Detailed analysis of the advantages and disadvantages of switching power supply topology

　　In order to characterize the quality of various voltage or current waveforms, the amplitude, average value, effective value, first harmonic and other parameters of voltage or current are generally compared with each other. In switching power supplies, the amplitude and average value of voltage or current are the most intuitive, so we use the ratio of the amplitude of voltage or current to its average value, called the ripple coefficient S; some people also use the ratio of the effective value of voltage or current to its average value, called the waveform coefficient K.

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

　　Sv = Up/Ua —— Voltage ripple coefficient (1-84)

　　Si = Im/Ia —— Current ripple coefficient (1-85)

　　Kv =Ud/Ua —— Voltage waveform factor (1-86)

　　Ki = Id/Ia —— Current waveform factor (1-87)

　　In the above 4 formulas, Sv, Si, Kv, and Ki represent: the ripple coefficient S of voltage and current, and the waveform coefficient K of voltage and current, respectively. In general, only the capital letters S or K are written when they can be distinguished. The ripple coefficient S and the waveform coefficient K are both indicators of the quality of voltage or current. Obviously, the smaller the value of S and K, the better. The smaller the value 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 supply

　　1 The output characteristics of voltage and current of the flyback switching power supply are worse than those of the forward switching power supply.

　　The flyback switching power supply does not provide power output to the load during the period when the control switch is turned on. It converts the stored energy into back electromotive force to provide output to the load only during the period when the control switch is turned off. However, when the duty cycle of the control switch is 0.5, the average value of the voltage output by the secondary coil of the transformer is approximately equal to half of the maximum voltage, and the current flowing through the load is exactly equal to one-fourth of the maximum current of the secondary coil of the transformer. That is, the voltage ripple coefficient is equal to 2, and the current ripple coefficient 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 current ripple coefficient is twice that of the forward switching power supply. It can be seen from this 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-voltage shock, the duty cycle is generally less than 0.5. At this time, the current flowing through the secondary coil of the transformer will be intermittent, the voltage and current ripple coefficients will increase, and its voltage and current output characteristics will become worse.

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

　　Since the flyback switching power supply only provides energy output to the load during the switch-off period, when the load current changes, the switching power supply cannot immediately respond to the output voltage or current, but needs to wait until the next cycle. Through the output voltage sampling and width modulation control circuit, the switching power supply begins to respond to what has already passed, that is, to change the duty cycle. 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 are consistent with the delay characteristics of the voltage output by the sampling and width modulation control circuit, the output voltage of the flyback switching power supply may jitter. This situation is most likely to occur in the switching power supply of a TV.

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

　　The iron core of the flyback switching power supply transformer generally needs to have a certain air gap. On the one hand, it is to prevent the iron core of the transformer from being easily saturated 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 regulator 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 back electromotive force, which is easy to break down the switch tube.

　　4 The advantages of the flyback switching power supply are that the circuit is relatively simple and the size 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, and it uses one less large energy storage filter inductor and a freewheeling diode than the forward switching power supply. At the same time, the volume of the flyback switching power supply is smaller than that of the forward switching power supply, and the cost is also lower. 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 that the error signal amplitude of the duty cycle regulation should be relatively low, and the gain and dynamic range of the error signal amplifier should also be small. Due to these advantages, the flyback switching power supply is still widely used in the field of home appliances.

　　5 Flyback switching power supplies are mostly used in situations with lower power or multiple outputs.

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

　　In a flyback switching power supply, when the switch tube is turned off, the transformer energy stored in the flyback converter is released to the load, and the magnetic core is naturally reset without the need for magnetic reset measures.

　　7. In the flyback switching power supply, the voltage regulator has the functions of energy storage, voltage transformation and isolation.

　　Advantages and disadvantages of forward switching power supply

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

　　The forward transformer switching power supply is just when the primary coil of the transformer is excited by the DC voltage, the secondary coil of the transformer provides power output to the load, and the amplitude of the output voltage is basically stable. At this time, although the output power keeps changing, the amplitude of the output voltage remains basically unchanged, which shows that the transient control characteristics of the output voltage of the forward transformer switching power supply are relatively good; only when the control switch is in the off period, the power output is fully provided by both the energy storage inductor and the energy storage capacitor. At this time, although the output voltage is affected by the load current, if the capacity of the energy storage capacitor is relatively large, the load current will have little effect on the output voltage.

