Application Note: STR-W6700 Series Off-Line Quasi-Resonant Switching Regulators

Product Information
STR-W6700 Series Off-Line
Quasi-Resonant Switching Regulators
Introduction
The Series STR-W6750 devices are hybrid integrated circuits (HICs) with a built-in power MOSFET and a control
IC designed for quasi-resonant type switch-mode power
supplies (SMPS). In normal operation, the HIC provides
high efficiency and low EMI noise with bottom-skip quasiresonant operation during light output loads. Low power
consumption is also achieved by blocking (intermittent)
oscillation during an auto-burst mode and reduced even
further in a manually triggered (clamping an output voltage)
standby mode.
The HIC is supplied in a seven-pin fully-molded TO-220style package with pin 2 deleted, which is suitable for downsizingand standardizing of an SMPS by reducing external
componentcount and simplifying circuit design.
Figure 1. STR-W6700 series packages are fully molded TO-220
package types. Pin 2 is deleted for greater isolation.
Features and benefits include the following:
▪ Blocking (or intermittent) oscillation operation by
reducing output voltage in the standby mode.
▪ In addition to the standard quasi-resonant operation, a
bottom-skip function is available for increased efficiency
from light to medium load.
▪ Soft-start operation at start-up.
▪ Reduced switching noise (compared to conventional
PWM hard-switching solution) with a step-drive
function.
▪ Built-in avalanche-energy-guaranteed power MOSFET
(to simplify surge-absorption circuit; no VDSS derating
is required).
▪ Overcurrent protection (OCP), overvoltage protection
(OVP), overload protection (OLP), and maximum
ON-time control circuits are incorporated. OVP and OLP
go into a latched mode.
▪ Able to save SMPS design time with present designs and
evaluation processes.
Table 1. Product Line-up
Type #
MOSFET
VDSS
(V)
RDS(on)
(Max)
(Ω)
STR-W6735
500
0.57
STR-W6753
STR-W6754
1.70
650
STR-W6756
STR-W6765
0.96
0.73
800
1.80
VAC
Input
(V)
POUT*
(W)
120
160
Wide
58
230
120
Wide
100
230
160
Wide
140
230
240
Wide
50
230
110
*The listed output power represents thermal ratings, and the peak
output power, POUT , is obtained by 120% to 140% of the thermal
rating value. In case of low output voltage and narrow on-duty
cycle, the POUT (W) becomes lower than the above.
Contents
Introduction
Pin functional descriptions
Operation description
Transformer parameters
General considerations
Design considerations
Package Dimensions, TO-220
Worldwide Contacts
28103.30
1
2
6
10
11
13
14
17
All performance characteristics given are typical values for
circuit or system baseline design only and are at the nominal
operating voltage and an ambient temperature of 25°C, unless otherwise stated.
SANKEN ELECTRIC CO., LTD.
http://www.sanken-ele.co.jp/en/
starts its operation by the start-up circuit, and supply current is
increased. Once the VCC pin voltage drops down to lower than
the Operation-Stop voltage 9.7 V, the UVLO circuit operates to
stop the control circuit, and the IC returns to its initial state.
Pin functional descriptions
VCC Supply (pin 4)
Start-up circuit The start-up circuit detects the VCC pin voltage, and makes the control IC start and stop operation. The power
supply of the control IC (VCC pin input) employs a circuit as
shown in figure 2. At start-up, C3 is charged through a start-up
resistor R2. The R2 value needs to be set for more than the hold
current of the latch circuit (140 μA max.) and to operate at the
minimum AC input.
If the value of R2 is too high, the C3 charge current will be
reduced. Consequently, it will take longer to reach the OperationStart voltage. The VCC pin voltage falls immediately after
the control circuit starts its operation. The voltage drop can be
reduced by increasing C3 capacitance. However, too large a
C3 capacitance will cause an improperly long time to reach the
Operation-Start voltage after the initial power turn on.
Bias/drive winding After the control circuit starts its operation,
the power supply is operated by rectifying and smoothing the
voltage of the bias winding. Figure 4 shows the start-up voltage
waveform of the VCC pin. The bias winding voltage does not
immediately increase up to the set voltage after the control circuit
starts its operation. That is why the VCC pin voltage starts dropping. The Operation-Stop voltage is set as low as 10.6 V (max),
the bias winding voltage reaches a stabilized voltage before it
drops to the Operation-Stop voltage, and the control circuit contiues its operation. The bias winding voltage, in normal power supply operation, is set for the voltage across C3 to be higher than
the Operation-Stop voltage, VCC(OFF) , 10.6 V (max.) and lower
than the OVP-operation voltage, VCC(OVP) , 25.5 V (min.).
In general, SMPS performs its start-up operation properly
with a value of C3 between 4.7 and 47 μF, and R2 between
47 and 150 kΩ for 120 V narrow or universal AC input, and
82 to 330 kΩ for 200 V narrow AC input.
I CC
As shown in figure 3, the circuit current is limited to 100 μA max
(VCC = 15 V, and resistor R2 with appropriate high resistance
value for the circuit) until the control circuit starts its operation.
