Use a low-cost PWM ramp generator in switch-mode power supplies

-April 08, 2010


The circuit in Figure 1 shows a PWM (pulse-width-modulated) ramp generator that you can use in low-cost switch-mode dc/dc power supplies. Its supply voltage can range from 5 to 35V dc, and you can set the output-ramp amplitude of 0.3 to 1V. You can also set a minimum off time that lets you set a maximum 50% duty cycle for magnetic components that need duty-cycle limiting.

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Design Ideas
The ramp generator (Figure 1) uses one-half of an LM393 dual comparator.
The other half of the comparator is available to generate the PWM portion of the converter. The ramp amplitude and frequency depend on the reference. An ordinary red LED can act as a low-cost reference. Its forward voltage of approximately 1.7V is reasonably constant over indoor temperature ranges. The ratio of R1 to R2 sets the ramp amplitude relative to the reference, and R1, R2, and C1 set the minimum off time. R3 and C2 establish a time constant, which sets the period. Note that the R1, R2, and C1 network also affects the period. Table 1 shows examples of various configurations.

Figure 1 a 70V-dc upconverter, employs the ramp generator. You can easily configure it at any output ranging from the highest input voltage to whatever the FET can handle. This example uses a 330-µH inductor, but you can easily change that value by choosing the appropriate PWM frequency.

Note that the output FET does not turn on quickly, and it doesn’t need to, but it does turn off quickly. You can enhance the turn-off speed by adding a 2N4403 PNP transistor between the output of the comparator and the pullup resistor. Connect the base to the comparator, the emitter to the FET gate, and the collector to ground. Add a 100Ω resistor from the base to the emitter.

The circuit has slow load-transient response, which you can adjust by altering the time constant that R5 and C3 form. Note that R9 and R5 form a voltage divider that ensures the lowest error voltage at the PWM comparator is above the ramp’s lowest point. The converter cannot operate without R9.

Spice files were developed with Linear Technology’s LTSpice/SwitcherCAD program. Click here for the files.

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