Design of Secondary Side Rectification using IR1167 SmartRectifier Control IC

Application Note AN-1087
Design of Secondary Side Rectification using
IR1167 SmartRectifier™ Control IC
By Maurizio Salato, Adnaan Lokhandwala, Marco Soldano
Table of Contents
Device Overview
SmartRectifier™ Concept
Operation and analysis in CCM & DCM Flyback
Operation in Resonant Converters
Typical System Schematics and Passive Components Nomenclature
Design Procedure & example
PCB Layout Guidelines
Symbols list
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Introduction
IR1167S is a smart secondary-side driver IC designed to drive N-Channel power
MOSFETs used as synchronous rectifiers in isolated Flyback converters. The IC
can control one or more paralleled MOSFETs to emulate the behavior of
Schottky diode rectifiers.
The drain to source voltage of the MOSFET is sensed differentially to determine
the level of the current and the device is turned on and off in close proximity of
the zero current transition. The pinout for the 8 pin device is shown below.
Figure 1: IR1167 SmartRectifier™ control IC pin assignment.
SmartRectifier™ Concept
The typical power stage schematic for a flyback converter with synchronous
rectification (low-side configuration) at the output is shown below.
Vin
XFM
Cs
Co
LOAD
Rs
Ci
M2
M1
Rtn
Figure 2: Typical schematic of a flyback converter with synchronous rectification.
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When the primary device M1 is turned off, the secondary current will flow through
the body diode of the MOSFET M2 (which is analogous to the current flowing
through the output diode when diode rectification is used).
The SmartRectifier™ Control Technique is based on sensing the voltage across
the MOSFET and comparing it with two negative thresholds to determine the turn
on and off transition for the device. A higher negative threshold, VTH2, detects
current through the body diode and hence, controls the turn on transition for the
power device. Similarly, a second externally programmable smaller negative
threshold, VTH1, determines the level of the current at which the device turns off.
VGate
VDS
VTH2
VTH1
VTH3
Figure 3: IR1167 SmartRectifier™ control IC differential voltage sensing thresholds.
Additional control logic has been incorporated to prevent false turn off and gate
chattering when the device current transitions between its body diode and
channel.
When the power device is turned on, the instantaneous sensed voltage reduces
to RDSon ⋅ I D and depending on the level of the device current, could fall below the
turn off threshold and cause false device turn off. Additionally, the device turn on
is also associated with some parasitic ringing between the transformer leakage
inductance and device output capacitance.
Operation and analysis in CCM & DCM Flyback
The IR1167 SmartRectifier™ IC can emulate the operation of diode rectifier by
properly driving a Synchronous Rectifier (SR) MOSFET. The rectifier current is
sensed by the input comparator using the power MOSFET RDSon as a shunt
resistance and the GATE pin of the MOSFET is driven accordingly depending on
the level of the sensed voltage with respect to the 3 thresholds shown in Figure
3. Internal blanking logic is used to prevent spurious transitions and guarantee
operation in continuous (CCM), discontinuous (DCM) and critical (CrCM)
conduction mode.
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The modes of operation for a Flyback circuit differ mainly for the turn-off phase of
the SR switch, while the turn-on phase of the secondary switch (which
correspond to the turn off of the primary side switch) is identical.
¾ Turn On Phase
When the conduction phase of the SR FET is initiated, current will start flowing
through its body diode, generating a negative VDS voltage across it. The body
diode has generally a much higher voltage drop than the one caused by the
MOSFET on resistance and therefore will trigger the turn-on threshold VTH2. At
that point, the IR1167 will drive the gate of MOSFET on which will in turn cause
the conduction voltage VDS to drop down.
This drop is usually accompanied by some amount of ringing, that can trigger
the input comparator to turn off; hence, an externally programmable Minimum On
Time (MOT) blanking period is used that will maintain the power MOSFET on for
a minimum amount of time.
The programmed MOT will limit also the minimum duty cycle of the SR MOSFET
and, as a consequence, the max duty cycle of the primary side switch.
Figure 4: MOT and tBLANK during operation in DCM.
Notice both Minimum On Time and Blanking time logic are allowed only once per
switching cycle; it is necessary that VDS reaches VTH3 (therefore primary turn on)
for them being enabled again (therefore ready for the next switching cycle).
