Discussion on the Peak Absorption Circuit of Rectifier Tube

The RC spike absorption problem of the secondary side rectifier diode of Flyback . I think everyone is still too traditional in dealing with such spike problems. In fact, using RCD absorption here will be better than using RC absorption. With RCD absorption, the rectifier tube spike voltage can be lower (with reasonable parameter matching, it can be completely absorbed and the spike voltage is almost invisible), and the absorption loss is also smaller.

Rectifier diode voltage waveform (RC absorption)

Rectifier diode voltage waveform (RCD absorption)

Judging from these two simulation figures, the absorption effect is equivalent. If the high voltage drop when the diode is turned on is not considered, it can be considered that the absorption is complete.

After the experiment, you will be surprised that the diodes can be SMD (fast switching diodes, 1N4148 is good if the parameters are suitable), and the resistors and capacitors can all be SMD.

If you are designing the parameters of the RC absorption circuit here , please refer to the post: http://bbs.dianyuan.com/topic/200377 , where there are more detailed instructions;

The RCD absorption design here can be considered as follows: in order to absorb the oscillation spike, C should have enough capacitance, so that after absorbing the spike energy, the voltage on the capacitor will not be too high. In order to balance the energy on the capacitor, the resistor R needs to consume the leakage inductance energy stored in the capacitor C. Therefore, the ideal parameter matching is that the energy consumed by the resistor is exactly equal to the energy in the leakage inductance spike (at this time, the voltage at the end of the capacitor C is exactly equal to Uin/N+Uo). Because there are many uncertain factors in the leakage inductance spike energy, the calculation method is difficult to work, so the following introduces an experimental method to design

1. Choose a larger capacitor (such as 100nF) as capacitor C, and choose an ultra-fast recovery diode (such as 1N4148) with a withstand voltage > 1.5*(Uin/N+Uo) for D;

2. You can choose a smaller resistor 10K, 1W resistor as the absorption R;

3. Gradually increase the load and observe the voltage at the capacitor C terminal and the peak voltage of the rectifier tube:

If the voltage ripple on C is greater than 20% of the average value, the C value needs to be increased;

If the voltage at the C terminal is much higher than Uin/N+Uo (more than 20%, depending on the withstand voltage of the rectifier tube) when fully loaded, it means that the absorption is too weak and the resistance R needs to be reduced;

If the voltage on C is lower than or equal to Uin/N+Uo when fully loaded, it means that the absorption is too strong and the resistance R needs to be increased;

If the voltage on C is slightly higher than Uin/N+Uo (5%~10%, depending on the withstand voltage of the rectifier tube) at full load, the design parameters can be considered reasonable;

Under different input voltages, verify whether the parameters are reasonable and finally select the appropriate parameters.

Let's take a look at the absorption loss problems corresponding to the two absorption circuits (taking Flyback as an example):

Adopt RC absorption: The voltage on C after the primary MOS is turned on and reaches the steady state is Vo+Ui/N, (Vo is the output voltage, Ui is the input voltage, and N is the transformer primary-to-secondary turns ratio). Because the time parameter of the RC we designed is much smaller than the switching cycle, it can be considered that within one absorption cycle, the RC charge and discharge can reach the steady state. Therefore, in each switching cycle, the energy lost in its absorption is: secondary leakage inductance peak energy + RC steady-state charge and discharge energy, which is approximately RC charge and discharge energy = C*(Vo+Ui/N)^2 (energy consumed on R, charging and discharging once in each cycle), so the energy consumed by RC absorption is fsw*C*(Vo+Ui/N)^2. Taking DC300V input, 20V output, transformer turns ratio of 5, switching frequency of 100K, and absorption capacitor of 2.2nF as an example, the energy lost is 2.2N*(20+300/5)^2*100K=1.4w;

RCD absorption is used. Because RCD absorption is used, the absorbed energy includes two parts. One part is the DC energy on the capacitor C, and the other part is the peak energy converted from the leakage inductance energy to C. Because the leakage inductance is very small, its peak current cannot be too large, so the energy is also very limited. Relatively speaking, only the DC energy consumed by R is considered. With the same parameters as above, the DC voltage on C is Vo+Ui/N=80V, and the resistor R is 47K, and its energy consumption is 0.14W. Compared with the above 1.4W, the "low carbon" effect is extraordinary.

Let's talk about the characteristics of these two absorption circuits and other absorption circuits:

RC absorption: While absorbing the peak, it also absorbs the square wave energy output by the transformer. The absorption efficiency is low and the loss is large, but the circuit is simple and the absorption cycle is consistent with the switching frequency. It can be used in low standby power consumption circuits.

RCD absorption: Suitable for all places where RC is used to absorb leakage inductance spikes (including forward , flyback , full-bridge , half-bridge and other topologies). The absorption efficiency is higher than that of RC, but there is a situation where the energy stored in the capacitor (usually large) is always consumed . It is not suitable for use in low standby power consumption circuits (including leakage inductance absorption of primary MOS tubes);

Let's discuss ZENER absorption: it can be applied to primary MOS leakage inductance spike absorption, secondary rectifier tube voltage spike absorption, and low standby power consumption circuit. It has the highest absorption efficiency and high cost, but ZENER voltage regulation parameters vary greatly and need to be carefully designed.

The reverse recovery of the rectifier tube will only occur in the continuous working mode. The power supply topology in the intermittent working mode will not have the reverse recovery problem of the rectifier tube;

The capacitance effect of the rectifier tube and the secondary stray capacitance and secondary leakage inductance will cause oscillation. Under the influence of the large dv/dt (voltage change rate between the transformer and the rectifier tube) and the reverse recovery current (continuous mode) of the rectifier tube , this oscillation is manifested as the resonance of the transformer output + output voltage through the secondary leakage inductance and the stray capacitance of the rectifier tube, thereby causing the reverse voltage spike of the rectifier tube.

Generally speaking, the reverse recovery of a diode refers to a dynamic process in which a conducting diode switches from a conducting state to a reverse cut-off state. There are two prerequisites: the diode must have a certain forward current before reverse cut-off (the current size affects the maximum peak current and recovery time of reverse recovery, and the originally cut-off state is not included in this list, so only the continuous mode has a reverse recovery problem); in order to allow the diode to quickly enter the cut-off state, a reverse voltage will be added to both ends of the diode (the size of this reverse voltage also affects the reverse recovery current and recovery time of the known diode). Therefore, to see if there is a reverse recovery problem, you can compare whether it meets these two conditions.

The advantage of the quasi-resonant circuit is that it changes the maximum terminal change voltage N*Uo+Uo of the discontinuous mode rectifier diode into N*Uo-Uo, reducing the voltage change rate of the rectifier diode when the primary MOS tube is turned on, thereby reducing the excitation source of the leakage inductance oscillation and reducing the oscillation spikes it produces. If the amplitude does not affect the voltage withstand safety of the rectifier tube, absorption circuits such as RC can be completely omitted.