SP6013 - Sync Power Corp.

SP6013
Synchronous Rectifier Driver
APPLICATION INFORMATION
Predictive Timing Operation
The essence of SP6013, the predictive timing
circuitry, is based on several U.S. patented
technologies.
This assures higher rectification
efficiency by a) eliminating high cross conduction
current under all operating conditions and b)
significantly reducing the body diode conduction losses
in the synchronous rectifier.
time sets the minimum size capacitor needed. Since it
is usually desirable to achieve proper operation down
to essentially zero duty cycle, the capacitor minimum
size can be represented as a function of frequency.
VDD Decoupling Capacitor
The IC is sensitive to large supply voltage ripple. If
the IC drives a MOSFET with significant input
capacitance, Ciss, the ripple due to gate drive energy
transfer can create large ripple. Therefore, it is most
suitable to add a high frequency decoupling ceramic
capacitor of 0.1μF~1.0μF between Vdd to ground, and
the capacitor should be placed as close proximity to the
driver as possible.
Cmin=2.66*10^-5/F
Ramp Capacitor Selection (Cramp)
The ramp capacitor selection is frequency dependent.
The capacitance selection should be made in according
to the plot below:
140
Frequency (kHz )
120
100
Cmin = Icharge * dT/dV = Icharge/F/dV;
Since Icharge = 80µA & dV=3.8V-0.7V,
A capacitor size should be selected to meet the
above criteria including the frequency tolerance and
the capacitor tolerance over temperature. The criterion
for the maximum suitable capacitance is less precise
because it is driven by noise constraints. If a larger
capacitor is chosen, the ramp voltage becomes lower
and there is more jitter on the pulse width due to
noise. It is usually advantageous to operate with as
high a ramp voltage as possible for this reason. The
minimum ramp amplitude occurs at the maximum
converter operating duty cycle because this is when
the MOSFET is conducting for the shortest period of
time. For converters that need to be able to operate to
a wider duty cycle, the capacitance should be close to
the minimum calculated above to prevent noise
problems.
80
60
40
20
0
8
15
20
30
40
50
Cram p (pF)
Fig. 1
The ramp capacitor selection is dependant on the range
of the On and OFF times of the synchronous rectifier.
In most application it is necessary to work to a narrow
duty cycle. The ramp capacitor voltage charges from
about 0.7 volts during the ON time. The charging
current is 80 μA. For proper operation of the internal
timing it is necessary to limit the ramp amplitude to 3.8
volts. This voltage limits and the maximum ramp ON
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Fig. 2
Figure 2 illustrates the characteristic waveforms of a
flyback converter with input voltage of 110VAC.
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SP6013
Synchronous Rectifier Driver
Fig. 3: Application Schematic
Channel 1 indicates the SYNC voltage on the
secondary side of a flyback transformer at 77 kHz,
channel 3 is the SP6013 output pin, MOSG-C, connects
to the gate of MOSFET, and channel 4 designates the
input current (ID). Notice the timing gap between the
falling edge of MOSG-C and the rising edge of SYNC
is 3.5μs with no ramp capacitor connected. Figure 3 is
the equivalent circuit for this particular application.
For typical flyback application with 50 kHz to 120
kHz operations, a ramp capacitor value of 10 pF to
47pF is recommended. This allows the gate ON time
to reach 10-12 microseconds for narrow pulse widths.
Timing Adjustment
The Timing pin provides adjustment of the patented
dv/dt filter circuit that differentiates between the real
power and ring-back no-power transformer secondary
voltage positive waveform.
Under light or no load
conditions, the output current will be discontinuous.
For that condition the transformer voltage “rings”
back positive. The SP6013 detects positive
transformer secondary voltage to establish power
transmission, and determines the SR MOSFET turn
ON time. However, it is not desirable to turn on the
MOSFET during the “ring-back”. The dv/dt filter
detects the true power pulse from the “ring-back”.
•
Fig. 4
With 33pF added to Cramp pin with respect to ground,
the timing gap immediately shortens and the ON time
of the gate increased from 8.2μs to 11.6μs, as shown in
figure 4.
Thus, the efficiency is improved by
approximately 0.655%. The output for this converter
is set at 5V/8A.
