Stacked buck converter for HID lamps

D.H.J.van Casteren, M.A.M. Hendrix Department of Electrical Engineering Technical University of Eindhoven

P.O. Box 513 5600MB Eindhoven

The Netherlands

Abstract—This paper presents a low-cost electronic lamp driver design for HID burners based on a stacked buck converter topology. This circuit is operated in zero voltage switching mode to improve the efficiency and miniaturization.

Keywords – electronic ballast; synchronous buck converter; synchrone commutation

I. INTRODUCTION The metal halide lamp has become very popular as a

practical light source for general and specific applications. Its application range has been greatly expanded, especially in the last ten years. This growth can be attributed to its high efficacy (measured in lumens per watt) as a general purpose light source. It is now widely used for interior and exterior lighting of large scale facilities. Another reason for its increased use is its applicability to interior retail applications through the creation of compact low-power lamps with superior colour rendering properties. As a result, metal halide lamps have been able to replace incandescent lamps, formerly the only option for many shop lighting applications.

Gas discharge lamps cannot be operated directly from the mains. Since the early days of discharge lighting magnetic ballasts have been used for lamp stabilisation. Above all such ballasts are simple and cheap. Progress in electronic components has opened the door for more advanced solutions of the stabilisation problem, now some decades ago, which allowed gas discharge lamps to be operated at frequencies different from that of the 50 or 60 Hz mains. Various options exist. For fluorescent lamps it has been clear from the beginning that HF operation at a frequency somewhere above 20kHz was far the most attractive approach. For metal halide lamps it is well known that instabilities of the arc over a wide frequency range exclude ballasts of simple design such as those used for fluorescent lamps.

For electronic HID lamp operation, LFSW (Low Frequent Square Wave) is the most used method. A LFSW current is, from circuit point of view, certainly not the most simple way to drive a HID lamp, but to operate a variety of existing HID lamps in the market, it is certainly the option with the smallest risks [1,2]. The square wave frequency is preferably chosen higher than the line frequency (mostly in the range 70 – 400 Hz).

The existing three-stage (boost, down, full bridge) electronic driver concept for HID lamps is more than a decade

old [3]. In the meantime a market pull, driven by the need for compactness and cheaper electronic ballast’s, initiated a new race between competitors in lighting electronics business.

Among several possible LFSW topologies two-stage drivers [4] provide excellent performance with high efficiency. Presented is a low-cost electronic lamp driver design for HID burners based on the stacked buck converter topology. This output stage combined with a boost stage operating as power factor corrector delivers the desired two stage HID driver as depicted in figure 1.

Figure 1. Two stage lamp driver topology

II. OPERATION MODE In HID drivers there is a need for improved efficiency.

Improving efficiency at higher frequencies requires both: reduction in conduction losses and minimization of the switching losses. Therefore, zero-voltage-switching (ZVS) topologies are interesting due to their extremely low switching losses during each switching transition [5,6]. The stacked buck topology is operated in critical discontinuous (transition) mode to allow zero voltage switching. The ZVS condition is only possible when relation (1) is fulfilled. Remark: Usupply equals the pre-conditioner output voltage.

( )supply21

lamp0 UUZVS ≤≤⇒ (1)

Thus zero voltage switching can be achieved within a lamp voltage range between zero and half the supply voltage, in practice this is always the case. In the basic topology, every low frequent half period one MOSFET is operated in combination with a (body) diode of the other side. The slow reverse recovery of the internal body diodes cause large

T1

T2

D2

Lignitor

Cs

Cs

lamp

T3

Lhbfc

Lup

Cr

Pre-conditioner(boost converter)

Lamp driver (stacked buck converter)Bridge rectifier+ RFI - filter

Cs = buffer capacitor

D1

T1D4

D3

D5

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switching losses [7,8]. To minimize the switching losses the slow body diode must be disabled. The classical way is to use schottky diodes in series with the MOSFETS in combination with fast anti-parallel diodes. Disadvantage of this construction is the extra number of power components and an increase in the conduction losses.

