OCP8155 is a high precision constant current LED driver IC designed for offline flyback , specially applying to
constant current LED lighting within 18W output power. OCP8155 utilizes primary-side feedback technology to
achieve excellent line regulation and load regulation without TL431, optical coupling and feedback circuit, greatly
saving the system cost and size.
OCP8155 only requires 32uA start-up current, than which VCC will rise as long as the current flowing through the
start-up resistor RST is higher. The chip starts up when VCC goes up to 14V (Typical). At this time the operating
current is usually higher than the current provided by the start-up resistor, which leads to VCC decrease. The start-up
process will proceed successfully so long as the auxiliary winding can provide normal power supply to the chip before
VCC deceases below under-voltage lockout threshold.
Turns Ratio Setting
Only when the chip operates in a current discontinuous conduction mode can it keep LED current constant.
When designing the system, the chip operating in a current discontinuous conduction mode should be ensured. In
another word, the designed maximum duty cycle must be less than the inherent maximum duty cycle (58%) in the
chip. Turns Ratio is limited by two factors: the inherent maximum duty cycle and power MOSFET breakdown voltage.
First, turns ratio is considered according to the maximum duty cycle. Then duty cycle is calculated on the basis of
continuous conduction mode:
D = VOR
VBULK + VOR
According to formula (1), the maximum duty cycle working situation happens to the system when the VBULK is
lowest. Maximum VOR which is limited by maximum duty cycle is gained when duty cycle is set to 58%.
Secondly, turns ratio is considered according to the power MOSFET breakdown voltage.
The Drain-Source breakdown voltage of the power MOSFET is:
VDS = VBULK + VRCD = VBULK + k *VOR < VBK
Where, VBK is the Drain-Source breakdown voltage of the power MOSFET. The k coefficient affects the leakage
inductance dissipation. Ultra-low value of k leads to large leakage inductance dissipation and low efficiency, while
ultra-high value of k results in requiring high Drain-Source breakdown voltage of the power, so that k is usually set to
According to formula (2), the power MOSFET endures the largest Drain-Source voltage when VBULK is largest.
Maximum VOR which is limited by breakdown voltage is gained according to the Drain-Source breakdown voltage of
the power MOSFET.
By considering these two aspects above, the lower VOR is selected.
Then turns ratio of primary side and secondary side is calculated according to the following formula:
n p = VOR
According to the chip operating input voltage VCC and output voltage, the turns ratio of auxiliary-side and
secondary-side is calculated by the following formula:
Constant Current Control
IC compares CS pin voltage with internal 1V threshold voltage to set the primary-side peak current Ipkp of the
The LED output current IO is gained according to the following formula:
1 * np
* I pkp
Where, Ipkp is the primary-side peak current of the transformer, np is the number of the primary-side turns of the
transformer, and ns is the number of the secondary-side turns of the transformer.
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Ver 1.0 April. 01, 2013