Discussion on Optimization of Buffer Circuit of High Frequency Power Module

Abstract: The noise of high-frequency power modules mainly comes from power conversion and output rectifier filter circuits. The ZVZCS PWM full-bridge converter realizes the soft switching of the switch tube, but its output rectifier diode does not work in the soft switching state. When the output rectifier diode is commutating, parasitic oscillation exists on the secondary side of the converter. This article discusses the causes and suppression methods.
Keywords: high frequency; spike; discussion

1 Reverse recovery process of the secondary rectifier diode
In fact, when a reverse voltage is suddenly applied to a diode that has been turned on, after the current drops to zero, it does not stop conducting immediately, but remains in a reverse low-resistance state. At this time, under the action of the reverse voltage, the carriers enter the recombination process, so the current continues to flow in the reverse direction; when the carrier recombination is completed, the reverse current rapidly decays to zero. This stage is the reverse recovery process of the diode, as shown in Figure 1.

During the reverse current decay process, the circuit produces a strong transient process, which generates extremely high overvoltage at both ends of the turn-off element, namely the commutation overvoltage; in addition, because there is both current and voltage on the turn-off element when the current decays, a huge amount of power is instantaneously generated in the element, namely the so-called turn-off power.
The equivalent circuit of the diode oscillation is shown in Figure 2.

In the figure, Lk is the leakage inductance of the transformer, Lp is the series parasitic inductance of the diode, Cp is the parallel parasitic capacitance of the diode, and VD is the ideal diode.
When the secondary voltage is zero, all four diodes in the full-bridge rectifier are turned on, and the output filter inductor current is in a natural freewheeling state. When the secondary voltage changes to a high voltage U2, two diodes in the rectifier bridge are turned off, and two diodes continue to be turned on. At this time, the leakage inductance of the transformer and the series parasitic inductance Lp of the rectifier begin to generate parasitic oscillations with the parallel parasitic capacitance Cp of the rectifier. The high-frequency oscillation waveform with exponential decay of the diode current and voltage waveform will generate a very high reverse voltage surge at the moment the diode is turned off. Its existence not only increases the power consumption of the diode, but also has a great impact on the output power quality. Especially in high-power applications, huge voltage spikes are likely to cause overvoltage breakdown of the diode. Therefore, special attention should be paid in the design.

2 Countermeasures to reduce voltage spikes
The reverse recovery time of the rectifier diode is determined by the performance of the device itself and is also affected by many circuit factors. These include the magnitude of the forward current flowing through it when it is turned on, the rate of decrease of the forward current, the magnitude of the reverse voltage, and the rate of increase of the reverse voltage.
The reverse current i is the source of the voltage spike, and reducing the value of i is undoubtedly the fundamental measure to suppress the spike. Select a suitable rectifier diode, such as a fast recovery diode. Although the reverse recovery time is short and the reverse recovery loss is small, the recovery characteristics are relatively hard, and the voltage spike is still large. A soft fast recovery diode with a relatively soft recovery characteristic (small tb/ta value) can be appropriately selected. In addition, appropriately increasing the diode current capacity or connecting multiple tubes in parallel to reduce the forward current passing through each tube can have a positive effect on suppressing voltage spikes. Reasonable layout and wiring, reducing the transformer leakage inductance and lead inductance, and thus reducing oscillation is also a fundamental method to suppress spikes.
When the device is selected and the wiring is completed, we can also suppress the voltage spike by adding an external buffer circuit. Commonly used snubber circuits are as follows:
(1) RC absorption circuit
The most common way to solve the reverse recovery problem of power diodes is to use an RC absorption circuit, which is a series branch of R and C connected in parallel to each diode. The RC absorption circuit is shown in Figure 3. When the diode is reversely turned off, the energy in the parasitic inductance charges the parasitic capacitance, and at the same time charges the absorption capacitor C through the absorption resistor R. When absorbing the same energy, the larger the absorption capacitor, the smaller the voltage on it; when the diode is quickly forward-conducted, C discharges through R, and most of the energy will be consumed on R. Although this absorption network can effectively suppress the reverse voltage spike, it is lossy, which is equivalent to transferring the turn-off loss of the rectifier diode to the RC absorption circuit, which is not conducive to improving the efficiency of the converter.
(2) Active clamping
In order to reduce losses, someone proposed an active clamping circuit, which consists of a clamping switch tube TVs, a clamping diode VDs and a clamping capacitor Cs, and the capacity of Cs is relatively large. As shown in Figure 4.

