One Powerful Decade: Next Generation Monolithic Off-Line Switcher Improves Performance, Flexibility
Consumer electronic systems require small size power supplies with a minimum number of components so that they represent a low cost to the overall system. For ac systems, the power supply should operate directly from the power line and switch at a high enough frequency to minimize component size. An added bonus is the inclusion of energy saving capability within the power supply.
The above requirements have been met in the new switchmode, 15-75W, TOPSwitch-FX family. It follows the original TOPSwitch, introduced in 1994, and the upgraded TOPSwitch-II of 1997. TOPSwitch technology for all three generations employs a patented high voltage CMOS technology that allows its 700V MOSFET and low voltage control circuits to be integrated onto a single monolithic chip. Table 1 compares the new TOP-Switch-FX with the older TOP-Switch-II. Figure 1 shows the functional block diagram of the FX version.
Two pins are added in the new FX version that is housed in a special 5-pin TO-220 package with additional creepage between pins. There is also an 8-pin DIP-8 and SMD-8 package with one additional pin; these two packages have a wider creepage between the Source (S) and Drain (D) pins because pin 6 is missing. Figure 2 shows the device configured as an off-line flyback converter. Figure 3 shows the 5-pin TO-220 package.
Added to all package versions is the Multifunction (M) pin usually used for line sensing to implement line overvoltage (OV)/undervoltage (UV) and line feed forward with DMAX (maximum duty cycle) reduction. As an alternative, the pin could be used to set an external current limit by connecting it to the Source pin through a resistor. This reduces the current limit to a value close to the operating peak current.
The Multifunction pin can also be used for remote on/off and as a synchronization input. Remote on/off provides the ability to put the chip to sleep to save energy. Other energy savings are derived from cycle skipping and half frequency operation.
Energy saving is becoming a bigger issue in power supply design. The TOPSwitch-FX provides the ability to put the chip to “sleep” to save energy. Designers are now trying to meet the standby or no-load power consumption specification of less than 1W that is being driven by new government regulations, such as Energy Star and Blue Angel.
The Control (C) pin receives the combined supply and feedback current. During normal operation, a shunt regulator separates the feedback signal from the supply current. The Control pin voltage is the supply voltage for the control circuits, including the MOSFET gate driver.
Available only in the TO-220 package is the added Frequency (F) pin that provides the half frequency option. The Frequency pin can be shorted to the Control pin to make the TO-220 packaged device operate at half frequency, 65kHz.
If the Multifunction pin is shorted to the Source it puts the device into a three-terminal mode. This mode is basically equivalent to the earlier generation parts, but incorporates new features, including: soft-start, cycle skipping, 130kHz switching frequency (providing the Frequency is also shorted to the Source), frequency jittering, wider DMAX, and hysteretic thermal shutdown.
NEW FEATURES
One new feature, soft-start, is activated at start-up with a 10msec (typical) duration. Maximum duty cycle starts from zero and linearly increases to the default 78% maximum at the end of the 10msec. Besides start-up, soft-start is also activated at each restart attempt during auto-restart and when restarting after being in hysteretic regulation of the Control pin voltage, due to remote off or thermal shutdown conditions.
Continue to next page.
The power supply operates in the cycle skipping mode when the load at its output consumes less power than the power that the FX delivers at minimum duty cycle, DMIN. No additional control is needed for the transition between normal operation and cycle skipping. As the load increases or decreases, the power supply automatically switches between normal operation and cycle skipping mode as Off-Line Switcher IC necessary.
An internal oscillator charges and discharges the internal capacitance between two voltage levels to create a sawtooth waveform for the PWM. The oscillator sets a PWM/current limit latch at the beginning of each cycle.
The nominal 130kHz switching frequency was selected to minimize transformer size while keeping the fundamental EMI frequency below 150kHz. To further reduce EMI, the frequency is jittered (frequency modulated) by approximately±4kHz at 250Hz (typical) rate, as shown in Figure 4.
When using the Multifunction pin (M) line sensing, for the same Control pin current, higher line voltage reduces the operating duty cycle. As an added safety measure, the maximum duty cycle, DMAX, is also reduced from 78% (typical) at a voltage slightly higher than the UV threshold to 38% (typical) at the OV threshold. DMAXat the OV threshold was chosen to ensure that the power capability of the TOPSwitch-FX is not restricted by this feature under normal operation.
Temperature protection is provided by a precision analog circuit that turns the output MOSFET off when the junction temperature exceeds the 135°C (typical) of thermal shutdown. When the junction temperature cools to below the hysteretic temperature, normal operation resumes. A large hysteresis of 70°C (typical) is provided to prevent overheating due to a repeating fault condition.
All critical TOPSwitch-FX parameters, such as frequency, current limit, PWM gain, etc. have tighter temperature and absolute tolerances than its predecessors. Higher current limit accuracy and larger DMAX, when combined with other features, allow 10-15% higher power than previous versions under the same input/output conditions.
30W SUPPLY
The circuit shown in Figure 5 takes advantage of several of the TOP-Switch-FX features to reduce system cost and power supply size and to improve efficiency. This design delivers 30W at 12V, from an 85 to 265Vac input, at an ambient of 50°C, in an open frame configuration. A nominal efficiency of 80% at full load is achieved using TOP234.
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 avoiding transformer core saturation during startup and output transient conditions. 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, allows the use of a low-cost RCD clamp (R3, C3 and D I) with a higher reflected voltage, by safely limiting the TOP-Switch-FX drain voltage, with adequate margin, under worst-case conditions.
The extended maximum duty cycle feature (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 that reduces the peak reverse voltage experienced by the secondary rectifier, D8. As a result, a low-cost 60V Schottky rectifier can be used for up to 15V outputs, which greatly improves power supply efficiency. The cycle skipping feature 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 for zener VR2 for typical regulation of ±5% at the 12V output level, over line and load and component variations.
BALU BALAKRISHNAN
Balu Balakrishnan has served as president and chief executive officer and as a director of Power Integrations, Inc. since January 2002. He joined the company in 1989 and subsequently served in engineering, marketing, and business development roles before becoming president and chief operating officer in April 2001. He is chief inventor of the company's TOPSwitch and TinySwitch technologies and holds more than 100 patents. He holds an M.S.E.E from the University of California, Los Angeles, and a B.S.E.E. from Bangalore University, India.