Improving energy efficiency has been a long-term goal of power supply manufacturers. This is a real “win-win” as this not only reduces operating costs but also reduces wasted energy in the form of heat, meaning less thermal management is required, reducing the size and cost of the power supply. But how we can improve energy efficiency?
Why the energy efficiency of power supply is not good enough?
According to the efficiency formula of the power supply: efficiency = (output power Po/input power Pi)*100 From the formula, we can know that there are only two ways to improve the efficiency, either increase the output power under the premise of the input power unchanged, or increase the output power Reduce the input power under the same premise; in fact, we have other methods to find the reasons for the decrease in efficiency, and then eliminate the cause of the decrease in efficiency, then our efficiency will naturally increase.
The following 6 points show us some factors why the energy efficiency of power supply is not good:
- The switching transistor driving method is not the best solution, there may be overdrive or underdrive, and the reverse bias current of the switch tube is insufficient; these factors will lead to an increase in power consumption, thereby reducing the efficiency of the switching power supply. In this case, we only need to modify the design parameters slightly to achieve the effect of improving efficiency.
- Poor transformer design; transformer problems include transformer saturation, large leakage inductance, improper selection of windings and magnetic cores, these factors will also cause the transformer to be in an imperfect working state, which will also reduce the efficiency of the product, and the transformer has a great impact on efficiency. Large, reasonable transformer design and unreasonable efficiency can differ by about 5% to 10%.
- The parameters of the RCD absorption circuit are inappropriate.
- Unreasonable choke design, such as improper inductance or too much loss of winding and magnetic core, will lead to increased power and reduced efficiency.
- The opearation performance of the rectifier device is not good enough, such as the large voltage drop and long time for recovery of the reverse diode.
- The power consumption of the auxiliary circuit is large, the dummy load current is too large, and the control circuit produces abnormal oscillations, which will also reduce the efficiency of the product.
Methods to improve the energy efficiency of power supply.
There are 5 typical methods to improve the energy efficiency of power supply.
Quasi-resonant circuit
This method is for reason 1, the switching transistor driving, to reduce frequency-dependent switching losses.
How Quasi-resonant circuit help improve the energy efficiency of power supply?
Quasi-resonant flyback topologies, LLC resonant converter topologies, and asymmetric half-bridge topologies. Quasi-resonant flyback topologies have been successfully used in the range from the lowest power levels to over 200W. In the 70W-100W range, LLC resonant converters are more efficient than quasi-resonant flyback topologies.
Both quasi-resonant and resonant topologies can reduce conduction switching losses in the circuit. Figure 1 compares the turn-on switching waveforms of continuous conduction mode (CCM) flyback, quasi-resonant flyback, and LLC resonant converters.
The switching losses in all cases are given by:
Here, PTurnOnLoss is the switching loss; ID is the drain current; VDS is the voltage across the switch; COSSeff is the equivalent output capacitance value (including stray capacitance effects); tON is the on-time, and FSW is the switching frequency.
The CCM flyback converter has the highest switching losses. For designs with a wide input voltage range, VDS is around 500V – 600V, which is the sum of the input voltage VDC and the reflected output voltage VRO. When entering discontinuous conduction mode (DCM), the leakage current drops to zero, and the first term of switching losses also drops to zero. In quasi-resonant converters, losses can be further reduced by turning on at the first (or last) valley of the voltage waveform. The dotted line in the figure shows the drain waveform of the quasi-resonant converter when the first valley is turned on.
If the quasi-resonant flyback converter has a turn ratio of 20 and the output voltage is 5V, VRO is equal to 100V, so for a bus voltage of 375V, the switch will turn on at 275V. If the effective output capacitance COSSeff is 73 pF and the switching frequency FSW is 66 kHz, the loss is 0.18W:
For standard CCM flyback converters, the switch is not synchronized with the drain voltage ringing. In the worst case, the drain voltage is greater than VDC. Then the loss will be 0.54W. So for the discontinuous mode flyback converter, the power consumption fluctuates between 0.18W and 0.54W depending on the timing.
Resonant converters could reduce the consumption of switching by using a different technique. Let’s go back and look at the conduction loss formula, which shows that if VDS is set to zero, there is no loss at all, a principle known as zero voltage switching (ZVS).
A resonant converter utilizes a resonant circuit to generate the delay. Two MOSFETs generate a square wave, which is loaded on the resonant circuit. By choosing a suitable resonant circuit and setting the operating point above the resonant point, the current flowing into the resonant circuit can be very close to a sine wave, since the higher order components are generally attenuated greatly. The sinusoidal current waveform lags the voltage waveform, so when the voltage waveform reaches its zero-crossing point, the current remains negative, enabling zero-voltage switching.
Choose a suitable Transformer
This is the method for reason2.
Active Clamp Circuit
This is the method for reason3.
Active Clamp Circuit Provides isolation between the input and output stages.
Since the flyback converter has the function of insulating isolation, the control circuit also needs to have the function of insulating isolation. The two most commonly used control modes are voltage feedback control and current feedback control. Both of these control modes need to transmit the signal from the secondary coil side to the primary coil side, usually using an optocoupler or adding a separate winding on the transformer core to achieve isolated signal transmission.
Other control methods include primary-coil-side control techniques [2], which use magnetically attached coils to directly monitor the transformer current waveform. Using primary winding side control, both current and voltage rectification accuracy can be improved
Power Factor Circuit
This method is for reason4 &reason6, please refer another article for this topic. https://www.xjkadapter.com/how-pfc-circuits-work/
Active Rectification
This method is for reason5.
Active rectification is a new technology that uses a very low resistance special power MOSFETs to replace rectifier diodes for reducing the rectification losses. It could make a good efficiency of the DC/DC converter and no dead-time voltage..
The traditional rectification technology is similar to a door that must be pushed open by someone. Therefore, every time the current passes through the door, a huge effort is required, and the loss of sweat will naturally be a lot.
The synchronous rectification technology is a bit similar to the induction door of the higher-end places we pass through: it looks closed, but when you walk in front of it and need to pass through, it opens by itself, and you don’t need to push it hard. , so naturally there is no loss.
Through the above analogy, we can know that the active rectification technology reduces the rectification loss at the output end of the power adapters.
