With ENERGY efficiency and environmental protection is becoming more and more important, people expect more and more high efficiency in shenzhen standby switch power supply, the customer request power supply manufacturers of power supply products can satisfy the BLUE ANGEL, ENERGY STAR, 2000 green ENERGY such as ENERGY, standards, and the European Union to shenzhen of switch power supply requirements are: by 2005, the rated power of 0.3 W ~ 15 W, 15 W ~ 50 W ~ 50 W and 75 W switching power supply, standby power consumption to be respectively less than 0.3 W, 0.5 W and 0.75 W. And at present most of the switch power supply by the rated load into light load and standby mode, the power efficiency fell sharply, standby efficiency cannot meet the requirements. This gives the power supply design engineers proposed the new challenge.

2, shenzhen switch power supply, power consumption analysis

To reduce the standby switch power loss, improve the efficiency of standby, first to analyse the composition of switch power loss. The flyback type power supply, for example, the loss of work main show is: MOSFET conduction loss

MOSFET parasitic capacitance loss

Switch loss of overlapping, PWM controller and start the resistance loss, loss of output rectifier tube, clamp protection circuit loss, feedback circuit loss, etc. The top three loss is proportional to the frequency relation, namely is proportional to the device switch frequency per unit time.

In standby mode, the main circuit current is small, MOSFET turn-on time ton is small, the circuit work in DCM mode, hence the conduction loss of relevant secondary rectifier losses small, such as the loss is mainly composed of parasitic capacitance loss and switch are overlapping loss and start the resistance loss.

3, improve the efficiency of standby

According to loss analysis, to cut off the starting resistance, reduce the switching frequency, reduce the number of switch standby loss can be reduced, improving the efficiency of standby. The specific methods are: lower clock frequency. Work by high Frequency mode switch to low Frequency mode, such as a Quasi Resonant mode (Quasi Resonant, QR) switch to the PWM (Pulse Width Modulation, PWM) and Pulse Width Modulation switch to the Pulse Frequency Modulation (Pulse Frequency Modulation, PFM); Controllable pulse Mode (Burst Mode).

3.1 cut the starting resistance

For the flyback type power supply, control chip by the auxiliary winding of power supply, since the start on the starting resistance pressure drop to about 300 v. Set to start the resistance value of 47 k Ω, consumed power nearly 2 w. To improve standby efficiency, must be started in the channel resistance after cut off. TOPSWITCH, ICE2DS02G with internal special starting circuit, can shut down the resistance after launch. If the controller is no specific starting circuit, also can be in the starting resistance concatenated capacitance, the loss can be gradually decreased to zero after start-up. Only drawback is that the power can restart, disconnect the input voltage, can start again after the capacitor discharge circuit. While starting circuit shown in figure 1, can avoid the above problems, and the circuit power consumption of only 0.03 W. But circuit to increase the complexity and cost.

3.2 lower clock frequency

The clock frequency can smooth down or sudden drop. Smoothing down when feedback quantity is more than a certain threshold, through specific module, realize the clock frequency of the linear decline. POWER of TOPSwitch - GX and SG company SG6848 chips built in this module, can adjust the frequency, depending on the load size is shown in figure 2 SG6848 clock frequency and its feedback current relationship.

When the power to enter standby mode, Q1 closing, for increasing clock cycle

The switching frequency. Switching loss down to a lower frequency

(less than 1). L5991 CoolSet F2 series and Infineon company has integrated the functions.

3.3 switch work mode

3.3.1 QR - PWM

To work in the high frequency switch power supply of work patterns, when in standby switch to low frequency working mode can reduce standby loss. For example, for quasi resonant switching power supply (working frequency for hundreds of kHz to a few MHz), can be in when the standby switch to low frequency PWM control mode of PWM (a few tens of kHz).

IRIS40xx chip is through QR and PWM switching to improve standby efficiency. Figure 4 is IRIS4015 constitute the flyback type switch power supply, overloading, the auxiliary winding voltage, R1 partial pressure is greater than 0.6 V, Q1 conduction, auxiliary quasi resonant signal through D1, D2, R3, arrive time delay circuit, composed of C2 IRIS4015 FB, internal comparator compares the signal circuit work in quasi resonant modes. When power is in the light load and standby, auxiliary winding voltage smaller, switch off Q1, resonance cannot transmit the signals to FB, FB voltage is less than the chip inside a threshold voltage, can't trigger a quasi resonant mode, the circuit is working on a low frequency PWM control mode.

3.3.2 rainfall distribution on 10-12 PWM and PFM

For rated power when working in PWM mode switching power supply, and can also be to switch to the PFM mode to improve standby efficiency, namely fixed opening time, adjust the turn off time, the lower the load, turn off the longer working frequency is lower. Figure 5 is controlled using NS company LM2618 Buck converter circuit and PWM and PFM control were used respectively to measure the efficiency curve. The figure shown, when the light load using PFM mode of the efficiency of the power supply is bigger than the efficiency of using PWM mode, and the lower the load, PFM, the more obvious efficiency advantage. Add the standby signal on its PW/pins, under the condition of rated load, this pin is high level, circuit works in PWM mode, when the load is below a certain threshold, the pin is pulled as low level, working circuit in PFM mode. Realization of PWM and PFM switch, also improve the efficiency of light load and standby power supply.

By lowering the clock frequency and switching work pattern implementation reduce standby working frequency, improve the efficiency of standby, can keep the controller has been in operation, in the whole load range, the output can be properly adjusted. Even if the load from zero surged under the condition of full load, quick response, and vice versa. Output voltage drop and the chongzhi remain within the scope of the permit.

3.4 controllable pulse Mode (Burst Mode)

Controllable pulse Mode, the control Mode can also be called jump Cycle (Skip Cycle Mode) refers to when in light load or standby condition than PWM controller clock Cycle by Cycle of signal control circuit, a link, the PWM output pulse of periodic or failure effectively, as shown in figure 6. This constant frequency can be realized by reducing the switch frequency, increasing the duty ratios to improve the efficiency of the light load and standby. The signal can be added in the feedback channel, the PWM signal output channel, PWM chip enable pin (such as LM2618 L6565) or internally, chip module (such as NCP1200 FSD200 L6565 and TinySwitch series chips).

NCP1200 module's internal cycle structure as shown in figure 7, when feedback detection feet FB voltage below 1.2 V, the value is programmable, skipping cycle comparator control Q triggers, to close the output number of clock cycles, which is over several cycles, the lighter load, skip the cycle. To avoid the audio noise, only in the peak current down to a set value, skipping cycle model is valid.