　　2 The load capacity of the forward transformer switching power supply is relatively strong.

　　Since the forward transformer switching power supply generally selects the weekly average value of the transformer output voltage, the energy storage inductor provides current output to the load during the on and off periods of the control switch. Therefore, the load capacity of the forward transformer switching power supply is relatively strong, and 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 larger adjustment rate, under normal load conditions, the duty cycle of the control switch is preferably selected at around 0.5, or slightly greater than 0.5. At this time, the current flowing through the energy storage filter inductor is a continuous current. When the current flowing through the energy storage filter inductor is a continuous current, the load capacity is relatively strong.

　　3 The voltage and current output characteristics of the forward transformer switching power supply are much better than those of the flyback transformer switching power supply.

　　When the duty cycle of the control switch is 0.5, the amplitude of the output voltage uo of the forward transformer switching power supply is exactly equal to twice the average voltage Ua, and the maximum value Im of the current flowing through the filter energy storage inductor is exactly twice the average current Io (output current). Therefore, the voltage and current ripple coefficients S of the forward transformer switching power supply are approximately equal to 2, which is almost half as small as the voltage and current ripple coefficient S of the flyback transformer switching power supply, indicating that the voltage and current output characteristics of the forward transformer switching power supply are much better than those of the flyback transformer switching power supply.

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

　　The disadvantages of the forward transformer switching power supply are also very obvious. One of them is that the circuit uses a large energy storage filter inductor and a freewheeling diode more than the flyback transformer switching power supply. In addition, the output voltage of the forward transformer switching power supply is modulated by the duty cycle, which is much lower than that of the flyback transformer switching power supply. This can be clearly seen from the comparison between (1-77) and (1-78). Therefore, the forward transformer switching power supply requires a relatively high amplitude of the error signal for regulating the 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 in size.

　　In order to reduce the excitation current of the transformer and improve working efficiency, the volt-second capacity of the forward transformer switching power supply is generally large (the volt-second capacity is equal to the product of the input pulse voltage amplitude and the pulse width, represented by US here), 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 transformer switching power supply has one more back electromotive force absorption winding than the transformer of the flyback transformer switching power supply. Therefore, the volume of the transformer of the forward transformer switching power supply is larger than that of the flyback transformer switching power supply.

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

　　Another greater disadvantage of the forward transformer switching power supply is that when the control switch is turned off, the back EMF voltage generated by the transformer primary coil is higher than the back EMF voltage generated by the flyback transformer switching power supply. This is because when the forward transformer switching power supply is working, the duty cycle of the control switch is generally around 0.5, while the duty cycle of the flyback transformer switching power supply control switch is relatively small.

　　7 The dual-tube forward converter can be used in situations where higher voltage input and larger power output are required.

　　Advantages and disadvantages of push-pull switching power supply

　　1. The push-pull switching power supply has a high output current transient response speed and good voltage output characteristics. The push-pull switching power supply has the highest voltage utilization rate among all switching power supplies.

　　Since the two control switches in the push-pull switching power supply work alternately, its output voltage waveform is very symmetrical, and the switching power supply provides power output to the load throughout the entire cycle. Therefore, its output current transient response speed is very high and the voltage output characteristics are very good. The push-pull switching power supply is the switching power supply with the highest voltage utilization rate among all switching power supplies. It can still maintain a large output power when the input voltage is very low, so the push-pull switching power supply is widely used in low input voltage DC/AC inverters and DC/DC converter circuits.

　　2 The push-pull switching power supply is a switching power supply with very good 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 very small value of energy storage filter capacitor or energy storage filter inductor is needed to obtain an output voltage with very small voltage ripple and current ripple. Therefore, the push-pull switching power supply is a switching power supply with very good output voltage characteristics.

