Advantages and disadvantages of various switching power topologies

Advantages and disadvantages of various switching power topologies

In order to characterize the quality of various voltage or current waveforms, the voltage, current amplitude, average value, effective value, and first harmonic are generally compared with each other. Among the switching power supplies, the magnitude and average value of the voltage or current are the most intuitive. Therefore, the ratio of the magnitude of the voltage or current to the average value is called the pulsation coefficient S; some people use the effective value of the voltage or current The ratio of the average values ​​is called the waveform coefficient K.

 

Therefore, the pulsation coefficients Sv and Si of the voltage and current and the waveform coefficients Kv and Ki are expressed as:

 

Sv=Up/Ua--Voltage pulsation coefficient

Si =Im/Ia --current ripple coefficient

Kv=Ud/Ua--voltage waveform coefficient

Ki=Id/Ia--current waveform coefficient

 

In the above formula 4, Sv, Si, Kv, and Ki respectively represent: the pulsation coefficient S of the voltage and current, and the waveform coefficient K of the voltage and current, and generally only the uppercase S or K is written in the case where the division is generally clear. The pulsation coefficient S and the waveform coefficient K are both indicators for characterizing the voltage or current, and the values ​​of S and K are obviously as small as possible. The smaller the values ​​of S and K, the more stable the output voltage and current, and the smaller the ripple of voltage and current.

 

Advantages and disadvantages of flyback switching power supplies

 

1. The output voltage of the flyback switching power supply is worse than that of the forward switching power supply.

 

The flyback switching power supply does not provide power output to the load during the control switch is turned on, but only converts the stored energy into the back electromotive force to provide output to the load during the off period of the control switch, but the duty ratio of the control switch is 0.5, the transformer The average value of the voltage output by the secondary coil is approximately equal to one-half of the maximum value of the voltage, and the current flowing through the load is exactly equal to one-fourth of the maximum current of the secondary winding of the transformer. That is, the voltage ripple factor is equal to 2 and the current ripple factor is equal to 4. The voltage ripple coefficient of the flyback switching power supply is basically the same as that of the forward switching power supply, but the ripple coefficient of the current is twice the current ripple coefficient of the forward switching power supply. It can be seen that the output characteristics of the voltage and current of the flyback switching power supply are worse than those of the forward switching power supply. In particular, when the flyback switching power supply is used, in order to prevent the power switch tube from over-pressing, the duty ratio is generally less than 0.5. At this time, the current flowing through the secondary winding of the transformer may be intermittent, voltage and current. The pulsation coefficient will increase and the output characteristics of the voltage and current will become worse.

 

2. The transient control characteristics of the flyback switching power supply are relatively poor.

 

Since the flyback switching power supply only supplies energy to the load during the off period of the switch, when the load current changes, the switching power supply cannot immediately react to the output voltage or current, but needs to wait until the next cycle, sampling through the output voltage. And the function of the widening control circuit, the switching power supply begins to react to the things that have already passed, that is, the duty cycle is changed. Therefore, the transient control characteristics of the flyback switching power supply are relatively poor. Sometimes, when the frequency and phase of the load current change and the delay characteristic of the voltage output by the sampling and widening control circuit are in phase, the output voltage of the flyback switching power supply may generate jitter, which is the case in the TV. Switching power supplies are the easiest to appear.

 

3. The leakage inductance of the primary and secondary coils of the flyback switching power supply transformer is relatively large, and the switching power supply transformer has low working efficiency.

 

The core of the flyback switching power supply transformer generally needs to leave a certain air gap. On the one hand, it is to prevent the magnetic core of the transformer from being excessively large due to excessive current flowing through the primary coil of the transformer. On the other hand, because the output power of the transformer is small, it is necessary to adjust the inductance of the primary coil of the transformer by adjusting the air gap of the voltage transformer and the number of turns of the primary coil. Therefore, the leakage inductance of the primary and secondary coils of the flyback switching power supply transformer is relatively large, which will reduce the working efficiency of the switching power supply transformer, and the leakage inductance will also generate a counter electromotive force, which easily breaks down the switching tube.

 

4. The advantage of the flyback switching power supply is that the circuit is relatively simple and the volume is relatively small. The output voltage of the flyback switching power supply is modulated by the duty cycle, which is much higher than that of the forward switching power supply.

