Akai A-5181, Руководство по обслуживанию, Страница: 24 / 50

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Repair of Power Board
I. Power switch working principles
TinySwitch-II(TNY267) maintains the simplicity of the TinySwitchtopology, while
providing a number of new enhancements tofurther reduce system cost and component count,
and topractically eliminate audible noise. Like TinySwitch, a 700 Vpower MOSFET,
oscillator, high voltage switched current source,current limit and thermal shutdown circuitry
are integrated onto amonolithic device. The start-up and operating power are derived directly
from the voltage on the DRAIN pin, eliminating the need for a bias winding and associated
circuitry. In addition, the TinySwitch-II devices incorporate auto-restart, line undervoltage
sense, and frequency jittering. An innovative design minimizes audio frequency components
in the simple ON/OFF control scheme to practically eliminate audible noise with standard
taped/varnished transformer construction. The fully integrated auto-restart circuit safely limits
output power during fault conditions such as output short circuit or open loop, reducing
component count and secondary feedback circuitry cost. An optional line sense resistor
externally programs a line under-voltage threshold, which eliminates power down glitches
caused by the slow discharge of input storage capacitors present in applications such as
standby supplies. The operating frequency of 132 kHz is jittered to significantly reduce both
the quasi-peak and average EMI, minimizing filtering cost.
1. Conversion from a.c. to d.c. circuit
220V a.c. current flows restrictively through F1 fuse, and through D301~D304 to
combine as bridge rectification. After C1 L2 C2 undergoes electrolytic filter, we can
obtain a 320V d.c. voltage (Uhv).
2. Process to start up the software
The TinySwitch-II does not require a bias winding to providepower to the chip, because
it draws the power directly from the DRAIN pin (see Functional Description above). This has
twomain benefits. First, for a nominal application, this eliminatesthe cost of a bias winding
and associated components.Secondly, for battery charger applications, the current-voltage
characteristic often allows the output voltage to fall close tozero volts while still delivering
power. This type of application normally requires a forward-bias winding which has many
more associated components. With TinySwitch-II, neither are necessary. For applications that
require a very low no-load power consumption (50 mW), a resistor from a bias winding to the
BYPASS pin can provide the power to the chip. The minimum recommended current supplied
is 750 µA. The BYPASS pin in this case will be clamped at 6.3 V. This method will eliminate
the power draw from the DRAIN pin, thereby reducing the no-load power consumption and
improving fullload efficiency. And check c5,it is very importance of starp up.
3. Bias winding
After starting the power, T P1~P2 bias winding supplies bias current and error current to
the internal of IC2 , through D7 and C4 rectifier filter and through R2 light-electric coupler.Check
IC2 BP pin (IC2 PIN1),it’s voltage is 6.3v.
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