# Formula for current vs duty cycle in step-up pwm circuit?

What is the formula for average current with step-up PWM circuit (and about 30 kHz frequency and 300 uH inductor) that charges battery at 72 volts, and source is 24 volts, relative to duty cycle? I tested this with CPU PWM signal and noticed that current begins to raise at some critical level very quickly, which caused a problem with controlling, because my PWM signal was based on 1 MHz clock and could not be adjusted finely enough. I'm trying to use continuous mode, although I noticed that discontinuous mode makes it easier to control the average current (by adjusting gap between known charge/discharge pulses)

The TI doc has the basic step up schematic, and Vin and Vout can be assumed to be fixed at 72 and 24V: The components can be assumed to be ideal, and since the current will be infinite after some critical point, where D > 1-(24/72) = 0.66, I'm wondering if there is function f for inductor current = f(D)? Anyway I'm interested in duty cycle range that is close to the maximum value, and probably will change PWM clock to 16 MHz to get more resolution. (16 MHz / 30 kHz = 533 time steps)

The problem with low resolution PWM is that I get for example about 500 mA average current with pwm value = 20, and 4 amps when pwm value = 21. So not very good control system. And would like to be able to calculate the currents for different D that are possible.

The inductor ripple current formula is rather close, but it does not say anything about what the final value of current will be after lots of cycles.

The infinite current can be understood for example by thinking what happens when duty cycle = 1, or switch is on always.

• ti.com/lit/an/slva372d/… Nov 29, 2022 at 9:20
• Show the schematic please. Nov 29, 2022 at 9:43
• Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
– Community Bot
Nov 29, 2022 at 10:41