Analysis of all buffer absorption circuits in switching power supplies
“Prevent device damage, absorb to prevent voltage breakdown, buffer to prevent current breakdown; keep power devices away from dangerous operating areas, thereby improving reliability; reduce (switching) device losses, or achieve some degree of soft-off.
The effect of absorption and buffering:
Prevents device damage, absorbs against voltage breakdown, and buffers against current breakdown
Improve reliability by keeping power devices out of hazardous operating areas
Reduce (switching) device losses, or achieve some degree of soft-opening
Reduce di/dt and dv/dt, reduce ringing, improve EMI quality
Improve efficiency (it is possible to improve efficiency, but it may also reduce efficiency if you do it wrong)
In other words, preventing device damage is only one of the functions of absorption and buffering, and other functions are also valuable.
Absorption is for voltage spikes.
Causes of voltage spikes:
Voltage spikes are caused by Inductor freewheeling.
The inductance that causes the voltage spike may be: transformer leakage inductance, line distributed inductance, inductive component in the equivalent model of the device, etc.
Currents that cause voltage spikes can be: topology currents, diode reverse recovery currents, inappropriate resonant currents, etc.
The main measures to reduce voltage spikes are:
Reduce inductances that can cause voltage spikes, such as leakage inductance, wiring inductance, etc.
Reduce currents that can cause voltage spikes, such as diode reverse recovery currents, etc.
If possible, divert the above inductive energy elsewhere.
After taking the above measures, the voltage spike is still unacceptable, and absorption is finally considered.Absorption is a technical measure of last resort
The switch tube Q1, the topological freewheeling diode D1 and a lossless topological capacitor C2 form an absorption loop as short as possible on the wiring.
Features of topological absorption:
At the same time, the voltage spikes and ringing of Q1 and D1 are reduced to a minimum.
Topological absorption is lossless absorption with higher efficiency.
The absorption capacitor C2 can take values in a wide range.
Topological absorption is hard switching because topology is hard switching.
Body diode reverse recovery absorption
The reverse recovery characteristics of the body diode of the switching device play a role in the rising edge of the turn-off voltage, and have the absorption effect of reducing the voltage spike.
The essence of RC absorption is damping absorption.
Some people think that R is the current limiting effect, and C is the absorption. The reality is just the opposite.
The most important role of the resistor R is to generate damping and absorb the resonance energy of the voltage spike, which is a power device.
The function of capacitor C is not to absorb voltage, but to provide energy channel for R damping.
RC absorption is connected in parallel with the resonant circuit, C provides the resonance energy channel, the size of C determines the absorption degree, and the ultimate purpose is to make R form power absorption.
Corresponding to a specific absorption environment and a specific size of capacitance C, there is an optimal size of resistance R, which forms the largest damping and obtains the lowest voltage spike.
RC absorption is non-directional absorption, so RC absorption can be used for absorption in unidirectional circuits, as well as absorption in bidirectional or symmetrical circuits.
RC Absorption Design
The difficulty of the design method of RC absorption is that absorption is related to too many factors, such as leakage inductance, winding structure, distributed inductance and capacitance, device equivalent inductance and capacitance, current, voltage, power level, di/dt, dv/dt, frequency, Diode reverse recovery characteristics, etc. And some of these factors are difficult to obtain accurate design parameters.
For example, the absorption of diode back pressure, even if the other conditions are exactly the same, the RC absorption parameters required by different diode models may vary greatly. It is difficult to derive a general calculation formula.
The power loss in R can be roughly estimated as:
Ps = FCU2
Among them, U is the reflected voltage of the absorption loop topology.
In engineering, after the preliminary parameters are obtained through calculation or simulation, the final design parameters must be debugged on the board according to the actual wiring.
The RCD absorption is not damped absorption, but relies on the nonlinear switch D to directly destroy the resonance conditions that form the voltage spike, controlling the voltage spike to any desired level.
The magnitude of C determines the absorption effect (voltage spike), and at the same time determines the absorption power (ie, the thermal power of R).
The role of R is to dissipate the absorbed energy in the form of heat. The minimum value of its resistance should meet the current limit of the switch tube, and the maximum value should meet the needs of the PWM reverse RC discharge cycle. The value within this range has little effect on the absorption effect.
RCD absorption will achieve a certain degree of soft turn-off on the protected switching device, because the voltage on the switching device at the moment of turn-off, that is, the voltage on the absorption capacitor C is equal to 0, and the turn-off action will form a charge on C. During the process, the voltage recovery is delayed, the dv/dt is reduced, and the soft turn-off is realized.
RCD absorption is generally not suitable for the absorption of flyback topologies because RCD absorption may conflict with flyback topologies.
RCD absorption is generally not suitable for the absorption of diode reverse voltage spikes, because the RCD absorption action may increase the diode reverse recovery current.