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¾ DCM/CrCM Turn Off Phase
Once the SR MOSFET has been turned on, it will remain on until the rectified
current will decay to the level where VDS will cross the turn-off threshold VTH1.
This will happen differently depending on the mode of operation.
dI
. Once the
dt
threshold is crossed, the current will once again flow through the body diode,
causing the VDS voltage to jump negative. Depending on the amount of residual
current, VDS may again trigger the turn on threshold: for this reason VTH2 is
blanked for an internally set blank time tblank (as shown in Figure 4) after VTH1 has
triggered. As soon as VDS crosses the positive threshold VTH3, this blanking time
is terminated and the IC is ready for next conduction cycle.
In DCM the current will cross the threshold with a relatively low
ID_PRIM
VDS_PRIM
T1
T2
T3
time
ID_SEC
VDS_SEC
time
Figure 5: DCM operating flyback converter simplified waveforms.
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ID_PRIM
VDS_PRIM
T1
time
T2
ID_SEC
VDS_SEC
time
Figure 6: CrCM operating flyback converter simplified waveforms.
¾ CCM Turn Off Phase
During the SR FET conduction phase the current will decay linearly, and so will
VDS on the SR FET. Once the primary switch will start to turn back on, the SR
FET current will rapidly decrease crossing VTH1 and turning the gate off. The turn
off speed is more critical here to avoid cross conduction on the primary side and
reduce switching losses. The blanking period is also applied in this case, but
given the very fast nature of this transition, it will be reset as soon as VDS crosses
VTH3.
ID_PRIM
VDS_PRIM
T1
time
T2
ID_SEC
VDS_SEC
time
Figure 7: CCM operating flyback converter simplified waveforms.
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Operation in Resonant Converters
Figure 8 shows the typical secondary-side schematic for a series resonant
converter with a capacitive output filter. Implementing synchronous rectification in
such applications would require 2 current sense transformers, 2 high speed
comparators and finally 2 high current, low propagation delay gate drivers
needed to drive the two power devices. Existing monolithic solutions are based
on PLL control techniques and rely on synchronizing signals from the primaryside to anticipate the turn off transition for the secondary MOSFETs; hence, they
cannot guarantee reliable operation when the converter operates in burst mode
during light and no load conditions. The SmartRectifier™ control technique
operates completely independent of the primary-side switching technique and the
dI
low
transitions in the resonant converter make the IR1167 an excellent
dt
candidate in such applications. Figure 9 illustrates this with waveforms.
M1
Ls 1
Lp
Ls 2
Cout
LOAD
M2
Figure 8: Series resonant converter secondary-side general schematic.
IR1167 operates completely independent of the primary-side switching technique
dI
and the low
transitions make it an excellent candidate for such applications
dt
as shown in Figure 9 below.
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VTH3
ID_SEC
VDS_SEC
T1
T2
time
VTH1
VTH2
Gate Drive
time
Blanking
MOT
tblank
Figure 9: Series resonant converter with IR1167 SmartRectifier™ control IC general waveforms.
In resonant converter applications, output voltage regulation can be achieved by
operating in fixed or variable frequency (50% duty cycle) operating modes. In
variable frequency applications, the converter operates at the minimum switching
frequency at low line-full load conditions and at the maximum frequency at high
line-no load conditions.
Hence, the MOT selection for resonant converters can simply be based on the
maximum switching frequency of the converter. MOT ensures proper gating
signals for the synchronous MOSFETs during light load conditions (i.e. operating
at maximum switching frequency), and the situation only improves when the
converter operates at heavier loads.
Typical System Schematics and Passive Components Nomenclature
Passive components needed for IR1167 operations are:
• C: supply decoupling capacitor
• Rg: synchronous MOSFET gate resistor
• RMOT: Minimum On Time setting resistor
Component not necessary but recommended:
• RCC: series resistor on supply
In all the low side configurations, power can be drawn directly from converters’
output whenever its regulated voltage falls in the recommended range (12-20V).
In all other cases, recommendation is to provide a dedicated supply through:
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• Auxiliary transformer winding if high side
• Transformer main winding tap if low side
Figure 10 to Figure 16 show typical systems schematics.
