TNY174-180 データシートの表示（PDF） - Power Integrations, Inc

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TNY174-180

TinySwitch-LT Family Energy Efficient, Offline Switcher with Enhanced Flexibility and Extended Power Range®

Power Integrations, Inc 

TNY174-180

BYPASS/MULTI-FUNCTION Pin Undervoltage

The BYPASS/MULTI-FUNCTION pin undervoltage circuitry

disables the power MOSFET when the BYPASS/MULTI-

FUNCTION pin voltage drops below 4.9 V in steady state

operation. Once the BYPASS/MULTI-FUNCTION pin voltage

drops below 4.9 V in steady state operation, it must rise back

to 5.85 V to enable (turn-on) the power MOSFET.

Over Temperature Protection

The thermal shutdown circuitry senses the die temperature. The

threshold is typically set at 142 °C with 75 °C hysteresis. When

the die temperature rises above this threshold the power MOSFET

is disabled and remains disabled until the die temperature falls

by 75 °C, at which point it is re-enabled. A large hysteresis of

75 °C (typical) is provided to prevent overheating of the PC

board due to a continuous fault condition.

Current Limit

The current limit circuit senses the current in the power MOSFET.

When this current exceeds the internal threshold (ILIMIT), the

power MOSFET is turned off for the remainder of that cycle. The

current limit state machine reduces the current limit threshold

by discrete amounts under medium and light loads.

The leading edge blanking circuit inhibits the current limit

comparator for a short time (tLEB) after the power MOSFET is

turned on. This leading edge blanking time has been set so that

current spikes caused by capacitance and secondary-side rectifier

reverse recovery time will not cause premature termination of

the switching pulse.

Auto-Restart

In the event of a fault condition such as output overload, output

short circuit, or an open loop condition, TinySwitch-LT enters

into auto-restart operation. An internal counter clocked by the

oscillator is reset every time the EN pin is pulled low. If the EN

pin is not pulled low for 64 ms, the power MOSFET switching

300

V

DRAIN

200

100

0

10

V

5

DC-OUTPUT

0

0

2500

Time (ms)

Figure 5. Auto-Restart Operation.

4A

08/07

5000

is normally disabled for 2.5 seconds. The auto-restart alternately

enables and disables the switching of the power MOSFET until

the fault condition is removed. Figure 5 illustrates auto-restart

circuit operation in the presence of an output short circuit.

Adaptive Switching Cycle On-Time Extension

Adaptive switching cycle on-time extension keeps the cycle

on until current limit is reached, instead of prematurely

terminating after the DCMAX signal goes low. This feature

reduces the minimum input voltage required to maintain

regulation, extending hold-up time and minimizing the size

of bulk capacitor required. The on-time extension is disabled

during the startup of the power supply, until the power supply

output reaches regulation.

TinySwitch-LT Operation

TinySwitch-LT devices operate in the current limit mode. When

enabled, the oscillator turns the power MOSFET on at the

beginning of each cycle. The MOSFET is turned off when the

current ramps up to the current limit or when the DCMAX limit is

reached. Since the highest current limit level and frequency of

a TinySwitch-LT design are constant, the power delivered to the

load is proportional to the primary inductance of the transformer

and peak primary current squared. Hence, designing the supply

involves calculating the primary inductance of the transformer

for the maximum output power required. If the TinySwitch-LT

is appropriately chosen for the power level, the current in the

calculated inductance will ramp up to current limit before the

DCMAX limit is reached.

Enable Function

TinySwitch-LT senses the EN pin to determine whether or not to

proceed with the next switching cycle. The sequence of cycles

is used to determine the current limit. Once a cycle is started,

it always completes the cycle (even when the EN pin changes

state half way through the cycle). This operation results in a

power supply in which the output voltage ripple is determined

by the output capacitor, amount of energy per switch cycle and

the delay of the feedback.

The EN pin signal is generated on the secondary by comparing

the power supply output voltage with a reference voltage. The

EN pin signal is high when the power supply output voltage is

less than the reference voltage.

In a typical implementation, the EN pin is driven by an

optocoupler. The collector of the optocoupler transistor is

connected to the EN pin and the emitter is connected to the

SOURCE pin. The optocoupler LED is connected in series with

a Zener diode across the DC output voltage to be regulated.

When the output voltage exceeds the target regulation voltage

level (optocoupler LED voltage drop plus Zener voltage), the

optocoupler LED will start to conduct, pulling the EN pin low.

The Zener diode can be replaced by a TL431 reference circuit

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