Once the VCC pin voltage reaches 18.2 V, the control circuit
100 μ A
( MAX)
9. 7 V
( TYP)
15V
18. 2 V
( TYP)
VCC
Figure 3. VCC pin current versus voltage
VCC
R2
P
18.2 V
(TYP)
Operation Start
Bias Winding Voltage
1
D2
D
VCC
4
STR-W6700
S/GND
10.6V
(MAX)
D
C3
Start-up
failure
time
3
AC on
Figure 2. External start-up circuit.
STRY6700-AN Rev. 2.0
Figure 4. VCC pin voltage after start-up, capacitor C3 installed
SANKEN ELECTRIC CO., LTD.
2
In an actual power supply circuit, the VCC pin voltage might
be changed by the value of secondary output current as shown
in figure 5. C3 is fully charged by the surge voltage generated
instantly after the MOSFET turns off. In order to prevent this, it
is effective to add a resistor (R7) of several ohms to tens of ohms
in series with the diode as shown in figure 5. The optimum value
of the additional resistor is determined in accordance with the
specifications of the transformer because the VCC pin voltage is
determined by construction of the transformer.
Furthermore, the variation ratio of the VCC pin voltage becomes
worse due to a loose coupling between primary and secondary
windings of the transformer (the coupling between the bias winding and the stabilized output winding for the constant voltage
control). Therefore, when designing a transformer, the winding
position of the bias winding needs to be studied carefully.
Overvoltage protection (OVP) circuit If VCC, referencing
the S/GND pin, exceeds 27.7 V, the OVP circuit of the control
IC starts its operation and the fault mode is latched by the latch
circuit, the control IC stopping its oscillation. Generally, the VCC
pin voltage is supplied from the bias winding of the transformer,
VCC
ut R7
Witho
With R7
and the voltage is in proportion to the output voltage; thus, the
OVP circuit also operates in the case of overvoltage output of the
secondary side, for example, when the voltage detection circuit
is open. The secondary output voltage for the OVP operation
(VO(OVP)) is obtained from the following formula:
VO(normal operation)
VO(OVP) =
(1)
27.7 V
VCC(normal operation)
Latch circuit OVP and OLP fault modes latch the oscillation
output low, which stops the power supply circuit operation. The
holding current of the latch circuit is 140 μA (max., TA = 25°C)
when the VCC pin voltage is at the Operation-Stop voltage minus
0.3 V.
In order to prevent malfunction caused by, for instance, noise,
a delay time is programmed into a timer circuit, which prevents
latch circuit operation until the OVP or OLP circuit keeps operating for more than a programmed time. During the latched mode,
the internal regulator circuit keeps running, the circuit current is
maintained at a high level, and the VCC pin voltage drops.
When the VCC pin voltage drops down to the Operation-Stop
voltage (9.7 V (typ.) ), the voltage starts rising again as the circuit
current becomes less than 140 μA. When the VCC pin voltage
reaches the Operation-Start voltage (18.2 V (typ.) ), the circuit
current increases, and the voltage drops again. Consequently,
the VCC pin voltage is maintained between 9.7 V and 18.2 V in
the latched mode. Figure 7 shows the voltage waveform in the
latched mode. The latched mode is released by decreasing the
VCC pin voltage to below 7.2 V, or in general, by shutting off the
AC input.
SS/OLP (Pin 5)
Through the SS/OLP pin, soft-start and overload protection is
enabled by connecting a 0.47 to 3.3 μF capacitor to the pin.
IO
Figure 5. VCC versus IO (secondary load)
S/GND
D
C3
3
Figure 6. VCC versus IO (secondary load)
STRY6700-AN Rev. 2.0
R7
Bias
D2
VCC
4
STR-W6700
Figure 7. VCC during latch mode
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Soft-start operation at start-up of power supply At the
power supply start-up, an external capacitor is charged up to the
soft-start operation threshold voltage, VSSOLP(SS) , by soft-start
operation charging current, ISSOLP(SS) , sourced from the SS/OLP
pin. Soft-start is activated at power supply start-up by means of
the SS/OLP pin voltage change from the initial 0 V level, up to
1.2 V. Timing is shown in figure 8 and the following table:
Gradual increase of drain current suppresses audible noise from
the transformer.
By comparing the oscillation waveforms of the SS/OLP pin and
that of the internal control, soft-start widening of the on-width is
activated. In addition, soft-start is operated every time in the burst
oscillation of Auto-standby mode and manual standby mode.
Overload protection (OLP) Figure 9 shows output characteristics of the secondary side when the OCP circuit is activated due
to an overload at the secondary side output. When the output voltage drops in an overload mode, the bias winding voltage of the
primary side drops proportionally, and the VCC pin voltage drops
below the Operation-Stop voltage to deactivate the IC. Then the
circuit current decreases, and the VCC pin voltage rises again by
way of the start-up resistor (R2) charge current to reactivate the
IC intermittently at the Operation-Start voltage. However, where
the transformer has multiple output windings and coupling is
not good enough, the intermittent operation might not be sensed
even if the output voltage drops in an overload mode, because
the primary bias winding voltage would not drop. Although the
intermittent operation is not realized, protection might still be by
means of the OLP activation.
