LinkSwitch-HP
The transient load response is dependent on the loop gain and
minimum switching frequency. The values of R7 and C7 shown
here typically give good transient response for most designs.
When the supply is at no-load, the minimum switching frequency
at no-load will create a delay to respond to any step load event
during the off-time. In the case above, the minimum frequency
is 250 Hz so there is a potential 4 ms delay to response. If a
faster response is desired from no-load initial condition there is
the option to use the LNK666x which has a minimum frequency
of 2 kHz. There is a trade-off in using this family as no-load
input power will be slightly higher and a smaller pre-load resistor
will be required.
BYPASS pin to provide external bias. The external bias current
should set via R9 to be at least 500 mA to guarantee the internal
current source of LinkSwitch-HP is turned off as this will allow
the supply to operate more efficiently, especially at light load.
For best no-load performance the external supply voltage
across C6 should be minimized (typically 8-9 V) and the current
into the BYPASS pin set by R9 should be as low as possible.
Input overvoltage protection is done through sensing the
negative forward pulse of feedback winding. When the negative
forward voltage is sufficiently high to produce more than 1.15 mA
current into the FEEDBACK pin, for 2 consecutive on-cycles the
device will stop switching for auto-restart delay period.
In order to have good efficiency, regulation performance and
stability, the transformer leakage inductance should be minimized.
Low leakage will minimize ringing on the sense winding which
can create an error in the feedback sampling. The example
above uses split primary winding technique to lower leakage
inductance. Leakage inductance should not be greater than
2% of nominal primary inductance and 1% is typically the
desirable target value.
Output overvoltage protection is achieved by sensing the flyback
pulse through the FEEDBACK pin. When the FEEDBACK pin
sees 2.5 V or greater for 16 consecutive cycles, the supply will
latch off. If non-latching OVP is desired then changing C5 from
0.47 mF to 4.7 mF will change fault mode accordingly (see Table
3 for details).
OCP protection is accomplished by sensing when the output
drops below 0.925 of nominal regulation value for a duration
greater than specified delay time. In the example above, the
total delay time is about 50 ms. Capacitor C20 extends the
default internal delay time of 35 ms (see Figure 6 for details).
The latching shut-off option is used in the design above.
Resistor R28 serves as a pre-load resistor to minimize output
voltage rising in no-load condition. The pre-load resistor should
be no smaller than is necessary to maintain output within
specification limits to minimize added dissipation. In this example,
the added pre-load dissipation is only 4.8 mW.
The primary current limit of LinkSwitch-HP can be adjusted by
selecting the value for R8 (see Table 4 for details). For this
design 60% of maximum current limit was chosen. A lower
current limit setting is typical for an adapter where lower RDS(ON)
is desirable for higher efficiency and also lower thermal rise of
LinkSwitch-HP.
LinkSwitch-HP provides an internal current source to bias the
BYPASS pin which is necessary for start-up. When the supply
is operating and in regulation an external bias is provided from
the rectified flyback voltage from the bias winding (D2 and C6).
Resistor R9 is sourced from the bias voltage across C6 into the
9
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