INDUSTRIAL LCD DISPLAYS / IGBT MODULES DISTRIBUTOR

Infineon / Mitsubishi / Fuji / Semikron / Eupec / IXYS

# Analysis of advantages and disadvantages of various switching power supply topologies

Posted Date: 2024-01-24

In order to characterize the quality of various voltage or current waveforms, the amplitude, average value, effective value, first harmonic and other parameters of the voltage or current are generally compared with each other. Among switching power supplies, the amplitude and average value of voltage or current are the most intuitive. Therefore, we use the ratio of the amplitude of voltage or current to its average value, which is called the pulsation coefficient S; some people also use the ratio of the effective value of voltage or current to its average value. 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 respectively expressed as:

Sv = Up/Ua - Voltage ripple coefficient (1-84)
Si = Im/Ia - Current ripple coefficient (1-85)
Kv =Ud/Ua - voltage waveform coefficient (1-86)
Ki = Id/Ia - current waveform coefficient (1-87)

In the above 4 formulas, 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. When the distinction can be made clearly, generally only the uppercase letter S or K is written. The pulsation coefficient S and the waveform coefficient K are both indicators of the quality of voltage or current. Obviously, the smaller the values ​​of S and K, the better. The smaller the values ​​of S and K, the more stable the output voltage and current are, and the smaller the ripples of voltage and current are.

1. The voltage and current output characteristics of flyback switching power supply are worse than those of 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 only converts the stored energy into counter electromotive force to provide output to the load during the period when the control switch is turned off. However, when the duty cycle of the control switch is 0.5, the transformer The average voltage output by the secondary coil is approximately equal to one-half of the maximum voltage, and the current flowing through the load is exactly equal to one-quarter of the maximum current of the secondary coil of the transformer. That is, the voltage pulsation coefficient is equal to 2, and the current pulsation coefficient is equal to 4. The voltage pulsation coefficient of the flyback switching power supply is basically the same as that of the forward switching power supply, but the current pulsation coefficient is twice that of the forward switching power supply. It can be seen that the voltage and current output characteristics 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 overvoltage, 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, and the voltage and current will vary. The ripple coefficient will increase, and its voltage and current output characteristics will become worse.

2. The transient control characteristics of 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 react immediately to the output voltage or current, but needs to wait until the next cycle to sample the output voltage. With the function of the width adjustment control circuit, the switching power supply begins to react to things that have passed, that is, changing 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 phase delay characteristics of the voltage output by the sampling and width adjustment control circuit, the output voltage of the flyback switching power supply may jitter. This situation is common in TV sets. It is most likely to occur in switching power supplies.

3. The leakage inductance of the primary and secondary coils of the flyback switching power supply transformer is 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 leave a certain air gap. On the one hand, it is to prevent the iron core of the transformer from easily causing magnetic saturation 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 transformer's primary coil 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 counter electromotive force, which can easily break 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 modulation amplitude of the output voltage of the flyback switching power supply is affected 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. Compared with the forward switching power supply, it uses one less large energy storage filter inductor and one freewheeling diode. Once, the flyback switching power supply is smaller than the forward switching power supply. The power supply should be small in size and low in cost. In addition, the modulation amplitude of the output voltage of the flyback switching power supply is much higher than that of the forward switching power supply. Therefore, the flyback switching power supply requires a lower error signal amplitude for regulating the duty cycle, and the error The gain and dynamic range of the signal amplifier should also be smaller. Due to these advantages, flyback switching power supplies are still widely used in the field of home appliances.

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

6. The flyback switching power supply does not need to add a magnetic reset winding.

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

7. In the flyback switching power supply, the voltage regulator not only has the function of energy storage, but also has the functions of voltage transformation and isolation.

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

Forward transformer switching power supply happens when the primary coil of the transformer is excited by the DC voltage, and 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, However, 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 off, the power output is completely controlled by the energy storage inductor and storage The energy storage capacitor provides both at the same time. 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 impact of the load current on the output voltage will be small.

2. The forward transformer switching power supply has relatively strong load capacity.

Since the forward transformer switching power supply generally selects the one-week average value of the transformer output voltage, the energy storage inductor provides current output to the load during the turn-on and turn-off periods of the control switch. Therefore, the load capacity of the forward transformer switching power supply is relatively It 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 large adjustment rate, under normal load conditions, the duty cycle of the control switch is best selected to be around 0.5, or slightly greater than 0.5. At this time, the energy storage filter inductor flows through The current is 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 also exactly the average current Io (output current) twice. Therefore, the pulsation coefficient S of the voltage and current of the forward transformer switching power supply is approximately equal to 2, which is almost small compared with the voltage and current pulsation coefficient S of the flyback transformer switching power supply. Double, 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 shortcomings of 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 than the flyback transformer switching power supply. In addition, the modulation amplitude of the output voltage of the forward transformer switching power supply by the duty cycle is much lower than that of the flyback transformer switching power supply. This is very clear from the comparison between (1-77) and (1-78). It can be clearly seen. Therefore, the forward transformer switching power supply requires a relatively high error signal amplitude for regulating the duty cycle, and a relatively large gain and dynamic range of the error signal amplifier.