Figure 8. Soft-start operation
Figure 9. Current-mode control
Soft-start Timing (Charging current: 550 μA)*
CSS
(μF)
0.47
1
2.2
3.3
4.7
Time
(ms)
1.0
2.2
4.8
7.2
10.3
*A large CSS value also results in a longer time from OLP
operation to latched mode.
Figure 10. Timing at overload
STRY6700-AN Rev. 2.0
SANKEN ELECTRIC CO., LTD.
4
In the overload mode, where drain current is controlled by OCP
operation, the secondary-side output voltage drops. Accordingly,
the error-amplifier and photocoupler on the secondary side are
cut off. The STR-W6700 series regards the signal absence with
continuous OCP operation as an overload status, and the SS/OLP
pin voltage starts rising by ISSOLP(OLP) as shown in figure 10 and
in the following table:
OLP timing (0 to 4.9 V, charging current: 11 μA)
CSS (μF)
0.47
1.0
2.2
3.3
4.7
Time (ms)
209
445
980
1470
2094
NOTE: A large CSS value also results in a longer soft-start time.
After the SS/OLP pin voltage keeps rising to the OLP-Operation
Threshold Voltage (VSSOLP(OLP) = 4.9 V), the oscillation stops,
and the IC goes into a latched mode.
The time from OLP activation to a latched mode should be
obtained from the following formula, assuming ISSOLP(OLP) is
from a constant-current circuit:
CSS × ∆V
t=
(2)
ISSOLP(OLP)
where ∆V is the capacitor charging voltage of approximately 4.9 V.
However, the ISSOLP(OLP) is dependent on the SS/OLP pin voltage,
and ISSOLP(OLP) drops as the SS/OLP pin voltage rises. The actual
SS/OLP
5
V
CSS
current value therefore does not exactly match the value calculated in the equation above, and the actual load conditions should
be carefully considered. Also, make sure that OCP operation at
power supply start-up does NOT place the IC in a latched mode.
Note: During this period, if VCC goes below the UVLO threshold voltage, the IC does not go into a latch mode, but goes into
intermittent operation. Where the CSS voltage rises to 4.9 V and
VCC does not go below the UVLO threshold voltage, the IC goes
into a latched mode.
Operation at power supply turn off At power supply turn
off, voltage on capacitor CSS, which is externally connected to
the SS/OLP pin, is discharged by way of an internal RESET circuit as shown in figure 11. The RESET circuit does not operate in
normal operation while the internal regulator circuit operates.
Deactivating the OLP circuit To deactivate the OLP circuit
while soft-start is active, connect either a 47 kΩ resistor or a
Zener diode to the SS/OLP pin (figure 12). By doing this, OLP
operation is deactivated at start-up or during an overload status.
FB (Pin 6)
The FB pin is used in either a normal mode (constant-voltagecontrol circuit operation) or in a standby mode. Refer to Standby
Operation section for controlling in the standby mode.
Constant voltage control circuit The STR-W6700 series
adopts the current-mode control circuit, which ensures stability
with a heavy load. The peak value of the MOSFET drain current
(at on-time) is changed by comparing the FB pin voltage with the
internal VOCPM. Off-time becomes quasi-resonant operation synchronized to the reset signal from a transformer. Where no reset
signal is input from the transformer, it becomes fixed oscillation
frequency (approximately 22 kHz) set by the internal oscillator
Figure 11. Reset circuit at power turn off
IDS
GND
SS/OLP
SS/OLP
5
5
VOCPM
47 kΩ
VOCPD(LIM)
GND
Overload
Figure 12. OLP deactivation circuit alternative configurations
STRY6700-AN Rev. 2.0
VFB
Normal Load
Light Load
Figure 13. Constant-voltage control at fixed oscillation frequency (quasiresonant signal not available)
SANKEN ELECTRIC CO., LTD.
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circuit. The timing chart is shown in figure 13, and the internal
circuit diagram of the constant voltage control circuit is shown in
figure 14.
The constant-voltage-control circuit feeds a control signal (FB
current) from an error amplifier into pin 6 by way of the isolating
photocoupler. The input FB current is transformed into feedback
voltage VFB by the internal resistor (SW1 is turned on during
normal status). The voltage waveform (VOCPM) from the drain
current waveform is input to the inverting input terminal of the
FB comparator.
Figure 13 shows the FB current is decreased to nearly zero in an
overload, when the drain current is restricted to below the current
value set by the overcurrent protection circuit. In the period from
normal load to light load in figure 13, the drain current decreases
because the FB current increases and VFB rises. When VFB
exceeds the FB pin threshold voltage (VFB(OFF) , 1.45 V) at light
load, the oscillation stops so as not to raise the secondary-side
output voltage.
rent sense resistor, ROCP, is connected externally along with R4
and C5. R4 and C5 are to prevent malfunction caused by surges
when the MOSFET switches on. When the MOSFET switches
on, switching current occurs, and a voltage is developed across
ROCP . After that, the MOSFET turns off when the OCP/BD pin
voltage reaches VOCPBD(LIM).
The threshold voltage of the OCP/BD pin, VOCPBD(LIM), is
–0.94 V. The OCP circuit adopts negative-detection, which creates the detecting voltage, VOCPM , in the control part by dividing
the voltage (V1 + VROCP) with RB1, RB2, and R4. Because RB1
and RB2 are resistors incorporated in the IC, taking the variance
of RB1 and RB2 (defined as IOCPBD in the specifications) into
consideration, the value of R4 should be small, between 100 Ω
and 330 Ω. Select capacitor C5 (100 to 680 pF target value) for
good thermal behavior type. A high capacitance value results in
slow response time, ending up with an increase in peak drain current during a transient and at start-up.
