IR2161 datasheet(11/20 Pages) IRF

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IR2161(S)&(PbF)

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convertor is close to maximum load. If the load is reduced,

the average level at which the ripple occurs (i.e. the DC

component) will be at a lower level.

Shut Down Circuit

The IR2161 contains a dual mode auto-resetting shutdown

circuit that detects both a short circuit or overload condition

in the convertor. The load current detected at the CS pin is

used to sense these conditions. If the output of the convertor

is short-circuited, a very high current will flow in the half

bridge and the system must shut down within a few mains

half cycles, otherwise the MOSFETs will rapidly be destroyed

due to excessive junction temperature. The internal CS pin

has an internal threshold (VCSSC). There also exists a lower

threshold so that if the voltage exceeds this level for more

than 50mS, the system will shut down.

A delay is included to prevent false tripping either due to

lamp inrush current at switch on (this current is still higher

than normal with the soft start operation) or transient

currents that may occur if an external triac based phase

cut dimmer is being used.

There also exists a lower threshold (VCSOL), which has a

much longer delay before it shuts down the system. This

provides the overload protection if an excessive number of

lamps is connected to the output or the output is short-

circuited at the end of a length of cable that has sufficient

resistance to prevent the current from being large enough

to trip the short circuit protection. Also under this condition

there is an excessive current in the half bridge that is

sufficient to cause heating and eventual failure but over a

longer period of time. The threshold for overload shutdown

is approximately 50% above maximum load with a delay of

approximately 0.5s. These timings are based on a current

waveform that has a sinusoidal envelope and a high

frequency square wave component with 50% duty cycle.

Both shutdown modes are auto resetting, which allows the

oscillator to start again approximately 1.5s after shutting

down. This is so that if the fault condition is removed the

system can start operating normally again without the line

voltage having to be switched off and back on again. It also

provides a good indication of overload to the end user as all

the lamps connected to the system will flash on and off

continuously if too many are connected.

The shut down circuit also uses the external CSD capacitor for

its timing functions. When the 0.5V threshold (VCSOL) is ex-

ceeded at CS the CSD is internally disconnected from the voltage

compensation circuit and connected to the shutdown circuit.

Figure 8, Shut Down Circuit.

The oscillator operates at minimum frequency when the

CSD capacitor is required for shutdown circuit timing.

During soft start or run mode, if the 0.5V threshold (VCSOL)

is exceeded the IR2161 charges CSD rapidly to approxi-

mately 5V (VCSDOL).

When the voltage at the CS input is greater than 0.5V, the

CSD capacitor is charged by current source IOL and when

the short circuit threshold of 1.2V is exceeded it is charged

by ISC as well. If 1.2V is exceeded CSD will charge from 5V

(VCSDOL) to 12V (VCSDSD), in approximately 50ms. When

0.5V is exceeded but 1.2V is not, CSD charges from 5V

(VCSDOL) to 12V (VCSDSD) in approximately 0.5s. It should

be remembered that, the timing accounts for the fact that

high frequency pulses with approximately 50% duty cycle

and a sinusoidal envelope appear at the CS pin.

The values of ISC and IOL take into account that only at the

peak of the mains will the comparator outputs go high and

effectively the capacitor will be charged in steps each line

half cycle. Once VCSD reaches 12V (VCSDSD), VCSD

discharges down to 2.4V (VCSDRS) and the system starts

up again. If the fault mode is still present, CSD starts

charging again.

If a fault is detected but goes away before CSD reaches

12V (VCSDSD), then CSD will discharge to 2.4V (VCSDRS)

and then the system will revert to compensation mode

without interruption of the output.

Following a shutdown, when the system starts up again

after the delay, the CSD capacitor will be internally switched

back to the voltage compensation circuit. However, if the

fault is still present the system will switch CSD back to the

shutdown circuit again.

1.2V

0.54V

12V

2.5V

CSD

Enable Outputs

I_OL

I_SC

Shutdown Function

Overload Function

Switch

I_RESET

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