Although the RCD clamp and RCD snubber circuit can be exactly the same, the component parameters and operating conditions are completely different. The RCD sinks RC time constant much smaller than the PWM period, while the RCD clamps the RC time constant much larger than the PWM period.
Different from the full charge and full discharge condition of the RCD absorption capacitor, the capacitor clamped by the RCD can be regarded as a voltage source, and the valley value of the RC charge and discharge amplitude should not be less than the topological reflection voltage, and the peak value is the clamp voltage.
Since the RCD clamp does not act on the rising and falling edges of the PWM voltage, it only acts when the voltage spike occurs, so the RCD clamp is a high-efficiency absorption.
Several forms of Zener clamps.
Zener clamping also works on voltage spikes and is also highly efficient at absorbing.
In some cases, the Zener clamp needs to consider the effect of the reverse recovery characteristics of the Zener diode on the circuit.
Zener absorption needs to pay attention to the absorption power matching, and if necessary, active power devices can be used to form a high-power equivalent circuit
Conditions for lossless absorption
Absorptive networks must not use resistors.
LD current loops must not be formed.
The sink loop must not be a topological current path.
The absorbed energy must be transferred to the input side or the output side.
Minimize the effect of the reverse recovery current of the sink loop diode.
Lossless absorption is a strong absorption that can not only absorb voltage spikes, but even topologically reflected voltages, such as:
Buffering is for inrush current spikes
The first situation that causes current spikes is diode (including body diode) reverse recovery current.
The second situation that causes current spikes is the charging and discharging current to the capacitor. These capacitances may be: circuit distributed capacitance, equivalent distributed capacitance of transformer windings, improperly designed snubber capacitors, improperly designed resonant capacitors, capacitive components in the equivalent model of the device, and so on.
The basic method of buffering:
Insert some type of inductance in series with the path of the inrush current spike, which can be of the following types:
Since the series-insertion of the snubber inductance will significantly increase the workload of the absorption, the snubber circuit generally needs to be used in conjunction with the absorption circuit.
The snubber circuit delays the on-current surge, enabling some degree of soft turn-on (ZIS).
Transformer leakage inductance can also act as a snubber inductance.
Absorptive circuit coordination may not be required.
The current stress of the snubber energy release diode is equal to or greater than that of the topological freewheeling diode.
The loss of the snubber energy release diode can be simply understood as the loss reduced by the switch.
Appropriate snubber inductance (L3) parameters can greatly reduce switch losses and achieve high efficiency.
A snubber circuit is required to transfer the residual energy of the inductor.
The loss of the buffer energy release resistor R is relatively large, which can be simply understood as the loss transferred from the switch tube.
The R and L parameters must be optimally matched, and it is difficult to master the parameter design and debugging.
High efficiency can still be achieved as long as the parameters are appropriate.
Saturated inductive buffer
The electrical performance of saturated inductance is sensitive to di/dt.
At the rising edge of an inrush current, a large impedance begins to appear, and gradually enters saturation as the current increases, thereby delaying and weakening the inrush current spike, that is, achieving soft turn-on.
After the current reaches a certain level, the saturable inductance presents a very low impedance due to saturation, which is conducive to high-efficiency power transfer.
When the current is turned off, the inductor gradually exits the saturation state. On the one hand, because the saturation inductance in the previous saturation state is very small, that is, the energy storage and the required energy release are small. On the other hand, the recovery of the inductance at the time of exit can slow down the rising speed of the voltage, which is conducive to the realization of soft turn-off.
Taking Ls2 as an example, 5u means that the magnetic circuit cross-sectional area is 5mm2, which is roughly equivalent to a small magnetic core made of PC40 material of 4*4*2.
Saturation Inductance Characteristics
Saturation inductors are power devices that absorb current spike energy through hysteresis losses (rather than eddy current losses or copper losses) entering and exiting the saturation process, with the main thermal power coming from the magnetic core.
On the one hand, it requires that the magnetic core should be a high-frequency material, and on the other hand, it requires that the temperature of the magnetic core should not exceed the Curie temperature under any circumstances. This means that the magnetic core of the saturated inductor should have the most favorable heat dissipation characteristics and structure, namely: higher Curie temperature, higher thermal conductivity, larger heat dissipation area, and shorter heat conduction path.
Obviously, the saturation inductance generally does not need to consider the use of air gaps or low permeability materials that are not easily saturated.
Initial Inductance Equivalent Characteristics
Under other conditions being the same, the initial inductance of the magnetic core with lower magnetic permeability with more turns is equivalent to that of the magnetic core with higher magnetic permeability with fewer turns of saturation inductance, and the buffering effect is roughly the same.