 

The advantage of the flyback switching power supply is that the circuit is relatively simple, using a large energy storage filter inductor and a freewheeling diode compared to the forward switching power supply. Once, the flyback switching power supply is larger than the forward switching power supply. The power supply is small in size and low in cost. In addition, the output voltage of the flyback switching power supply is modulated by the duty cycle, which is much higher than that of the forward switching power supply. Therefore, the flyback switching power supply requires a lower error signal amplitude for adjusting the duty cycle. The gain and dynamic range of the signal amplifier are also small. Due to these advantages, flyback switching power supplies are still widely used in the field of home appliances.

 

5. Flyback switching power supplies are often used in applications with low power or multiple outputs.

 

6. The flyback switching power supply does not require a magnetic reset winding.

 

In the flyback switching power supply, when the switching transistor is turned off, the transformer energy storage of the flyback converter is released to the load, and the magnetic core is naturally reset, and no magnetic resetting measures are required.

 

7. In flyback switching power supplies, the voltage regulator has both energy storage and variable voltage and isolation functions.

 

Advantages and disadvantages of forward switching power supplies

 

1. The transient control characteristics of the output voltage of the forward transformer switching power supply are relatively good.

 

The forward-type transformer switching power supply happens to be when the primary coil of the transformer is excited by the DC voltage. The secondary coil of the transformer supplies power to the load, and the amplitude of the output voltage is basically stable. In this case, although the output power constantly changes, However, the amplitude of the output voltage is basically unchanged, which indicates that the transient control characteristic of the output voltage of the forward-slewing transformer switching power supply is relatively good; only when the control switch is turned off, the power output is all stored by the energy storage inductor and the storage. Both the energy and the capacitor are provided at the same time. Although the output voltage is affected by the load current, if the capacity of the storage capacitor is relatively large, the influence of the load current on the output voltage is small.

 

2. Forward transformer switching power supply load capacity is relatively strong.

 

Since the forward-type transformer switching power supply generally selects the average value of the transformer output voltage for one week, the energy storage inductor supplies current output to the load during the control switch is turned on and off. Therefore, the load capacity of the forward-type transformer switching power supply is relatively In comparison, the ripple of the output voltage is relatively small. If the output voltage of the forward transformer switching power supply is required to have a large adjustment rate, in the case of normal load, the duty ratio of the control switch is preferably selected to be about 0.5, or slightly larger than 0.5, at which time the energy storage filter inductor flows. The current is the continuous current. When the current flowing through the energy storage filter inductor is a continuous current, the load capacity is relatively strong.

 

3. The forward voltage transformer's switching power supply has much better voltage and current output characteristics than the flyback transformer switching power supply.

 

When the duty ratio of the control switch is 0.5, the amplitude of the output voltage uo of the forward-transition transformer switching power supply is exactly equal to twice the voltage average value Ua, and the maximum value Im of the current-storing energy storage inductor current is also the average current Io. It is twice the output current. Therefore, the pulsation coefficient S of the voltage and current of the switching power supply of the forward transformer is about 2, which is almost the same as the pulsation coefficient S of the voltage and current of the flyback switching power supply. Double, indicating that the voltage and current output characteristics of the forward-slewing transformer switching power supply are much better than the flyback-type transformer switching power supply.

 

4. The forward switching power supply uses a large energy storage filter inductor and a freewheeling diode instead of the flyback transformer switching power supply.

 

The shortcomings of the forward-type transformer switching power supply are also very obvious. One of them is that the circuit uses a larger energy storage filter inductor than the flyback transformer switching power supply, and a freewheeling diode. In addition, the output voltage of the forward-type transformer switching power supply is modulated by the duty cycle, which is much lower than that of the flyback transformer switching power supply. This comparison between (1-77) and (1-78) is very Obviously can be seen. Therefore, the excitation transformer switching power supply requires a higher error signal amplitude of the regulation duty cycle, and the gain and dynamic range of the error signal amplifier are also relatively large.

 

5. The forward switching power supply is relatively large.

 

Forward transformer switching power supply In order to reduce the excitation current of the transformer and improve the working efficiency, the volt-second capacity of the transformer is generally larger (the volt-second capacity is equal to the product of the input pulse voltage amplitude and the pulse width, which is represented by US), and In order to prevent the back electromotive force generated by the primary coil of the transformer from breaking down the switching tube, the transformer of the forward-type transformer switching power supply has one more back-electromotive force absorption winding than the transformer of the flyback-type transformer switching power supply. Therefore, the transformer of the forward-type transformer switching power supply The volume is larger than the transformer of the flyback transformer switching power supply.

