October 9, 1997

Half-bridge inverter has variable frequency

Thierry Castagnet, SGS-Thomson Microelectronics, Schaumburg, IL

Modern electronic ballasts require added features, such as high-frequency preheat for filaments, nominal lamp-power control, and light-dimming capability. Because the output inverter operates in resonant mode, you can control the power or current of these ballasts simply by varying the switching frequency. To vary the frequency, you can build a simple VCO that uses a half-bridge MOSFET driver (Figure 1). You can use this VCO as the core of a controller for a supply or ballast that requires a variable frequency.

As the input voltage varies from 2 to 9V, the switching frequency ranges from 30 to 120 kHz and maintains a duty cycle of 50% with a ±2% overall tolerance. For ballasts or resonant converters, the linearity and frequency range are not critical, but the duty cycle must stay within 50% ±5%. IC₁'s high-voltage half-bridge driver has a 555-timerlike oscillator; an additional integrator transforms IC₁ into a VCO.

The integrator, which replaces the resistor and capacitor of typical single-frequency applications, comprises C₁ and op amp IC₂. A square-wave alternating current, the amplitude of which is proportional to V_IN, controls the charge and discharge of C₁. The internal oscillator in IC₁ operates as a Schmitt trigger with thresholds at one-third and two-thirds of V_CC. IC₁'s output buffer drives Q₁. When the buffer output is high, Q₁ pulls R₂ to ground; when the output is low, R₂ is an open circuit.

Because R₁ is twice R₂ and their common-node volt age is set at V_IN/2 by the CMOS IC₂ (neglecting the 5-mV offset), Q₁'s switching changes the capacitor current's sign but not its magnitude. Thus, the VCO frequency is theoretically proportional to the input voltage as follows:

The frequency depends on the ratio V_IN/V_CC and is insensitive to variations in V_CC when V_IN is proportional to V_CC.

Changing the input voltage from 1.35 to 8.9V corresponds to a change in the switching frequency and duty cycle from 20 to 120 kHz. The dynamic range of the frequency is at least 5. You can adapt the frequency profile to each application by changing C₁. The linearity of the frequency is within 4% when V_IN varies from 2 to 9V.

In practice, the switching frequency is slightly different from the theoretical one because the chain made by the op amp, oscillator timer, and the current commutating transistor, Q₁, adds a propagation delay to the total period. So, the real frequency is:

For electronic ballasts, the duty cycle should be well-controlled and equal to 50%, because it directly influences the lamp current's crest factor. As V_IN varies, the current in R₂ and the turn-off time of Q₁ vary. When compared with the half-period, this variation introduces a duty-cycle variation of less than ±1%. Because the on-resistance of Q₁ is negligible compared with the current source's resistance, the whole circuit is not very sensitive to the ambient temperature. Overall, the duty-cycle variation is less than ±2%.

In one application of the VCO, Q₁ is off at start-up, and the 6V input to the VCO sets a frequency of 85 kHz (Figure 2). Q₁ turns on after the delay programmed on its gate, and the frequency gradually shifts to 50 kHz under the filtering effect of R₁ and C₁ on the VCO voltage. (DI #2093)

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