Electronic Design

Multiple Modes Flatten Flyback Efficiency Curves

One trouble with most power converters is that they achieve their best efficiencies only at high load levels. At moderate and low loads, their efficiencies can fall to as low as 40%. This is unacceptable when systems being powered spend most of their on-time drawing less than full power. Dealing with this leads to power supplies that incorporate extra transformer windings, rectifiers, and control circuitry for standby operation.

More elegantly (and economically), Power Integrations' TOPSwitch-HX substantially improves moderate- and low-load performance compared to existing solutions - for example, 57% versus 37% efficiency at 1-W output with a 265-V ac source and 62% versus 52% at 120 V.

The HX family members achieve their flatter efficiency curves through a new fourmode control scheme. At maximum load, the modulator operates in full-frequency pulse-width modulation (PWM) mode. As the load requirements decrease, the modulator transitions to a variable-frequency PWM mode and then to a low-frequency PWM mode. Under light loads, the modulator operates in multicycle mode.

In full-frequency mode, the average switching frequency is kept constant. (This mode is the standard operating mode for Power Integrations' other TOPSwitch chips.) At a preset load reduction, peak drain current is held constant while switching frequency drops toward a 30-kHz rate. In this mode, duty-cycle reduction is accomplished by extending off-time.

When the switching frequency reaches its minimum, the PWM modulator transitions to a mode in which switching frequency is held constant while duty cycle is reduced further by cutting on-time. When peak drain current drops even further, the modulator transitions to multicycle-modulation mode in which, at each turn-on, the modulator bursts four or five consecutive pulses at 30 kHz. This mode not only keeps peak drain current low, it also minimizes harmonic frequencies between 6 and 30 kHz, avoiding the transformer resonant frequency.

Pricing in 1000-piece quantities for the TOP258PN, a 35-W part in a DIP-8 package, is $1.57 each.

Power Integrations www.powerint.com

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