 

6. The forward voltage of the transformer primary coil of the forward switching power supply is higher than the back electromotive voltage generated by the flyback switching power supply.

 

A further disadvantage of the forward-type transformer switching power supply is that when the control switch is turned off, the back-EM voltage generated by the primary winding of the transformer is higher than the back-EM voltage generated by the flyback switching power supply. Because the general forward-type transformer switching power supply works, the duty cycle of the control switch is taken to be about 0.5, and the duty cycle of the flyback-type transformer switching power supply control switch is relatively small.

 

7. Dual-tube forward converters can be used for higher voltage inputs and larger power outputs.

 

Advantages and disadvantages of push-pull switching power supply

 

1. Push-pull switching power supply output current transient response speed is very high, voltage output characteristics are very good. The push-pull switching power supply is the switching power supply with the highest voltage utilization among all switching power supplies.

 

Since the two control switches in the push-pull switching power supply alternately work in turn, the output voltage waveform is very symmetrical, and the switching power supply supplies power to the load throughout the cycle, so the output current transient response speed is high. The voltage output characteristics are very good. The push-pull switching power supply is the switching power supply with the highest voltage utilization among all switching power supplies. It maintains a large output power even at low input voltages, so push-pull switching power supplies are widely used in low input voltage DC/AC inverters, active DC/DC converter circuits.

 

2. Push-pull switching power supply is a switching power supply with excellent output voltage characteristics.

 

After the push-pull switching power supply is bridge rectified or full-wave rectified, its output voltage ripple coefficient and current ripple coefficient are very small. Therefore, a small value of the energy storage filter capacitor or the energy storage filter inductor can be used to obtain a voltage. Ripple and current ripple are small output voltages. Therefore, the push-pull switching power supply is a switching power supply with excellent output voltage characteristics.

 

3. Push-pull switching power supply transformer leakage inductance and copper resistance loss are much smaller than unipolar magnetization transformer, switching power supply efficiency is high.

 

The transformer of the push-pull switching power supply belongs to the bipolar magnetization pole, the magnetic induction voltage transformation range is more than twice that of the unipolar magnetization pole, and the transformer core does not need an air gap, therefore, the magnetic core of the push-pull switching power supply transformer core The permeability is many times higher than the magnetic permeability of the transformer core of the forward or flyback switching power supply of the unipolar magnetization pole, so that the number of turns of the primary and secondary coils of the push-pull switching power supply transformer can be more than that of the unipolar magnetization The number of turns of the primary and secondary coils of the pole transformer is more than doubled. Therefore, the leakage inductance and the copper resistance loss of the push-pull switching power supply transformer are much smaller than that of the unipolar magnetization transformer, so the switching power supply has a high efficiency.

 

4. The drive circuit of the push-pull switching power supply is simple.

 

The two switching devices of the push-pull switching power supply have a common ground terminal, and the driving circuit is much simpler than the half-bridge or full-bridge switching power supply.

 

5. Push-pull switching power supplies do not have the possibility of two control switches colluding at the same time as half-bridge, full-bridge switching power supplies.

 

6. The main disadvantage of the 6 push-pull switching power supply is that the two switching devices require high withstand voltage values.

 

The main disadvantage of push-pull switching power supplies is that the two switching devices require a high withstand voltage and must withstand more than twice the operating voltage. Therefore, push-pull switching power supplies are rarely used in 220V AC powered equipment. In addition, the adjustment range of the output voltage of the DC output voltage adjustable push-pull switching power supply is much smaller than the adjustment range of the output voltage of the flyback switching power supply, and a storage filter inductor is required. Therefore, the push-pull switching power supply is not suitable for the requirements. When the load voltage varies too much, especially when the load is very light or often open.

 

7. Push-pull switching power supply transformers have two sets of primary coils, which is a disadvantage for low-power output push-pull switching power supplies, which is an advantage for high-power output push-pull switching power supplies. Since the coils of the high-power transformer are generally wound by multiple strands, the two sets of primary coils of the transformer of the push-pull switching power supply are not different from the multi-strand winding, and the two coils are connected to the single coil. The ratio can be reduced by half the current density.

 

8. The push-pull converter can be thought of as a combination of two forward converters, which alternately operate during one switching cycle. If the two forward converters are not completely symmetrical or balanced, DC bias will occur. After several periods of accumulated bias, the core will enter saturation and cause the excitation current of the high-frequency transformer to pass. Large and even damaged the switch tube.

