Independent current control for a multi-string system

AN3151

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Doc ID 17051 Rev 1

4

Independent current control for a multi-string system

The aim of this control is to implement an average current control on four independent

strings in order to cover the entire power rating requirement.

The total power, 120 W, is covered using four strings up to 30 W with ten 3 W LEDs

(Everlight, EHP-AX08EL-V4) [4] connected in series.

The ADC peripheral, in terms of maximum conversion time, doesn't allow instantaneous

conversion of 4 channels and in order to minimize the error introduced by the sample and

hold circuitry a solution based on single-shot conversion is considered.

The advanced timer TIM1 starts the counting in Up/Down mode. The overflow event (crest

of carrier waveform) is used for an Update event which triggers the ADC conversion.

The single-shot conversion is enabled on a selected ADC channel. Once the analog to

digital conversion is performed, the end of conversion interrupt (EOC ISR) is served and the

PI controller changes the duty cycle according to the control law.

Moreover, the new channel to be converted at the next Update event is set and this is

repeated cyclically until the four strings are controlled.

The PI controllers, related to the other 3 channels, are not performed at this time. The four-

string control is executed in 4 different Update events.

Figure 8

shows a simple block diagram highlighting the four-string control:

Figure 8.

Multi string current control methodology

The current control loop is executed as soon as the end of conversion interrupt service

routine (EOC ISR) is served.

The Update event, which triggers the ADC for the single shot conversion, occurs once every

five PWM Periods.

Although this decreases the system bandwidth, it leaves some time to other important tasks

such as communication for remote control.