Current-fed Resonant Ballasts

A. Current-fed Resonant Ballasts

22.2.2 Classification of Electronic Ballast

These ballasts are supplied with a dc current source, usu-

ally obtained by means of a choke inductor in series with the Typical topologies used to supply discharge lamps at high fre- input dc voltage source. The dc current is transformed into an

Topologies

quency can be classified into two main groups: nonresonant alternating square current waveform by switching power tran- ballasts and resonant ballasts.

sistors. Typical topologies of this type of ballasts are shown in Fig. 22.11.

The topology shown in Fig. 22.11a corresponds to a class E

inverter. Inductor Le is used to obtain a dc input current with These topologies are usually obtained by removing the out- low current ripple. This current supplies the resonant tank put diode of dc-to-dc converters, in order to supply alternat- through the power switch formed by Q1–D1. The resonant ing current to the lamp. Current mode control is normally tank used in this topology can vary from one ballast to another; employed to limit the discharge lamp current. The lamp is the circuit shown in Fig. 22.11a is the one, which is nor- supplied with a square current waveform, which can exhibit mally used. The main advantage of this topology is that zero

22.2.2.1 Nonresonant Ballasts

a dc level in some cases. A small capacitor is used to initially voltage switching (ZVS) can be attained in the power switch, ignite the lamp, but its effect at steady-state operation can be thus reducing the switching losses and making possible the neglected.

operation at very high frequencies, which can reach several Examples of nonresonant electronic ballasts are shown in megahertz. This allows to drastically reduce the size and weight Fig. 22.10. Figures 22.10a and 22.10b illustrate a boost-based of the ballast. However, the adjustment of the circuit parame- and a flyback-based ballast, respectively. Other topologies, ters to obtain the optimum operation results is quite difficult, which can supply symmetric alternating current through the specially for mass production. Another important drawback is

FIGURE 22.10 Nonresonant electronic ballasts.

582 J. M. Alonso

C2 V Lamp L

(c) FIGURE 22.11 Two typical current-fed resonant inverters: (a) class E inverter; (b) current-fed push–pull inverter; and (c) current-fed full-bridge

(a)

(b)

resonant inverter.

the high voltage stress across the switch, which can reach values advantage of the voltage-fed series resonant circuit is that the of three times the dc input voltage. For these reasons, the main starting voltage can be easily obtained without using extra igni- applications of this circuit are battery-supplied ballasts with tion capacitors by operating close to resonant tank frequency. low input voltage and low lamp power, as those used in emer- Figure 22.12 shows electrical diagrams of typical voltage-fed gency lighting and portable equipment. Typical power range of resonant ballasts. this ballast varies from 5 to 30 W. Applications of this circuit

The voltage-fed version of the push–pull inverter is illus- can be found in [9, 10].

trated in Fig. 22.12a. This inverter includes a transformer, Another typical topology in this group is the current-fed which can be used to step up or down the input voltage in order push–pull inverter shown in Fig. 22.11b. In this circuit, a dc to obtain an adequate rms value of the output square wave volt- input current is obtained by means of choke inductor Le. Tran- age. This not only provides higher design flexibility but also sistors are operated with a 50% duty cycle, thus providing a increases the cost. One disadvantage is that the voltage across current square wave, which supplies the current-fed parallel transistors is twice the input voltage, which can be quite high resonant circuit formed by the mutual inductance of the trans- for line applications. Therefore, this inverter is normally used former and capacitor C. This circuit has the advantage of being for low-voltage applications. Another important drawback of relatively easy to implement in a self-oscillating configuration, this voltage-fed inverter is that any asymmetry in the two pri- avoiding the use of extra control circuits and thereby reduc- mary windings (different number of turns) or in the switching ing the cost. Also, ZVS can be obtained in the power switches. times of power transistors would provide an undesirable dc However, the switches also present a high voltage stress, about level in the transformer magnetic flux, which in turn could sat- three times the dc input voltage, which makes this topology urate the core or decrease the efficiency due to the circulation unsuitable for power line applications. This circuit is also nor- of dc currents. mally used in battery-operated applications in a self-oscillating

Figures 22.12b and 22.12c illustrate two possible arrange- arrangement. The typical power range is 4–100 W. Applications ments for the voltage-fed half-bridge resonant inverter. The based on this circuit can be found in [11, 12].

former is normally referred as asymmetric half-bridge, and it Finally, Fig. 22.11c shows a current fed full-bridge resonant uses one of the resonant tank capacitors (C1 in the figure) to inverter, which can be used for higher power rating. Also, this block the dc voltage level of the square wave generated by the circuit allows to control the output power at constant fre- bridge. This means that capacitor C1 will exhibit a dc level quency by switching the devices of the same leg simultaneously, equal to half the dc input voltage superimposed to its nor- generating a quasi-square current wave through the resonant mal alternating voltage. A transformer can also be used in this tank [13].