 

9. Push-pull, half-bridge, full-bridge converters are DC-AC-DC converters. Since the DC-AC converter increases the operating frequency, both the size and weight of the transformer and output filter can be reduced.

 

 

 

Advantages and disadvantages of half-bridge switching power supply

 

1. Half-bridge transformer switching power supply has large output power and high working efficiency

 

The half-bridge transformer switching power supply is the same as the push-pull transformer switching power supply. Since the two switching tubes alternately work in turn, equivalent to the simultaneous output power of the two switching power supplies, the output power is approximately equal to twice the output power of the single switching power supply. Therefore, the half-bridge transformer switching power supply has a large output power and high working efficiency. After bridge rectification or full-wave rectification, the voltage ripple coefficient Sv and the current ripple coefficient Si of the output voltage are both small, requiring only a small filtering. Inductance and capacitance, the output voltage ripple and current ripple can be very small.

 

2. The switching voltage of the half-bridge switching power supply has a low withstand voltage.

 

The biggest advantage of the half-bridge transformer switching power supply is that the withstand voltage requirement for the two switching devices can be reduced by half compared to the push-pull transformer switching power supply. Because the operating voltage of the two switching devices of the half-bridge transformer switching power supply is only half of the input power source Ui, the highest withstand voltage is equal to the sum of the working voltage and the back electromotive force, which is about twice the power supply voltage. This result is just a push-pull type. Transformer switching power supply two switching devices with half the withstand voltage. Therefore, the half-bridge transformer switching power supply is mainly used in the case where the input voltage is relatively high. Generally, the high-power switching power supply with the grid voltage of 220 volts is mostly a half-bridge transformer switching power supply.

 

3. The half-bridge switching power supply transformer primary winding requires only one winding, which is also its advantage, which brings some convenience to the coil winding of the small power switching power supply transformer. However, there is no advantage in coil winding of a high-power switching power supply transformer because the coil of the high-power switching power supply transformer needs to be wound with multiple strands.

 

4. The disadvantage of the half-bridge transformer switching power supply is that the power utilization rate is relatively low. Therefore, the half-bridge transformer switching power supply is not suitable for occasions with low operating voltage. In addition, the two switching devices in the half-bridge transformer switching power supply are connected without a common ground, and it is troublesome to connect with the driving signal.

 

5. The disadvantage of the half-bridge switching power supply is that a semi-conducting area occurs and the loss is large.

 

The biggest disadvantage of the half-bridge switching power supply is that when the two control switches K1 and K2 are in the alternate switching state, the two switching devices will simultaneously have a short-time semi-conducting region, that is, the two control switches are simultaneously On state. This is because the switching device is equivalent to charging the capacitor when it starts to conduct. It needs a transition process from the off state to the fully on state. When the switching device switches from the on state to the off state, it is equivalent to the capacitor. Discharge, it also requires a transition from the on state to the fully off state.

 

When the two switching devices are in the on-and-off transition process respectively, that is, when the two switching devices are in the semi-conducting state in the semi-conducting state, the two control switches are simultaneously turned on, and they cause the power supply voltage to be generated. Short circuit; at this point, a large current will appear in the series circuit of the two control switches, and this current does not pass through the transformer load. Therefore, during the transition process of the two control switches K1 and K2, the two switching devices will generate a large power loss. In order to reduce the loss caused by the control switch transition process, generally in the half-bridge switching power supply circuit, it is intentional to make the switching on and off times of the two control switches staggered for a short period of time.

 

6. Single-capacitor half-bridge transformer switching power supply saves one capacitor compared to dual-capacitor half-bridge transformer switching power supply, which is its advantage. In addition, when the single-capacitor half-bridge transformer switching power supply is just starting to work, the output voltage is almost twice as high as the output voltage of the dual-capacitor half-bridge transformer switching power supply. This feature is most suitable for use as a fluorescent power source, for example, an energy-saving lamp or Fluorescent lamps and backlights for LCD displays.

 

Fluorescent lamps generally require a high voltage when they start to light up, about a few hundred volts to several kilovolts. After lighting, the working voltage requires tens of volts to more than one hundred volts. Therefore, almost all energy-saving lamps are used. Single capacitor half bridge transformer switching power supply.

 

7. Single-capacitor half-bridge transformer switching power supply also has the disadvantage that the withstand voltage requirement of the switching device is higher than that of the dual-capacitor half-bridge transformer switching power supply.