5. The forward switching power supply is relatively large.

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

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 bigger disadvantage of the forward transformer switching power supply is that when the control switch is turned off, the back electromotive force voltage generated by the primary coil of the transformer is higher than the back electromotive force voltage generated by the flyback transformer switching power supply. Because generally when the forward transformer switching power supply is working, the duty cycle of the control switch is set at about 0.5, while the duty cycle of the flyback transformer switching power supply control switch is relatively small.

7. The two-tube forward converter can be used in situations with higher voltage input and larger power output.

1. The push-pull switching power supply has a very high transient response speed of output current and good voltage output characteristics. The push-pull switching power supply has the highest voltage utilization 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 cycle. Therefore, its output current transient response speed is very high. , the voltage output characteristics are very good. The push-pull switching power supply has the highest voltage utilization 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 good output voltage characteristics.

After the push-pull switching power supply undergoes bridge rectification or full-wave rectification, its output voltage ripple coefficient and current ripple coefficient are very small. Therefore, a very small energy storage filter capacitor or energy storage filter inductor is needed to obtain a voltage Output voltage with very small ripple and current ripple. Therefore, the push-pull switching power supply is a switching power supply with good output voltage characteristics.

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

The transformer of the push-pull switching power supply has bipolar magnetization poles. The magnetic induction strain range is more than twice that of the unipolar magnetization pole, and the transformer core does not require an air gap. Therefore, the magnetic field of the push-pull switching power supply transformer core is The permeability is many times higher than the magnetic permeability of the transformer core of the forward or flyback switching power supply with unipolar magnetization. In this way, the number of turns of the primary and secondary coils of the push-pull switching power supply transformer is comparable to that of the unipolar magnetization The number of turns in the primary and secondary coils of the transformer is more than doubled. Therefore, the leakage inductance and copper resistance loss of the push-pull switching power supply transformer are much smaller than that of the unipolar magnetized pole transformer, so the working efficiency of the switching power supply is higher.

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

5. Push-pull switching power supply does not have the possibility of two control switches colluding at the same time like half-bridge and full-bridge switching power supply.

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

The main disadvantage of the push-pull switching power supply is that the two switching devices require a high withstand voltage, which must be greater than twice the operating voltage. Therefore, push-pull switching power supplies are 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 applications requiring When the load voltage changes within a large range, especially when 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 push-pull switching power supplies with low power output and an advantage for push-pull switching power supplies with high power output.

Because the coils of high-power transformers are generally wound with multi-strand wires, there is no difference at all between the two sets of primary coils of a push-pull switching power supply transformer and those wound with multi-strand wires, and the two coils are in phase with a single coil. The current density can be reduced by half.

8. A push-pull converter can be regarded as a combination of two forward converters. During a switching cycle, the two forward converters work alternately.

If the two forward converters are not completely symmetrical or balanced, DC bias will occur. The accumulated bias after several cycles will cause the magnetic core to enter a saturated state and cause the excitation current of the high-frequency transformer to overshoot. Large, even damage to 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.

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 the output power of two switching power supplies at the same time. Its output power is approximately twice the output power of a single switching power supply. Therefore, the output power of the half-bridge transformer switching power supply is very large and the working efficiency is very high. After bridge rectification or full-wave rectification, the voltage pulsation coefficient Sv and current pulsation coefficient Si of the output voltage are very small, and only a small amount of filtering is required. For inductors and capacitors, their output voltage ripple and current ripple can be very small.

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

The biggest advantage of the half-bridge transformer switching power supply is that the withstand voltage requirements for the two switching devices can be reduced by half compared to the push-pull transformer switching power supply for the two switching devices. Because the working 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 working voltage and the back electromotive force, which is about twice the power supply voltage. This result is exactly the push-pull type The withstand voltage of the two switching devices of the transformer switching power supply is half. Therefore, half-bridge transformer switching power supplies are mainly used in situations where the input voltage is relatively high. Generally, most high-power switching power supplies with a grid voltage of AC 220 volts use half-bridge transformer switching power supplies.

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

4. The main disadvantage of the half-bridge transformer switching power supply is the low power utilization rate.

Therefore, the half-bridge transformer switching power supply is not suitable for applications with low operating voltage. In addition, the two switching devices in the half-bridge transformer switching power supply have no common ground connection, making the connection with the driving signal more troublesome.

