TOP254EN datasheet(13/50 Pages) POWERINT

13 / 50 page

Rev. M 12/21

TOP252-262

www.power.com

Line Feed-Forward with DC

MAX Reduction

The same resistor used for UV and OV also implements line voltage

feed-forward, which minimizes output line ripple and reduces

power supply output sensitivity to line transients. Note that for the

same CONTROL pin current, higher line voltage results in smaller

operating duty cycle. As an added feature, the maximum duty

cycle DC

MAX is also reduced from 78% (typical) at a voltage slightly

lower than the UV threshold to 36% (typical) at the OV threshold.

MAX of 36% at high line was chosen to ensure that the power

capability of the TOPSwitch-HX is not restricted by this feature

under normal operation. TOPSwitch-HX provides a better fit to the

ideal feed-forward by using two reduction slopes: -1% per

mA for all

bus voltage less than 195 V (typical for 4 M

W line impedance) and

-0.25% per

mA for all bus voltage more than 195 V. This dual

slope line feed-forward improves the line ripple rejection

significantly compared with the TOPSwitch-GX.

Remote ON/OFF

TOPSwitch-HX can be turned on or off by controlling the

current into the VOLTAGE-MONITOR pin or out from the

EXTERNAL CURRENT LIMIT pin (Y, E/L and M packages) and

into or out from the MULTI-FUNCTION pin (P and G package,

see Figure 12). In addition, the VOLTAGE-MONITOR pin has a

1 V threshold comparator connected at its input. This voltage

threshold can also be used to perform remote ON/OFF control.

When a signal is received at the VOLTAGE-MONITOR pin or the

EXTERNAL CURRENT LIMIT pin (Y, E/L and M packages) or the

MULTI-FUNCTION pin (P and G package) to disable the output

through any of the pin functions such as OV, UV and remote

ON/OFF, TOPSwitch-HX always completes its current switching

cycle before the output is forced off.

As seen above, the remote ON/OFF feature can also be used as

a standby or power switch to turn off the TOPSwitch-HX and

keep it in a very low power consumption state for indefinitely

long periods. If the TOPSwitch-HX is held in remote off state for

long enough time to allow the CONTROL pin to discharge to the

internal supply undervoltage threshold of 4.8 V (approximately

32 ms for a 47

mF CONTROL pin capacitance), the CONTROL

pin goes into the hysteretic mode of regulation. In this mode,

the CONTROL pin goes through alternate charge and discharge

cycles between 4.8 V and 5.8 V (see CONTROL pin operation

section above) and runs entirely off the high voltage DC input,

but with very low power consumption (160 mW typical at

230 VAC with M or X pins open). When the TOPSwitch-HX is

remotely turned on after entering this mode, it will initiate a

normal start-up sequence with soft-start the next time the

CONTROL pin reaches 5.8 V. In the worst case, the delay from

remote on to start-up can be equal to the full discharge/charge

cycle time of the CONTROL pin, which is approximately 125 ms

for a 47

mF CONTROL pin capacitor. This reduced

consumption remote off mode can eliminate expensive and

unreliable in-line mechanical switches. It also allows for

microprocessor controlled turn-on and turn-off sequences that

may be required in certain applications such as inkjet and laser

printers.

Soft-Start

The 17 ms soft-start sweeps the peak drain current and

switching frequency linearly from minimum to maximum value

by operating through the low frequency PWM mode and the

variable frequency mode before entering the full frequency

mode. In addition to start-up, soft-start is also activated at each

restart attempt during auto-restart and when restarting after

being in hysteretic regulation of CONTROL pin voltage (V

C), due

to remote OFF or thermal shutdown conditions. This effectively

minimizes current and voltage stresses on the output MOSFET,

the clamp circuit and the output rectifier during start-up. This

feature also helps minimize output overshoot and prevents

saturation of the transformer during start-up.

Shutdown/Auto-Restart

To minimize TOPSwitch-HX power dissipation under fault

conditions, the shutdown/auto-restart circuit turns the power

supply on and off at an auto-restart duty cycle of typically 2% if

an out of regulation condition persists. Loss of regulation

interrupts the external current into the CONTROL pin. V

regulation changes from shunt mode to the hysteretic auto-

restart mode as described in CONTROL pin operation section.

When the fault condition is removed, the power supply output

becomes regulated, V

C regulation returns to shunt mode, and

normal operation of the power supply resumes.

Hysteretic Over-Temperature Protection

Temperature protection is provided by a precision analog circuit

that turns the output MOSFET off when the junction temperature

exceeds the thermal shutdown temperature (142 °C typical).

When the junction temperature cools to below the lower

hysteretic temperature point, normal operation resumes, thus

providing automatic recovery. A large hysteresis of 75 °C

(typical) is provided to prevent overheating of the PC board due

to a continuous fault condition. V

C is regulated in hysteretic

mode, and a 4.8 V to 5.8 V (typical) triangular waveform is

present on the CONTROL pin while in thermal shutdown.

Bandgap Reference

All critical TOPSwitch-HX internal voltages are derived from a

temperature-compensated bandgap reference. This voltage

reference is used to generate all other internal current

references, which are trimmed to accurately set the switching

frequency, MOSFET gate drive current, current limit, and the line

OV/UV/OVP thresholds. TOPSwitch-HX has improved circuitry

to maintain all of the above critical parameters within very tight

absolute and temperature tolerances.

High-Voltage Bias Current Source

This high-voltage current source biases TOPSwitch-HX from the

DRAIN pin and charges the CONTROL pin external capacitance

during start-up or hysteretic operation. Hysteretic operation

occurs during auto-restart, remote OFF and over-temperature

shutdown. In this mode of operation, the current source is

switched on and off, with an effective duty cycle of approxi-

mately 35%. This duty cycle is determined by the ratio of

CONTROL pin charge (I

C) and discharge currents (ICD1 and ICD2).

This current source is turned off during normal operation when

the output MOSFET is switching. The effect of the current

source switching will be seen on the DRAIN voltage waveform

as small disturbances and is normal.

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