Series STR-W6750 Off-Line
Quasi-Resonant Switching Regulators
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 effi-
ciency and low EMI noise with bottom-skip quasi-resonant
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-220-style
package with pin 2 deleted, which is suitable for downsizing
and standardizing of an SMPS by reducing external component
count and simplifying circuit design.
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
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
All performance characteristics given are typical values for
circuit or system baseline design only and, unless otherwise
stated, are at the nominal operating voltage and an ambient
temperature of +25°C, unless otherwise stated.
VCC (Pin 4)
The start-up circuit detects the VCC pin voltage (pin 4), 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 1. 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.
Figure 1 – External start-up circuit
If the value of R2 is too high, the C3 charge current will be
reduced. Consequently, it will take longer to reach the
operation start-up voltage. The VCC pin voltage falls immedi-
ately after the control circuit starts its operation. The voltage
drop can be reduced by increasing C3’s capacitance. There-
fore, to maintain the start-up operation, even if the rise of the
bias winding voltage is slow, the VCC pin voltage would not
fall to the operation-stop voltage. However, too large a C3
capacitance will cause an improperly long time to reach the
operation start after the initial power turn on.
In general, SMPS performs its start-up operation properly with
a value of C3 between 4.7 µF and 47 µF, and R2 between
47 kΩ and 150 kΩ for 120 V narrow or universal ac input, and
82 kΩ to 330 kΩ for 200 V narrow ac input.
Sanken Power Devices
from Allegro MicroSystems
As shown in Figure 2, 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 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 prior to start-up.
Figure 3 – VCC after start-up
Figure 2 – ICC vs. VCC
Figure 4 – VCC vs. IO (secondary load)
After the control circuit starts its operation, the power supply is
operated by rectifying and smoothing the voltage of the bias
winding. Figure 3 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
continues 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)
In an actual power supply circuit, the Vcc pin voltage might be
changed by the value of secondary output current as shown in
Figure 4. Because of the low circuit current of the STR-
W6750, 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
Figure 5 – VCC peripheral circuit with R7
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
Furthermore, the variation ratio of the VCC pin voltage be-
comes 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.
115 Northeast Cutoff, Box 15036
2 Worcester, Massachusetts 01615-0036
Copyright © 2005 Allegro MicroSystems, Inc.
Overvoltage protection (OVP) circuit
If VCC, reference 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, and the voltage is in propor-
tion to the output voltage; thus, the OVP circuit also operates
in the case of overvoltage output of the secondary side, e.g.,
when the voltage detection circuit is open.
The secondary output voltage (VO) for the OVP operation is
obtained from the following:
VO in normal operation
VCC in normal operation
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 “Operation-stop voltage – 0.3 V”.
In order to prevent malfunction caused by, for instance, noise,
a delay time is programmed into a timer circuit, which will
prohibit the latch circuit operation until the OVP or OLP
circuit keep operating for more than a programmed time.
During the latched mode, the regulator circuit (or constant
voltage circuit) keeps running, the circuit current being
maintained at a high level, and the VCC pin voltage dropping.
When the VCC pin voltage drops down to the operation-stop
voltage (9.7 V), 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), 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 6 indicates the voltage waveform in the
latched mode. The latched mode is released by decreasing the
VCC pin voltage to below 7.2 V, in general, by restarting.
Figure 6 – VCC during latch mode
SS/OLP (Pin 5)
Through the SS/OLP pin, soft-start and overload protection is
realized by connecting a 0.47 µF to 3.3 µF capacitor to the pin.
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 0 V to 1.2 V.
Timing is shown in Figure 7 and the next table.
Figure 7 – Soft-start operation
By comparing the oscillation waveforms of the 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 standby mode. Gradual increase of drain current sup-
presses magnetostriction noises from the transformer.
Soft-start timing (charging current: 550 mA)
2.2 3.3 4.7
Time (ms) 1.0 2.2 4.8 7.2 10.3
NOTE: A large CSS value also results in a longer time from
OLP operation to latched mode.
Overload protection (OLP)
Figure 8 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-up 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
Figure 8 – Current-mode control
drop. Although the intermittent operation is not realized,
protection might still be by means of the OLP activation.
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 Series STR-W6750 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 9. After the SS/OLP pin voltage keeps
rising to ‘OLP-Operation Threshold Voltage’ (VSSOLP(OLP) =
4.9 V), the oscillation stops, and the IC goes into a latched
Figure 9 – Timing at overload
115 Northeast Cutoff, Box 15036
4 Worcester, Massachusetts 01615-0036
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