An active clamp flyback converter that achieves ZVS over the full load range

0 Introduction
　　The traditional flyback converter has been widely used in DC/DC applications because of its relatively simple circuit structure and its ability to realize buck-boost functions. However, since the transformer of the flyback converter also serves as an inductor in the circuit, the air gap is large and the inevitable leakage inductance is also large. When the primary switch tube of the circuit is turned off, the leakage inductance will produce parasitic oscillation with the junction capacitance on the primary switch tube, thereby generating a voltage spike on the primary switch tube, causing it to withstand high voltage stress. At the same time, the Oscillation is also an EMD source, causing EMI problems to the circuit. The traditional RCD clamp circuit consumes all the energy stored in the leakage inductance of the transformer on the clamp resistor, which relieves this pressure to a certain extent, but reduces the efficiency of the circuit. This problem can be solved well if an active clamping circuit is used to replace the traditional RCD clamping circuit.

1 Active clamp circuit
　　A typical active clamp circuit is shown in Figure 1.

　　In addition to feeding back the energy on the leakage inductance to the output and improving circuit efficiency, the active clamp flyback converter also has the following advantages: first, the voltage clamping effect is good and can reduce the voltage stress on the switching tube; Secondly, both the main and auxiliary tubes on the primary side of the circuit can achieve ZVS, thereby reducing the switching loss of the circuit. This feature is particularly important for high-voltage input applications. Since the voltage on the switch tube resonates to zero, this not only limits the dv/dt when the voltage is turned off, but also the resonance of the clamping capacitor and the primary side resonant inductance of the transformer also limits the di/dt when the secondary rectifier is turned off. dt; By properly designing the value of the clamping capacitor, the ZCS of the secondary rectifier diode can also be achieved, thereby reducing or eliminating the switching loss of the rectifier and the switching noise caused by the diode reverse recovery, thereby effectively reducing the circuit's EMI.
　　It is precisely because of these advantages that the active clamp flyback converter has received much attention from the industry. This circuit can not only be used as an ordinary DC/DC converter, but also can be used as a PFC circuit with excellent performance.
　　There are two different working states in the traditional flyback converter: "continuous inductor current (CCM)" and "discontinuous inductor current (DCM)". These two different working states also reflect different working characteristics in the active clamp flyback converter. The active clamp flyback converter of CCM is the same as the traditional flyback converter. During a switching cycle, the excitation current on the primary side of the transformer is always greater than zero; while in the active clamp flyback converter of DCM, the primary side of the transformer However, the excitation current will appear in an intermittent state. When the excitation current reaches zero, under the action of the clamping capacitor, the excitation current on the primary side of the transformer will flow in the opposite direction, thus manifesting as a positive and negative alternating current within a switching cycle. The amount of change. Literature [4] analyzes in detail the working process of the active clamp flyback converter in the CCM state and the design considerations. It can be seen from this that the active clamp flyback converter in the CCM state, like the traditional flyback converter, has the advantages of small current ripple, small circuit conduction loss, and is suitable for high-power output occasions. However, this working state requires an external resonant inductor to realize the ZVS of the primary switch tube (Lr in Figure 1), and the realization of soft switching is related to the load. This can only be achieved within a certain load range.
　　However, it is of great practical significance to ensure that the circuit realizes soft switching in the full range, because the full range soft switching can ensure that the working state of the entire circuit is consistent, especially the EMI performance of the circuit is consistent, thereby reducing the EMI of the entire circuit. filter. To this end, this article optimizes the design of the active clamp flyback converter to ensure that the entire circuit can achieve soft switching from no load to full load.
　　The article first conducts a detailed analysis of the working status of the circuit, and then provides the design basis for the key components in the circuit. Finally, a 100W/100kHz prototype is used to verify the circuit's high efficiency and excellent performance within the full load range. Soft switching characteristics.

