FA5546 Datasheet, PDF (25/27 Pages) Fuji Electric – Switching Power Supply Control IC

FA5546/47
(9) Reduction of dependency of overload
detection level on input voltage
Since the gradient of the inductor current of a transformer
varies depending on the input voltage in the overload
protection function, the current value determined to be
overload also varies. The higher the input voltage, the
higher the output current that causes overload shutdown to
occur.
As shown in Fig.34. resistor R9 can be connected between
the auxiliary winding and the IS terminal to minimize the
dependency of the overload detection level on input voltage
(IS terminal line correction).
If R7 is 1k, the resistance of R9 should be approximately
several 10k to several 100k. Note that if the input
voltage is made low by correction, the power output current
for overload shutdown decreases. Also note that the
correction described above affects the input power under
light load.
Since the polarity of the detection voltage of the IS terminal
is negative, the loss of correction resistance can be lower
than that of the positive-polarity system (correction made
based on AC rectification voltage).
(11) Loss calculation
To use the IC within its ratings, the loss of the IC may have
to be found. However, it is not feasible to measure loss
directly. The following is an example of finding a rough
value of loss by calculation.
The rough value of the total loss of the IC, Pd, can be
calculated using the following expression:
Pd  VCC  (ICCop1  Qg  fsw)  VVH  IHrun
where,
VVH: voltage to be applied to the VH terminal,
IHrun: current fed to the VH terminal during operation,
VCC: power voltage,
ICCop1: Consumption current of the IC
Qg: electrical charge to be input to the MOSFET gate
used, and
Fsw: switching frequency.
A rough value can be found using the above expression,
and the total loss found by the calculation, Pd, is slightly
larger than the actual value.
Be sure to take into consideration that each characteristic
value varies depending on temperatures and other factors.
VCC
6
34
IS GND
R9
C2
Rs
R7
Fig. 34 Reduction of dependency of overload
detection level on input voltage
(10) Prevention of malfunction due to negative
potential of the terminal
If large negative voltage is applied to each terminal of the
IC, the parasitic element within the IC may be actuated,
thus causing malfunction to occur. Be sure to maintain the
voltage to be applied to each terminal within the maximum
absolute ratings.
Example:
When the VH terminal is connected to a half-wave rectifier
circuit with 100VAC input, the average voltage to be applied
to the VH terminal is calculated to be approximately 45V,
and the average current to be fed to the VH terminal is
approximately 130A;.
Furthermore, assuming that Tj = 25C, VCC = 18V, and Qg
= 80nC, and based on
IHrun = 130A,
ICCop1 = 1.3mA (typ),
and fsw = 60kHz (typ) (product having Fosc = 60kHz),
the loss of the IC having standard characteristics can be
calculated as follows:
Pd  18V  (1.3mA  80nC  60kHz)  45V  130A
 112mW
Fuji Electric Co., Ltd.
AN-027E Rev.0.4
April -2011
25
http://www.fujielectric.co.jp/products/semiconductor/