Improve the Efficiency and Reliability of Switching Power Supply: Half Bridge Resonant LLC CoolMOS
1. SummaryRecently, LLC topology is favored by power design engineers for its high efficiency and high power density, but the requirements of this soft switching topology for MOSFET exceed any previous hard switching topology. Especially in the case of power startup, dynamic load, overload, short circuit, etc. CoolMOS with its fast recovery body diode, low QG and coss can fully meet these needs and greatly improve the reliability of the power supply system.For a long time, improving the power density, efficiency and reliability of power supply system has been a major issue faced by R & D personnel. Increasing the switching frequency of power supply is one of the methods, but the increase of frequency will affect the switching loss of power devices, so the effect of increasing frequency is not very obvious for hard switching topology, which has reached its design bottleneck. At this time, soft switching topology, such as LLC topology, is sought after by the majority of design engineers because of its unique characteristics. However... This topology puts forward new requirements for power devices.2. Characteristics of LLC circuitThe following characteristics of LLC topology make it widely used in various switching power supplies:1. LLC converter can realize zero voltage switching in a wide load range.2. It can adjust the output when the input voltage and load change in a large range, and the change of switching frequency is relatively small.3. Frequency control is adopted, and the duty cycle of upper and lower tubes is 50%4. Lower voltage MOSFET can be used to reduce the voltage stress of secondary synchronous rectifier MOSFET, so as to reduce the cost.5. No output inductance is required, which can further reduce the system cost.6. The efficiency can be further improved by using lower voltage synchronous rectifier MOSFET.3. Basic structure and working principle of LLC circuitFigure 1 and Figure 2 show the typical circuit and working waveform of LLC resonant converter respectively. As shown in Fig. 1, the LLC converter includes two power MOSFETs (Q1 and Q2) with a duty cycle of 0.5; Resonant capacitance Cr, center tap transformer tr with equal secondary side turns, equivalent inductance LR, excitation inductance LM, full wave rectifier diodes D1 and D2 and output capacitance Co.Fig. 1 typical circuit of LLC Resonant ConverterFig. 2 working waveform of LLC Resonant ConverterLLC has two resonant frequencies, Cr and LR determine the resonant frequency FR1; LM, LR and Cr determine the resonant frequency FR2.When the load of the system changes, the working frequency of the system will change. When the load increases, the switching frequency of MOSFET decreases. When the load decreases, the switching frequency increases.3.1 working sequence of LLC Resonant ConverterThe steady-state working principle of LLC converter is as follows.1ï¼ãt1,t2ãQ1 turns off, Q2 turns on, inductors LR and Cr resonate, secondary D1 turns off, D2 turns on, diode D1 is about twice the output voltage, and the energy is converted from Cr and LR to secondary. Until Q2 is off.2ï¼ãt2,t3ãQ1 and Q2 are turned off at the same time. At this time, the inductor LR and LM current charge the output capacitor of Q2 and discharge the output capacitor of Q1 until the voltage of Q2 output capacitor is equal to VINSecondary D1 and D2 turn off VD1 = VD2 = 0. When Q1 is turned on, the phase ends.3ï¼ãt3,t4ãQ1 on, Q2 off. D1 is on and D2 is off. At this time, VD2 = 2voutCr and LR resonate at FR1. At this time, the current of LS returns to VIN through Q1 until the current of LR is zero order phase.4ï¼ãt4,t5ãQ1 on, Q2 off, D1 on, D2 off, VD2 = 2voutCr and LR resonate at FR1, and the current of LR flows back to power ground through Q1 in reverse. The energy is converted from the input to the secondary until Q1 turns off the end of the phase5ï¼ãt5,t6ï¼Q1 and Q2 are turned off at the same time, D1 and D2 are turned off, and the primary side current I (LR LM) charges the coss of Q1 and discharges coss2 until the coss voltage of Q2 is zero. At this time, Q2 diode begins to turn on. The phase ends when Q2 is turned on.6ï¼ãt6,t7ãQ1 turns off, Q2 turns on, D1 turns off, D2 turns on, Cr and LS resonate at frequency FR1, and LR current returns to ground through Q2. When LR current is zero, the phase ends.3.2 abnormal state analysis of LLC Resonant ConverterThe above description is that LLC works in resonant mode. Next, we analyze the operation of LLC converter under startup, short circuit and dynamic load.3.21 startup status analysisThrough LLC simulation, we get the waveform shown in Figure 3. In the first switching cycle of startup, the upper and lower tubes will have a short peak current ids1 and ids2 at the same time. Since MOSFET Q1 will charge the output capacitor coss of lower tube Q2 when it is turned on, the charging ends when VDS is high. The peak currents ids1 and ids2 are also generated by VIN charging the Q2 junction capacitor coss through MOSFET Q1.Fig. 3 LLC simulation waveformWhen we focus on the second switching cycle, as shown in Figure 4, we find that there will be a peak current similar to the first switching cycle, and the peak will be higher. At the same time, MOSFET Q2 VDS also has a high DV / dt peak voltage. Is this peak current still caused by coss? Let's do further research.Fig. 4 waveform of the second switching cycleEngineers who have a certain