2 Working principle of the circuit
　　Figure 1 is the basic schematic diagram of the active clamp flyback converter. In the figure, Lr is the leakage inductance of the transformer, Lm is the magnetizing inductance of the primary side of the transformer, Cr is the sum of the equivalent junction capacitances of the main and auxiliary tubes, Cc is the active clamping capacitance of the circuit, Vin is the input DC voltage, and Vo is Output voltage, Vcc is the voltage of the clamping capacitor during steady-state operation.
　　Figure 2 is the equivalent working state diagram of the active clamp flyback converter. Figure 3 shows several key waveforms during steady-state operation of the active clamp flyback converter. The working status of the circuit is described below.
　　Mode1[t0, t1] At time t0, the main tube S1 is turned on and the auxiliary tube S2 is turned off. Output rectifier diode D1 withstands reverse voltage. The anti-parallel diode in S2 is also reverse biased. The current on Lm and Ln increases linearly under the action of Vin.
　　Mode 2[t1, t2] At time t1, Sl is turned off. Lm and Lr resonate with Cr together, and use the excitation current (at this time the excitation current is equal to the current flowing through the leakage inductance) to charge Cr. S2 is in the off state and the diode in S2 continues to be reverse biased.
　　Mode 3[t2, t3] At time t2, Cr is charged to vDS1=Vin+Vcc (Vcc≈nVo) is the voltage when the clamping capacitor is working in a steady state); at this time, the internal diode of S2 begins to conduct, Lm and Lr resonates with Cc and uses the excitation current to charge Cr. Since almost all the excitation current of Cc Yuandagan Cr flows to the clamping capacitor through the diode, and Lm and Lr divide the voltage at the same time, the excitation voltage, that is, the primary voltage Vpri of the transformer is

　　
　　Mode, 4[t3, t4] At time t3, vpri is small enough, and D1 is forward-conducting. The primary voltage of the transformer is clamped at nVo. At this time, Lr and Cc resonate and use the excitation current to charge Cr. In order to achieve ZVS of S2, S2 must be triggered to conduct before the resonant current reverses.
　　Mode 5[t4, t5] At time t4, S2 is turned off, causing Cr to be quickly disconnected from the circuit. At the same time, Lr and Cr resonate, and the primary voltage of the transformer is still clamped at nVo. When the current on Lr is equal to the current on Lin, the secondary current reduces to zero, D1 reverses and the voltage on the primary side of the transformer begins to reverse.
　　Mode 6[t5, t6] At time t5, the energy stored in Lr and Lm is greater than the energy stored in Cr, the charge on Cc will be discharged, and at the same time, the body diode of S1 begins to conduct; if at this time S1 in the segment is triggered to conduct, then ZVS can be achieved. At the same time, for Lin and Lr, the voltage at both ends is Vin, and the current on the inductor begins to rise linearly again. At time t6, S1 is turned on and enters the next switching cycle, the switching cycle Ts=t6-t0.
　　From the above analysis, we can draw the following conclusion: This circuit uses the active-clamped flyback converter to work in the DCM state, using the primary side excitation inductance of the transformer to participate in the resonance of the circuit. Before S1 is turned on, the transformer is used to The energy on the primary side magnetizing inductor resonates the voltage on the junction capacitance Cr to zero, thereby achieving the ZVS of the circuit. Read this. This circuit does not require an additional resonant inductor to achieve ZVS. Therefore, in the literature [4], Lr is the sum of the leakage inductance of the transformer and the external resonant inductance, while in this article, Lr is only the leakage inductance of the transformer.

　　In low-power, high-voltage applications, the advantages of this circuit are not only limited to full-range soft switching, but also eliminates the need for external resonant inductance. Because, if you want to resonate the voltage on the junction capacitance Cr to zero in a low-power, high-voltage situation, the value of the resonant inductor may be as high as several hundred μH.
　　However, like the working state of the DCM of the traditional flyback circuit, the biggest disadvantage of this circuit is that its current ripple is relatively large, because there is always an AC component in the primary current of the circuit that has nothing to do with the output power. This AC component Unnecessary conduction loss will be generated on the primary switching tube, and since the design is in an intermittent working state, the peak-to-peak value of this AC component is higher than that of CCM, which will affect the efficiency of the circuit to a certain extent.

3 Working characteristics of the circuit and design of main components
　　In order to ensure that the circuit is in good working condition and thus achieve soft switching in the full range, the design of key components in the circuit is very important.
3.1 Design of the transformer (design of the magnetizing inductor Lm)
　　The transformer transmits energy in the circuit and also acts as an energy storage element. In addition, the magnetizing inductor also participates in the resonance with the junction capacitance Cr, which ensures that the circuit can be realized within the full range. important factor in soft switching.
　　In order to ensure that the active clamp flyback converter operates in the DCM state, the value of the exciting inductance cannot be too large. Its design idea is completely consistent with the transformer design of the traditional OCM flyback converter.
　　——The maximum DC average value of the primary side magnetizing inductor current of the DCM active clamp flyback circuit is

　　?
In the formula: Iin is the input current;
　　Iin is the output current;
　　D is the duty cycle of S1;
　　n is the turns ratio of the primary and secondary sides of the transformer.
　　- In order for the circuit to achieve soft switching, the energy stored in Lm must be able to ensure that the voltage on Cr resonates to zero when S2 is turned off. Therefore, the minimum current Icri in the magnetizing inductor must satisfy equation (3).

