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In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

In the current form of the global energy crisis, improving the energy efficiency of Electronic equipment, achieving high performance while reducing energy consumption, has become a new focus of the industry. To comply with this trend, many electronics manufacturers in the world hope to improve energy efficiency standards in product specifications. In terms of power management, it is difficult to achieve new energy efficiency standards with traditional hard-switching converters. Therefore, power supply designers have turned their development direction to soft-switching topologies to improve the energy efficiency of the power supply and achieve higher operating frequencies.

In the current form of the global energy crisis, improving the energy efficiency of electronic equipment, achieving high performance while reducing energy consumption, has become a new focus of the industry. To comply with this trend, many electronics manufacturers in the world hope to improve energy efficiency standards in product specifications. In terms of power management, it is difficult to achieve new energy efficiency standards with traditional hard-switching converters. Therefore, power supply designers have turned their development direction to soft-switching topologies to improve the energy efficiency of the power supply and achieve higher operating frequencies.

LLC resonant converter is a soft-switching topology that allows zero-voltage switching of the main power switch, significantly reducing switching losses and greatly improving power efficiency. In this topology, in order to realize the ZVS switch, the parasitic body diode of the power switch must have a very short reverse recovery time. If the body diode cannot recover all the carriers, hard switching operations may occur during the load change from low to high, and the parasitic bipolar transistor may be turned on.

1 Introduction

In consumer application markets such as telecommunications equipment power supplies, mainframe computers/servers, electric welding machines, and steel cutting machines, the demand for power density is increasing every year. To increase power density, it is necessary to reduce the number of components, reduce power loss, and reduce the size of heat sinks and passive components. Currently, hard-switching half-bridges are the typical topology for these applications, and LLC resonant half-bridges are emerging alternatives. The LLC topology ensures that the voltage of the switch tube is zero before turning on (or the current of the switch tube is zero during the turn-off period), thereby eliminating the power loss caused by the overlap of current and voltage during each switch.

The use of this switching technology in high-frequency applications can also reduce switching losses, thereby helping to reduce the size of passive devices. Obviously, the reduction of switching power loss makes it possible to select a smaller radiator in the application design. The zero voltage condition occurs due to the conduction of the MOSFET parasitic body diode. During rapid load changes, the MOSFET switches from zero voltage switching to zero current switching. In this case, a high dv/dt value can turn on the parasitic bipolar transistor and burn the MOSFET.

2. Introduction to Topology

The basic half-bridge circuit of LLC topology is composed of two switching tubes, the high-side switching tube (Q1) and the low-side switching tube (Q2) are connected to the transformer through the Inductor Lr and the capacitor Cr (see Figure 1). The switch tube is connected in parallel with the parasitic body diodes (D1 and D2) and the parasitic output capacitors (C1 and C2). In order to clarify their role in the global function, we mark them separately in the figure.

In Figure 1, we noticed an extra Lm inductor. In fact, Lm is the transformer leakage inductance, and its rules are very important in LLC topology.
  

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

Figure 1: LLC half-bridge circuit

If the primary inductance Lm of the transformer is large and will not affect the resonant network, the converter shown in the figure above is a series resonant converter.
  

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

figure 2

In a resonant unit, when the input signal frequency (fi) is equal to the resonant frequency (fr)-that is, when the LC impedance is zero, the gain is maximum. The operating frequency range of the resonant converter is defined by two specific resonant frequency values, which are related to the circuit. The drive controller sets the switching frequency (fs) of the MOSFET equal to the circuit resonance frequency to ensure the important advantage of resonance.

Now we will see how to change the load to change the resonant frequency from the minimum value (fr2) to the maximum value (fr1):
  

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

When in LLC topology, what is the reason for choosing a MOSFET with fast body diode recovery?

