S ecial Re crt - Li htin S stems Simple and cost effective dimming solutions Dimming of fluorescent lamps will normally require the incorporation of a complex, high pin count controller IC into the circuit design. As a result, dimming circuits are more difficult to create than nondimming alternatives, needing a larger number of components, taking up more board space, and raising the system's total price tag. By Tom Ribarich, Director, Lighting Systems, International Rectifier, EI Segundo, California T hese issues are compounded still further when Bcotwap vee 11--.----------------------,FET.----, considering compact fluorescent lamps, due 15.6'1 eOM 2 to the small form factors involved and the need to be highly cost effective. Here we look at a non-dimming ballast control based on an 8-pin controller Ie and 5 LO explain how dimming functionality can be added without needing to increase the pin count. With an existing J '!N 8-pin, non-dimming lighting ballast control- 5V 'FIotI""~ n.RrI.UN FMIN 3 ler (for example, the IR2520D from Interna- All values are typical Figure 1: Block diagram of the IR2520D non-dimming ballast IC tional Rectifier, shown in Figures 1 and 2) only two pins are cuit, current charges the vce until it for the IC, maintaining the VCC at preheat, ignition and running require- reaches the internal UVLO+ threshold. the internal clamp voltage. A small ments of the fluorescent lamp. The re- When vee goes past this threshold, internal current source at the VCO maining pins perform standard func- the IR2520D enters frequency sweep pin slowly charges up an external used (FMIN and VeO) to deal with tions such as Ie supply and ground mode, the gate driver outputs (LO capacitor causing the voltage on the (pins vee and COM), plus high- and and HO) and the half-bridge circuit VCO pin to ramp up linearly. This in low-side gate drive for the half-bridge then starts oscillating at the maximum turn ramp downs the frequency of the (pins LO, VS, HO and VB). frequency. The charge-pump circuit gate driver outputs (LO and HO), and then becomes the main supply circuit the half-bridge switching circuit from Within a non-dimming ballast cirwww.powersystemsdesign.com 27 S ecial Re art - Li htin S stems Symbol Description vee Supply voltage COM Ie power and signal ground FMIN Minimum frequency setting veo Voltage controlled oscillator input LO Low-side gate driver output VS High-side floating return HO High-side gate driver output VB High-side gate driver floating supply vee [[ ~ [[J :::0 I\) II COM - UI VB mHO I\) FMIN [I veo ~ -0 C en till VS m LO Figure 2: Lead assignment for IR2520D its maximum starting value. The lamp can be achieved by using operating of the AC+DC signal at the DIM pin, voltage increases as the frequency frequency to control the current being until the valley reaches COM again. ramps down towards the resonance applied to the lamp. As the frequency If the DC reference is decreased, the frequency of the high-Q, under- of the half-bridge is increased, the valley will decrease below COM. The damped output stage. The VCO pin gain of the resonant tank circuit de- feedback circuit will then increase the voltage continues to increase and the creases and the lamp current lowers. frequency to lower the resonant tank frequency keeps decreasing until the It is possible to regulate the lamp gain until the valley reaches COM lamp ignites. The output circuit then current to a dimming reference level again. The IR2520D's FMIN pin, used becomes an over-damped, low-Q by continuously adjusting the half- to program a single running frequen- circuit. The VCO voltage increases, bridge frequency through closed-loop cy, has now been replaced with a causing the IC to enter run mode. feedback circuit. Dimming is enabled DIM pin, which measures the AC+DC The frequency level stops decreasing by combining the AC lamp current signal for dimming. once the VCO pin surpasses 5V and measurement with a DC reference stays at the minimum frequency as voltage at a single node. The AC lamp programmed by an external resistor current is measured across sensing offers a complete 8-pin solution that on the FMIN pin. resistor RCS and coupled onto the contains all dimming ballast functions. DC dimming reference via feedback The VCO pin includes the frequency capacitor CFB and resistor RFB. sweep timing control for preheat and With demand for dimming functionality becoming ever greater, but