Basic principles and working methods of flyback switching power supply

Basic principles and working methods of flyback switching power supply

Basic principles and working methods

Fundamental

When the transistor Trton is switched, the primary Np of the transformer has a current Ip and stores energy in it (E=LpIp/2). Since Np and Ns have opposite polarities, the diode D is reversely biased and cut off at this time, and no energy is transferred to Load. When switching Troff, according to Lenz's law: (e=-N△Φ/△T), the primary winding of the transformer will generate a reverse potential. At this time, the diode D is forward-conducting, and the load has current IL flowing. Steady state waveform of flyback converter

The size of the conduction time ton will determine the amplitude of Ip and Vce:

Vcemax=VIN/1-Dmax

VIN: input DC voltage; Dmax: maximum working cycle

Dmax=ton/T

It can be seen that in order to obtain a low collector voltage, Dmax must be kept low, that is, Dmax<0.5. In practical applications, Dmax=0.4 is usually taken to limit Vcemax≦2.2VIN.

The collector operating current Ie when switching the tube Tron, that is, the primary peak current Ip is: Ic=Ip=IL/n. Because IL=Io, when Io is constant, the size of the turns ratio n determines the size of Ic , the above formula is derived based on the principle of power conservation and the number of primary and secondary ampere turns is equal to NpIp = NsIs. Ip can also be expressed by the following method:

Ic=Ip=2po/(η*VIN*Dmax)η: Converter efficiency

The formula is derived as follows:

Output power:po=LIp2η/2T

Input voltage: VIN=Ldi/dt, assuming di=Ip, and 1/dt=f/Dmax, then:

VIN=LIpf/Dmax or Lp=VIN*Dmax/Ipf

Then po can be expressed as:

po=ηVINfDmaxIp2/2fIp=1/2ηVINDmaxIp

∴Ip=2po/ηVINDmax

In the above formula:

VIN: Minimum DC input voltage (V)

Dmax: maximum conduction duty cycle

Lp: Transformer primary inductance (mH)

Ip: transformer primary side peak current (A)

f: conversion frequency (KHZ)

Way of working

Flyback transformers generally work in two modes:
1. Inductor current discontinuous mode DCM (DiscontinuousInductorCurrentMode) or "complete energy conversion": all the energy stored in the transformer at ton is transferred to the output during the flyback period (toff).

2. Inductor current continuous mode CCM (ContinuousInductorCurrentMode) or "incomplete energy conversion": part of the energy stored in the transformer is retained at the end of toff until the beginning of the next ton cycle.

DCM and CCM are very different in terms of small signal transfer functions. Their waveforms are shown in Figure 3. In fact, when the converter input voltage VIN changes within a large range, or the load current IL changes within a large range When , it must span two working modes. Therefore, the flyback converter is required to work stably in DCM/CCM. But it is more difficult to design. Usually we can use the DCM/CCM critical state as the design basis. Coupled with current mode control pWM. This method can effectively solve various problems in DCM, but it does not eliminate the inherent instability problem of the circuit in CCM. CCM can be solved by adjusting the control loop gain to separate the low frequency band and reduce the transient response speed. The instability is caused by the "right half-plane zero" of the transfer function.

DCM and CCM are very different in terms of small signal transfer functions.

DCM/CCM primary and secondary current waveform diagram

In fact, when the converter input voltage VIN changes within a large range, or the load current IL changes within a large range, it must span two operating modes. Therefore, the flyback converter requires DCM/CCM Both can work stably. But it is more difficult to design. Usually we can use the DCM/CCM critical state as the design basis, and use current mode control pWM. This method can effectively solve various problems in DCM, but in There is no inherent instability problem in the circuit during CCM. The instability caused by the "right half plane zero point" of the transfer function in CCM can be solved by adjusting the control loop gain to separate the low frequency band and reduce the transient response speed.

In a stable state, the change in magnetic flux increment ΔΦ at ton must be equal to the change at "toff", otherwise the magnetic core will be saturated.

therefore,

ΔΦ=VINton/Np=Vs*toff/Ns

That is, the volts/second value of each turn of the primary winding of the transformer must be equal to the volts/second value of each turn of the secondary winding.

Comparing the current waveforms of DCM and CCM in Figure 3, we can know that during the Trton period in the DCM state, the entire energy transfer waveform has a higher primary peak current. This is because the primary inductance value Lp is relatively low, making Ip sharply The negative effect caused by the increase is to increase the winding loss (winding loss) and the ripple current of the input filter capacitor, which requires the switching transistor to have a high current carrying capacity in order to work safely.

In the CCM state, the peak current of the primary side is low, but the switching crystal has a high collector current value in the ton state. This results in high power consumption of the switching crystal. At the same time, in order to achieve CCM, a higher primary voltage of the transformer is required. The side inductance value Lp and the residual energy stored in the transformer core require the volume of the transformer to be larger than that of DCM, while other coefficients are equal.

To sum up, the design of DCM and CCM transformers is basically the same, except for the definition of the primary side peak current (Ip=Imax-Imin in CCM).