This means that it is always possible to directly use a 1-turn feedthrough inductance, since any multi-turn inductance can always find a higher permeability core with 1-turn equivalent. This also means that the maximum permeability of the core is limited, and if a suitable core has a 1-turn saturation inductance, there will be no possibility of using a higher permeability core with fewer turns.
Core Volume Equivalent Characteristics
Under the same other conditions, the saturation inductance buffering effect of the same volume of the magnetic core is roughly the same. In this case, the magnetic core can be designed according to the magnetic circuit that is most conducive to heat dissipation. For example, a slender tubular magnetic core has a larger heat dissipation surface area than a ring-shaped magnetic core, a plurality of small magnetic cores are concentrated in a large magnetic core, and a feed-through inductance is obviously larger than a multi-turn inductance.
Sometimes, the magnetic core of a single material cannot achieve the buffering effect required by the project, and the use of magnetic cores of multiple materials may be able to meet the needs of the project.
Passive lossless buffer absorption
If the snubber inductance itself is lossless (non-saturated inductance), and its inductive energy storage is processed by lossless absorption, it constitutes a passive lossless snubber absorption circuit, which is actually a passive soft-switching circuit.
The existence of snubber inductance delays and weakens the turn-on inrush current to achieve a certain degree of soft turn-on.
The existence of the lossless absorption circuit delays and reduces the dv/dt of the turn-off voltage, realizing a certain degree of soft turn-off.
The conditions for achieving passive soft switching are roughly the same as for lossless absorption. Not all topologies can create a passive soft-switching circuit. Therefore, in addition to the classic circuits, many passive soft-switching circuits are popular with patents.
The efficiency of passive lossless soft-switching circuit is significantly higher than that of other buffer absorption methods, which is almost the same as that of active soft-switching circuit. Therefore, as long as a passive soft-switching circuit can be realized, it is not necessary to use an active soft-switching.
Absorptive snubber circuit performance vs.
The electrolytic capacitors in the circuit generally have a large ESR (the typical value is in the order of hundreds of milliohms), which causes two problems: one is that the filtering effect is greatly reduced; the other is that the ripple current produces a large loss on the ESR, which not only reduces the Efficiency, but also reliability and life problems directly caused by the heating of electrolytic capacitors.
The general method is to connect high-frequency lossless capacitors in parallel with electrolytic capacitors. In fact, this method cannot fundamentally change the above problems. This is because high-frequency lossless capacitors still have a large impedance in the frequency range commonly used in switching power supplies. for the sake of.
The proposed method is to separate the electrolysis and the CBB with an inductor, the CBB is located on the high-frequency ripple current side, and the electrolysis is located on the DC (power frequency) side, each of which undertakes the corresponding filtering task.
Design principle: The resonant frequency Fn of the Π-shaped filter network should be staggered from the PWM frequency Fp. Desirable Fp = (1.5 ~ 2) Fn.
This design idea can be extended to bidirectional buffering of DC bus filtering, or other circuit structures with large filtering stress.
Dangers of ringing:
The MEI test is prone to exceed the standard at the ringing frequency.
Ringing will cause losses in the ringing loop, causing device heating and reducing efficiency.
Ringing voltage amplitude exceeding the critical value will cause ringing current, break the normal working condition of the circuit, and greatly reduce the efficiency.
Causes of ringing:
Ringing is mostly caused by the resonance of the junction capacitance and some equivalent inductance. For a particular frequency of ringing, the cause can always be found. Capacitance and inductance can determine a frequency, and frequency can be observed. The capacitance is mostly the junction capacitance of a device, and the inductance may be the leakage inductance.
Ringing is most likely to occur in lossless (no resistance) loops. For example: the resonance between the secondary diode junction capacitance and the secondary leakage inductance, the resonance between the stray inductance and the device junction capacitance, the resonance between the absorption loop inductance and the device junction capacitance, and so on.
Magnetic bead absorption, as long as the magnetic bead behaves as a resistance at the ringing frequency, the ringing energy can be greatly absorbed, but inappropriate magnetic beads may also increase the ringing.
RC absorption, where C can be roughly equivalent to the ringing (junction) capacitance, and R is selected according to the RC absorption principle.
Change the resonant frequency, for example: as long as the ringing frequency is reduced to a frequency close to the PWM frequency, the ringing on the PWM can be eliminated.
In particular, improper design of the input and output filter circuits may also cause resonance, and it is also necessary to adjust the resonance frequency or other measures to avoid it.
Absorb buffer energy for reuse
RCD absorption energy recovery circuit
As long as the positive and negative circuits of the absorption circuit are separated to form positive and negative current channels relative to 0 potential, positive and negative voltage outputs can be obtained. Its design points are:
The parameters of the RCD absorption circuit should mainly meet the absorption needs of the main circuit. It is not recommended to increase the DC output power by increasing the absorption power.