Operation description
OCP/BD (Pin 7)
The OCP/BD pin functions in overcurrent detection, bottom-skip,
and quasi-resonant-operation control. Bottom-skip and quasi-resonant features are described in the Operation Description section.
Negative-detection type OCP circuit The OCP circuit of
the STR-W6700 series is a pulse-by-pulse type, which detects
the peak value of the MOSFET drain current for each pulse and
inverts the oscillator output. As shown in figure 15, the overcur-
IDS
Quasi-resonant operation Quasi-resonant operation matches
the timing of the MOSFET turn on to the bottom point of the
voltage resonant waveform, namely, ½ cycle of the resonant
frequency after the transformer discharges energy.
As shown in figure 16, the voltage resonant capacitor, C4, is
connected between the drain and source, and the delay circuit,
C10, D3, D4, and R9 are connected between the bias winding and
D
D
Control
V CC
BURST
FB
OSC
FB
3
S/GND
SW 1
㧗
㧙
P
DRIVE
VFB
Filter
Reg.V1
RB1
㧗
㧙
3 S/GND
LOGIC
6
㧗
㧙
1
VOCPM
OCP
7
OCP/BD
OCP
VOCPM
ROCP
STA RT 18V
BUR ST 11 V
㧗
D
4
㧙
1
RB2
OCP/BD
C5
V4
7
R4
ROCP
V5
Figure 14. REG circuit functional block diagram
STRY6700-AN Rev. 2.0
Figure 15. OCP functional block diagram
SANKEN ELECTRIC CO., LTD.
6
the OCP/BD pin. When the MOSFET turns off, a quasi-resonant
signal is generated from the flyback voltage in the bias winding,
and the BD comparator inside the IC operates, enabling quasiresonant operation. Even after the energy of the transformer is
discharged by way of the delay circuit, the quasi-resonant signal
imposed on the OCP/BD pin does not drop immediately. This is
because C10 is discharged by R4, and the voltage drops to the
threshold voltage, VOCPBD(TH1), at 0.4 V after a certain period.
The delay time needs to be set by adjusting C10, monitoring the
operating waveform, so that the MOSFET turns on when the VDS
of the MOSFET hits the lowest point.
In addition to the quasi-resonant operation, the IC incorporates a
bottom-skip mode in order to suppress the increase of oscillating
frequency during a light-to-medium load. It lengthens the off time
in accordance with the load status. Change-over timing between
the quasi-resonant and bottom-skip modes is described below.
When the quasi-resonant signal voltage imposed on the OCP/BD
pin is below VOCPBD(TH2) at 0.8 V, the IC goes into PWM operation with a fixed oscillating frequency of 22 kHz.
P
D
VCC
1
C4
S/GND
D
4
D3
Control
R9
7
㧟
OCP
/BD
C10
ROCP
R5
C5
R4
Figure 16. Quasi-resonant and delay circuit
STRY6700-AN Rev. 2.0
D4
PWM operation is also activated at power supply start-up or at
low bias winding voltage due to a winding short, which lowers oscillating frequency and reduces MOSFET stress. After the
quasi-resonant signal exceeds VOCPBD(TH2) at 0.8 V, the MOSFET
remains off during VOCPBD(TH1) at 0.4 V and higher. A voltage difference between VOCPBD(TH1) and VOCPBD(TH2) prevents malfunction.
In setting R9 and R4, the quasi-resonant signal imposed on the
OCP/BD pin needs to be 5 V or less. In a normal condition, it
should be approximately 1.5 V. The value of R4 is 100 to 330 Ω
and ROCP is small enough to be ruled out. The bias winding output voltage is set at 18 V. To make the OCP/BD pin voltage 1.5 V
or higher, R9 value is to be 1 to 3.3 kΩ. However, R9 needs to be
considered together with C10 capacitance relative to setting up
the delay time. R9 determines the time constant with C10 capacitance. Assuming the time constant is 2.2 μs, R4 is 220 Ω, R9 is
2.2 kΩ and C10 is 1000 pF, then proper selection should be done
while looking at the quasi-resonant signal and VDS waveform in
the actual application.
Bottom-skip mode (shift from quasi-resonant operation) The basic bottom-skip mode is activated at light load by
judging secondary load status by means of the drain current value
(actually OCP/BD pin voltage). If the load status evaluates as
heavy load, the IC goes into quasi-resonant operation. Judging is
made by reading the OCP/BD pin voltage during the falling edge
of the MOSFET gate voltage. Also, the count of falling edges
(OCP/BD pin voltage is less than VOCPBD(TH1)) of quasi-resonant
signal is counted to be utilized to turn the MOSFET on in accordance with the load status described above.
• Quasi-resonant operation → bottom-skip mode
Quasi-resonance is operated under the absolute value of VOCP
greater than VOCPBD(BS2) . When the load becomes lighter and
the drain current drops to make VOCP less than VOCPBD(BS2) , the
operation is shifted to the bottom-skip mode, and the reference
voltage is automatically changed to VOCPBD(BS1) . Figure 17
shows shift timing from quasi-resonant operation to bottom-skip
mode.
