Achieving higher efficiency in battery chargers through primary side regulation

In recent developments, the demand for consuming electronic product market is growing rapidly. Because the international energy regulations e.g. Energy Star specifications, power supplies are required to meet the requirements. Particularly, for battery chargers that has been widely used to power the cell phone, PDA, MP3 and digital cameras, etc. However, the traditional approach of the battery charger used a secondary feedback circuitry for reaching the constant voltage output regulation (CV) and constant current output regulation (CC). The secondary feedback circuit increases the cost and size of the charger. This article shows a patented technology “Primary Side Regulation” (PSR). The PSR controller precisely regulates CV/CC of the charger in the primary side of the transformer without secondary feedback circuitry. It includes the frequency hopping mechanism to reduce EMI and the green mode function to reduce standby power loss. According to the experiment results, the PSR charger can achieve a smaller form factor, lower standby power and higher efficiency compared with conventional designs, such as Ringing Chock Converter (RCC) and traditional PWM. Furthermore, the PSR provides the most cost effective solution in the market.

Figure 1 shows a traditional flyback power supply developed for battery charging. It includes a secondary feedback circuit achieving a constant voltage control and a constant current control. The optocoupler is coupled to transfer a secondary control signal to a PWM controller in the primary side. The PWM controller is used for switching the transformer, controlling the power delivered through the transformer and achieving the output regulations. The drawback of this traditional approach is high device count, which will cause higher cost and lower reliability. Besides, the optocoupler provides a potential risk for electrical leakage. The current-sense resistor RO for constant current regulation increases the power loss of power supply.

THE BASIC CONCEPT OF PRIMARY SIDE REGULATION CONTROLLER
Figure 2 is the circuit schematic of a PSR flyback power converter. The PSR controller uses an innovative method to detect the output voltage VO information through an auxiliary winding of the transformer. Figure 3 shows the principal operation waveforms.

When the MOSFET of the PSR controller is turned on, the transformer current iP will increases linearly from zero to ipk. The ipk is shown in Equation (1). During the turn-on period of the MOSFET, the energy is stored into the transformer. When the MOSFET is turned off (toff), the energy stored in transformer will be delivered to the output of the power converter through the output rectifier. During this toff period, the output voltage VO and diode forward voltage VF will be reflected to the auxiliary winding NAUX, the voltage on the auxiliary winding NAUX can be expressed by Equation (2). A proprietary sampling technology is applied to sample the reflected voltage. The correlated output voltage information can be obtained because the forward voltage of the output rectifier becomes a constant. After that, the sampled voltage compares with a precise reference voltage to develop a voltage loop for determining the on-time of the MOSFET and regulating an accurate constant output voltage.

(1) i_pk = V_in/L_p x t_on

(2) v_aux = N_aux/N_s x (V_O + V_F)

wherein L_p is the primary winding inductance of the transformer; V_in is the input voltage of the transformer; ton is the on time period of the MOSFET; N_aux/N_s is the turn ratio of the auxiliary winding and secondary output winding; V_O is the output voltage; the V_F is the forward voltage of the output rectifier.

This sampling approach also duplicates a discharge time (t_dis) of the transformer as shown in Figure 3, the output current I_O is related to secondary side current of the transformer. It can be calculated by the signal i_pk, t_dis as Equation (3). The PSR controller uses this result to determine the on-time of the MOSFET and regulate a constant output current. The current-sense resistor RSENSE is utilized to adjust the value of the output current.



wherein tS is the switching period of the PSR controller; NP/NS is the turn ratio of the primary winding and secondary output winding; RSENSE is the sense resistance for converting the switching current of the transformer to a voltage VCS.

Realization and experimentation is a 5W charger. Its output is 5V/1A. The PSR controller is Fairchild semiconductor’s FSEZ1216, it integrates with a 600V MOSFET. To minimize standby power loss, the proprietary green-mode function provides off-time modulation to decrease the PWM frequency linearly at light-load and no load conditions, which can easily meet the most of power conservation requirements. A built-in frequency hopping function further improves EMI performance. Furthermore, a cable compensation function compensates the voltage drop at the output cable significantly improves the load regulation.

All the experimentation result as shown in Figure 4 to 7, CV regulation achieves 2.88 percent at 90VAC to 264VAC input and no load to full load. The CC regulation can achieve 1.75 percent with fold-back voltage at 1.5V. The CC regulation range is controlled by 5V~28V VDD range. This CC output performance prevents an unregulated output current when the output voltage is low. The average efficiency is 72.3 percent at 115V and 71.5 percent at 230V that meets Energy Star 2.0 Level V specification (68.17 percent average efficiency). It offers an enough margin for the mass production tolerances. The built-in frequency hopping reduces EMI and can easily pass EN55022 class B specifications.

CONCLUSION
PSR controller uses proprietary analog signal process and sampling technologies to achieve CV and CC regulations through an auxiliary winding of the transformer, which eliminates the need of the secondary feedback circuit, optocoupler and secondary current-sense resister. By using the PSR solution can reduces these secondary circuits and increase more than 1 percent efficiency in general. From experiment results using PSR controller, the charger can achieve a smaller form factor, lower standby power, higher efficiency and higher reliability.

CAPTIONS
Figure 1. Conventional secondary side regulation flyback converter.

Figure 2. Basic circuit diagram of flyback converter using primary side controller (PSR).

Figure 3. The principal operation waveform of the flyback converter.

Figure 4. CC and CV curve by using PSR controller.

Figure 5. Efficiency by using PSR controller.

Figure 6. No load standby power loss by using PSR controller.

Figure 7. EMI by using PSR controller at 230V 50Hz(maximum load).

Click here for the illustrations:

Figure1, Figure2, Figure3, Figure4, Figure5, Figure6, Figure7