One Powerful Decade: Next Generation Monolithic Off-Line Switcher Improves Performance, Flexibility

Apr 1, 2010 12:00 PM
BALU BALAKRISHNAN Power Integrations, Inc., Sunnyvale, California

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.

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