PWM Controller Sizing Calculator

Formula used

• Duty cycle: D ≈ Vout / Vin (clamped to a safe range).
• PWM steps: Steps = 2^N, where N is resolution bits.
• Timer clock: f_clk ≈ f_sw × 2^N (edge-aligned estimate).
• Gate peak current: I_pk ≈ Q_g / (t_r + t_f).
• Gate average current: I_avg ≈ Q_g × f_sw.
• Gate-drive power: P_gate ≈ Q_g × V_drv × f_sw.
• Switching loss: P_sw ≈ 0.5 × Vin × Iout × (t_r+t_f) × f_sw.
• Conduction loss: P_cond ≈ Iout² × R_ds(on) (split by parallel devices, roughly).

How to use this calculator

1. Enter Vin, Vout, and Iout to set the switching conditions.
2. Choose switching frequency and resolution that match your control goal.
3. From your MOSFET datasheet, enter total gate charge at your drive voltage.
4. Set rise and fall targets you can realistically achieve on your layout.
5. Submit to see required timer clock, drive current, and loss estimates.
6. Export CSV or PDF for design notes and comparisons.

Sizing targets

Controller sizing starts with operating conditions: input voltage, target output, and expected current. The duty estimate D≈Vout/Vin sets how much on-time the PWM must deliver. Values near 0% or 100% reduce control margin, so many designs keep duty between 5% and 95%. Switching frequency defines the energy moved per cycle and strongly influences component size and switching stress.

Clock and resolution tradeoffs

PWM resolution translates directly into timing demand. For an edge-aligned timer, the clock requirement is fclk≈fsw×2^N. At 200 kHz, 10 bits implies about 204.8 MHz, while 11 bits doubles that. If your platform’s maximum clock is lower, reduce frequency, reduce resolution, or use alternative modulation features such as center-aligned counting or fractional dithering.

Gate-drive capability

Gate-drive sizing is driven by MOSFET gate charge. Peak current is approximated by Ipk≈Qg/(tr+tf) for the chosen transition time. Average gate current is Iavg≈Qg×fsw, and gate-drive power is Pgate≈Qg×Vdrv×fsw per device. Higher Qg, higher drive voltage, or higher frequency increases driver dissipation and may require a dedicated driver stage.

Loss and thermal budgeting

Loss estimation supports thermal decisions early. A rough hard-switching estimate uses Psw≈0.5×Vin×Iout×(tr+tf)×fsw. Conduction loss can be approximated with Pcond≈Iout^2×Rds(on), reduced when devices are paralleled. These simplified numbers help compare options and highlight when switching loss dominates, guiding you toward different devices, snubbing, or lower frequency.

Design workflow and validation

Use the calculator as a checklist: enter timing limits, add a realistic margin, and verify the recommended timer clock and gate peak current are achievable. Then cross-check with datasheets, scope waveforms, and temperature rise under load. Adjust deadtime, drive strength, and layout to balance efficiency, EMI, and reliability. Record your chosen fsw, bits, Qg, and margins in the CSV, so reviews stay consistent across revisions and multiple candidate controllers during prototype tuning and qualification testing.

FAQs

What does “timer clock requirement” mean?

It is the approximate internal clock rate needed to create your switching frequency at the chosen resolution. If the required clock exceeds your available limit, reduce frequency, reduce resolution, or change the timer mode.

Why can high resolution be impractical at high frequency?

Each extra bit doubles PWM steps. With edge-aligned counting, fclk grows as fsw×2^N, so clocks quickly exceed typical MCU or controller limits at hundreds of kilohertz.

How do I pick rise and fall time targets?

Start from datasheet driver capability and layout quality. Faster edges reduce switching loss but raise EMI and ringing risk. Use realistic targets, then verify with scope measurements at full load.

Is gate peak current the same as driver output current rating?

Not exactly. The Ipk estimate is a simplified demand during transitions. Driver datasheets specify peak source/sink currents and thermal limits. Ensure both peak and average gate-drive power are acceptable.

Why are the loss numbers labeled “rough”?

They use simplified formulas and assume hard switching. Real losses depend on device capacitances, current waveforms, deadtime, temperature, and gate resistance. Use these results for comparison, then refine with measurements.

What safety margin should I use?

Common early-stage margins range from 10% to 30% for timing and drive recommendations. Increase margin if input data is uncertain, temperature varies widely, or the design must tolerate component aging.