R
C
Rg
Vgate
Vcc
GND
OVT
Vs
MOT
Vd
EN
IR1167
+
OUTPUT
Rmot
Figure 10: single ended, low side rectification, supply from winding tap (Voutput < 12V).
Rcc
C
Rg
Vgate
Vcc
GND
OVT
Vs
MOT
Vd
EN
IR1167
+
OUTPUT
Rmot
Figure 11: single ended, low side rectification, supply from output voltage (Voutput = 12-20V).
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C
R
Rg
Vcc
Vgate
OVT
GND
MOT
Vs
EN
Vd
+
-
IR1167
Rmot
OUTPUT
Figure 12: Single ended, high side rectification, supply through aux winding (output voltage
independent).
IR1167
Rg
Vd
EN
Vs
MOT
GND
OVT
Vgate
Vcc
Rmot
C
R
C
Rg
Vgate
Vcc
GND
OVT
Vs
MOT
Vd
EN
IR1167
+
OUTPUT
Rmot
Figure 13: Center tap, low side rectification (Half or Full bridge resonant), supply from extra
winding (Voutput < 6V).
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IR1167
Rg
Rmot
Vd
EN
Vs
MOT
GND
OVT
Vgate
Vcc
C
R
C
Rg
Vgate
Vcc
GND
OVT
Vs
MOT
Vd
EN
IR1167
+
OUTPUT
Rmot
Figure 14: Center tap, low side rectification (Half or Full bridge resonant), supply from winding
tap (6V < Voutput < 10V).
IR1167
Rg
Vd
EN
Vs
MOT
GND
OVT
Vgate
Vcc
Rmot
C
Rcc
Rcc
C
Vgate
Rg
Vcc
GND
OVT
Vs
MOT
Vd
EN
IR1167
+
OUTPUT
Rmot
Figure 15: Center tap, low side rectification (Half or Full bridge resonant), supply from output
voltage (Voutput = 12-20V).
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C
R
Rg
Vcc
Vgate
OVT
GND
MOT
Vs
EN
Vd
+
-
IR1167
Rmot
OUTPUT
C
R
Rg
Vcc
Vgate
OVT
GND
MOT
Vs
EN
Vd
Rmot
IR1167
Figure 16: Center tap, high side rectification (Half or Full bridge resonant), supply from extra
winding (output voltage independent).
Required System Parameters
Fundamental values to be captured on the system if not known by design are
1. maximum switching frequency f SWmax and minimum operating switching
frequency f SWmin
2. secondary minimum conduction time, also called (in SmartRectifier™
terminology) Minimum On Time ( MOT )
3. mode of operations: Continuous (CCM), Discontinuous (DCM) or Critical
(CrCM) conduction mode of operations
4. The maximum temperature of the environment in which the IR1167S IC
will operate, TICamb (this is normally the maximum PCB temperature)
5. The available supply voltage Vsupply . It can be the converter output voltage
(for low side rectification systems) or a dedicated supply (i.e.: transformer
extra winding or tap)
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¾ Minimum On Time (MOT) Determination
In order to properly capture the minimum on time on an existing system, the
following procedure is recommended. First, probe the output rectifier voltage (or
current) and set the oscilloscope in order to trigger the conduction waveform.
Second, identify the condition (AC main line and load) which visually shows
reducing the conduction waveform at minimum (normally small or no load, high
line). Then, using measuring tools, monitor the conduction pulse width
statistically. Most equipments include that function. Due to variable frequency
nature of most flyback controllers, width of that pulse can vary significantly.
Target is therefore to capture the lowest possible value while slightly varying
both, load and main line, around the identified working point (if the condition is at
no load, there is no need to apply some load).
Figure 17 shows the acquired statistic -
Figure 17: Minimum On Time determination on an existing system.
Notice that the minimum value (low) showed in the scope statistic could easily
mislead. It can be due to several reasons not directly linked to reality (trigger
issues, measuring issues, etc.) and it’s normally the minimum absolute recorded.
In this case, the recommendation is to properly use the statistical data, assuming
the average minus 6 times the standard deviation reasonably represents the
minimum possible value (3ppm). In the present example
MOT = 2.32 − 6 ⋅ 0.0987 = 1.73µs
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¾ Maximum Switching Frequency Determination
Like in MOT determination, also max frequency can be variously located around
different power supply working conditions. A simple sweep of AC line and load
can easily show around which working point this condition occurs. Then using
statistical tool, max frequency is easily measured around the identified working
point (slight variation of load and main AC line).