5. The disadvantage of the half-bridge switching power supply is that there will be a semi-conducting area and high loss.

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 state, the two switching devices will have a short-term semi-conducting area at the same time, that is, the two control switches are in the alternating state at the same time. On state. This is because when the switching device starts to turn on, it is equivalent to charging the capacitor. It requires a transition process from the off state to the fully on state; and when the switching device transitions from the on state to the off state, it is equivalent to charging the capacitor. Discharge also requires a transition process from the conductive state to the completely cut-off state.

When the two switching devices are in the on-and-off transition process respectively, that is, when both switching devices are in the semi-conducting state or the semi-conducting state, it is equivalent to two control switches being turned on at the same time, which will cause an impact on the power supply voltage. Short circuit; 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, during the transition period of the two control switches K1 and K2 at the same time, the two switching devices will produce a large power loss. In order to reduce the loss caused by the transition process of the control switch, generally in the half-bridge switching power supply circuit, the turn-on and turn-off times of the two control switches are intentionally staggered by a short period of time.

6. The single-capacitor half-bridge transformer switching power supply saves one capacitor than the dual-capacitor half-bridge transformer switching power supply. This is its advantage. In addition, when the single-capacitor half-bridge transformer switching power supply first 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 for use as a fluorescent lamp power supply, such as energy-saving lamps or Fluorescent lamps and backlights for LCD displays, etc.

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

7. The single-capacitor half-bridge transformer switching power supply also has disadvantages, that is, the withstand voltage requirements of the switching device are higher than those of the dual-capacitor half-bridge transformer switching power supply.

1. The 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 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 output power of the full-bridge transformer switching power supply is very large and the working efficiency is very high. After bridge rectification or full-wave rectification, the voltage pulsation coefficient Sv and current pulsation coefficient Si of the output voltage are very small, and only a small With a small value of energy storage filter capacitor or energy storage filter inductor, an output voltage with very small voltage ripple and current ripple can be obtained.

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

The biggest advantage of the full-bridge transformer switching power supply is that the withstand voltage requirements for four switching devices can be reduced by half compared to the voltage withstanding requirements for two switching devices of the push-pull transformer switching power supply. Because the four switching devices of the full-bridge transformer switching power supply are divided into two groups, and the two switching devices are connected in series with each other during operation. When turned off, the voltage endured by each switching device is only half of the voltage endured 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. 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 situations where the input voltage is relatively high. When the input voltage is very high, the output power of a full-bridge transformer switching power supply is larger than that of a push-pull transformer switching power supply. a lot of.

Therefore, most 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, the total voltage drop when the two switching devices are turned on is higher than that of a single switching device. The voltage drop when turned on is twice as large; but the power utilization rate is much higher than that of the half-bridge transformer switching power supply.

Therefore, the full-bridge transformer switching power supply can also be used in situations where the working power supply voltage is relatively low.

5. Like the half-bridge switching power supply, the primary coil of the transformer of the full-bridge switching power supply only requires one winding, which is also its advantage. This brings some convenience to the coil winding of the small-power switching power supply transformer. However, there is no advantage in winding the coil of a high-power switching power supply transformer, because the coil of a 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 relatively large power loss. Therefore, the full-bridge transformer switching power supply is not suitable for applications with low operating voltage, otherwise the working efficiency will be very low. In addition, the four switching devices in the full-bridge transformer switching power supply do not have a common ground connection, making the connection with the drive signal more troublesome.

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

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 state, the four switching devices will have a short-term semi-conducting area at the same time, that is, the two sets of The control switch is on at the same time. This is because when the switching device starts to turn on, it is equivalent to charging the capacitor. It requires a transition process from the off state to the fully on state; and when the switching device transitions from the on state to the off state, it is equivalent to charging the capacitor. Discharge also requires a transition process from the conductive state to the completely cut-off state.

When two sets of switching devices are in the on-and-off transition process respectively, that is, when both sets of switching devices are in a semi-conducting state, it is equivalent to two sets of control switches being turned on at the same time, which will cause a short circuit to the power supply voltage; at this time, A large current will appear in the series circuit of 4 control switches, and this current does not pass through the transformer load. Therefore, during the transition period of the four control switches K1, K4 and K2, K3 at the same time, the four switching devices will produce a large power loss. In order to reduce the loss caused by the transition process of the control switch, generally in the full-bridge switching power supply circuit, the turn-on and cut-off times of the two sets of control switches are intentionally staggered by a short period of time.

Double-ended isolated PWM DC/DC converter, during a switching cycle, power is input alternately from one end and the other end of the primary winding of the isolation transformer, 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.