• Bottom-skip mode → quasi-resonant operation
Bottom-skip is operated under the absolute condition of VOCP
less than VOCPBD(BS2) . When the load becomes heavier and the
drain current increases to make VOCP greater than VOCPBD(BS2) ,
operation is shifted to quasi-resonant operation mode, and the
SANKEN ELECTRIC CO., LTD.
7
reference voltage is automatically changed to VOCPBD(BS2) . VOCP
is the OCP/BD pin voltage at the falling edge of the MOSFET
gate voltage.
As described above, the reference voltages for bottom-skip mode,
VOCPBD(BS1) and VOCPBD(BS2) , has hysteresis to make a stable
operation shift as shown in figure 18.
Standby modes The STR-W6700 series devices incorporate
standby modes to reduce power consumption. Two modes are
available. One is Auto-Standby mode and the other is manual
standby mode (external trigger).
Auto-Standby mode The Auto-Standby mode is started by internally sensing the drain current pulse. Because the minimum drain
pulse width is limited to the minimum on-time pulse width of
tON(MIN) , at light load the power supply can not lower its output
power any more, and the output voltage starts increasing. When
the FB pin voltage exceeds the the FB terminal threshold voltage,
VFB(OFF) , the IC stops working until VFB drops and then the power
supply starts working again.
Quasi- Resonant
0V
Quasi-resonant
Bottom-skip
VOCPBD(BS2)
Hysteresis
VOCPBD(BS1)
VOCPBD(LIM)
Figure 18. Operation mode shift
BottomSkipQuasi- Resonant
VDS
IDS
Detection
level
V OC PB D(TH2 )
OCP/BD V OC PB D(TH1 )
V OC PB D(BS2)
V OC PB D(BS1)
V OCP
Hysteresis
V OC PB D(LIM)
MOSFET Gate
(Power IC‫ޓ‬internal)
Bottom detect signal
(Power IC internal)
State signal
(Power IC internal)
Figure 17. Quasi-resonant to bottom-skip operation timing
STRY6700-AN Rev. 2.0
SANKEN ELECTRIC CO., LTD.
8
Manual standby mode (external trigger) The manual standby
mode is remotely controlled by a clamp on the secondary side to
reduce the output voltage. Then, the transformer winding voltage
drops and it reduces the bias winding voltage and the VCC pin
(pin 4) voltage decreases. When the VCC pin voltage reaches the
Operation-Stop voltage (9.7 V), the IC stops its operation, and
current consumption of the IC becomes the Standby Non-Operation Circuit Current, ICC(S) . The IC will not restart its operation
until the VCC pin voltage rises to the Standby Operation Start
Voltage, VCC(S) , by charging the start-up capacitor (C3) through
the start-up resistor (R2). By repeating this cycle, the IC maintains a UVLO intermittent oscillation mode. This is illustrated in
figure 19.
NOTE: During transitions between the manual standby and
normal operation, because the STR-W6700 series is not pumping
energy, make sure that normal output load is not applied, otherwise output voltage will drop significantly and will affect the
entire system operation.
In order to eliminate the transformer audible noise in the UVLO
intermittent oscillation mode, the voltage difference between the
Standby Operation Start Voltage, VCC(S) , and the Operation Stop
Voltage, VCC(OFF) , is designed to be small. By doing this, the
operating frequency is increased without increasing the losses in
the startup resistor, and the IC is in a mode where switching current is reduced to as low a level as possible.
Control IC
DRIVE
REG
RG2
D
1
㧝
Delay
RG1
RG3
S/GND
3
㧟
Figure 20. Step drive circuit
Secondary
Output Voltage
Feedback
VCC
Stand-by
detection level
Start
Voltage
Stop
Voltage
V FB(S)
Start Voltage
at stand-by
VCC(S)
V CC(ON)
V CC(OFF)
Power MOSFET
Waveform
Normal Operation
Standby Operation
Normal Operation
Figure 19. Quasi-resonant to bottom-skip operation timing
STRY6700-AN Rev. 2.0
SANKEN ELECTRIC CO., LTD.
9
LP =
Derr = D × (1 – [fOSC × td ] )
( [2×PO×fOSC / H] 1/2
+VIN× ×fOSC×D×C14/2 ) 2
(3)
(6)
Idp = 2 × IIN / Derr
(7)
NP = (LP / AL)
(8)
NS = NP (VO + Vf ) / Ef
(9)
where:
IIN = average DC input current,
Idp = peak switching current,
C4 = voltage resonance capacitance,
η2 = power supply efficiency (0.85-0.9 in case of CRT TV),
LP = primary inductance
NP = primary turns
NS = secondary turns, and
Vf = forward voltage of the secondary rectifier.
Winding
P1
where:
PO = maximum output power,
fOSC = minimum oscillating frequency,
D = On duty cycle at minimum VIN(AC)
= Ef /(VIN +Ef ), given Ef = flyback voltage,
η = transformer conversion efficiency (0.9 in the case of
CRT TV , and 0.75 to 0.85 in the case of low output
voltage), and
VIN = rectified and smoothed DC input voltage at minimum
VIN(AC) .