Figure 18: maximum switching frequency determination on an existing system.
Also in this case, low and high value shown on scope could easily misled. Having
this parameter thermal impact on IR1167 design only and being this averaged in
nature (see electrical procedure), we recommend keeping average plus 3 sigma
(0.3%) as a good rule for determining the maximum switching frequency. In the
present example:
f SWmax = 66.14 + 3 ⋅ 2.48 = 73.6kHz
¾ Mode of Operation Determination
Mode of operation can be easily checked sweeping both, AC main line and load
across their range.
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Detailed Design Procedure
Following procedure assumes the synchronous MOSFET has been already
identified as well as the above mentioned systems parameters
a. OVT setting
Table 1 shows the guidelines in properly choosing the right OVT (turn on and off
comparator Offset Voltage Trimming) threshold.
System mode of operation
OVT connected to (typical values)
DCM or CrCM
Ground, VTH1= -3.5mV
Boundary CCM
Floating, VTH1= -10.5mV
CCM
VCC, VTH1= -19mV
Table 1: OVT setting guideline as function of system mode of operations
The basic idea behind this is the need to ideally approximate a rectifier behavior,
having the voltage sense as a sole input to the controller. In DCM or CrCM, it is
obvious the use of the comparator threshold closer to zero, in order to maximize
the conduction through the MOSFET channel.
In CCM conditions, the reverse voltage normally appears quickly across the
rectifier, requiring the controller to turn it off with the minimum possible delay.
The two lower comparator thresholds enable earlier detection of current fall.
Ideally, the optimized system would have one of those two thresholds as closest
as possible to the MOSFET VDS at the end of the conduction cycle as shown in
Figure 19.
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Figure 19: OVT design for CCM operations.
Further requirements for CCM operations are explained at the end of this design
procedure.
In boundary CCM (CCM during ac main or load boundary conditions only,
therefore rarely occurring in normal system operating conditions) the floating
OVT threshold generally represents a good compromise.
b. IC current consumption calculation
First, from selected synchronous MOSFET total gate charge Qg and gate to drain
charge Qgd data have to be identified, together with the corresponding gate
voltage Vgs. Because of the IR1167 mode of operations, secondary current
initially flows through the body diode, therefore the turn on gate characteristic
doesn’t include the Miller charge. Figure 20 shows how a regular gate
characteristic (black continuous line) changes when the switch is turned on at
zero or slightly negative drain to source voltage (red discontinuous line)
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VGS
QG
Qgd
Qg
Qg-Qgd
Figure 20: MOSFET gate characteristic when driven by SmartRectifier™ control IC.
It is evident much less charge is required and the behavior can satisfactorily be
modeled as a capacitor:
Csync =
Qg − Qgd
Vgs
If more parts are paralleled, the above capacitance have to be multiplied for the
number of devices.
The maximum IC required current can then be calculated as following:
(
I CC = f SWmax CsyncVghigh + I QCC + 7 ⋅10 −9 f SWmax
)
where Vg high is the IR1167 gate driver output voltage and fSW_max is the converter
maximum switching frequency as previously identified. The first term is entirely
due to synchronous MOSFET gate drive while the second term accounts for the
IC internal logic consumption in regular operations (the factor 7 ⋅10 −9 accounts
the frequency dependent current requirements for the internal logic).
Notice this term is independent from VCC voltage.
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c. Gate and supply series resistors design and thermal verification
IR1167S based synchronous rectification has the prerogative to turn the switch
on and off at VDS levels close to zero. The gate resistor hasn’t therefore impact
on transitions and can be designed on a different basis.
In order for the gate loop to be optimized, oscillations have to be avoided in
regular operations. Assuming the total gate trace loop inductance is known (a
very first estimation can be 1nH/mm of physical trace length), the minimum
recommended gate resistor will be
Rg loop > 2
Lg
Ciss
Where Ciss is the switch input capacitance (from switch datasheet).
Figure 21 shows how this critical resistance value varies with the overall gate
loop inductance for some popular International Rectifier MOSFETs.