The active clamping snubber circuit can clamp the voltage on the rectifier bridge to an appropriate voltage. And because there is no resistance in the snubber circuit, there is no loss. At the same time, TVs has zero voltage switching and no switching loss, so the loss of the active clamping snubber circuit is much smaller than that of the RC absorption circuit. However, this method requires the addition of a set of control circuits and an active device TVs, which increases the complexity of the system and reduces reliability.
(3) Series saturated inductor (spike suppressor)
Series saturated inductor (spike suppressor) is another common method to solve the reverse recovery problem of diodes, as shown in Figure 5.

During normal flow, the core that suppresses noise is saturated, has very low inductance, and stores almost no energy. When the current decreases and tries to cross zero, the core of the rectangular hysteresis loop exits saturation and exhibits a large inductance. This large inductance prevents the current from changing in the opposite direction, suppresses the reverse current, and eliminates the spike caused by the reverse current. Spike suppression is usually achieved using a spike suppressor made of rectangular hysteresis loop material.

When the diode is turned on, current Io flows (“I” in Figure 6(a)), the spike suppressor is saturated (“I” in Figure 6(b)), the magnetic permeability is the air magnetic permeability μo, and the equivalent inductance of the spike suppressor is very small, equivalent to the inductance of the wire.

When the diode is turned off, its forward current decreases from Io to zero ("II" in Figure (a)), and the core is demagnetized along the magnetization curve "II" until the Br value on the ordinate. The core still presents low impedance. Since the diode is still in the on state due to the stored charge, and there is a reverse voltage in the circuit, it attempts to flow a reverse current. If there is no spike suppressor, a large reverse recovery current will flow under the action of the reverse voltage (as shown by the dotted line in Figure (a)). This large current stores energy in the parasitic inductance, and then enters the reverse recovery time trr, and the reverse current of the diode decreases. When this reverse recovery current decreases, it causes a large voltage spike and circuit noise. When the spike suppressor is connected in series, the diode starts to try to flow reverse current under the action of reverse voltage. The spike suppressor exits saturation and presents a large impedance. Only a very small reverse current (the zero-crossing shaded part "III" in Figure (a)) causes the core to demagnetize along the "III" section of the magnetization curve. Here, the magnetic permeability is very high and the apparent inductance is very large, which effectively blocks the reverse recovery current with high di/dt, turning hard recovery into soft recovery, greatly reducing the noise. Most of the magnetization energy is converted into hysteresis loss and eddy current loss.
If the volt-second of the core is large enough during the reverse recovery time of the diode, that is, there is no reverse saturation (point "IV" in Figure (b)) before the diode reverse blocks ("IV" in Figure (a)), and the diode is fully recovered, the noise can be basically eliminated.
When the diode is turned on again ("V" in Figure (a)), the core is still in high impedance, reducing the rate of rise of the forward current of the diode. In high-power diodes, it is beneficial to improve the forward recovery characteristics of the diode. The core is saturated and magnetized by the forward current through "V". Repeat the process of "I" to "V" later. From the working principle, we can see that the magnetic beads have excellent noise suppression performance. To suppress the noise in the circuit, the following formula must be satisfied:

3 Conclusion
The above schemes suppress voltage spikes and reduce the loss of the buffer circuit, but increase the number of magnetic components.