Keywords: booster circuit; Soft switch; synchronous rectification
introduce
Lightweight and miniaturization are the goals pursued by power products at present. Increasing the switching frequency can reduce the volume of inductors, capacitors and other components. However, the bottleneck of improving switching frequency is the switching loss of devices, so soft switching technology came into being. Generally, in order to achieve ideal soft-switching effect, it is necessary to have one or more auxiliary switches to create soft-switching conditions for the main switch, and it is hoped that the auxiliary switches themselves can also achieve soft-switching.
As a basic DC/DC topology, Boost circuit is widely used in various power products. Because the boost circuit only contains one switch, it is often necessary to add many active or passive additional circuits to realize soft switching, which increases the cost of the converter and reduces the reliability of the converter.
The boost circuit has a diode besides the switch tube. In the case of low voltage output, it is best to use MOSFET instead of diode (synchronous rectification) to obtain higher efficiency. If this synchronous switch can be used as the auxiliary tube of the main switch, it will be a better scheme to create soft switching conditions and realize soft switching at the same time.
A method of soft switching using Boost circuit is proposed. This scheme is suitable for low output voltage.
The working principle of 1
Figure 1 shows a synchronous boost circuit with two switches. The two switches are complementary, and there is a certain dead zone in the middle to prevent the * * * state from conducting, as shown in Figure 2. Usually, the current on the inductor in the design is unidirectional, as shown in the fifth waveform of Figure 2. Considering the junction capacitance and dead time of the switch, a cycle can be divided into five stages, and the equivalent circuit of each stage is shown in Figure 3. The following briefly introduces the working principle of synchronous boost circuit with constant inductor current direction. In this design, S2 can realize soft handover, but S 1 can only work in hard handover state.
1) stage 1 [t0 ~ T 1] At this stage, S 1 is turned on, the input voltage is applied to L, and the current on L increases linearly. At time t 1, S 1 is closed, and the phase ends.
2) After the second stage [T 1 ~ T2] S 1 is turned off, the junction capacitance of S 1 is charged by the inductor current, which discharges the junction capacitance of S2, and the drain-source voltage of S2 can be approximately regarded as a linear decline until it drops to zero, and this stage is over.
3) Stage 3 [T2 ~ T3] When the drain-source voltage of S2 drops to zero, the parasitic diode of S2 is turned on, which clamps the drain-source voltage of S2 in a zero-voltage state, creating conditions for the zero-voltage conduction of S2.
4) In stage 4 [T3 ~ T4], the gate of S2 becomes high level, and the zero voltage of S2 is turned on. The current on the inductor L flows through S2 again. L bears the difference between the output voltage and the input voltage, and the current linearly decreases until S2 is turned off, and this stage is over.
5) In the fifth stage [T4 ~ T5], the current direction on the inductor L is still positive, so the current can only be transferred to the parasitic diode of S2, and the junction capacitance of S 1 cannot be discharged. Therefore, S 1 is in a hard handoff state.
Then S 1 opens and enters the next cycle. From the above analysis, we can see that S2 realizes soft switching, while S 1 does not. The reason is that after S2 is turned off, the current direction on the inductor is positive, and the junction capacitance of S 1 cannot be discharged. However, if L is designed to be small enough so that the inductance current is negative when S2 is turned off, as shown in Figure 4, the junction capacitance of S 1 can be discharged to realize the soft switching of S 1.
In this case, a cycle can be divided into six stages, and the equivalent circuit of each stage is shown in Figure 5. Its working principle is described as follows.
1) stage 1 [t0 ~ T 1] At this stage, S 1 is turned on, the input voltage is applied to L, and the current on L increases linearly from negative to positive. At time t 1, S 1 is closed, and the phase ends.
2) After the second stage [T 1 ~ T2] S 1 is turned off, the inductor current is positive, and the junction capacitor of S 1 is charged, which discharges the junction capacitor of S2, and the drain-source voltage of S2 can be approximately considered as a linear decrease. This phase ends until the drain-source voltage of S2 drops to zero.