IR1167 driving stage: minimum required gate resistor vs. gate loop inductance for circuit
damping using some popular IR HexFET (0.7Ω IR1167 driver impedance and 1.2Ω FET
internal gate resistance included)
Critical gate resistance for loop damping [ Ω]
8
IRF7853 Ciss = 1.64nF
IRF6644 Ciss = 2.21nF
IRFB4610 Ciss = 3.55nF
IRFB4410 Ciss = 5.15nF
IRFB4110 Ciss = 9.62nF
7
6
5
4
3
2
1
0
5
8
11
14
17
20
23
26
29
32
35
Total gate loop inductance [nH]
Figure 21: minimum external gate resistor vs. gate loop inductance for some MOSFETs.
It is evident how a good layout practice can dramatically reduce this requirement.
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Now, let’s consider the well known series RC network transient: the energy
dissipated by the resistor is exactly equal to the energy stored in the capacitor.
IR1167S internal gate driver is of course always in series with the external gate
resistor, which means they will linearly share the power dissipation.
First, let’s calculate the energy stored in the MOSFET gate:
Eg =
1
CsyncVg2high
2
The power dissipated by the driver buffer AND the total gate resistance will
therefore be
Pdr = 2 f SW max E g
The driver buffer and the total gate resistance will linearly share this power
dissipation as described in the following relationship:

Rg
Rg
+
PRg = 
R +R
Rg + RSink
Source
 g
 Pdr
⋅
 2

Rearranging this last relationship
Rg
Rg
1
= 
+

Pdr 2  Rg + RSource Rg + RSink
PRg




and solving it with respect to Rg (which includes the external gate resistor and the
MOSFET internal gate resistance), it is possible to plot the percentage of the
total driving power dissipated into the gate resistor as a function of its value. This
is also useful for proper dimension the gate resistor itself. Notice on IR1167S
datasheet, pull up and pull down resistances are defined; while RSink = rdown ,
RSource = 1.1 rup in order to account for some extra energy dissipated for voltage
clamping.
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IR1167 driving stage: percentage of the required driving power Pdr dissipated in
the gate resistor as a function of the gate resistor value
Percentage of Pdr dissipated in gate resistor [W]
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Gate resistor value [Ω]
Figure 22: percentage of gate driver power dissipated in the gate resistor as function of its value.
It is evident the asymptotic nature of the curve (it would require an infinite gate
resistor for dissipating all the power in it).
The final step is the thermal verification for the chosen value. Using the
maximum thermal resistance junction to ambient, the maximum temperature
(where ambient refers to the environment in which the IC will work , i.e. box, PCB
etc.) and the IC maximum junction temperature, it is now possible to calculate the
maximum allowable IC power dissipation
PICmax =
TJ max − TIC _ amb
RϑJA
where, according to IR1167 datasheet, RθJA=128ºC/W.
Because PRg is known and supply current has already been calculated, this will
imply to limit the VCC supply voltage (therefore the maximum input power for
IR1167)
VCCmax =
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PICmax + PRg
I CC
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Figure 23 shows maximum allowable IR1167A VCC vs. maximum switching
frequency for some popular International Rectifier 100V MOSFET, assuming 1Ω
external gate resistor and 85ºC environment
IR1167AS maximum VCC vs. synchronous rectifier switching frequency
TIC_J=125ºC, TIC_amb=85ºC external RG=1Ω, HexFET 1Ω gate resistance included
20
Maximum allowable VCC voltage [V]
19
18
17
16
IRF6644 Csynch = 2.4nF
15
IRFB4610 Csynch =5.4nF
IRFB4410 Csynch = 7.6nF
14
IRFB4110 Csynch = 10.8nF
13
12
11
50
100
150
200
250
300
350
400
450
500
Maximum synchronous HexFET switching frequency [kHz]
Figure 23: Max VCC supply voltage vs. switching frequency for some chosen MOSFET, IR1167AS
IC ∆TJ=40ºC.
In order to avoid UVLO issues, VCC designs below 12V should be avoided.
It is clear how supply voltage and gate resistor play a major role in the design
trade off. In most commercial systems, the minimum gate resistor value for loop
damping will satisfy the thermal requirements.
If not, the procedure has to be iterated taking the following steps
Step 1: decrease the VCC to the lowest possible value through a series resistor:
RCC =
Vsupply − VCC
I CC
If this allows VCC to comply the thermal limit, then gate resistor value can be kept
as designed.