 

Advantages and disadvantages of full-bridge switching power supply

 

1. Full-bridge transformer switching power supply has a large output power and high working efficiency.

 

The full-bridge transformer switching power supply is the same as the push-pull transformer switching power supply. Since the two switching devices alternately work in turn, it is equivalent to the simultaneous output power of two switching power supplies, and its output power is approximately equal to twice the output power of a single switching power supply. Therefore, the full-bridge transformer switching power supply has a large output power and high working efficiency. After bridge rectification or full-wave rectification, the voltage ripple coefficient Sv and current ripple coefficient Si of the output voltage are small, only a small The value of the energy storage filter capacitor or the energy storage filter inductor, you can get a voltage ripple and current ripple are very small output voltage.

 

2. The advantage of a full-bridge switching power supply is that the withstand voltage of the switching tube is particularly low.

 

The biggest advantage of the full-bridge transformer switching power supply is that the withstand voltage requirement for the four switching devices can be reduced by half compared to the push-pull transformer switching power supply. Because the four-switching device of the full-bridge transformer switching power supply is divided into two groups, two switching devices are connected in series with each other during operation. When turned off, each switching device is subjected to a voltage that is only half of the voltage that a single switching device is subjected to. Its maximum withstand voltage is equal to half the sum of the operating voltage and the back EMF. This result is exactly half of the withstand voltage of the two switching devices of the push-pull transformer switching power supply.

 

3. Full-bridge transformer switching power supply is mainly used in applications where the input voltage is relatively high. When the input voltage is high, the full-bridge transformer switching power supply is used, and its output power is much larger than that of the push-pull transformer switching power supply. . Therefore, most of the high-power switching power supplies with a grid voltage of 220 VAC are mostly full-bridge transformer switching power supplies. In the case of a low input voltage, the output power of the push-pull transformer switching power supply is much larger than that of the full-bridge transformer switching power supply.

 

4. The power utilization of the full-bridge transformer switching power supply is lower than that of the push-pull transformer switching power supply because the two sets of switching devices are connected in series, and the total voltage drop when the two switching devices are turned on is connected to a single switching device. The voltage drop across time is doubled; however, the power utilization of the half-bridge transformer switching power supply is much higher. Therefore, the full-bridge transformer switching power supply can also be used in applications where the operating power supply voltage is relatively low.

 

5. Like the half-bridge switching power supply, the transformer primary winding of the full-bridge transformer switching power supply requires only one winding, which is also its advantage, which brings some convenience to the coil winding of the small power switching power supply transformer. However, there is no advantage in coil winding of a high-power switching power supply transformer because the coil of the high-power switching power supply transformer needs to be wound with multiple strands.

 

6. The disadvantage of full-bridge transformer switching power supply is that the power loss is relatively large. Therefore, the full-bridge transformer switching power supply is not suitable for occasions with low operating voltage, otherwise the working efficiency will be very low. In addition, the four switching device connections in the full-bridge transformer switching power supply have no common ground, and it is troublesome to connect with the driving signal.

 

7. The disadvantage of full-bridge switching power supply is that there will be a semi-conducting area and the loss is large.

 

The biggest disadvantage of the full-bridge switching power supply is that when the two sets of control switches K1, K4 and K2, K3 are in the alternate switching state, the four switching devices will simultaneously have a short-time semi-conducting area, ie two groups. The control switch is simultaneously turned on. This is because the switching device is equivalent to charging the capacitor when it starts to conduct. It needs a transition process from the off state to the fully on state. When the switching device switches from the on state to the off state, it is equivalent to the capacitor. Discharge, it also requires a transition from the on state to the fully off state.

 

When the two sets of switching devices are in the on-and-off transition process respectively, that is, when the two sets of switching devices are in a semi-conducting state, the two sets of control switches are simultaneously turned on, which may cause a short circuit to the power supply voltage; A large current will appear in the series loop of the four control switches, and this current does not pass through the transformer load. Therefore, during the transition process of the four control switches K1, K4 and K2, K3, the four switching devices will generate a large power loss. In order to reduce the loss caused by the control switch transition process, in the full-bridge switching power supply circuit, it is intentional to make the switching-on and off-time of the two sets of control switches staggered for a short period of time.

 

A double-ended isolated PWM DC/DC converter, in one switching cycle, the power is alternately input from one end and the other end of the primary winding of the isolation transformer, so it is called double-ended. The core of the double-ended isolated PWM DC/DC converter operates in the first and third quadrants of the B-H plane coordinate system, so the core can be fully utilized.