Product Pages:
 BUL704 BUL705 BUL804 BUL805
The low-cost lighting market requires PFC converters to provide simple, cost-effective solutions without sacrificing PF and THD levels.
Passive solutions assure neither Vout regulation against supply voltage variations nor protection on the DC/AC converter during start up and overload conditions. ST is now offering an easy to implement solution which guarantees the previous performance and also targets the low-cost HF ballast market up to 80W, making the PF correction without using any IC to generate the PWM signal. |
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Bipolar PFC in HF ballast |
| Simple Cost Effective Solution |
The proposed active PFC with Bipolar transistors adopts the Boost topology working in discontinuous conduction mode. This is the most simple and cost effective solution for 220V and 120V mains at low to medium power levels. In this case no IC is used to generate a PWM signal, but the physical relation (tS, IC) of any power bipolar transistor is exploited when the base current IB value is kept constant.
| PWM by Storage Time Modulation |
The following graph shows two different storage time values at two different input Vac values: at t = t1 the bipolar reaches a higher saturation level than that at t = t2, and this means ts1> ts2. The overall switch on time is given by the sum of “IBON time” plus the storage time, therefore keeping the “IBON time” constant, the duty cycle changes according to the ts modulation.
Storage time modulation effect
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This natural duty cycle variation generates an appropriate PWM signal used to control the PFC stage and reduces the Imain distortion.
The negative feedback network further controls the duty cycle modulation by modifying the total charge injected into the base of the main transistor.
This duty is accomplished through the low voltage/high hFE STX724 which reduces the Q1 IBon amplitude and duration, mainly close to the peak value of the VAC wave.
The of the solution proposed achieves satisfactory THD values, (about 10%) inside the range ±20% Vmains.
THD versus AC mains variation
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| Start-Up and Overload Protection |
The feedback block also performs the lamp power regulation and the system protection phase thanks to a smart combination of three inputs:
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A voltage signal coming from the line (Input 1). |
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A voltage signal coming from PFC Vout (Input 2). |
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A voltage signal proportional to lamp current (Input 3). |
The Output signal drives the Q1 main switch.
Input 1 is used to reduce the THD by improving the storage time modulation.
Input 2 helps to regulate the PFC Vout against supply voltage variations.
Input 3 is useful as start up/overload protection. |
Feedback block
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Low spread of hFE and tS parameters |
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Large RBSOA |
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High BVceo |
As a main characteristic, these devices show a hFE curve optimized to perform the proper modulation of the duty cycle necessary to achieve the sinusoidal input current.
In addition, since the BUL804 and BUL805 are designed for a 220V mains, they guarantee high breakdown voltage (BVceo = 450V) and large RBSOA to sustain high energy levels in turn-off during lamp start-up at high Vac values.
Vmains
Power |
120
[V] |
220
[V] |
| Up to 40W |
BUL704 |
BUL804 |
| Up to 80W |
BUL705 |
BUL805 |
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