A solution is to use the low ohmic MOSFET channel bi-directionally. The MOSFET channel now replaces the body diode function and the stacked buck is operated in synchronous switching mode. Remark: A MOSFET can be switched off very fast compared with the body diode reverse recovery and the losses are eliminated.

The topological stages of the transition mode within one switching cycle are showed in figure 2 and described as follows:

Stage 1: MOSFET T2 starts conducting and the inductor current increases.

Stage 2: When the on time is expired or peak current is reached T2 is switched off and the parallel capacitor Cp takes over the inductor current. The capacitor voltages rise fast.

Stage 3: Bode diode D1 take over the inductor current when the capacitor voltage equals the supply voltage for a short time.

Stage 4: Then MOSFET T1 can be switched on (ZVS) while the body diode is conducting. In this phase the inductor current decreases further and crosses zero. The control circuitry must switch off the MOSFET now and an inherent time delay cause a small negative current.

Stage 5: When the MOSFET is switched off the capacitor Cp takes over the inductor current. The capacitor discharges and the voltage across it decrease rapidly.

Stage 6: When the capacitor voltage equals zero the MOSFET T2 body diode takes over the negative current. Now MOSFET T2 can be switched on again with ZVS (Zero Voltage Switching) and stage 1 start again.

Stage 1

Stage 2

Stage 3

Stage 4

Stage 5

Stage 6

Figure 2. Topological stages of the transition mode stacked buck converter within one switching cycle.

III. LF COMMUTATION To produce a low frequent square wave current on the

output terminals the role of the two switches must be changed every half low frequent period part. To be sure of correct ZVS during the low frequent commutation, to minimize the switching losses, this process is synchronized with the high frequency switching action. Zero current is the optimal moment to change the role of the MOSFETS. This eliminates hard switching during a low frequency commutation and make the current reversal as fast as possible, without an additional dead time [9]. Most control functionality is implemented in digital building blocks to make the control part applicable for high frequency operation.

Figure 3. Current waveform

T1

T2

D1

D2Cp

L R

C r

C s

Cs

T1

T2

D1

D2Cp

L R

C r

C s

Cs

T1

T2

D1

D2Cp

L R

C r

C s

Cs

T1

T2

D1

D2Cp

L R

C r

C s

Cs

T1

T2

D1

D2Cp

L R

C r

C s

Cs

T1

T2

D1

D2Cp

L R

C r

C s

Cs

Current I [A]

T-on

T-off

T1 T2 T1 T2 T1 MOSFET T1/T2

T-off

T-on Current I [A]

T-on

T-off

T1 T2 T1 T2 T1 MOSFET T1/T2

T-off

T-on

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The inductor current waveforms for both parts of a low frequency period are shown in figure 3. Between the zero current detection (ZCD) moment and the switching action a small delay is introduced mainly by the control circuitry. This small delay causes a small negative current at the end of a high frequent cycle. In contrast with a normal buck converter this topology doesn’t need this negative current to provide ZVS switching. It can be concluded that for optimum efficiency this logic delay must be as small as possible.

IV. CYCLE BY CYCLE SWITCHING To operate the proposed topology in transition mode with

ON time control theoretical only one input signal is needed, but in practice two input signals are necessary. First an accurate ZCD signal is necessary to achieve transition mode operation. Secondly a coarse peak current measurement is needed. Directly after commutation very high current peaks flow through the coil, caused by recharging of the output filter capacitor. To avoid saturation of the buck coil, additional peak current control is required. Further the current must be reduced extra in the run up phase and during a short-circuit situation. The lamp current is sensed indirectly to avoid differential measurements. An auxiliary winding on the buck inductor is used to reduce the ON time and related current.

