TNY274-280
Over Temperature Protection
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.
The thermal shutdown circuitry senses the die temperature.
The threshold is typically set at ꢀ42 °C with 71 °C hysteresis.
When the die temperature rises above this threshold the power
MOSFET is disabled and remains disabled until the die
temperature falls by 71 °C, at which point it is re-enabled. A
large hysteresis of 71 °C (typical) is provided to prevent over-
heating 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.
Line Undervoltage Sense Circuit
The DC line voltage can be monitored by connecting an
external resistor from the DC line to the EN/UV pin. During
power up or when the switching of the power MOSFET is
disabled in auto-restart, the current into the EN/UV pin must
exceed 21 μA to initiate switching of the power MOSFET. During
power up, this is accomplished by holding the BYPASS/MULTI-
FUNCTION pin to 4.9 V while the line undervoltage condition
exists. The BYPASS/MULTI-FUNCTION pin then rises from
4.9 V to 1.81 V when the line undervoltage condition goes
away. When the switching of the power MOSFET is disabled in
auto-restart mode and a line undervoltage condition exists, the
auto-restart counter is stopped. This stretches the disable time
beyond its normal 2.1 seconds until the line undervoltage
condition ends.
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-III enters
into auto-restart operation. An internal counter clocked by the
oscillator is reset every time the EN/UV pin is pulled low. If the
EN/UV pin is not pulled low for 64 ms, the power MOSFET
switching is normally disabled for 2.1 seconds (except in the
case of line undervoltage condition, in which case it is disabled
until the condition is removed). The auto-restart alternately
enables and disables the switching of the power MOSFET until
the fault condition is removed. Figure 1 illustrates auto-restart
circuit operation in the presence of an output short circuit.
The line undervoltage circuit also detects when there is no
external resistor connected to the EN/UV pin (less than ~2 μA
into the pin). In this case the line undervoltage function is
disabled.
TinySwitch-III Operation
TinySwitch-III 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-III 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-III 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.
In the event of a line undervoltage condition, the switching of
the power MOSFET is disabled beyond its normal 2.1 seconds
until the line undervoltage condition ends.
V
300
DRAIN
200
100
0
Enable Function
TinySwitch-III senses the EN/UV 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/UV
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.
10
V
DC-OUTPUT
5
0
2500
5000
0
The EN/UV pin signal is generated on the secondary by
comparing the power supply output voltage with a reference
voltage. The EN/UV pin signal is high when the power supply
output voltage is less than the reference voltage.
Time (ms)
Figure 5. Auto-Restart Operation.
4
Rev. I 01/09
www.powerint.com
POWERINT [ Power Integrations ]