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It is worth mentioning the additional benefit of adding some series resistance to
supply: an enhanced filtering effect with local decoupling capacitor. For systems
powered from the output (no dedicated power through windings, etc.) this can
result in smoother operations
Step 2: increase the gate resistor value. This can be of some effect if a small
resistance has been used, according to Figure 22.
d. Decoupling capacitor
Several techniques are possible for decoupling capacitor sizing, depending upon
system topology and or special requirements. As general guideline a capacitor of
at least 100nF should be used.
The two most common cases are IR1167S powered directly form the output or
from a dedicated winding.
In the first case, in order to reduce the voltage ripple and possible noise, a good
criteria is to use a series resistor on supply (if not already used for thermal
management reasons) and size the capacitor in order to obtain a low pass filter
with pole frequency a couple of octaves below the minimum operating switching
frequency (not stand-by)
2
C min =
π ⋅ f SWmin ⋅ RCC
Figure 24 chart shows some obtained values as examples. A minimum value of
100nF limits the curves.
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Decoupling capacitor value vs minimum switching frequency (4 RCC values) for 10dB attenuation (two octaves on frequency domain)
2.0E-06
1.8E-06
Rcc = 10ohm
Rcc = 33ohm
Rcc = 66ohm
Rcc = 100ohm
Decoupling capacitor value [C]
1.6E-06
1.4E-06
1.2E-06
1.0E-06
8.0E-07
6.0E-07
4.0E-07
2.0E-07
0.0E+00
10
60
110
160
210
260
Converter minimum operating frequency [kHz]
Figure 24: decoupling capacitor value vs. min. switching frequency, 10dB attenuation on supply.
In case of operations through aux winding or winding tap, decoupling capacitor
should be sized in order to allow one switching period operation even in absence
of main supply, within an acceptable voltage ripple ∆VCC
C min =
I CC
f SWmin ⋅ ∆VCC
e. MOT resistor
Being the MOT setting linear with the resistor value, the following relationship
can be used
RMOT = 2.5 ⋅ 1010 t MOT
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f. Rectifier turn off maximum current slope calculation for CCM systems
dI SEC
dt turnoff
when the SmartRectifier™ turns off, in order to maximize efficiency. Figure 25
shows turn off waveforms, where at given times:
• t1: primary switch turns on
• t2: secondary VDS hits VTH1 threshold
• t3: secondary VDS and ID reach zero
In CCM systems, it is highly recommended to control secondary slope
dI SEC
dt turn off
ID_SEC
time
VTH1
VDS_SEC
tDoff + 3
gate off
SR Gate Drive
t1
t2
t3
time
Figure 25: SmartRectifier™ turn off waveforms in dI/dt controlled CCM conditions.
It is evident, the optimal condition is to have synchronous FET turned off when
dVDS
the current approaches zero. In order to obtain this,
should be designed
dt
such as enough time is allowed to internal logic to react (tDoff on IR1167
datasheet) and to gate driver to completely discharge the gate (3 times the gate
loop time constant at turn off, i.e. τ gate off = ( Rg FET + Rg + rdown ) ⋅ Csync , where
resistances are internal MOSFET gate resistance, external gate resistor and
driver pull down resistance). This normally ends up to be in the range of 55 –
100ns.
The secondary current slope at turn of should therefore be designed according to
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dI SEC
VTH 1
≤
dt turnoff RDS on (t Doff + 3τ gate off )
In order to obtain this, a small saturable core in series with the primary winding
could be used or a sufficiently rugged primary transistor with slow turn on for
small power systems.
The primary maximum current slope requirement at turn on will be easily
calculated using transformer turns ratio:
dI PRI
N
dI
= SEC ⋅ SEC
dt turnon N PRI
dt turnoff
If this design condition is met, the reverse current through the Synchronous FET
will be minimal and only needed for charging its output capacitance up to the
reverse voltage.
