In today's era of power management and energy conservation, it is increasingly important to reduce the power consumption of power supplies during standby. Some practical methods have been proposed to reduce the switching losses and other rated losses of switching power supplies at very light or no loads. This paper discusses pulse skipping, burst mode and off time modulation. In addition, techniques to reduce starting current and other losses are also introduced.
Loss analysis
Take the typical flyback converter in Figure XNUMX as an example to analyze the losses of the power converter. Because of its low price and wide input range, flyback converters are popular in practical applications. For a flyback converter, the main losses include conduction loss and switching loss, as well as losses caused by the control circuit. Tables XNUMX, XNUMX, and XNUMX respectively list approximate estimates and commonly used solutions for these major losses, including major conduction losses, switching losses, and losses caused by control circuits.
It can be clearly found that both conduction loss and switching loss are closely related to the switching frequency. Reducing the switching frequency can effectively reduce losses, especially at light loads. However, the wavewidth generated by the wavewidth modulation generator must be controlled to avoid saturation of the magnetic components. Moreover, the output energy of the flyback converter can be expressed as Po = (Vdc2 × Ton2 ) / (2 × Lp × T) × eta, where eta represents the conversion efficiency. At light load, the on-time (Ton) is very short, and it is a very intuitive idea to increase the switching period (T) or reduce the switching frequency (fs).
Reduce switching frequency
Reducing the switching frequency can effectively reduce power loss. Recently many methods have been proposed to actually reduce the frequency
[2-7]. SGS-Thompson[2] and National Semiconductor[3] proposed pulse skipping technology, which determines whether to omit switching pulses according to the severity of the load. Figure 2 expresses the concept of pulse skipping mode, which effectively reduces switching number of pulses to meet low loss requirements at light loads. As shown in Figure 2, the main disadvantage of pulse omission technology is that the output voltage may suddenly drop or rise due to feedback delays caused by changes in dynamic loads. For a flyback power converter, the output power Po = fs × Lp × Ip2 /2, where Lp is the inductance of the primary side of the transformer, and Ip is the primary side current, Ip = (Vin/ Lp) × Ton, the output power It can be expressed as Po = (fs × Vin2 × Ton1 ) / (XNUMX × Lp), and the switching period T (=XNUMX/fs) is suddenly extended by multiples. The output power and voltage also drop suddenly. After that, the on-time Ton will increase due to the feedback of the output voltage to stabilize the output voltage. At the same time, it will also cause a sudden drop in the output voltage. In addition, when the omitted pulse is restored, the on-time Ton is also relatively suddenly extended, causing a sudden rise in the output voltage. In addition, the pulse omission technology determines whether to insert or omit pulses based on specific output load changes. When the output voltage is similar due to small changes in the load, the power saving is difficult to detect.
The technology of burst mode [4] is also called hiccup mode or omitted period mode. As shown in Figure XNUMX, when the load suddenly drops, the control loop requires Ton to be shortened. At a certain load level, the pulse mode control circuit begins to prevent the on-time Ton from being reduced, and then also begins to periodically block the wave width modulation. pulse. The power supply can achieve the purpose of saving energy under different loads by reducing the pulse group width or increasing the shielding period length. This technology has two obvious shortcomings, that is, low-frequency interference will appear in resonance with the shadowing period, and sudden changes in the load will also cause a sudden drop in the output voltage. Figure XNUMX shows a possible sudden drop in the output voltage.
Off time modulation[5-7]. Figure XNUMX shows the basic concept of off-time modulation. When the output voltage drops below the critical level, the off time increases linearly as the load decreases, and the switching frequency decreases linearly. Therefore, under light conditions Power loss can be reduced when loading and no-loading. Figure XNUMX provides a plot of switching frequency versus output power. The dynamic response of non-conduction time modulation should be better than the pulse omission mode because the non-conduction time is adjusted cycle by cycle.
Reduce startup losses
The traditional starting circuit is shown in Figure 1(a), where VSTA is the starting critical voltage of the wave width modulation controller, TD_ON is the start-up delay time, TD_ON = (C1 x VSTA)/IC1 Large Input resistance (Rin) can effectively reduce resistive losses, but the startup delay time will be extended. Figure 2(b) is a proposed circuit in order to reduce the initial loss caused by the input resistance (Rin). The large input resistor can be used together with the small capacitor CXNUMX to ensure that the startup delay time is short enough, while the large capacitor (CXNUMX) is used to provide a stable voltage to VDD. Using this kind of starting circuit, the starting losses can be very low and the starting delay time can be very short.
in conclusion
This article discusses ways to reduce the standby power loss of power supplies. First, a mathematical description is used to roughly estimate the main conduction, switching and control circuit losses, and then it is confirmed that reducing the switching frequency is the main method to reduce standby power loss. Then, various patented frequency reduction technologies and their possible deficiencies are introduced. In addition, a low-loss starting circuit is also introduced to reduce the loss of the starting resistor. Finally, using the method proposed in this article, an experimental AC converter was produced with an output voltage and current specification of 12V/5A. When the AC input is 240V and the output is unloaded, the input power is only 0.1W.
