In order to characterize the quality of various voltage or current waveforms, the parameters such as amplitude, average value, effective value, and first harmonic of voltage or current 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 ripple factor (1-84)
Si = Im/Ia - current ripple factor (1-85)
Kv = Ud / Ua - voltage waveform coefficient (1-86)
Ki = Id/Ia - current waveform coefficient (1-87)
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, and 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 quarter of the maximum current of the secondary winding 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 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 flyback switching power supplies are relatively poor.
Since the flyback switching power supply only supplies energy to the load during the switch off period, 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 iron 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 the flyback switching power supply, the voltage device has both energy storage functions and functions of voltage transformation and isolation.
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. At this time, 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 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 on and off. Therefore, the load capacity of the forward-type 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, 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.
The forward voltage transformer and switching power supply have 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 transformer switching power supply are much better than the flyback transformer switching power supply.
The 4 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 forward transformer switching power supply requires a higher error signal amplitude for adjusting the duty cycle, and the gain and dynamic range of the error signal amplifier are also relatively large.
5 Forward switching power supplies are relatively large.
Forward-type 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 relatively large (the volt-second capacity is equal to the product of the input pulse voltage amplitude and the pulse width, which is 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-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.
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 the counter-electromotive voltage generated by the primary coil of the transformer is higher than the counter-electromotive voltage generated by the flyback switching power supply when the control switch is turned off. 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.
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