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

LLC is like a series RC resonator; this function appears under high load conditions, that is, when Lm is connected in parallel with low impedance; when in LLC topology, what is the reason for choosing a MOSFET with fast body diode recovery, LLC is similar For parallel RC resonators, this function appears under low load conditions. The system usually does not work in this area because it can operate under ZCS conditions. If the frequency fi is in the range of fr2 << fi << fr1, the two functions exist at the same time.
If we use graphs to represent the gain of the resonant unit, we get the curve shown in Figure 3. It is not difficult to see that the graph change is related to the Q value.

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

image 3

The operating range of the LLC resonant converter is limited by the peak gain. It is worth noting that the peak voltage gain neither occurs at fr1 nor fr2. The peak gain frequency corresponding to the peak gain is the maximum frequency between fr2 and fr1. As the Q value decreases (as the load decreases), the peak gain frequency shifts to fr2, and a higher peak gain is obtained. As the Q value increases (the load increases), the peak gain frequency shifts to fr1, and the peak gain decreases. Therefore, full load should be the worst working condition for resonant network design.

From the perspective of MOSFET, as mentioned earlier, the soft switching of MOSFET is an important advantage of resonant converters including LLC. For the entire system, since the output current is a sine wave, EMI interference is reduced. Figure 4 shows the typical waveform characteristics of LLC converters.

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

Figure 4: Typical waveform of LLC converter

We noticed in Figure 4 that the drain current Ids1 swings in the negative current region before becoming positive. A negative current value indicates that the body diode is conducting. At this stage, due to the voltage drop across the diode, the voltage across the drain and source of the MOSFET is very small. If the MOSFET is switched during the body diode conduction period, ZVS switching occurs, and the switching loss is reduced. This feature can reduce the size of the radiator and improve the energy efficiency of the system.

If the MOSFET switching frequency fs is less than fr1, the shape of the current on the power device will change. In fact, if the duration is long enough to produce a discontinuous current on the output diode, the shape of the primary current will deviate from the sinusoidal waveform.
 

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

Figure 5: Typical waveforms of LLC converter at fs

In addition, if the parasitic output capacitances C1 and C2 of the MOSFET have the same capacitance value as Cr, the resonance frequency fr will also be affected by the device. It is for this reason that in the design process, choosing a Cr value greater than C1 and C2 can solve this problem and make the fr value unaffected by the devices used.

3. Freewheeling and ZVS conditions

Analyzing the resonant frequency equation will find that when the frequency is higher than the peak gain frequency, the input impedance of the resonant network is inductive reactance, and the input current (Ip) of the resonant network lags the input voltage (Vd) of the resonant network. Below the peak gain frequency, the input impedance of the resonant network becomes capacitive reactance, and Ip leads Vd. When working in the capacitive region, the body diode performs a reverse recovery operation during the MOSFET switching period.

When the system works in the capacitive region, the MOSFET will face a great risk of potential failure. In fact, as shown by the green circle in Figure 6, the reverse recovery time of the parasitic body diode becomes very important.
 

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

Image 6

According to this, in the process of load from low to high (Figure 7), the drive circuit should force the MOSFET to enter the ZVS and positive turn-off current region. If it cannot be guaranteed, the working area of ​​the MOSFET may be dangerous.
 

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

Figure 7

Under the low-load steady-state condition, the system works near the lower resonant frequency fr2, and then ZVS is turned on, and the drain current is guaranteed to be turned off. After the load changes (from low to high), the switching frequency should become the new resonant frequency. If this does not happen (as shown by the green line in Figure 8), the system state passes through zone 3 (ZCS zone) and ZVS is turned on, and the positive turn-off drain current will not appear. Therefore, when the MOSFET is turned off, current also flows through the parasitic body diode.

Analyzing the process of the load from low to high on the gain chart, it is not difficult to find:
  

In LLC topology, what is the reason for choosing MOSFET with fast body diode recovery?

Figure 8

The black dashed line represents the ideal path during load changes, while the green dashed line represents the actual path. In the process of changing the load from low to high, it can be seen that the system passes through the ZCS area. Therefore, the performance of the parasitic body diode becomes very important. For this reason, the trend in new LLC designs is to use power devices with very short body diode recovery times.


"In the current form of the global energy crisis, improving the energy…