engineers not wanting to sacrifice ignition, and also programs the loop The feedback circuit regulates the speed for the dimming feedback cir- the advantages of compact low pin valley of the AC+DC signal to COM count devices like the IR2520D, a way as the DC dimming level is raised or to control dimming through the pins lowered by continuously adjusting the When a voltage is first applied to cuit during dim mode. already available needed to be found. half-bridge frequency. This causes the VCC (14V, typical) the IC exits UVLO As the VCO pin is required to perform amplitude of the lamp current to then mode and enters preheat/ignition the necessary frequency sweep for increase or decrease so that dimming mode. The half-bridge begins oscil- preheat and ignition, the FMIN pin was can be carried out. If the DC reference lating at the maximum frequency and left as the only viable option through is increased, the valley of the AC+DC the internal current source at the VCO which this could be accomplished. signal will rise above COM and the pin begins charging up an external feedback circuit will lower the fre- capacitor (CVCO) linearly from COM. Dimming control through a single quency in order to enlarge the gain of The output frequency decreases as pin the resonant tank. This will raise the the VCO voltage increases and the lamp current, as well as the amplitude lamp filaments are preheated by sec- The dimming of a fluorescent lamp 28 The IRS2530D dimming control IC Power Systems Design Europe May 2010 S ecial Re crt - Li htin S stems 220V If AC Line I~H '''A Input N ICI . ,, cr CRUS "IN 35O\f ", LAESc4 UrnH EF20 :xl en f',.) COIM 1(1nF .---+- -+ -+-...J CDC 47f1F 400V c.:n w o DIM 3 RMAX airlrsp::lmm o CVC02.2nF .-II--<t+-f":_Q_ _-Tr,Li"l----<l........IVv--li+t ~~~ +-~ ... 'IXIK I~ Potentiometer POIM Dimming 10K POI ~ ..t+---____f----+-t"""--""'--<t----ll-...A,f-,M SlnogleTl/tI'l Input CfB RFB RMIN o.luf .3Ofl I~ ,K ALMPI "'" Figure 3: Dimming mini-ballast circuit using the IRS2530D ondary windings from the resonant NUB, DCP1 and DCP2) takes over as tank inductor. As the VCO voltage the IC supply once the half-bridge charges up, the frequency decreases begins to oscillate. The resonant tank nent count. There is a clear need for simple towards the resonance frequency circuit (LRES and CRES) provides and cost effective dimming solu- of the resonant tank circuit and the the transfer function for generating tions which take up the minimum of output voltage across the lamp in- the high voltages needed for lamp real estate and do not require a large creases. The lamp ignites when the ignition and low-pass filtering for number of components. The 8-pin output voltage exceeds the lamp igni- dimming. Secondary windings from IRS2530D offers the means to de- tion threshold voltage, lamp current the resonant inductor (LRES: A, B) velop dimming circuits in a quick and begins to flow, and the IC enters dim are used to heat the filaments of the unproblematic manner. Furthermore, mode. lamp during preheat and dimming, it has the potential to bring dimming and also separate the lamp current features to a broader spectrum of from the filament current allowing applications, thus allowing marked dimming mini-ballast circuit is de- for a single current-sensing resis- energy savings to be realized. scribed in Figure 3. It is designed tor (RCS) to be utilized for sens- to run from a 220VAC line and to ing the lamp current. The AC lamp drive a 25W compact fluorescent current measurement across RCS A schematic showing a complete lamp. The 220VAC/50Hz line input is coupled to the DIM pin through a voltage is full-wave rectified (BR1) feedback capacitor and resistor (CFB and then goes through the EMI filter and RFB). A potentiometer dimming (CF and LF) before being smoothed input circuit is used (PDIM, RMIN, by the DC bus capacitor (CBUS). RMAX) to convert the potentiometer The half-bridge switches (MHS and resistance to the dimming reference MLS), which are controlled by the voltage for the IRS2530D through IRS2530D, allow preheating, ignit- the DIM pin. Protection against bal- ing and dimming of the lamp. RVCC1 last fault conditions (failure to strike, and RVCC2 provide the micro-power open filament, and low AC linel start-up current for the IC's VCC brown-out) are incorporated into the supply, and the charge pump (CS- IRS2530D to further reduce compo- www.powersystemsdesign.com www.irf.com 29