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Connect resistor to VDD.
The first option is the nominal setting. For this
condition, the Timing pin is left open or no
connection. In result, the ON time of MOSG-C
should be turned ON at every cycle. If not, the timing
needs to be adjusted.
Figure 5 illustrates an
example of several ON time cycles missing:
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SP6013
Synchronous Rectifier Driver
Fig. 5
Fig. 7
The Timing adjustment should be performed under
light load circumstance.
A variable resistor,
0~100KΩ, is connected from Timing pin to VDD and
adjust the variable resistor until the waveform at
MOSG-C pin appears in every cycle. If this is unable
to achieve, connect the variable resistor to ground and
follow the same procedures. But be careful, once the
correct resistor value is exceeded, more ON time cycles
will appear during oscillation. It is best to eliminate
these.
Once the variable resistor is adjusted to the correct
value, the output waveform of pin MOSG-C should be
ON at every cycle as shown in Figure 6. Then
measure the resistance of the variable resistor. The
typical value is between 10KΩ to 30KΩ.
SYNC Voltage
The Sync input voltage stands between 5V and
5.5V. It is necessary to use a resistor divider if the
Sync voltage is much higher than 5 volts. R4 and R5
function as a voltage divider, in which the voltage
from the secondary side of the transformer might
reach as high as 40V-60V. The maximum allowable
voltage for this pin is 7.5 volts. When the voltage is
reached above this limit, the IC may suffer permanent
damage. Therefore, it is recommended to insert a
1-5KΩ resistor in between Sync, Pin 1, to ground so
the voltage can be lowered.
Here is another example of a 3.3V/3.69A output
converter with everything adjusted at no load:
Fig. 6
Fig. 8
For light load condition, the equivalent waveform with
correct resistor value connected to Timing pin is
demonstrated in figure 7. The output of MOSG-C pin
is turned ON at every cycle. Observe during
“ring-back”, the MOSFET remains OFF. The dv/dt
filter will differentiate between the true power pulses
from the “ring-back” pulses.
In this application, the resistor for Timing pin is
adjusted at 8.80KΩ to VDD and no Cramp capacitor is
needed. The converter is using a green-mode PWM
IC at primary side, and figure 8 demonstrates the
output, MOSG-C (channel 3), during burst mode
(channel 1).
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SP6013
Synchronous Rectifier Driver
Rapid Adjustment
The internal “Rapid Adjustment” circuit initiates a
delay to assure that the drive circuit will not false
trigger when there is a very fast reduction in pulse
width required in the pulse width modulation. This
can occur in regulators for very high di/dt load changes
demanded by such loads as microprocessors. False
triggering could result in a high cross conduction
current for a few nanoseconds. A capacitor can be
connected among Rapid-Adj to ground that sets the
amount of filtering. For most applications, a capacitor
of 100 pF is suitable.
The goal is to keep prediction time as short as possible
to keep minimum body diode conduction. The
Prediction logic uses the previous cycles to control SR
operation in the present cycle. The predictive turn
OFF time is adjusted by the capacitance on Pred pin.
When the actual prediction time is more than that set
by the capacitance on Pred pin, the IC reduces the
Prediction time in the next cycle. When the actual
prediction time is less than that set by the capacitance
on Pred pin, the IC increases the prediction time in the
next cycle. (Fig.10)
Alternate Power Source
For applications with single set of transformer
output, a way to deliver power to the IC can be derived
from the isolated secondary voltage via a voltage
regulator KA7812 or 78L12 as shown in figure 3.
KA7812 is selected in this application.
An input
(4.7uF) and output (10uF) capacitor is also required for
the voltage regulator. This is an acceptable approach
for voltages from 6.5 volts minimum ripple valley
voltage to 35 volts maximum peak voltage.
Predictive timing circuit operation ( Pred )
The IC operates on the principle of “prediction”. For
freewheeling (catch) control applications, the prediction
time is defined as time interval from the falling edge of
Vgs to the rising edge of the synchronizing signal as
shown at below.
