SLUA110 Datasheet, PDF (14/19 Pages) Texas Instruments – PRACTICAL CONSIDERATIONS IN CURRENT MODE POWER SUPPLIES

APPLICATION NOTE
Current Mode ICs Used in Voltage Mode
Most of today’s current mode control ICs are second and
third generation PWMs. Their features include high current
output driver stages, reduced internal delays through their
protection circuitry, and vast improvements in the refer-
ence voltage, oscillator and amplifier sections. In compari-
son to the first generation ICs (1524), numerous advan-
tages can be obtained by incorporating a second or third
generation IC (18XX) into an existing voltage mode design.
In duty cycle control (voltage mode), pulse width modula-
tion is attained by comparing the error amplifier output to
an artificial ramp. The oscillator timing capacitor Ct is used
to generate a sawtooth waveform on both current or vol-
tage mode ICs. To utilize a current mode chip in the voltage
mode, this sawtooth waveform will be input to the current
sense input for comparison to the error voltage at the PWM
comparator. This sawtooth will be used to determine pulse
width instead of the actual primary current in this method.
U-111
VI. FULL DUTY CYCLE (100%) APPLICATIONS
Many of the higher power (>500 watt) power supplies
incorporate the use of a fan to provide cooling for the mag-
netic components and semiconductors. Other users lo-
cate fans throughout a computer mainframe, or other
equipment to circulate the air and keep temperatures from
skyrocketing. In either case, the power supply designer is
usually responsible for providing the power and control.
The popularity of low voltage DC fans has increased
throughout the industry due to the stringent agency safety
requirements for high voltage sections of the overall circuit.
In addition, it’s much easier to satisfy dual AC inputs and
frequency stipulations with a low cost DC fan, powered by
a semi-regulated secondary output.
The most efficient way to regulate the fan motor speed
(hence temperature) is with pulse width modulation. An
error signal proportional to temperature can be used as the
control voltage to the PWM error amplifier. While nearly full
duty cycle can be easily attained, the circumstances may
warrant full, or true 100% duty cycle.
This condition is highly undesirable in a switch-mode
power supply, therefore most PWM IC designs have gone
to great extent to prevent 100% duty cycle from occurring.
There are simple ways to over-ride these safeguards, how-
ever. One method, presented below, “freezes” the oscilla-
tor and holds the PWM output in the ON, or high state
when the circuit is activated. Feedback from the output is
required to guarantee that the oscillator is stopped while
the output is high. Without feedback, the oscillator can be
nulled with the output in either state.
Figure 36. Current Mode PWM Used as a
Voltage Mode PWM
Compensation of the loop is similar to that of voltage mode,
however, subtle differences exist. Most of the earlier PWMs
(15xx) incorporate a transconductance (current) type
amplifier, and compensation is made from the E/A output
to ground. Current mode PWMs use a low output resis-
tance (voltage) amplifier and are compensated accord-
ingly. For further reference on topologies and compensa-
tion, consult “Closing the Feedback Loop” listed in this
appendix.
Figure 37. Full Duty Cycle Implementation
3-119