Turn-on delay results in duty cycle change in a quasi-resonant
operation, therefore, duty cycle correction is necessary. From the
following formulas, the number of turns, the peak switching cur-
IIN = PO / ( η2 × VIN )
1/2
P2
(VIN × D)2
D
(Bias)
D
B+
S1
S2
LoB
S3
S3
S1
P1
Audio
S2
Bobbin
(A)
Winding
D
P2
S3
S3
S2
D
(Bias)
P2
S2
P1
I
DS
DS
P2
Core
P1
V
(5)
Barrier
Basically, the same type of transformer is recommended as that
for a conventional quasi-resonant circuit. Examples are shown
in figure 22. The primary inductance, Lp, is determined by the
following:
(4)
Barrier
Transformer parameters
td = π × (LP × C4 )1/2
Barrier
Maximum on-time control function The MOSFET on-time
is limited during transients such as at low input voltage and at
turn on and turn off of AC input. This is illustrated in figure 21.
The maximum on-time is set at about 70% (approximately 32 μs)
of the oscillation cycle (1/fosc = 45 μs). In designing a power
supply, the MOSFET on-time at maximum load and at minimum
input voltage should be considered.
rent (IDP), corrected duty cycle (Derr), delay time (td), and others
can be obtained:
Barrier
Gate step-drive circuit The STR-W6700 series incorporates
a step-drive circuit (figure 20) for driving the MOSFET, which
reduces noise when the MOSFET turns on. The drive current,
when the MOSFET turns on, is at first limited only by RG1,
and the gate voltage is increased gradually, and then rapidly in
approximately 0.9 μs via (RG1 / RG2) . Drive voltage then uses
the constant-voltage drive circuit, maintained at VDRM = 7.5 V,
which is not affected by VCC. The MOSFET gate charge is rapidly discharged through RG3 when the MOSFET turns off.
S1
S1
Bobbin
Core
Maximum On-time
Figure 21. Maximum on-time
STRY6700-AN Rev. 2.0
(B)
Figure 22. Example of recommended transformers: (A) CRT TV
transformer, (B) low output-voltage transformer.
SANKEN ELECTRIC CO., LTD.
10
In addition, in the design of the transformer, using 130% of the
estimated peak switching current is recommended to ensure that
the transformer is not saturated, based on the plot of N × I-limit
(AT) versus AL-value (nH / N2).
Instead of performing the calculations above, software that provides a complete flyback transformer design tool is available.
CRT TV application concerns:
• Rather than winding with a single thick wire, a thin and
bifilar or trifilar winding across the entire width of bobbin is
recommended.
• For windings where NP and +B are a large number of turns,
divisional sandwich winding is recommended.
• For an output where a tight regulation is required, winding
with good coupling with S1 (+B) is recommended.
• For the +B winding, better coupling by use of litz wire is
required. In case the litz wire does not fit into a the bobbin
winding width, reduce the wire size, and use 2 to 3 of them in
strands.
• For improved thermal design:
▫ Leakage flux of wires close to the core center becomes large.
Eddy current can be reduced by the use of litz wire.
▫ In case the entire winding does not fit into the available
winding thickness, reduce the size of wires from outer side.
▫ Wire diameter is determined based on actual current and
should be less than 4 A / mm2.
Single and/or low-voltage output concerns:
• Wind so that wires are parallel and with good coupling.
• Sandwich winding is recommended.
General considerations
Universal AC input correction in OCP
With a universal AC input application, as described in the
Overload Protection (OLP) section, the load conditions for OCP
activation vary according to input voltage level, 110 V or 230 V.
Figure 23 illustrates a solution.
In the loop surrounded by the dashed line, the negative voltage of
the bias winding, which is in proportion to the input voltage level
when the MOSFET switches on, works for the input correction
during OLP.
The Zener diode is set to be on with a 230 VAC input, but not to
be activated with a 110 VAC input. When the bias winding output
voltage is 18 V, the resistor, Zener, and diode within the dashed
line are recommended.
Input
Smoothing
Capacitor
P
Start-up
Resistor
Bias
D
VCC
STR-W6700
S/GND
OCP SS/
/BD OLP FB
OCP
Sense
Filter
Bottom-Detection
Delay Circuit
D
Feedback Circuit
CR
Voltage
Resonant
Capacitor
CurrentSense
Resistor
P
Snubber Circuit
200 V
Fast
Recovery
Diode
Bottom-Detection
Delay Circuit
10 to 22 kΩ
16 V
Loop for OCP Matching to Input Voltage
Figure 23. Reference circuit for general application considerations
STRY6700-AN Rev. 2.0
SANKEN ELECTRIC CO., LTD.
11
OCP on the FB pin
As shown in the feedback circuit portion of figure 23, a Zener
diode is connected in series with the photocoupler. This is a countermeasure against an FB-pin voltage rise over 9 V in the manual
standby mode. The absolute maximum FB-pin voltage is 9 V, and
a Zener diode voltage of 5.6 to 6.2 V is recommended.
Output regulation and transformer noise during
standby and automatic modes
Figure 24 presents a simplified circuit of the secondary output
and manual standby circuit by VO drop.