inverter to step up or down the input voltage to the required level for each application. In this case, the use of the series

capacitor C1 prevents any dc current from circulating through At present, electronic ballast manufactures mostly use the primary winding, thus avoiding transformer saturation. voltage-fed resonant ballasts, specially for applications supplied This topology is widely used by ballast manufacturers to sup- from the ac mains. The circuit is fed from a dc voltage source, ply fluorescent lamps, especially in the self-oscillating version, normally obtained by line voltage rectifying. A square wave which allows to drastically reduce the cost. When supplying voltage waveform is then obtained by switching the transistors hot-cathode fluorescent lamps, the parallel capacitor C2 is nor- with a 50% duty cycle, and is used to feed a series resonant mally placed across two electrodes, as shown in Fig. 22.12b, in circuit. This resonant tank filters the high/order harmonics order to provide a preheating current for the electrodes and and supplies the lamp with a sine current waveform. One achieve soft ignition. Figure 22.12c shows other version of the

B. Voltage-fed Resonant Ballasts

22 Electronic Ballasts 583

L1 Q1

D1

C1 Lamp

Lamp Q2 D2 D3 D2

(c)

(d)

FIGURE 22.12 Typical voltage-fed resonant inverters: (a) push–pull; (b)–(c) half-bridge; and (d) full-bridge.

half-bridge topology, using two bulk capacitors to provide a

50 Hz. As can be seen, the reignition voltage spike is nearly 50% floating voltage level equal to half the input voltage. In this higher than the normal discharge voltage, which is constant case, capacitor C1 is no longer used to block a dc voltage, thus during the rest of the half-cycle. showing lower voltage stress.

When lamps are operated at higher frequencies (above Finally, for the high power range (>200 W), the full-bridge

5 kHz), electrons and ions do not have enough time to recom- topology shown in Fig. 22.12d is normally used. The transistors bine. Therefore, charge carrier density is sufficiently high at of each half-bridge are operated with a 50% duty cycle, and each current reversal and no extra power is needed to reignite their switching signals are phase-shifted by 180 ◦ . Thus, when the lamp. The result is an increase in the luminous flux com- switches Q1 and Q2 are activated, direct voltage V in is applied pared with that at low frequencies, which is especially high for to the resonant tank; when switches Q3 and Q4 are activated, fluorescent lamps (10–15%). the reverse voltage −V in is obtained across the resonant circuit.

Figure 22.13b shows the lamp waveforms and I–V charac- One of the advantages of this circuit is that the switching signals teristics for the same 150-W HPS when supplied at 50 kHz. It of the two branches can be phase-shifted by angles between 0 also shows how the reignition voltage spikes dissapear, and the and 180 ◦ , thus controlling the rms voltage applied to the res- lamp behaviour is nearly resistive. onant tank ranging from 0 to V in . This provides an additional

Figure 22.14 illustrates how the voltage waveforms change parameter to control the output power at constant frequency, in a fluorescent lamp when increasing the supply frequency. which is useful to implement dimming ballast.

As can be seen, at a frequency of 1 kHz, the voltage is already nearly sinusoidal and the lamp exhibits a resistive property.

Therefore, a resistor can be used to model the lamp at high

22.3 Discharge Lamp Modeling

frequencies for ballast design purposes. However, most lamp manufactures provide only lamp data for operating at low fre-

The low frequency of the mains is not an adequate power quencies, where the lamp acts as a square wave voltage source. source for supplying discharge lamps. At these low frequencies, Table 22.2 shows the low-frequency electric data of different electrons and ionized atoms have enough time to recombine discharge lamps provided by the manufacturer and the mea- at each current reversal. For this reason, the discharge must be sured values at high frequency for the same lamps. As can be reignited twice within each line period. Figure 22.13a illustrates seen, a power factor close to unity is obtained at high frequency. the current and voltage waveforms and the I–V characteristics

The equivalent lamp resistance at high frequencies can be of a 150-W HPS lamp operated with an inductive ballast at easily estimated from the low-frequency data. Lamp power at

584 J. M. Alonso 200

Time (sec)

Time ( µsec)

Vla (V)

(b) FIGURE 22.13 150-W HPS lamp waveforms and I − V characteristics at: (a) 50 Hz and (b) 50 kHz.

any operating frequency can be expressed as follows: P LA =V LA I LA FP LA

(22.2) where V LA and I LA are the rms values of lamp voltage and

current, and FP LA is the lamp power factor. At line frequencies, the lamp power factor is low (typically 0.8), due to the high distortion in the lamp voltage waveform. However, at high frequencies the lamp power factor reaches nearly 1.0. Then, lamp voltage and current at high frequency

(V LA,hf

,I LA,hf

) can be estimated from the following equation:

I LA,hf V LA,hf =P LA (22.3) where P LA is the nominal lamp power provided by the manu-

facturer.

As can be seen in Table 22.2, fluorescent lamps tend to main- tain nearly the same rms current at low and high frequency, whereas high-pressure discharge lamps tend to maintain nearly

FIGURE 22.14 Voltage waveforms for a 36-W linear fluorescent lamp the same rms voltage. Based on these assumptions, the equiva- supplied through a resistive ballast at nominal power and different oper-

lent lamp resistance at high frequency estimated from the low ating frequencies. Vertical scale: 100 V/DIV.

frequency values is shown in Table 22.3.

