IRPLCFL5E
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9. Design Procedure to adapt the design to different lamps types
Designing with the IR2520D is very simple because it only has 2 control pins: VCO (0-5VDC oscil-
lator voltage input) and FMIN (minimum frequency setting). To modify the design for a higher lamp
power, you will need to modify RFMIN, CVCO, LRES and CRES. Make sure that the FETs and
inductors are rated for the current you need with the new lamp and that VCC is stable. To modify the
design to a lower lamp power, you will need to decrease RFMIN and, in some cases, to also modify
CVCO, LRES and CRES. In most cases you can use FETs and inductors with lower current ratings.
The FMIN pin is connected to ground through a resistor (RFMIN). The value of this resistor programs
the minimum frequency (fmin) of the IC and the starting frequency of the IC (about 2.5xfmin). The
IR2520D will work in run mode at the minimum frequency unless non-ZVS is detected. Generally, to
work with constant frequency, the minimum frequency needs to be chosen above the resonant
frequency of the low-Q R-C-L circuit. In this case, one can increase the value of RFMIN to decrease
the frequency and increase the lamp power, or, decrease the value of RFMIN to increase the run
frequency and decrease the lamp power.
The VCO pin is connected to ground through a capacitor (CVCO). The value of this capacitor
programs the time the frequency needs to ramp down from 2.5 times fmin (fmax) to fmin. One can
increase the capacitor value to increase the preheat time, or, decrease the capacitor value to de-
crease the preheat time.
The suggested design procedure is as follows:
1) Use the Ballast Design Assistant (BDA) software to calculate LRES and CRES.
Select the input configuration without PFC, select the IR2156 IC and select single lamp current
mode configuration. Select the new lamp in the database or add the lamp parameters by hand
selecting the “Advanced” option.
Calculate the operating point and chose the right values of L and C that satisfy:
1.1) Run frequency (best working range) 40-50KHz
1.2) C as small as possible to minimize losses (suggested value 4.7nF)
1.3) L values you have available
2) While measuring LO, apply 15V between the VCC pin and the COM pin and adjust the value
of RFMIN to obtain the right minimum frequency (it is suggested set fmin = run frequency
obtained with the BDA software). Increase RFMIN to decrease the minimum frequency or
decrease RFMIN to increase the minimum frequency.
3) Apply the AC input and check preheat, ignition and run states of the lamp.
3.1) If the lamp ignites during preheat, the preheat current is too small or the starting voltage
across the lamp is too big, increase the value of CRES to decrease the voltage across
