduction time, the other channel SR
conduction is prohibited. This added logic provides
more robust operation despite system noises.
Summary
With all the built-in intelligence, together with TI
UCC25630 series LLC controllers, UCC24624
provides a high efficiency, cost effective solution for
SR control in LLC converter designs.
Boosting LLC Resonant Converter Efficiency by Using UCC24624
Synchronous Rectifier Controller
Copyright © 2018, Texas Instruments Incorporated
SLUA922 – December 2018
Submit Documentation Feedback
IMPORTANT NOTICE AND DISCLAIMER
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE
DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS”
AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY
IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD
PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate
TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable
standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you
permission to use these resources only for development of an application that uses the TI products described in the resource. Other
reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third
party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims,
damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on
ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable
warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2018, Texas Instruments Incorporated
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Related manuals

Download PDF

advertising