TNY263-268
large hysteresis of 70
°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 (I
LIMIT
), 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 (t
LEB
) 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-II
enters
into auto-restart operation. An internal counter clocked by the
oscillator gets reset every time the EN/UV pin is pulled low. If
the EN/UV pin is not pulled low for 50 ms, the power MOSFET
switching is normally disabled for 850 ms (except in the case of
line under-voltage 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 5 illustrates auto-restart circuit
operation in the presence of an output short circuit.
In the event of a line under-voltage condition, the switching of
the power MOSFET is disabled beyond its normal 850 ms time
until the line under-voltage condition ends.
Line Under-Voltage 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 49
µA
to initiate switching of the power MOSFET.
During power-up, this is accomplished by holding the BYPASS
pin to 4.8 V while the line under-voltage condition exists. The
BYPASS pin then rises from 4.8 V to 5.8 V when the line under-
voltage condition goes away. When the switching of the power
MOSFET is disabled in auto-restart mode and a line under-
voltage condition exists, the auto-restart counter is stopped.
This stretches the disable time beyond its normal 850 ms until
the line under-voltage condition ends.
The line under-voltage 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 under-voltage function is
disabled.
TinySwitch-II
Operation
TinySwitch-II
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 DC
MAX
limit is
reached. Since the highest current limit level and frequency of
a
TinySwitch-II
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-II
is
appropriately chosen for the power level, the current in the
calculated inductance will ramp up to current limit before the
DC
MAX
limit is reached.
Enable Function
TinySwitch-II
senses the EN/UV pin to determine whether or
not to proceed with the next switching cycle as described
earlier. 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.
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.
In a typical implementation, the EN/UV pin is driven by an
optocoupler. The collector of the optocoupler transistor is
connected to the EN/UV pin and the emitter is connected to the
300
200
100
0
10
5
0
V
DRAIN
V
DC-OUTPUT
0
1000
2000
Time (ms)
Figure 5. TinySwitch-II Auto-Restart Operation.
4
E
4/04
PI-2699-030701
POWERINT [ POWER INTEGRATIONS, INC. ]
