Using a Modified Charge-Pump Technique to Reduce Line Harmonics

M. Selim Göksu

A solution is introduced to solve the problem of high power dissipation in the charge-pump method.

Since January 2001, EN 61000-3-2 has regulated the European line-current harmonic levels of equipment up to 16 A per phase from the public 230-V supply.¹ Active and passive power factor correction (PFC) circuits are used to comply with this standard.

When the charge-pump PFC circuit is used in flyback convertors, it provides a very cost-effective solution with enough performance to fulfill the Class D requirements of EN 61000-3-2, the class applicable to TV sets.² Because of their low cost and simplicity, conventional dc-to-dc flyback convertors are commonly found in consumer electronic products consuming less than 250 W, such as TV sets, digital versatile disc players, and satellite receivers.^{3, 4}

The basic charge-pump circuit has some heat problems related to power consumption in the switching element. The pump current that flows through the PFC capacitor (PFC cap in Figures 4 and 6) influences the voltage-current (V-I) characteristic of the metal-oxide semiconductor field-effect transistor (MOSFET) switching element and increases the temperature (caused by the switching loss) to an unacceptable level, leading to reliability problems.

The Existing Charge-Pump PFC Technique

In classic ac-to-dc convertors without PFC, the current is drawn through the mains during the charging process of the bulk capacitor, which is connected after the bridge rectifier to filter the rectified line signal (see Figure 1). This current has a pulse shape that provides a power factor of less than 1/2.

Figure 1. Basic bridge rectifier and current-voltage waveforms.

The mains voltage is rectified by the bridge rectifier and smoothed by the bulk capacitor to obtain the dc bus voltage. The dc bus voltage is switched across the primary winding of the switch-mode transformer (SMT) via the power switch Q in the flyback switch-mode (FBSM) power supply topology generally used in TV sets. A general block scheme of a one-output FBSM power supply without PFC application is shown in Figure 2.

Figure 2. General block diagram of a one-output FBSM power supply without PFC application.

An FBSM power supply used in a large-screen TV set includes an SMT with multiple windings, allowing the generation of various dc output voltages as shown in Figure 3.³ This type of switch-mode power supply is known as a multiple-output ac-to-dc flyback convertor.

Figure 3. Block diagram of an FBSM power supply for TVs.

The basic charge-pump PFC circuit shown in Figure 4 can be used to fulfill EN 61000-3-2 for large-screen TV sets with nominal power consumption (>75 W).² However, the power loss of the switching element Q₁ is quite high. According to measurements on a 33-in. TV set that consumes 230 W and uses a multiple-output FBSM power supply, the body temperature of Q₁ was 100°C, with the measurement conditions shown in Table I.

Figure 4. Flyback convertor primary-side schematic diagram with basic charge-pump circuitry.

The switch current I_Q is composed of I_p and I_{PFC cap}, as shown in Figure 4. The V-I waveform of Q₁ is shown in Figure 5 and is directly influenced by the momentary current spikes of PFC capacitor C₁. Especially at the beginning of the MOSFET power-on cycle, C₁ has almost no charge inside. Therefore, the amplitude of current spikes is more or less equal to the steady-state peak current amplitude. This behavior leads to an increase in the turn-on loss of Q₁ and its body temperature.

Figure 5. V-I characteristic of the switching element with basic charge-pump circuitry. Channel 1 shows the drain voltage of Q1 at 100 V per division, and Channel 2 shows the drain current of Q1 at 1 A per division.

Of course, the temperature of Q₁ can be limited using a variety of methods, such as increasing the size of the heat sink surface or using a switching element with a larger drain-to-source current. Investigation of the known charge-pump circuitry showed that solutions often involved changing the charging path of C₁, which would increase the required printed circuit board (PCB) surface area and the final cost of the product. To overcome the temperature rise problem of Q₁ without increasing the cost and PCB surface area, the following technique was developed.

Improving Switching Performance

To provide the same current flow to the PFC capacitor, an additional winding is added to the SMT, and the PFC capacitor is coupled to this so-called PFC winding as shown in Figure 6. The resulting switching-element current (see Figure 7) is completely independent of the PFC capacitor current.

Figure 6. Flyback convertor primary-side schematic diagram with modified charge-pump circuitry.

Figure 7. V-I characteristic of the switching element with modified charge-pump circuitry. Channel 1 shows the drain current at 2 A per division, and Channel 2 shows the drain voltage at 100 V per division.

The required current was obtained easily because of the voltage difference at the terminals of capacitor. When the switch is on, current flows from the PFC winding to the PFC coil. When the switch is off, the current flows in the reverse direction (see Figure 8).

Figure 8. PFC capacitor current (Channel 1 at 2 A per division) and VPFC voltage (Channel 2 at 100 V per division) waveforms.

According to measurements on the same 33-in. TV set, the body temperature of Q₁ was 88°C, with the measurement conditions shown in Table II. This measurement demonstrates a reduced body temperature despite a 20°C rise in the ambient temperature.

In terms of the turn-on loss of the switching element, the current spikes from the modified circuitry (see Figure 7) are negligible in comparison with the original circuitry (see Figure 5).

The redirection of I_{PFC cap} removes the need to use diode D₃, which is required in basic charge-pump circuits to prevent I_{PFC cap} from flowing from the Q₁ drain to the SMT during the charging of the PFC capacitor. At the same time, to keep standby power consumption to less than 3 W, the connection pin of the PFC capacitor to the PFC coil is changed to the cathode pin of D₁. Due to this modification in the connection of the PFC capacitor, it is necessary to use a PFC capacitor of a larger value with the same physical dimensions to retain the same current harmonic performance within the same PCB surface area. The mains current harmonic values are provided in Table III for a power consumption of 180 W, which is a standard condition.

Conclusion

The problem with the basic charge-pump circuit used in multiple-output flyback power supplies for PFC is the excessive turn-on loss of the switching element. The proposed technique contains an additional winding on the SMT and a new coupling to the so-called PFC winding (see Figure 6). By employing this topology, the V-I waveform of the switching element is reshaped, drastically reducing the turn-on loss. In addition, the proposed technique reduces the cost by removing one diode, which is not necessary for the new circuitry, and keeps standby power consumption to less than 3 W for a 33-in. TV set. Therefore, the improved version of the basic charge-pump circuit appears to be a good option for using PFC circuits to limit line-current harmonics. This technique minimizes high power dissipation and reduces line harmonics to meet EN 61000-3-2 requirements.

References

1. EN 61000-3-2, "Electromagnetic Compatibility—Part 3-2: Limits—Limits for Harmonic Current Emissions," European Committee for Electrotechnical Standardization (CENELEC), Brussels, 2000.

2. Peter Preller, "A Controller Family for Switch Mode Power Supplies Supporting Low Power Stand By and Power Factor Correction (PFC)," Infineon Technologies Application Note on AN-TDA1684X Version 1.2 (Munich: Infineon Technologies AG, 2000).

3. Keith H Billings, Switchmode Power Supplies Handbook (Maidenhead, Berkshire, UK: McGraw-Hill, 1989).

4. M Selim Göksu, "Passive Power Factor Correction (PFC) Considerations," Vest Report 1 (Manisa, Turkey: Vestel Electronics A.S, 2000).

M. Selim Göksu joined the Vestel Electronics Co. R&D hardware engineering team as a project manager in 1998. His work on the power supply design for 100-Hz TV sets is the subject of a patent application.