　　3. The leakage inductance and copper resistance loss of the push-pull switching power supply transformer are much smaller than those of the unipolar magnetizing pole transformer, and the working efficiency of the switching power supply is higher.

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

　　4 The driving 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. Compared with the half-bridge or full-bridge switching power supply, the driving circuit is much simpler.

　　5 The push-pull switching power supply will not have the possibility of two control switches being connected in series at the same time as the half-bridge and full-bridge switching power supplies.

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

　　The main disadvantage of the push-pull switching power supply is that the two switching devices require a very high withstand voltage, which must be greater than twice the operating voltage. Therefore, the push-pull switching power supply is rarely used in 220V AC power supply equipment. In addition, the adjustment range of the output voltage of the push-pull switching power supply with adjustable DC output voltage is much smaller than that of the flyback switching power supply, and an energy storage filter inductor is required. Therefore, the push-pull switching power supply is not suitable for occasions where the load voltage variation range is required to be too large, especially in occasions where the load is very light or the circuit is often open.

　　7 The transformer of the push-pull switching power supply has two sets of primary coils, which is a disadvantage for the push-pull switching power supply with low power output, but an advantage for the push-pull switching power supply with high power output. Because the coils of high-power transformers are generally wound with multiple strands of wire, the two sets of primary coils of the transformer of the push-pull switching power supply are no different from those wound with multiple strands of wire, and the two coils can reduce the current density by half compared with a single coil.

　　8 The push-pull converter can be regarded as a combination of two forward converters. In one switching cycle, the two forward converters work alternately. If the two forward converters are not completely symmetrical or balanced, DC bias will occur. After several cycles of accumulated bias, the magnetic core will enter a saturated state, causing the excitation current of the high-frequency transformer to be too large, and even damaging the switch tube.

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

　　Advantages and disadvantages of half-bridge switching power supply

　　1. The half-bridge transformer switching power supply has a 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 work alternately, it is equivalent to two switching power supplies outputting power at the same time. Its output power is about twice the output power of a 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 current ripple coefficient Si of the output voltage are very small, and only small filter inductors and capacitors are needed, and its output voltage ripple and current ripple can be very small.

　　2 The voltage resistance of the switching tube of the half-bridge switching power supply is relatively low.

　　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 withstand voltage requirement for the two switching devices of 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 supply Ui, its maximum withstand voltage is equal to the sum of the operating voltage and the back electromotive force, which is about twice the power supply voltage. This result is exactly half of the withstand voltage of the two switching devices of the push-pull transformer switching power supply. Therefore, the half-bridge transformer switching power supply is mainly used in occasions with relatively high input voltage. Generally, most of the high-power switching power supplies with a grid voltage of AC 220V use half-bridge transformer switching power supplies.

　　3 The primary coil of the transformer of the half-bridge switching power supply only needs one winding, which is also its advantage, which brings some convenience to the coil winding of the transformer of the small power switching power supply. But it has no advantage for the coil winding of the transformer of the high power switching power supply, because the coil of the transformer of the high power switching power supply needs to be wound with multiple strands of wire.

　　4 The main 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 working voltage. In addition, the two switching devices in the half-bridge transformer switching power supply are connected without a common ground, which makes it more troublesome to connect with the drive signal.

　　4 The disadvantage of the half-bridge switching power supply is that there will be a semi-conduction area, resulting in large losses.

　　The biggest disadvantage of the half-bridge switching power supply is that when the two control switches K1 and K2 are in the alternating switching working state, the two switch devices will simultaneously appear in a very short semi-conduction area, that is, the two control switches are in the on state at the same time. This is because when the switch device starts to conduct, it is equivalent to charging the capacitor, and it takes a transition process from the off state to the fully on state; and when the switch device switches from the on state to the off state, it is equivalent to discharging the capacitor, and it also takes a transition process from the on state to the fully off state.

　　When the two switch devices are in the transition process of on and off, that is, when both switch devices are in the semi-on state, it is equivalent to that the two control switches are turned on at the same time, and they will cause a short circuit to the power supply voltage; at this time, 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, when the two control switches K1 and K2 are in the transition process at the same time, the two switch devices will generate a large power loss. In order to reduce the loss generated by the transition process of the control switch, generally in the half-bridge switching power supply circuit, the turn-on and turn-off time of the two control switches are intentionally staggered for a short period of time.

