Power Electronics

A New PFC Control Method for Electronic Ballasts

Electronic ballasts for fluorescent lighting deliver high efficiency and long lamp lifetimes. A new and simplified control circuit demonstrates complete control of the ballast, including all necessary PFC and lamp functionality.

Adding power factor correction (PFC) to electronic ballasts offers additional benefits by enabling the ballast to work like a “resistive” load on the AC mains. A novel circuit controls the entire electronic ballast, including both active PFC at the input and full lamp management at the output.

To understand operation of the ballast controller we first have to review power factor. It is basically a measurement of how well the shape of the AC mains input current flowing through a given load matches the shape of the AC mains voltage. For a purely “resistive” load, the voltage and current will be in phase with each other and have the same AC shape (Fig. 1). This will produce the maximum power factor (PF=1) and the lowest peak current. In reality, electronic ballast loads typically include a rectifier bridge at the input followed by a large DC bus storage capacitor. This causes a large pulse of current to be delivered near the peaks of AC line voltage to replenish the DC bus capacitor (Fig. 1). Since all of the RMS current needed to supply the necessary power is delivered during a narrow window of time, the peak of this current pulse can be 4 or 5 times higher (depending on the value of the DC bus capacitor) than compared with a resistive load. When many electronic ballasts are connected to the AC mains, the total peak current becomes a significant problem for power companies and results in higher power line losses, voltage fluctuations, higher capacity power delivery equipment, and wasted electricity. Power factor correction (PFC) is required to “correct” these loads so that they have a high power factor, are more resistive and require less peak currents.

A typical electronic ballast circuit without power factor correction includes (Fig. 2) an EMI filter to block ballast switching noise, a full-bridge rectifier and DC bus capacitor for AC/DC conversion, and a half-bridge fed resonant tank to ignite and run the lamp at high-frequency. A popular active PFC method is to insert a boost-type switching converter in between the rectifier and the DC bus capacitor (Fig. 2).

The boost PFC circuit replenishes the DC bus capacitor while shaping the input current for high power factor. The on-time of the boost switch is set by the control loop to maintain a constant DC bus voltage level. During the on-time, the boost inductor is charged up to a peak current level determined by the inductor value and the level of the instantaneous voltage at the output of the rectifier. The off-time of the boost switch is the time it takes to discharge the inductor current back down to zero and is determined by the inductor value and the peak current value reached during the previous on-time. During the off-time, the boost inductor current flows through the boost diode to the DC bus capacitor. The total switching time period is the sum of the on-time and off-time and sweeps from a high frequency near the valleys of the rectifier output voltage to a lower frequency at the peaks. The shape of the inductor current during each switching period is triangular with the peaks following the envelope of the low frequency rectifier output voltage. When these “triangles” of current are smoothed by the EMI filter at the input of the ballast, the result is a sinusoidal RMS current that matches the input voltage (Fig. 3).

Ballast Control Circuit

The main control circuit blocks for the electronic ballast include (Fig. 4) the on/off time control blocks for the boost PFC circuit as well as the frequency and timing control blocks for the half-bridge resonant tank. The PFC circuit monitors the DC bus voltage using a high-voltage resistor divider circuit and feeds this measurement back to the regulation circuit. The regulation circuit compares the DC bus measurement to a reference voltage and uses the error to set the on-time during each switching period. The off-time control is achieved using a circuit that detects when the inductor current has reached zero. Both the on-time and off-time blocks then feed into the PFC logic and gate drive blocks in order to turn the boost switch on and off correctly. An over-current protection (OCP) circuit is included to protect the boost switch against high currents that can occur during saturation of the boost inductor. An over-voltage protection (OVP) circuit is also included to prevent the DC bus voltage from exceeding the maximum allowable voltage rating of the DC bus capacitor during minimum load conditions.

The control blocks for the half-bridge resonant tank include high- and low-side gate drive circuits to turn the half-bridge MOSFETs (M1 and M2) on and off. The high-side gate driver requires a 600V level-shift circuit since the source of the high-side MOSFET (M1) is equal to the DC bus voltage during the time when M1 is on. An oscillator circuit block then controls the frequency, dead-time, and duty-cycle of the half-bridge. An analog input voltage that is used to steer the frequency to the correct operating frequencies to preheat, ignite and run the lamp controls the oscillator. Initially, the frequency is set to a high level above the resonance frequency of the tank circuit to preheat the lamp filaments. After the preheat time has ended, the frequency then sweeps down to a lower level towards resonance to ignite and run the lamp.

Electronic Ballast Design

Fig. 5 shows the schematics for a fully functional 2x54W/T5 electronic ballast designed around the IRS2580DS PFC/Ballast Control IC. The circuit includes control for both the boost PFC at the input and the half-bridge resonant tank at the output. The PFC pin of the IC controls the boost PFC circuit. The internal PFC control loop (Fig. 4) determines the on-time and off-time of the PFC pin. The VCO pin of the IC sets the frequency of the half-bridge gate driver output pins (HO and LO). Resistors R1 and R2 form a voltage divider that programs the desired DC voltage levels at the VCO pin. These voltage levels control the frequency of the internal oscillator (Fig. 4). The internal oscillator signal then feeds into the internal high- and low-side gate driver logic circuitry of the IC to generate the desired preheat, ignition, and running frequencies for the half-bridge and resonant output stage. Upon normal power up of the IC at the VCC pin, the PFC circuit boosts the DC bus voltage up and maintains it at a constant level, and, the half-bridge transitions through the programmed frequencies such that the resonant tank preheats, ignites and runs the lamp. Resistors RVL1, RVL2 and RVL3 form a voltage divider across the lamp for measuring the lamp voltage. This measurement is used for lamp voltage feedback during ignition and for lamp end-of-life (EOL) detection. Finally, the lamps are connected in a series configuration with secondary windings (LRES:B,C,D) from the resonant inductor used for preheating the lamp filaments.

Experimental Results

The evaluation results from the functional ballast show that the boost PFC stage and resonant output stage are both working properly. Fig. 6 shows the voltage and current at the AC line input (top) during running, and, the half-bridge and lamp voltage at the output (bottom) during running. Table 1 summarizes the ballast electrical data measured at a standard 230VAC/50Hz mains input voltage.

The circuit’s simplicity allows the complete control circuit to contained in a small 8-pin IC. This offers further cost and size reduction of electronic ballasts, as well as increased reliability and manufacturability. The control circuit allows for control of all fluorescent lamp types (T5, T8, T12, CFL), and exhibits excellent performance over a wide input voltage and/or output load range. Dimming can also be implemented using additional external circuitry.

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TAGS: Lighting
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