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collector node is then connected via R18 to CDIV and R19 to 0 V (COM). This averages the PWM signal

and provides a DC reference to connect to the IRS2158D, which varies between 0V and the maximum

voltage set. The range can be changed if necessary by adjusting the value of R19.

The value of the lamp current sensing resistor RLS will determine the feedback voltage provided

to the op-amp feedback circuit of the IRS2158D. The value of RLS should be selected to provide as large a

feedback as possible without dissipating too much power at maximum light output. It is recommended to

keep the power dissipation in RLS below 0.25 W.

A delay is usually incorporated into dimming ballasts to provide a smooth fade from one dimming

level to the next. For example, if the control inputs were to be shorted together, the lamp would fade down

to minimum brightness over a period determined by the values used. This neatly avoids any flicker that

may be caused by sudden changes in load in the PFC boost regulator section. In this case delay is provided

via the time constant of R18 and CDIV. As the frequency of the PWM signal from the opto-isolator is of

the order of tens of Hz, the delay has been designed to be long enough to ensure minimal ripple on the DC

control voltage input to the IC. This can be increased if a longer fade time is desired by making CDIV

larger.

Since the lamp arc current is being sensed with a resistor, it is necessary to use voltage mode

preheating to avoid detecting the sum of the current in the arc added to the resonant output capacitor

(CRES) current. This has an additional advantage that during preheat and prior to ignition of the lamp the

arc current will always be zero and consequently the feedback circuit will not influence the oscillator

frequency at all until the lamp is running. This means that by setting the value of RPH the preheat will

occur in exactly the same way independent of the dimming control voltage achieving optimum preheat and

ignition under all conditions.

Design of the Ballast

The initial step in designing the ballast is to determine values required for the resonant output

inductor LRES and capacitor CRES. The Ballast Design Assistant (BDA) software, which can be

downloaded from the International Rectifier web site, is an extremely useful tool in doing this. The BDA

software used should be Version 4.2.x or above, this can be determined by clicking the Help button and

clicking About Ballast Designer.

The following inputs were selected:

IC

IRS2158D

Input

90 to 265VAC/500VDC

Lamp

T5 28W

Configuration

Single lamp/Voltage mode heating

Table 2: Initial BDA inputs. Lamp type: TL5/28W, Line Input Voltage: 90-305 VAC

The next step is to click “Calculate”. This displays the operating point for the ballast and

calculates the values of the tank components, i.e. LRES = 5.9 mH and CRES = 3.3 nF.