　　5 The single capacitor half-bridge transformer switching power supply saves one capacitor compared to the dual capacitor half-bridge transformer switching power supply, which is its advantage. In addition, when the single capacitor half-bridge transformer switching power supply starts working, 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 as a fluorescent lamp power supply, such as energy-saving lamps or fluorescent lamps and LCD display backlights.

　　Fluorescent lamps generally require a very high voltage when they start to light up, about several hundred volts to several thousand volts, and after lighting up, the operating voltage only requires tens of volts to more than 100 volts. Therefore, almost all energy-saving lamps use a single-capacitor half-bridge transformer switching power supply.

　　6 The single-capacitor half-bridge transformer switching power supply also has a disadvantage, that is, the voltage resistance 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 The full-bridge transformer switching power supply has 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 sets of switching devices work alternately, it is equivalent to two switching power supplies outputting power at the same time. Its output power is approximately 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 its output voltage are very small. Only a very small value of energy storage filter capacitor or energy storage filter inductor is needed to obtain an output voltage with very small voltage ripple and current ripple.

　　2 The advantage of the 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 withstand voltage requirement for the two switching devices of the push-pull transformer switching power supply. This is because the four switching devices of the full-bridge transformer switching power supply are divided into two groups. When working, the two switching devices are connected in series. When turned off, the voltage borne by each switching device is only half of the voltage borne by a single switching device. Its maximum withstand voltage is equal to half of the sum of the operating voltage and the back electromotive force, which 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 occasions with relatively high input voltage. When the input voltage is very high, the full-bridge transformer switching power supply is used, and its output power is much greater than that of the push-pull transformer switching power supply. Therefore, most of the high-power switching power supplies with a general grid voltage of 220 volts AC use full-bridge transformer switching power supplies. When the input voltage is low, the output power of the push-pull transformer switching power supply is much greater than that of the full-bridge transformer switching power supply.

　　4 The power utilization rate 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 twice as large as the voltage drop when a single switching device is turned on; but it is much higher than the power utilization rate of the half-bridge transformer switching power supply. Therefore, the full-bridge transformer switching power supply can also be used in occasions where the working power supply voltage is relatively low.

　　5. Like the half-bridge switching power supply, the full-bridge transformer switching power supply only needs one winding for the primary coil, which is also its advantage. It brings some convenience to the coil winding of the small-power switching power supply transformer. But it has no advantage for the coil winding of the high-power switching power supply transformer, because the coil of the high-power switching power supply transformer needs to be wound with multiple strands of wire.

　　6 The main disadvantage of the 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 working voltage, otherwise the working efficiency will be very low. In addition, the four switching devices in the full-bridge transformer switching power supply are connected without a common ground, which makes it more troublesome to connect with the drive signal.

　　7 The disadvantage of the full-bridge switching power supply is that there will be a semi-conduction area, resulting in large losses.

　　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 alternating switching working state, the four switch devices will simultaneously appear in a very short semi-conduction area, that is, the two sets of control switches are in the on state at the same time. This is because when the switch device starts to conduct, it is equivalent to charging the capacitor, and it takes a transition process from the off state to the fully on state; and when the switch device switches from the on state to the off state, it is equivalent to discharging the capacitor, and it also takes a transition process from the on state to the fully off state.

　　When the two sets of switch devices are in the on and off transition process respectively, that is, when the two sets of switch devices are in the semi-on state, it is equivalent to that the two sets of control switches are turned on at the same time, and they will cause a short circuit to the power supply voltage; at this time, a large current will appear in the series circuit of the four control switches, and this current does not pass through the transformer load. Therefore, when the four control switches K1, K4 and K2, K3 are in the transition process at the same time, the four switch devices will generate a large power loss. In order to reduce the loss generated by the transition process of the control switches, generally in the full-bridge switching power supply circuit, the turn-on and turn-off time of the two sets of control switches are intentionally staggered for a short period of time.

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