ST AN2349 APPLICATION NOTE

AN2349

Application note

Simple cost-effective PFC using Bipolar Transistors

for low-to-medium power HF Ballasts

Introduction

This note deals with the implementation of a Power Factor Correction (PFC) in a Discontinuous-mode Boost Converter where a PFC stage is achieved with a power bipolar transistor driven in self oscillating configuration. The new solution proposed exploits the physical relation (t (PWM) signal in a Boost Converter.

, IC) of any bipolar transistor to achieve the Pulse Width Modulation

June 2006 Rev 1 1/30

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Contents AN2349

Contents

1 PFC solutions for low-medium power HF Ballasts . . . . . . . . . . . . . . . . 5

1.1 Application description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Feedback block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Selection of boost output inductor L1 . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1 Selection of boost output capacitor C4 . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4 PFC driving network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.1 Feed-Back block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5 T Transformer and L1 inductor specifications . . . . . . . . . . . . . . . . . . 23

5.1 220V design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.2 120V design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

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AN2349 List of tables

List of tables

Table 1. 40W Demoboard 220V bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Table 2. 40W Demoboard 120V bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Table 3. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3/30

List of figures AN2349

List of figures

Figure 1. Valley Fill circuit schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 2. Valley Fll input current waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 3. Active PFC with IC and MOSFET in boost topology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 4. Base schematic of Bipolar PFC in HF ballast voltage Fed . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 5. Ts modulation in bipolar PFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 6. Imain achieved using the basic Bipolar PFC shown in Figure 4 . . . . . . . . . . . . . . . . . . . . . . 7

Figure 7. Detail of storage time value and Ic in t2 istant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 8. Detail of storage time value and Ic in t1 istant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 9. Complete electrical schematic of the Bipolar PFC in HF Ballast . . . . . . . . . . . . . . . . . . . . . 9

Figure 10. PFC stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 11. Feed-back block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 12. PFC waveforms with Feedback block working . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 13. Imain achieved by the proposed bipolar PFC solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 14. Detail of Storage time value in t2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 15. Detail of storage time value in t1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 16. Pre-heating @ 220V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 17. Current on the electrolytic capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 18. Inductor current with di/dt>0 and transformer voltage shape . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 19. Inductor current with di/dt=0 and transformer voltage shape . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 20. Inductor current with di/dt<0 and transformer voltage shape . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 21. Transformer Vout shape and base current shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 22. Collector current and base current shape. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 23. Detail of T1 total charge during Ton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 24. 40W demoboard electrical schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 25. 40W demoboard PCB layout and mounting components. . . . . . . . . . . . . . . . . . . . . . . . . . 25

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AN2349 PFC solutions for low-medium power HF Ballasts

1 PFC solutions for low-medium power HF Ballasts

The Valley Fill circuit is an example of a low-cost passive PFC available on the market.

Figure 1. Valley Fill circuit schematic diagram

DC-AC

CONVERTER/

INPUT

RECTIFIER+PFC+DC

FILTER BLOCK

Figure 2. Valley Fll input current waveform

CONVERTER/

BALLAST

LAMP

The capacitors are charged in serie, and discharged, via the two diodes, in parallel. Current is drawn from the line from 30° to 150°, and then from 210° to 330°. Discontinuities occur from 150° to 210° and from 330° to 360°, and then the cycle repeats itself.

Disadvantages of this PFC solution are spikes on input current waveform and large zero current gaps between the half sinusoidal wave and the next one (meaning a lower power factor and high input current distortion), and high ripple in the DC output voltage that causes poor performance in High Power Lamps. On the other hand, high performances can be achieved by IC driver optimized for controlling PFC regulators in boost topology as shown in

Figure 3.

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PFC solutions for low-medium power HF Ballasts AN2349

Figure 3. Active PFC with IC and MOSFET in boost topology

The proposed Bipolar PFC solution targets the low-cost HF Ballast market up to 80 W as it provides a simple cost-effective solution without sacrificing THD and PF levels. It does not need any ICs to achieve the PWM signal since it uses just a power bipolar transistor and a closed-loop feedback that performs the duty cycle modulation and a satisfactory output power regulation.

1.1 Application description

The active PFC solution with Bipolar transistor adopts the Boost topology working in Discontinuous Conduction mode. This is the most simple and cost-effective solution for 220V and 120V mains and low\medium power.

Figure 4. Base schematic of Bipolar PFC in HF ballast voltage Fed

No IC is used to generate a PWM signal, but the physical relation (t

, IC) of any power

bipolar transistor is exploited when the base current IB value is kept constant.

Figure 5 shows two different storage time values at two different input V

bipolar reaches a higher saturation level than in t

The overall switch on time is given by the sum of "I therefore, if the "I

time" is constant, the duty cycle changes according to the ts

BON

modulation. This natural duty cycle variation generates an appropriate PWM signal to

6/30

, and this means tS1>tS2.

time" plus the storage time,

BON

values: in t1 the

AN2349 PFC solutions for low-medium power HF Ballasts

control the PFC stage and reduces the Imain distortion achieving a THD in the range of about 30%, with a shape of the current drawn from the main as shown in Figure 6.

Figure 5. Ts modulation in bipolar PFC

IIN

IL=I

Figure 6. Imain achieved using the basic Bipolar PFC shown in Figure 4

Imain

Vce

Figure 7 and Figure 8 show in a real situation, what has been explained before.

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PFC solutions for low-medium power HF Ballasts AN2349

Figure 7. Detail of storage time value and Ic

istant

in t

Injected charges

Storage time

Vce

The PWM signal acts on T1 bipolar transistor base through an auxiliary winding T on the transformer normally used in the ballast.

Figure 8. Detail of storage time value and Ic

in t1 istant

Injected charges

Storage time

Vce

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AN2349 Feedback block

2 Feedback block

The duty cycle modulation performed by the Basic Solution shown in Figure 4 is not enough effective to achieve high THD values and no protection task can be implemented against overoload or high VAC values.

A negative feedback network has been introduced to further control the duty cycle modulation by modifying the total Q

Chapter Figure 9. on page 9 shows the complete solution of the proposed PFC stage.

Figure 9. Complete electrical schematic of the Bipolar PFC in HF Ballast

charge which is injected into the T1 base.

The feed-back block in Figure 11 changes the T amplitude and duration through the intervention of the transistor T proposed network by the T duty cycle of the main switch (T

conduction reduces the base current permitting to reduce the

) performing a further THD correction and output power

charge by modifying both the I

1 QON

. In particular the

regulation.

Figure 10. PFC stage Figure 11. Feed-back block

Feed-Back

Block

Input 2

Input 3

Input 1

Output

R13

Ds D8

Dz3

Dz1

Input 1

Output

R14

Input 2

Input 3

BON

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+ 21 hidden pages