Cuk Converter: How to Resolve Current Overshoot During Cuk PWM Dimming?

Semiconductor Integrated Circuits (ICs)
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PWM Dimming is the most common brightness control method for industrial lighting, automotive lighting, stage lighting and smart LED systems.

Continued from previous article: Cuk Converter: Analysis of Cuk LED Driver Implementation (MOSFET Version)

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(Images sourced from Microchip)

For low-power LEDs, dimming can be realized simply via PWM switching with voltage drive and series current-limiting resistors. Nevertheless, this scheme brings drawbacks for high-power LEDs:

  • Extra power dissipation across series resistors
  • LED color temperature drift with varying current
  • Reduced system efficiency

Accordingly, switching constant-current LED drivers are widely adopted for high-power LED applications.

The Cuk topology is popular for LED drivers thanks to these merits:

  • Continuous input and output currents
  • Favorable EMI performance
  • Supports both boost and buck operation
  • Low output current ripple

However, one typical issue frequently emerges under PWM dimming conditions for Cuk converters:

Current overshoot and high-frequency ringing occur the instant LEDs re-energize.

Consequences of current overshoot:

  • Degraded LED service life
  • Visible light flicker
  • Deteriorated EMC performance
  • Inconsistent brightnessMCP1633 in precision lighting systems

Therefore, it is critical to understand the root cause and implement proper countermeasures.

Cuk LED PWM Dimming Circuit

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(Images sourced from Microchip)

  • Q1: Main power switch
  • L1, L2, C1: Cuk power stage
  • Q3: LED string disconnect switch
  • Q2, Q4, D2: Drive and level-shift circuitry for Q3

Under load-side PWM chopping dimming, the Cuk converter may exhibit output current overshoot and high-frequency ringing immediately after the LED turns on. See measured waveform below:

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(Images sourced from Microchip)

The waveform shows obvious output current overshoot and ringing upon PWM turn-on (Duty Cycle = 1%, fPWM=100 Hz), followed by stabilization to steady-state current.

Waveform Channel Definition

Blue: PWM dimming signal

Red: SW switching node voltage

Pink: IOUT output current

Observation

At the PWM rising edge (LED turn-on instant):

  1. Output current spike
  2. Ringing
  3. Current gradually settles to the target constant-current level

This phenomenon fundamentally stems from the energy storage characteristics inherent to the Cuk topology.

Primary sources of oscillation:

  • Output capacitor COUT
  • Parasitic inductance of LED wiring
  • Energy variation on coupling capacitor Ccouple

Effective LED dimming requires current chopping technology.

When implementing current chopping with switching-mode constant-current drivers, the following points must be addressed:

  1. The error signal shall be held (frozen) at the present current value while LEDs are OFF.
  2. The LED load must be disconnected; otherwise, charge on the output capacitor will dissipate and trigger current overshoot when PWM turns ON.
  3. Switching operation must be suspended to preserve charge on the output capacitor. Without this measure, excessive output voltage overshoot will occur and induce current spikes and flicker once LEDs are reconnected.

The MCP1633 integrates all above functions, greatly simplifying PWM dimming of high-power LEDs via current chopping. Specifically, a PWM dimming signal can be fed directly into the DIMM pin to control the duty cycle and switching frequency of LED current.

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Additional References

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