understanding of the structure of MOSFET know that MOSFET is different from IGBT. In fact, there is a parasitic diode inside MOSFET. Like ordinary diodes, it needs to neutralize carriers in the cut-off process to reverse recovery. Only reverse voltage is added at both ends of the diode can make this reverse recovery complete quickly, The energy required for reverse recovery is related to the amount of charge qrr of the diode, and the reverse recovery of the body diode also needs to add a reverse voltage at both ends of the body diode. During startup, the voltage added to both ends of the diode VD = Id2 x Ron. While Id2 is almost zero during startup, and it takes a long time for the diode to reverse recover when VD is low. If the dead time setting is not enough, as shown in Figure 5, a high DV / dt will directly trigger the BJT in the MOSFET and breakdown the MOSFETFigure 5Through the actual test, we can repeat the similar waveform. The peak current in the second switching cycle is much higher than that in the first switching cycle. At the same time, when the MOSFET is started, the DV / DT is 118.4v/ns higher, and the VDS voltage exceeds the maximum value of 600V. MOSFET is at risk when starting up.Figure 63.22 abnormal state analysisNext, we continue to analyze the potential risks to the LLC topology when the load changes dramatically.When the load changes dramatically, such as short circuit, dynamic load and so on, the key device MOSFET of LLC circuit is also facing challenges.Generally, LLC will experience the following three states when the load changes. We call it hard turn off, and in the figure on the right, we can compare the differences in carrier changes between traditional MOSFET and CoolMOS and the risks to MOSFET.In sequence 1, Q2 zero voltage is turned on, and the reverse current passes through MOSFET and body diode. At this time, the secondary diode D2 is turned on and D1 is closed.-In traditional MOSFET, the electron current passes through the channel region, so as to reduce the number of holesAt this time, -CoolMOS is simultaneous interpreting the electron current through the channel and the hole as traditional MOSFET. The difference is that the structure of P well in CoolMOS is beginning to be established.In sequence 2, Q1 and Q2 are turned off at the same time, and the reverse current passes through the MOSFET Q2 body diode.When Q1 and Q2 are turned off, the internal electron and hole paths and flow directions are not much different from those of traditional MOSFET and CoolMOS.In sequence 3, Q1 starts to turn on at this time. Due to the change of load, the body diode of MOSFET Q2 needs a long time to reverse recover. When the diode reverse recovery is not completed, the MOSFET Q2 is hard turned off. At this time, Q1 is turned on, and the voltage applied to the Q2 body diode will form a large current in the diode, triggering the BJT inside the MOSFET and causing an avalanche.-At this time, the carriers of traditional MOSFET are extracted, the electrons gather around the PN node, and the hole current is blocked at the edge of the PN node.-The electron current and hole current of CoolMOS go their own way. At this time, the hole current flows in the established p-well structure without electron congestion.To sum up, under the conditions of overload, short circuit and dynamic load of LLC circuit, once the diode fails to reverse recover in time in the dead time, the huge composite current will trigger the BJT inside the MOSFET and make the MOSFET invalid.Some CoolMOS adopt super juction structure. When the MOSFET is hard turned off, the carriers will compound along the vertically constructed p-well, basically without lateral current, which greatly reduces the chance of triggering BJT.4. How to implement ZVS more easilyThrough the above analysis, it can be seen that increasing the dead time of MOSFET can provide sufficient diode reverse recovery time and reduce the risk of high DV / dt and di / dt to LLC circuit. But is increasing the dead time the only option? Let's further analyze how to reduce risks and improve system efficiency.Figure 7For LLC circuits, the initial current for dead time isAnd LLC can realize ZVS, which must meetThe minimum excitation inductance isAccording to the above three equations, we can make LLC realize ZVS in the following three waysFirst, add ipkSecond, increase the dead time.Third, reduce the equivalent capacitance CEQ, that is, CossFrom the above situations, it is not difficult for us to analyze. Increasing ipk will increase the inductance size and cost, and increasing the dead time will reduce the voltage during normal operation. Undoubtedly, the best choice is to reduce coss, because there is no need to make any adjustment to the circuit, just replace a MOSFET with a relatively small Coss.5. ConclusionLLC topology is widely used in various switching power supplies, and this topology not only improves the efficiency, but also puts forward new requirements for MOSFET. Different from the hard switching topology, the soft switching LLC resonant topology not only requires the on resistance (on loss) and QG (switching loss) of MOSFET, but also has higher requirements on how to effectively realize the soft switching, how to reduce the failure rate, improve the system reliability and reduce the system cost. CoolMOS has fast bulk diode, low coss, and some can reach a breakdown voltage of 650V, making LLC topology switching power supply more efficient and reliable.