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VCO

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DRIVER

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Reference

Lamp Arc Current

Regulated Lamp Current Control System

Error Amplifier

and Compensation

Fig 7. Closed Loop Dimming Control

ballasts. The input internally biased at 10 V if nothing is connected and can be reduced to 0 V by sinking

current only. Isolation between the dimming input and the ballast circuitry is provided by means of an opto-

isolator (IC4) and an additional isolated winding of the PFC boost inductor LPFC.

This winding must be isolated from the other two windings and from the ferrite cores and should

be flash tested to 4 kV. In order to obtain sufficient voltage, this winding will need to have several more

turns than the zero current detection winding. It is necessary to ensure that the voltage across the 18 V

zener diode (D3) never falls below 18 V when the ballast output is dimmed and also over the range of line

supply voltage. The next part of the circuit, centered around the programmable unijunction transistor Q1,

generates an approximate ramp waveform across capacitor C7, which peaks at approximately 10 V

guaranteeing maximum output when 10 V input is applied at the dimming input.

A trigger voltage provided by a potential divider of 9 V is applied to the gate of Q1. C7 charges

through R15 until the voltage reaches a point one diode drop above the 9 V trigger voltage. Q1 then fires

and remains switched on until the current drops below the valley current of the device which is small,

discharging C7 again. This produces a continuously oscillating ramp waveform. It is important that R15 is

sufficiently large that it does not supply a current larger than the valley current, otherwise Q1 remains on

indefinitely and the circuit does not oscillate. It necessary for this oscillator to run at a frequency of several

hundred Hz to prevent it from being a source of visible flicker.

This sawtooth signal is fed into the inverting input of a comparator. Note that this circuit makes

use of a dual comparator IC by placing both comparators in parallel so as to provide greater current sinking

capability to drive the input of IC4. However, there is no need for this provided the comparator used is

capable of sinking the opto-isolator diode current, which in this case is around 13 mA. The opto-isolator

diode current should be chosen to be as low as possible to guarantee saturation of the transistor when it

sinks 1mA and consequently R14 is as large as possible. The non-inverting input is connected to the control

input and pulled high via R20, which provides a sink current of 1 mA. C8 removes any noise that may be

picked up at the comparator input.

The comparator output is open collector providing current to the input of IC4 when low. The diode

will be continuously off when the input is at maximum and on when it is zero. Hence the opto-isolator

output transistor will be fully on when the control voltage is at minimum and fully off when it is at

maximum, providing a reasonably linear change in the duty cycle at intermediate levels. It is important in

production to ensure that the opto-isolator used is rated to the correct voltage and has been certified to the

necessary safety standards. The transistor side of the opto-isolator has the emitter connected to 0 V (COM)

and the collector connected to the IRS2158D VCC supply of 15.6 V via a pull up resistor R22. The