22 Electronic Ballasts 585

TABLE 22.2 Electric data of different discharge lamps Lamp ∗

Manufacturer @ 50 Hz

Measured @ H.F.

rms I (A ) P (W) PF Fluorescent (TLD-36 W)

0.44 36 0.79 83.2 0.46 36 0.94 Compact fluorescent (PLC-26 W)

0.31 26 0.80 82 0.32 26 0.99 Low-pressure sodium (SOX-55 W)

0.59 55 0.86 75 0.76 56 0.98 Mercury vapor (HPLN-125 W)

0.92 120 0.99 Metal-halide (MHN-TD-150 W)

0.93 92 1.63 146 0.97 High-pressure sodium (SON-T-150 W)

1.42 148 0.99 ∗ Lamps aged for 100 h.

TABLE 22.3 Estimated electric data of discharge lamps at high frequency

V 0 =100 V Fluorescent lamps

Lamp V LA,hf

I LA,hf

R LA,hf

R LA

P 0 =1 W High-pressure lamps

P LA /I LA,lf

I LA,lf

P LA /I 2 LA,lf (Ω) 7500

V LA,lf

P LA /V LA,lf

V LA,lf 2 /P LA

Low-pressure sodium lamps neither maintain voltage nor 2500 current constant at high frequency, and they also exhibit

an equivalent resistance quite dependent on the frequency.

0 5 10 15 Therefore, their equivalent resistance can only be obtained by 20 laboratory testing.

P LA (W) Note that the values given in Table 22.3 are only an approxi-

FIGURE 22.15 Lamp resistance versus lamp power characteristic. mation to the real value, which should be obtained by mea-

surement at the laboratory. This can be used as the first starting point in the design of the electronic ballast, but final adjustments should be made at the laboratory.

characteristic of lamp in a SPICE-based simulation program. Another important issue is that the lamp equivalent resis- The voltage-controlled voltage source EL is used to model the

tance is strongly dependent on power delivered to the lamp, resistive property of the lamp. The voltage source Vs is used to which is specially important for designing electronic ballasts measure the lamp current, so that the instantaneous and aver- with dimming feature. The characteristic lamp resistance ver- age lamp current can be calculated; for this reason, its voltage sus lamp power is different for each discharge lamp type and value is equal to zero. GP is a voltage-controlled current source must be obtained by laboratory testing. One of the best possi- used to calculate the instantaneous lamp power, which is then bilities to fit the lamp resistance versus power characteristic is filtered by RP and CP in order to obtain the averaged lamp the hyperbolic approximation. For example, Mader and Horn power. Finally, the hyperbolic relationship between the lamp propose in [14] the following simple approximation:

resistance and power is implemented by means of the voltage- controlled voltage source EK. The time constant τ = RP · CP is

R LA (P LA )

(22.4) related to the ionization constant of the discharge. P LA +P 0 Figure 22.17 illustrates some simulation results at low fre-

quency when the lamp is supplied from a sinusoidal voltage where R LA is the equivalent lamp resistance, P LA is the average source and stabilized with an inductive ballast.

The Mader–Horn model can also be used at high frequen- on each lamp. This characteristic has been plotted in Fig. 22.15 cies, which makes the lamp resistive. The equivalent lamp

lamp power, and V 0 and P 0 are two parameters which depend

resistance at high frequency will also exhibit a hyperbolic vari- This model can be implemented very easily in circuit simu- ation with the averaged lamp power and with a time constant lation programs, such as SPICE-based programs. Figure 22.16 given by τ . This model is then useful to simulate electronic shows the electric circuit and the description used to model the ballast with dimming feature.

for a particular lamp with V 0 = 100 V and P 0 = 1 W.

586 J. M. Alonso

.subckt lamp 10 20 + params: V o =100 P o =1 Tau=0.3 m

+ EL 10 15 Value={V(30,20)*I(VS)} EL

VS 15 20 0 30 40 EK 30 20 Value={V o *V o /(V(40,20)+P o )}

RK 30 20 1

GP 20 40 Value={V(10,20)*I(VS)} VS

RP 40 20 1 CP 40 20 {Tau}

FIGURE 22.16 (a) Mader–Horn linear model for discharge lamps and (b) SPICE description of the model.

Rb

Lb

1 2a 2 Vg 1 0 SIN(0 325 50)

LAMP

Rb 1 2a 100

V 0 =100 V

Lb 2a 2 500mH

Vg P

τ 0 =1 W

XLA 2 0 LAMP

0 = 0.3 ms

.tran 0.1 m 60 m 0 0.1 m

50 V 100 V 150 V v(2)

i(lb)*50 v(1)

Time

i(lb)

FIGURE 22.17 (a) Example of simulation with an inductive ballast at low frequency; (b) operating waveforms; and (c) lamp I–V characteristics.

Discharge lamp modeling has become an important subject, implemented in two basic ways: current-fed resonant inverters since its results are very useful to optimize the electronic ballast and voltage-fed resonant inverters. performance. Some improvements on the Mader–Horn model and other interesting models can be found in the literature [14–16].

22.4.1 Current-fed Resonant Inverters

One of the most popular topologies belonging to this cate- gory is the current-fed push–pull resonant inverter, previously