Features: • Features eliminate or reduce cost of external components
• Fully integrated soft-start for minimum stress/overshoot
• Externally settable accurate current limit
• Wider duty cycle for more power, smaller input capacitor
• Line under-voltage (UV) detection: no turn off glitches
• Line overvoltage (OV) shutdown extends line surge limit
• Line feed forward with maximum duty cycle (DCMAX) reduction rejects ripple and limits DCMAX at high line
• Single resistor sets OV/UV thresholds, DCMAX reduction
• Frequency jittering reduces EMI and EMI filtering costs
• Regulates to zero load without dummy loading
• 132 kHz frequency reduces transformer/power supply size
• Half frequency option for video applications
• Hysteretic thermal shutdown for automatic recovery
• Large thermal hysteresis prevents PC board overheating
• Standard packages with omitted pins for large creepage
• Active-on and active-off remote ON/OFF capability
• Synchronizable to a lower frequencyApplicationA High Efficiency, 30 W, Universal Input Power Supply The circuit shown in Figure 24 takes advantage of several of the TOPSwitch-FX features to reduce system cost and power supply size and to improve efficiency. This design delivers 30 W at 12 V, from an 85 to 265 VAC input, at an ambient of 50 °C, in an open frame configuration. A nominal efficiency of 80% at full load is achieved using TOP234.
The current limit is externally set by resistors R1 and R2 to a value just above the low line operating peak current of approximately 70% of the default current limit. This allows use of a smaller transformer core size and/or higher transformer primary inductance for a given output power, reducing TOPSwitch-FX power dissipation, while at the same time avoiding transformer core saturation during startup and output transient conditions. The resistor R1 provides a feed forward signal that reduces the current limit with increasing line voltage, which, in turn, limits the maximum overload power at high input line voltage. The feed forward function in combination with the built-in soft-start feature of TOPSwitch-FX, allows the use of a low cost RCD clamp (R3, C3 and D1) with a higher reflected voltage, by safely limiting the TOPSwitch-FX drain voltage, with adequate margin, under worst case conditions. The extended maximum duty cycle feature of TOPSwitch-FX (guaranteed minimum value of 75% vs. 64% for TOPSwitch-II) allows the use of a smaller input capacitor (C1). The extended maximum duty cycle and the higher reflected voltage possible with the RCD clamp also permit the use of a higher primary to secondary turns ratio for T1 which reduces the peak reverse voltage experienced by the secondary rectifier D8. As a result, a 60 V Schottky rectifier can be used for up to 15 V outputs,
which greatly improves power supply efficiency. The cycle skipping feature of the TOPSwitch-FX eliminates the need for any dummy loading for regulation at no load and reduces the no load/standby consumption of the power supply. Frequency jitter provides improved margin for conducted EMI meeting the CISPR 22 (FCC B) specification.
A simple Zener sense circuit is used for low cost. The output voltage is determined by the Zener diode (VR2) voltage and the voltage drops across the optocoupler (U2) LED and resistor R6. Resistor R8 provides bias current to Zener VR2 for typical regulation of ±5% at the 12 V output level, over line and load and component variations.
SpecificationsDRAIN Voltage ............................................ -0.3 to 700 V
DRAIN Peak Current: TOP232 ................................. 0.8 A
TOP233 ................................. 1.6 A
TOP234 ................................. 2.4 A
CONTROL Voltage ........................................... -0.3 to 9 V
CONTROL Current .................................................100 mA
MULTI-FUNCTION Pin Voltage ......................... -0.3 to 9 V
FREQUENCY Pin Voltage................................... -0.3 to 9 V
Storage Temperature ............................... -65 to 150 °C
Operating Junction Temperature(2) .......... -40 to 150 °C
Lead Temperature(3) .......................................... 260 °C
Notes:
1. All voltages referenced to SOURCE, TA = 25 °C.
2. Normally limited by internal circuitry.
3. 1/16" from case for 5 seconds.DescriptionTOPSwitch-FX uses the proven TOPSwitch topology and cost effectively integrates many new functions that reduce system cost and, at the same time, improve design flexibility, performance and energy efficiency. Like TOPSwitch, the high voltage power MOSFET, PWM control, fault protection and other control circuitry of TOPSwitch-FX are all integrated onto a single CMOS chip, but with two added terminals. The first one is a MULTIFUNCTION (M) pin, which implements programmable line OV/UV shutdown and line feed forward/DCMAX reduction with line voltage. The same pin of TOPSwitch-FX can be used instead to externally set an accurate current limit. In either case, this pin can also be used or remote ON/OFF or to synchronize the oscillator to an external, lower frequency signal. The second added terminal is the FREQUENCY (F) pin and is available only in the Y package. This pin provides the half frequency option when connected to CONTROL (C) instead of SOURCE (S). The features on the new pins can be disabled by shorting them to the SOURCE, which allows the device to operate in a three terminalTOPSwitch mode, but with the following new transparent features: soft-start, cycle skipping, 132 kHz switching frequency, frequency jittering, wider DCMAX, hysteretic thermal shutdown and larger creepage. In addition, all critical parameters of TOPSwitch-FX such as frequency, current limit, PWM gain, etc. have tighter temperature and absolute tolerances compared to the TOPSwitch-II family. Higher current limit accuracy and larger DCMAX, when combined with other features allow for a 10% to 15% higher power capability on the TOPSwitch-FX devices compared to equivalent TOPSwitch-II devices for the same input/output conditions.
TOP234 Data Sheet