Design example with IR1167AS (10.7V gate output voltage)
System data:
• f SWmax = 250kHz
•
MOT = 1.2µs
•
•
Critical Conduction Mode
f SWmin = 18kHz
•
TICamb = 80º C
•
Low side rectifier system, output voltage 19V (direct supply from
converter output possible)
Synchronous MOSFET: IRFB4110, 100V 4.5mΩ max
• Qg = 150nC @ Vgs = 10V
•
Qgd = 43nC @ Vgs = 10V
•
Ciss = 9.62nF ; Rg FET = 1.3Ω
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a. OVT setting: ground
b. IC current consumption calculation
Csync =
Qg − Qgd
Vgs
= 10.7nF
(
)
I CC = f SWmax CsyncVghigh + I QCC + 7 ⋅10−9 f SWmax = 32.8mA
c. Gate and supply series resistors design and thermal verification
Assuming the total gate loop trace length is 15mm (0.6inch) therefore Lg ≈ 15nH
Rg loop > 2
Lg
Ciss
= 2.5Ω
From MOSFET datasheet internal gate resistance is 1.3Ω, from IR1167S
datasheet driver pull down resistance is 0.7Ω, for a total of 2Ω. It looks therefore
reasonable to chose an external gate resistor for the missing part
Rg = 0.5Ω
According to the procedure, let’s now verify the system thermally:
Pdr = 2 f SW max E g = 306mW
Therefore

Rg + Rg FET
Rg + Rg FET
+
PRg + PRg _ FET = 
 Rg + Rg + RSource Rg + Rg + RSink
FET
FET

 Pdr
⋅
= 155mW
 2

Assuming an acceptable IC maximum junction temperature of 130ºC
PICmax =
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TJ max − TIC _ amb
RϑJA
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= 390mW
26
Which means a maximum VCC voltage
VCCmax =
PICmax + ( PRg + PRg _ FET )
I CC
= 16.6V
This value is far away from the available 19V supply, and a series resistor would
probably dissipate too much power. According to Figure 22, it is possible to
slightly increase the gate resistor and get significant benefits. Let assume it is
increased in value to
Rg = 1.1Ω
The power dissipated in the gate loop resistance external to the IC will become

Rg + Rg FET
Rg + Rg FET
+
PRg + PRg _ FET = 
 Rg + Rg + RSource Rg + Rg + RSink
FET
FET

 Pdr
⋅
= 172mW
 2

The maximum supply voltage will change to
VCCmax =
PICmax + ( PRg + PRg _ FET )
I CC
= 17.2V
It is reasonable now to “fill the gap” with 19V through a series resistor:
RCC = 55Ω
This resistor will dissipate a maximum of 60mW, which is considered acceptable.
d. Decoupling capacitor
Being the system powered from output, filtering criteria is the preferred one,
therefore
Cmin =
2
π ⋅ f SW ⋅ RCC
= 643nF
min
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from which the next standardized value is chosen
C = 660nF
e. MOT resistor
From the given relationship
RMOT = 2.5 ⋅1010 t MOT = 30kΩ
Layout guidelines and examples
¾ IC placement
Due to the nature of the control, based on fast and accurate voltage sensing, it is
mandatory to layout the circuit in order to keep the IR1167 as closest as possible
to the SR MOSFET. As a general guideline, the physical distance between the
two devices should never exceed 10mm (0.4 inches).
¾ IC Decoupling Capacitor
The key element to proper bypassing of the IC is the physical location of the
bypass capacitor and its connections to the power terminals of the control IC. In
order for the capacitor to provide adequate filtering, it must be located as close
as physically possible to the VCC and COM pins and connected thru the shortest
available path.
¾ Differential Sensing for VD/VS
IR1167S provides accurate differential voltage sensing across synchronous
MOSFET. It is recommended to minimize the traces lengths and to keep it
separated from the power ground as much as possible. In case of through hole
devices, the device pin would be the best place where VS and GND traces
connect.
If SO8 MOSFET is used, traces should be kept separated until the copper area
where source pins are soldered.
For low side configurations, when a sensing resistor is used for current feedback
in the rectifier power loop, it is highly recommended not to include it in the driving
and sensing loops. This will cause some noise on the VCC, which will be properly
filtered by the decoupling capacitor and the RCC series resistor, as shown in the
Figure 26 schematic.
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Rcc
C
Rg
Vgate
Vcc
GND
OVT
Vs
MOT
Vd
EN
IR1167
+
OUTPUT
Rmot
Rsense
Figure 26: output current sense resistor placement (if present).