Output Smoothing
Capacitors
After the output voltage is shifted over to a lower level, the IC
goes into a manual standby mode on the primary side. In this
mode, sufficient power is not obtained, resulting in audible noises
from the transformer, and deep ripple output voltage is generated, a sharp drop of output voltage, and unsustainable regulation,
although a larger output smoothing capacitor reduces this issue.
Load in an actual manual standby mode ranges between tens of
milliwatts and 0.2 W.
In regard to the audible noise from the transformer, contact a
transformer manufacturer as a precaution against possible varnish
dissolving and ferrite core attaching.
Output
S
Ground
External Signal
for Standby
Sanken Error Amplifier
Type SExxx
Figure 25. High frequency, high current loops
Manual Standby
by VO drop
Figure 24. Output circuit
STRY6700-AN Rev. 2.0
SANKEN ELECTRIC CO., LTD.
12
Design considerations
Component placement considerations in SMPS circuits
As pattern layout and component position may cause malfunctions of the IC, EMI noises, or power losses, the following guidelines should be followed:
• Traces where high frequencies and high current levels flow
should be kept thick and short to lower line impedance.
• The hatched area illustrated in figure 25, where high
frequencies and high currents create a loop, should be kept as
small as possible.
• S/GND and earth ground lines should be kept as thick and
short as possible.
• In off-line SMPS (switch-mode power supply) circuitry,
because traces and paths of high voltage exist, component
layout and trace length should be carefully considered, as
required by safety standards.
• Take into account the positive thermal coefficiency of the
MOSFET RDS(on) when preparing the thermal design.
Layout considerations
In order to reduce or eliminate common impedance lines, the
S/GND pin (pin 3) and its peripheral components should be
located as close together as possible, as illustrated in figure 26.
The trace from the overcurrent sense resistor to the input smoothing capacitor should be kept as short and thick as possible.
VCC
FB
SS/
OLP
STR-W6700
S/Gnd
D
Bias
OCP
/BD
Figure 26. High frequency, high current loops
Terminal List Table
Number
Name
1
D
Description
Drain
Functions
MOSFET drain
2
NC
Clipped
No connection
3
S/GND
Source/ground terminal
MOSFET source and ground
4
VCC
Power supply terminal
Input of power supply for control circuit
5
SS/OLP
Soft Start/Overload Protection terminal
Input to set delay for Overload protection and Soft Start operation
6
FB
Feedback terminal
Input for Constant Voltage Control and Burst (intermittent) Mode
oscillation control signals
7
OCP/BD
Overcurrent Protection/Bottom Detection
Input for overcurrent detection and bottom detection signals
STRY6700-AN Rev. 2.0
SANKEN ELECTRIC CO., LTD.
13
Package Dimensions, TO-220
2.8 ±0.2
Ø3.2 ±0.2
2.6 ±0.1
5.0 ±0.5
+0.2
(5.4)
0.65 –0.1
1)
×R
(2
0.74±0.15
Terminal dimension at case surface
10.4 ±0.5
XXXXXXXX
XXXXXXXX
XX
XX
2.8
Branding
16.9 ±0.3
Gate Burr
7.9 ±0.2
4.2 ±0.2
0.5
4 ±0.2
10.0 ±0.2
6×P1.27 ±0.15 = 7.62 ±0.15
Terminal dimensions at case surface
+0.2
0.45 –0.1
5.08 ±0.6
Terminal dimension at lead tips
1 2 3 4 5 6 7
0.5
View A
Gate burr: 0.3 mm (max.)
Terminal core material: Cu
Terminal treatment: Ni plating and solder dip
Leadform: 2003
Weight (approximate): 2.3 g
Dimensions in millimeters
0.5
View B
Drawing for reference only
Branding codes (exact appearance at manufacturer discretion):
1st line, type:
STR
2nd line, subtype: W6735
3rd line, lot:
YM DD
Where: Y is the last digit of the year of manufacture
M is the month (1 to 9, O, N, D)
DD is the 2-digit date
Leadframe plating Pb-free. Device composition
includes high-temperature solder (Pb >85%),
which is exempted from the RoHS directive.
STRY6700-AN Rev. 2.0
SANKEN ELECTRIC CO., LTD.
14
Because reliability can be affected adversely by improper
storage environments and handling methods, please observe
the following cautions.
Cautions for Storage
• Ensure that storage conditions comply with the standard
temperature (5°C to 35°C) and the standard relative
humidity (around 40% to 75%); avoid storage locations
that experience extreme changes in temperature or
humidity.
• Avoid locations where dust or harmful gases are present
and avoid direct sunlight.
• Reinspect for rust on leads and solderability of the
products that have been stored for a long time.
Cautions for Testing and Handling
When tests are carried out during inspection testing and
other standard test periods, protect the products from
power surges from the testing device, shorts between
the product pins, and wrong connections. Ensure all test
parameters are within the ratings specified by Sanken for
the products.
Remarks About Using Silicone Grease with a Heatsink
• When silicone grease is used in mounting the products on
a heatsink, it shall be applied evenly and thinly. If more
silicone grease than required is applied, it may produce
excess stress.
