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