Design Ideas: August 17, 1995

Precharged capacitor ensures high-side start-up

Christophe Basso,
European Synchrotron Radiation Facility, Grenoble, France

When you use a high-voltage bridge driver, such as the IR2110 or the newer IR2155, a complete driver circuit can exhibit erratic behavior when you apply power. The main problem arises from the bootstrap capacitor that discharges when the power appears. Thus, the floating supply is not ready to fulfill its primary function of driving the high-side MOSFET. The circuit needs a few commutations to rapidly charge the bootstrap capacitor to allow it to supply the upper driver.

Unfortunately, this short period of unstable power conditions produces erratic driver behavior. For example, in fluorescent ballasts, in which a frequency sweep implements a preheat operation, the erratic start-up behavior creates a brief illumination in the tube. This illumination diminishes the benefits of a hot-cathode start. The simple configuration in Fig 1, which joins a high-voltage resistor and a 400-mW, 16V zener diode, cures the problem of erratic start-up.

The main supply directly feeds the IR2155. However, you can extend the method to other topologies, such as a ballast supplied by a power-factor-corrected auxiliary supply. At power-up, R₁ precharges C_BOOT via the R₂ resistive path to ground. Zener diode D₁ limits the voltage across C_BOOT to 16V. This limiting action protects the upper driver from damage by overvoltage. Thus, when the IR2155's V_CC pin reaches the upper threshold level, C_BOOT is already charged and the MOSFET bridge delivers square waves without exhibiting erratic behavior.

The fluorescent tube's cathode, thus, receives a proper preheat before the complete strike occurs. Your only caveat is to ensure that the R_1βC_BOOT time constant is lower than the RC time constant of the IR2155's power-supply network.