A. Zero Current measurement A fast-saturated small ferrite “toroid” transformer delivers

pulses for the ZCD as displayed in figure 4.

Figure 4. Saturated transformer signals

• Ch2 [10V/div.] = Transformer output voltage (pulse signal) • Ch4 [50mA/div.] = Buck inductor current = primary transformer current

(diagonal line)

One benefit of this method is the presence of the signal flank before the actual zero crossing. This is used to eliminate most of the inherent control delay. Further the detection signal can be galvanic isolated coupled with the control circuitry.

The ZCD must function for both positive and the negative part of the low frequent commutation period. An additional

circuit is needed to create a positive flank independent of the current direction as showed in figure 5. Also the maximum voltage is clamped by means of a zener diode to protect the control input against too high voltages.

Figure 5. ZCD rectifier circuit

B. T-on Control In full electronic ballasts to drive HID lamps the power

level during steady state and the run-up current must be controlled. More precisely, in steady state the power must be held in a narrow power band independent of the lamp voltage during steady state operation. Also in the run-up phase the current must be limited to fulfill the given lamp specification. Normally a feedback loop is used to control the lamp power and run-up current [10]. In this stacked buck topology the power control is based on a nearly constant ON time of the active switch. The lamp power is then defined as follows:

( )

constant2

22

lamplampsupply21

=

⋅−⋅=

Lt

LtUUUP

on

onlamp (2)

The T-on control gives a parabolic power curve. Although the power is not constant over the voltage axis it is acceptable flat within the normal lamp voltage range. A practical measurement in section VI will validate the control behavior.

V. CONTROL CIRCUIT The control part to achieve transition mode operation is

build up around a standard flip-flop (latch1), see also figure 6. The ZCD signal is connected with the positive edge triggered clock input, which in combination with a logic “1” on the D input delivers a set signal. The maximum OFF time is connected with the asynchrony set input. Finally the ON-time and the current peak input are combined in a logic OR function and linked with the asynchrony dominant reset pin.

The ON-time timer is build up around a comparator, the OFF-time timer is an equivalent design. The peak current detect and zero current detect inputs as described are located on the right side of the control scheme.

Imain [A]

D1

D2

Z

R

Zero current detect signal

Toroidcore

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Figure 6. Control circuit part 1: input

The output pin of the latch1 is connected with a glitch suppression circuit as given in figure 7. During a low frequent commutation a glitch appears on the output of latch 1. The glitch prevention circuit locks the output and suppress this unwanted spike. An oscillator for the low frequency commutation is also implemented and is synchronized with the switching process to get an impeccable low frequency commutation process.

& =1

1

=1

> CLK D Q

nQ

> CLK D Q

nQ

nS

nR

> CLK D Q

nQ

& Q

nQR S

1

Output signal1

HI/LO

commutator

latch1

delay

latch2

D-flipflop

Q_extern

Q_intern

Commutationsignal

oscillator

Figure 7. Control circuit part 2: commutation

VI. EXPERIMENTAL RESULTS A prototype is build up to verify the stacked buck converter

topology in ZVS mode, and measure the overall efficiency during steady state. The front-end pre-conditioner operates in critical mode at 150kHz average. The same switching frequency is chosen for the lamp driver. This stage delivers the power to the lamp at a very low frequency of about 130 Hz. Finally the igniter is designed straight-forward with a ignition transformer and a SIDAC [11].

In the first oscilloscope plot (figure 7) the saturated toroid zero current measurement principle is displayed. Visible is the double zero crossing in the buck inductor current. This double zero crossing also delivers two zero current signal pulses. The edge-triggered flip-flop only responds on the first positive

edge. This positive flank is used to set the flip-flop, which results in an increasing main current. After the ON-time is expired the cycle starts over again. Further the commutation process shows a smooth transition, without hard switching, as result of the synchronized commutation process.

