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STR-X6700 Schematic ( PDF Datasheet ) - Sanken

Teilenummer STR-X6700
Beschreibung STR-X6700 Series
Hersteller Sanken
Logo Sanken Logo 



Gesamt 19 Seiten
		
STR-X6700 Datasheet, Funktion
Product Information
STR-X6700 Series Off-Line
Quasi-Resonant Switching Regulators
Introduction
The STR-X6700 series integrates a quasi-resonant control
IC and a MOSFET with avalanche guarantee. In normal
operation, the device provides high efficiency and low EMI
noise with bottom-skip quasi-resonant operation during
light output loads. Low power consumption is also achieved
by Auto-standby mode (not available in the STR-X6729,
STR-X6759N, or STR-X6768N) or manual standby mode
(external trigger).
The device is supplied in a seven-pin fully-molded TO-3P-
style package, which is suitable for downsizing and stan-
dardizing of an SMPS by reducing external component
count and simplifying circuit design.
Features and benefits include the following:
Auto standby mode (burst oscillation) or manual standby
mode (UVLO intermittent oscillation) in the standby
mode.
In addition to the standard quasi-resonant operation, a
bottom-skip mode 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.
The product lineup for the STR-X6700 series provides the
options shown in table 1.
Contents
Introduction
Pin functional descriptions
Operation description
Transformer parameters
General considerations
Design considerations
Package dimensions, TO-3P
Worldwide Contacts
1
2
6
10
11
13
14
19
LF1901
LF1902
LF1905
Figure 1. STR-X6700 series packages are fully molded TO-3P
package types: LF1901 (STR-X6737 and STR-X6769), LF1902
(STR-X6729 and STR-X6768N), and LF1905 (STR-X6757 and
STR-X6759N).
Table 1. Product Line-up
Type #
STR-X6729b
MOSFET
VDSS
(V)
450
RDS(on)
(Max)
(Ω)
0.189
VAC
Input
(V)
120
POUTa
(W)
460
STR-X6737 500 0.36 120
280
STR-X6757
STR-X6759Nb
650
0.62
0.385
Wide
230
Wide
230
165
320
250
460
STR-X6768Nb
STR-X6769
800
Wide
1.00
230
Wide
0.66
230
150
220
210
310
aThe 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.
bAuto-standby mode not included.
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, un-
less otherwise stated.
28103.3013






STR-X6700 Datasheet, Funktion
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.
OCP/BD (Pin 7)
The OCP/BD pin functions in overcurrent detection, bottom-skip,
and quasi-resonant-operation control. Bottom-skip and quasi-res-
onant features are described in the Operation Description section.
Negative-detection type OCP circuit The OCP circuit of
the STR-X6700 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-
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 cre-
ates 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 cur-
rent during a transient and at start-up.
Operation description
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
IDS
1
D
4 V CC
STA RT 18V
BUR ST 11 V
BURST
OSC
FB
FB
6
SW 1
VFB
OCP
2 S 3 GND
RO CP
VO CPM
7 OCP/BD
D
Control
D1
P
LOGIC DRIVE
Reg.V1
RB1
OCP
RB2
32
S
3
GND
VOCPM
7
OCP/BD
Filter
C5
V4
R4
V5
Figure 14. REG circuit functional block diagram
Figure 15. OCP functional block diagram
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
28103.3013

6 Page







STR-X6700 pdf, datenblatt
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 coun-
termeasure 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.
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 gener-
ated, 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 Smoothing
Capacitors
S
Output
Ground
Sanken Error Amplifier
Type SExxx
External Signal
for Standby
Manual Standby
by VO drop
Figure 25. High frequency, high current loops
Figure 24. Output circuit
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
12
28103.3013

12 Page


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