　　
In the formula: fs is the operating frequency of the circuit.
　　Because the current passing through the primary side of the converter can be decomposed into two parts: DC component and AC component. The size of the DC component is directly proportional to the output power, while the size of the AC component is only related to the level of the input voltage and the size of the primary side magnetizing inductance of the transformer. This part of the energy only circulates on the primary side. The size of this value determines the original value of the circuit. The current peak on the edge switch tube and the resulting conduction loss of the switch tube. Therefore, when the input voltage is constant, the primary side excitation inductance value should be as large as possible while ensuring the normal working conditions of the full-range soft switching of the circuit.
3.2 Design of clamping capacitor Cr
　　Since the resonance slope of Cc and Lm also determines the di/dt of the secondary rectifier turn-off; therefore, the ZCS of the secondary rectifier diode can be realized through the appropriate design of Cc, thereby reducing the The purpose of minimizing the switching loss of the rectifier diode, eliminating the switching noise caused by the reverse recovery of the diode, and reducing the EMI of the circuit. Through analysis, we know that as long as equation (5) is satisfied, the ZCS of the rectifier diode can be achieved,

　　
In the formula: toff is the off time of S1.
　　However, for the same output current, the earlier the resonant current reaches zero means the peak current through the diode will be higher. This also increases the current stress on the diode, increases the ripple of the circuit output current, increases the current stress on the output capacitor, and brings a certain conduction loss to the circuit. Therefore, in order to make full use of the toff period and reduce the circuit's output current ripple, it is best to design the circuit's resonant period at 2toff (toff when the output power is maximum).
　　In addition to determining the ZCS of the rectifier diode, it can be known from equation (6) that the size of Cc also determines the voltage stress of S1 and S2 to a certain extent.
3.3 Selection of main switching power transistor S1
3.3.1 Voltage stress of S1

3.4 Selection of auxiliary switching power transistor S2
　　The voltage and current tolerance of S2 are equal to S1. In practical applications, in order to overcome the shortcomings of the slow switching characteristics of the MOSFET's body diode, a fast recovery diode can be connected in parallel to the switching tube to speed up the switching speed.
3.5 Determination of dead time
　　The effective realization of soft switching of the circuit also depends on the determination of appropriate dead time.
　　1) During the period from S2 turning off to S1 turning on, there must be enough time for the resonant inductor to drain the energy from Cr. This time is

　　
　　If the energy in the magnetizing inductor is large enough at that time, equation (9) is more applicable. Therefore, the actual required dead time is often much smaller than the calculated value of equation (8), and a compromise value between equation (8) and equation (9) is usually taken.

　　2) During the period from when S1 is turned off to when S2 is turned on, what is shown on the circuit is that the exciting inductor Lm and the resonant inductor Lr resonate with Cr, and the exciting current is used to charge Cr. Since the energy on Cr is very small compared to the excitation current, this can be equivalent to the process of charging Cr with a current source with a size of the peak value of the excitation current. The requirements for this dead time are

　　?
4 Experimental results
　　A 100W prototype verified the working principle and advantages of the converter.
　　The specifications and main parameters of the converter are as follows:
　　input voltage Vin AC220 (1±20%)V;
　　output voltage Vo DC24V;
　　output current Io 0~4A;
　　output power Po 100W;
　　operating frequency fs 100kHz;
　　main switch tube S1 SPP07N60S5;
　　Clamp switch tube S2 5PP07N60S5;
　　rectifier diode D1 MBR20200CT;
　　transformer T E140 primary and secondary turns ratio is 80:8;
　　clamp capacitor Cc 630nF/600V;
　　active clamp control chip IC UCC3580-4.
4.1 Soft switching of S1
　　Figure 4(a) shows the gate waveform of S1 and the voltage waveform across DS. It can be seen that before the gate signal is turned on, the voltage across DS of S1 has reached zero, thus achieving ZVS. Figure 4(b) shows the voltage waveform across DS of S1 and the current waveform passing through it. It can be seen that when the voltage across DS reaches zero, the current through S1 is in the negative direction, which shows from another angle The body diode of S1 conducts before the gate signal of the power tube, thus achieving ZVS.
4.2 Soft switching of S2
　　Figure 5 shows the ZVS waveform of S2.