Fig.10 : No Load , SYNC = 300KHZ , Vdd = +12V
( Ch1 - SYNC waveform ; Ch3 - Catch waveform )
Tpre d vs Cpre d
800 ns
600 ns
Tpred
MOSG-C
Since the output of this IC delivers a square wave of
0~VDD volts at various frequencies, a logic level
MOSFET is most applicable for the Synchronous
Rectifiers. R3 is recommended at 5Ω and D2 is a
small signal Schottky diode, such as BAT54, 1N5817,
or 1N5819 or switching diode 1N4148.
400 ns
200 ns
ns
0
20
40
60
80
Cpre d (pF)
Fig.11 : Tpred vs Cramp
( VDD =12V ; Frequency = 70KHZ )
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SP6013
Synchronous Rectifier Driver
Reference Design Board
To familiar with SP6013, the reference design board
should be checked by using function generator before
connecting to the converter. Do not connect the
MOSFET onto the reference board. Force 12 Vdd at C6
of the board and force square wave at R5 which is
connected to the SYNC pin of SP6013. The square
wave should be adjusted to VH> 3.9V and VL<0.9V.
MOSG-C pin will have a square wave shown.
There are a couple of necessary steps when installing
the reference design board onto an existing flyback
converter:
Step 1: Short out the existing Schottky diode.
Step 2: Find the ground connection from the output of
the transformer and split it to two segments. When
doing so, be as close to the transformer’s output as
possible , to avoid contacts with any other components.
Step 5: Remove the MOSFET from step 4 and place
the reference design board by attaching the drain
and source of the MOSFET from the reference board
to the converter.
Step 6: Voltage divider. In most application, the peak
input voltage is more than 25 volts into pin 8, SYNC;
the recommended voltage at this pin is 5-7.5 volts.
R4 & R5 are used as voltage divider. Two resistors are
used for default settings.
Step 7: Timing adjustment. The Timing adjustment
should be performed under light load circumstance.
A variable resistor, 0~100KΩ, is connected from
Timing pin to VDD and adjust the variable resistor.
Changing the resistor value will change dv/dt slope
Fig.14. This resistor value normally is from 10Kohm
to 30Kohm. The typical relation between resistor
value and the Delta T is shown in Fig.13.
Step 3: Place any MOSFET with drain connecting to
the output of the transformer and source to the other
segment that was split from step 2. It is best if
attached directly onto the PCB/converter directly. At
this point, the gate should connect to source.
Step 4: Try turn on the converter and it should able to
generate output, even though it is using the body diode
of the MOSFET to conduct.
Step 5: Remove the MOSFET from step 4 and place the
reference design board by attaching the drain and
source of the MOSFET from the reference board to the
converter.
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Fig.12
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SP6013
Synchronous Rectifier Driver
(ns)
Delta T vs. Resistor Value
300
Step 8: Cramp selection. The size of this capacitor is
frequency dependent.
The recommended value
ranges from 10pF to 100pF. Please refer to page 1
for more details.
Delta T
250
200
150
100
50
10
15
20
25 (Kohm)
Fig.13
Once the resistor value is adjusted correctly, the
SP6013 gate will be triggered as shown in Fig.14 .
Fig. 14
Ch 1 : Drain Waveform ; Ch 3 : Output Gate Waveform
Ch 4 : Drain Current
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SP6013
Synchronous Rectifier Driver
Information provided is alleged to be exact and consistent. SYNC Power Corporation presumes no responsibility for the
penalties of use of such information or for any violation of patents or other rights of third parties, which may result from its use.
No license is granted by allegation or otherwise under any patent or patent rights of SYNC Power Corporation. Conditions
mentioned in this publication are subject to change without notice. This publication surpasses and replaces all information
previously supplied. SYNC Power Corporation products are not authorized for use as critical components in life support
devices or systems without express written approval of SYNC Power Corporation.
©The SYNC Power logo is a registered trademark of SYNC Power Corporation
©2004 SYNC Power Corporation – Printed in Taiwan – All Rights Reserved
SYNC Power Corporation
9F-5, No.3-2, Park Street
NanKang District (NKSP), Taipei, Taiwan, 115, R.O.C
Phone: 886-2-2655-8178
Fax: 886-2-2655-8468
http://www.syncpower.com
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