• Volatile-type silicone greases may crack after long periods
of time, resulting in reduced heat radiation effect. Silicone
greases with low consistency (hard grease) may cause
cracks in the mold resin when screwing the products to a
heatsink.
Our recommended silicone greases for heat radiation
purposes, which will not cause any adverse effect on the
product life, are indicated below:
Type
Suppliers
G746
Shin-Etsu Chemical Co., Ltd.
YG6260
Momentive Performance Materials Inc.
SC102
Dow Corning Toray Co., Ltd.
Cautions for Mounting to a Heatsink
• When the flatness around the screw hole is insufficient, such
as when mounting the products to a heatsink that has an
extruded (burred) screw hole, the products can be damaged,
even with a lower than recommended screw torque. For
mounting the products, the mounting surface flatness should
be 0.05 mm or less.
STRY6700-AN Rev. 2.0
•
Please select suitable screws for the product shape. Do not
use a flat-head machine screw because of the stress to the
products. Self-tapping screws are not recommended. When
using self-tapping screws, the screw may enter the hole
diagonally, not vertically, depending on the conditions of hole
before threading or the work situation. That may stress the
products and may cause failures.
• Recommended screw torque: 0.588 to 0.785 N●m (6 to 8
kgf●cm).
• For tightening screws, if a tightening tool (such as a driver)
hits the products, the package may crack, and internal
stress fractures may occur, which shorten the lifetime of
the electrical elements and can cause catastrophic failure.
Tightening with an air driver makes a substantial impact.
In addition, a screw torque higher than the set torque can
be applied and the package may be damaged. Therefore, an
electric driver is recommended.
When the package is tightened at two or more places, first
pre-tighten with a lower torque at all places, then tighten
with the specified torque. When using a power driver, torque
control is mandatory.
Soldering
• When soldering the products, please be sure to minimize
the working time, within the following limits:
260±5°C 10±1 s
(Flow, 2 times)
380±10°C 3.5±0.5 s (Soldering iron, 1 time)
• Soldering should be at a distance of at least 2.0 mm from
the body of the products.
Electrostatic Discharge
• When handling the products, the operator must be
grounded. Grounded wrist straps worn should have at
least 1 MΩ of resistance from the operator to ground to
prevent shock hazard, and it should be placed near the
operator.
• Workbenches where the products are handled should be
grounded and be provided with conductive table and floor
mats.
• When using measuring equipment such as a curve tracer,
the equipment should be grounded.
• When soldering the products, the head of soldering irons
or the solder bath must be grounded in order to prevent
leak voltages generated by them from being applied to the
products.
• The products should always be stored and transported in
Sanken shipping containers or conductive containers, or
be wrapped in aluminum foil.
SANKEN ELECTRIC CO., LTD.
15
• The contents in this document are subject to changes, for improvement and other purposes, without notice. Make sure that this is the
latest revision of the document before use.
• Application and operation examples described in this document are quoted for the sole purpose of reference for the use of the products herein and Sanken can assume no responsibility for any infringement of industrial property rights, intellectual property rights or
any other rights of Sanken or any third party which may result from its use.
• Although Sanken undertakes to enhance the quality and reliability of its products, the occurrence of failure and defect of semiconductor products at a certain rate is inevitable. Users of Sanken products are requested to take, at their own risk, preventative measures
including safety design of the equipment or systems against any possible injury, death, fires or damages to the society due to device
failure or malfunction.
• Sanken products listed in this document are designed and intended for the use as components in general purpose electronic equipment or apparatus (home appliances, office equipment, telecommunication equipment, measuring equipment, etc.).
When considering the use of Sanken products in the applications where higher reliability is required (transportation equipment and
its control systems, traffic signal control systems or equipment, fire/crime alarm systems, various safety devices, etc.), and whenever
long life expectancy is required even in general purpose electronic equipment or apparatus, please contact your nearest Sanken sales
representative to discuss, prior to the use of the products herein.
The use of Sanken products without the written consent of Sanken in the applications where extremely high reliability is required
(aerospace equipment, nuclear power control systems, life support systems, etc.) is strictly prohibited.
• In the case that you use Sanken products or design your products by using Sanken products, the reliability largely depends on the
degree of derating to be made to the rated values. Derating may be interpreted as a case that an operation range is set by derating the
load from each rated value or surge voltage or noise is considered for derating in order to assure or improve the reliability. In general,
derating factors include electric stresses such as electric voltage, electric current, electric power etc., environmental stresses such
as ambient temperature, humidity etc. and thermal stress caused due to self-heating of semiconductor products. For these stresses,
instantaneous values, maximum values and minimum values must be taken into consideration.
In addition, it should be noted that since power devices or IC's including power devices have large self-heating value, the degree of
derating of junction temperature affects the reliability significantly.
• When using the products specified herein by either (i) combining other products or materials therewith or (ii) physically, chemically
or otherwise processing or treating the products, please duly consider all possible risks that may result from all such uses in advance
and proceed therewith at your own responsibility.
• Anti radioactive ray design is not considered for the products listed herein.
• Sanken assumes no responsibility for any troubles, such as dropping products caused during transportation out of Sanken's distribution network.
• The contents in this document must not be transcribed or copied without Sanken's written consent.
STRY6700-AN Rev. 2.0
SANKEN ELECTRIC CO., LTD.
16