Figure 8. Measurement ZCD operation

• Ch1 [5V/div.] = Synchronized commutation signal • Ch2 [5V/div.] = Logical ZCD signal • Ch3 [5V/div.] = Rectified input ZCD signal • Ch4 [2A/div.] = Buck inductor current

Practical measured output power and current as function of the output voltage are displayed in figure 8 (the ballast characteristics is measured with a variable resistive load).

Figure 9. Practical power / current measurement

• Power curve Plamp = f(Ulamp) • Current curve Ilamp = f(Ulamp)

Finally measurements are done with a Philips CDM 70W/830 burner, the lamp current and voltage are given in figure 9. Measured is an overall efficiency of the circuit at 220 Vac input of 92.5%.

C2

C1

> CLK D Q

nQ

nS

nR

1

I1

+ -

CP1

+ -

CP2 I2

1

Current Peak input

1 +

+

latch1 Q1

nQ1

Q_intern

T-off time

T-on time

Zero current detect input

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Figure 10. Lamp current and voltage

• Ch3 [50V/div.] = lamp voltage • Ch4 [1A/div.] = lamp current

VII. CONCLUSION The stacked buck topology combined with a pre-

conditioner stage enables the miniaturization of full electronic drivers for HID. Smart MOSFET control creates permanent zero voltage switching and result in an overall efficiency of 92.5%. This enables high power density.

ACKNOWLEDGEMENT This work is supported by Philips Lighting (Lighting

Electronics).

REFERENCES [1] R. Keijser, “Electronic operation of HID lamps”, LS9, Ithaca 2001, pp.

103-111. [2] H.J. Faehnrich, E. Rasch, “Electronic ballast for metal halide lamps”,

Journal of the Illuminating Engineering Society, vol. 17, Summer 1988, pp 131-140.

[3] J. Melis, “A power controlled current source, circuit and analysis”, IEEE APEC Conf. Rec. 9th, vol.2, 13-17 Feb. 1994, pp. 856-861.

[4] M. Shen, Z. Qian, F. Z. Peng, “Design of a two-stage low-frequency square-wave electronic ballast for HID lamps”, IEEE Transactions on Industry Applications, vol. 39 Issue: 2, March-April 2003, pp. 424-430.

[5] V. Vorpérian, “Quasi-square-wave converters: topologies and analysis”, IEEE Transactions on power electronics, vol. 3, april 1988, pp. 183-191.

[6] J. Zhou, F. Tao, F.C. Lee, N. Onishi, M. Okawa, “High power density electronic ballast for HID lamps”, IEEE Conf. Rec. 37th IAS, Vol. 3, Oct. 2002, pp. 1875-1880.

[7] L. Saro, K. Dierberger, R. Redl, “High-voltage MOSFET behavior in soft-switching converters: analysis and reliability improvements”, IEEE INTELEC. Conf. Rec. 20th, Oct. 1998, pp. 30-40.

[8] A. Fiel, T. Wu, ”MOSFET failure modes in the zero-voltage-switched full-bridge switching mode power supply applications”, IEEE APEC 16th, Vol. 2, March 2001, pp. 1247-1252.

[9] J. Zhao, M. Shen, M. Chen, Z. Qian, “A novel low-frequency square wave electronic ballast for low-wattage HID lamps”,IEEE IAS Conf. Rec. 38th, vol. 1, Oct. 2003, pp. 321-324.

[10] Y. Jiang, J. Zhou, Z. Qian, ”A novel single stage single switch PFC converter with constant power control for ballast for medium HID lamps”, IEEE Industry Applications Conference, vol.5, Oct. 2000, pp 3415-3418.

[11] J. Garcia-Garcia, M. Rico-Secades, E.L. Corominas, J.M. Alonso, J. Ribas, J. Cardesin, A.J. Calleja, “Using solid-state over-voltage protection devices for high intensity discharge lamps ignition”, IEEE IAS Conf. Rec. 37th, vol. 1, Oct. 2002, pp. 363-368.

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