¾ Gate Drive Loop
Minimal gate drive loop will reduce requirements for damping, enhancing system
robustness. Gate loop inductance plays a major role in damping requirements as
shown in Figure 21. Once layout is finalized, then a “rule of thumb” estimation
consists in measuring the physical loop trace length in assuming each millimeter
(1mm = 39.37mils) accounts for 1nH. Other methods include measurement (low
frequency RCL meters or current slope for a given voltage pulse) or FEM
simulations.
¾ Single layer board layout examples
to supply
Figure 27: Single side board, TO220 MOSFET, OVT grounded, solder side view, through hole gate
resistor.
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Figure 28: Single side board, TO220 MOSFET, OVT grounded, solder side view, SMD gate resistor.
Figure 29: Single side board, TO220 MOSFET, OVT to VCC, solder side view, through hole gate
resistor.
Figure 30: Single side board, SO8 MOSFET, OVT to VCC, solder side view.
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Figure 31: Single side board, SO8 MOSFET, OVT grounded, solder side view.
Symbols list
VTH1: IR1167 turn off threshold
VTH2: IR1167 turn on threshold
VTH3: IR1167 periodic logic (reset) threshold
RDSon: synchronous rectifier MOSFET channel ON resistance
ID: synchronous rectifier MOSFET drain current
VDS: synchronous rectifier MOSFET drain to source voltage
MOT: IR1167 minimum ON time parameter
tblank: IR1167 turn off blanking time
T1: primary switch ON time in flyback converter
T2: secondary current duration in flyback converter
T3: primary AND secondary circuit inactivity time in DCM flyback converter
C: IR1167 decoupling capacitance value on supply
Rg: SR MOSFET gate drive loop resistance external to IR1167 IC
RMOT: Minimum ON Time programming resistor value
RCC: supply voltage series resistor value (Vsupply to VCC)
fSWmax: converter maximum operating switching frequency
fSWmin: converter minimum operating switching frequency
TICamb: ambient temperature with reference to IC (most cases is PCB temperature
where IC is soldered)
Vsupply: available voltage source for IC power up
OVT: Offset Voltage Trimming of turn on and off comparator
Qg: SR MOSFET total gate charge
Qgd: SR MOSFET gate to drain (Miller) charge
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Vgs: SR MOSFET gate to source voltage
Vg_high: IR1167 gate drive output voltage
IQCC: IR1167 quiescent current
Rg loop: total gate loop resistance
Lg: total gate loop parasitic inductance
Ciss: SR MOSFET input capacitance
Eg: SR MOSFET stored in gate
Pdr: total power dissipated by the gate drive function
RSource: gate driver source resistance
RSink: gate driver sink resistance
PRg: gate resistance dissipated power
PICmax: IR1167 IC maximum power dissipation
TIC_amb: IC environment temperature (PCB temperature)
RθJA: IR1167 IC junction to ambient thermal resistance
VCC: supply voltage on IR1167 pin
ICC: supply current on IR1167 pin
UVLO: IR1167 Under Voltage Lock Out
Vsupply: system available supply voltage for SR function
Cmin: minimum calculated decoupling capacitance
∆VCC: supply peak to peak ripple voltage on IR1167 VCC pin
tMOT: obtained minimum on time
τgate off: gate loop turn off time constant
NSEC: transformer secondary winding number of turns
NPRI: transformer primary winding number of turns
References
[1] IR1167S Smart Rectifier control IC datasheet, International Rectifier, February 2006
[2] M.T. Zhang, M. Jovanovic, F. Lee, “Design considerations and performance evaluations of
Synchronous Rectification in Flyback converters”, IEEE Transactions on Power Electronic,
VOL.13, N.3, May 1998
[3] Smart Rectifier™ increases Power Density in Flyback Topologies while reducing System
Complexity, Maurizio Salato, Adnaan Lokhandwala, Marco Soldano, and Helen Ding, PCIM
China 2006
[4] Novel Output Rectification Control IC Improves Efficiency & Thermal Performance in External
AC-DC Power Supplies, Adnaan Lokhandwala, Maurizio Salato and Marco Soldano, Portable
Power Developers Conference 2006
[5] US Patent Application Publication N. 2005/0122753 A1, Jun. 9, 2005
Rev. 1.1 – March 2006
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