3) Stage 3 [T2 ~ T3] When the drain-source voltage of S2 drops to zero, the parasitic diode of S2 is turned on, which clamps the drain-source voltage of S2 in a zero-voltage state, creating conditions for the zero-voltage conduction of S2.
4) In stage 4 [T3 ~ T4], the gate of S2 becomes high level, and the zero voltage of S2 is turned on. The current on the inductor L flows through S2 again. L bear the difference between the output voltage and the input voltage, and the current is linear? Very small, until it becomes negative, then S2 closes, and this stage ends.
5) At stage 5 [T4 ~ T5], the current direction on the inductor L is negative, which can just discharge the junction capacitance of S 1 and charge the junction capacitance of S2. The drain-source voltage of S 1 can be approximately considered as a linear decrease. Until the drain-source voltage of S 1 drops to zero, this stage ends.
6) When the drain-source voltage of S 1 drops to zero in stage 6 [T5 ~ T6], the parasitic diode of S 1 is turned on, which clamps the drain-source voltage of S 1 in a zero-voltage state, creating conditions for the zero-voltage conduction of S 1.
Then S 1 zero voltage is turned on, and the next cycle is entered. It can be seen that in this scheme, both switches S 1 and S2 can achieve soft handover.
2 parameter design of softswitch
Soft-switching of Boost circuit is realized by synchronous rectification and inductor current reversal, and the soft-switching difficulty of the two switches is different. The peak-to-peak value of the inductor current can be expressed as:
δI =(VinDT)/L( 1)
Where: d is the duty ratio;
T is the switching period.
Therefore, the maximum and minimum values of the current on the inductor can be expressed as:
IMAX =δI/2+Io(2)
imin =δI/2-Io(3)
Where: Io is the output current.
Substitute formula (1) into formula (2) and formula (3) to get it.
Imax=(VinDT)/2L+Io (4)
Amin = (Venter) /2L- Io (5)
From the above principle analysis, it can be seen that the soft-switching condition of S 1 is realized by charging the junction capacitance of S2 by Imin and discharging the junction capacitance of S 1. The soft-switching condition of S2 is realized by Imax charging the junction capacitance of S 1 and discharging the junction capacitance of S2. In addition, usually in the case of full load |Imax|? |Imin|. So the soft handover difficulty between S 1 and S2 is different, and S 1 is much more difficult than S2. Here, S 1 is called weak tube, and S2 is called strong tube.
The soft-switching limit condition of strong transistor S2 is that the junction capacitance of L and S 1 resonates with the junction capacitance C2 of S2, and the condition that the voltage on C2 can resonate to zero can be expressed by formula (6).
Substituting Formula (4) into Formula (6) can be obtained.
In fact, Equation (7) is easy to satisfy, and the dead time cannot be very large. Therefore, it can be approximately considered that the current on the inductor L remains constant in the dead time, that is, the constant current source is charging the junction capacitance of S2, so that the junction capacitance of S 1 is discharged. The ZVS condition in this case is called sufficient condition, and the expression is formula (8).
(C2+C 1)Vo≤ (Venter /2L+Io)tdead2 (8)
Where: tdead2 is the dead time before S2 is turned on.
Similarly, the soft-switching conditions of the weak transistor S 1 are as follows
(c 1+C2)Vo ≤( VinDT/2L-Io)tdead 1(9)
Where: tdead 1 is S 1 dead time before opening.
In actual circuit design, the soft-switching condition of strong transistor is very easy to realize, so the key is to design the soft-switching condition of weak transistor. First, determine the maximum allowable dead time, and then calculate the inductance L according to formula (9). Because, under the premise of soft switching, L can't be too small, so as not to cause the effective current value on the switch tube to be too large, and thus the on-loss of the switch is too large.
3 experimental results
A synchronous Boost converter with switching frequency of 200kHz and power of 100W further verifies the correctness of the above soft-switching implementation method.
The specifications and main parameters of the frequency converter are as follows:
Input voltage Vin24V
Output voltage Vo40V
Output current Io0~2.5A
Working frequency f200kHz
Main switches S 1 and S2IRFZ44
Inductance L4.5μH
Figs. 6(a), 6(b) and 6(c) are experimental waveforms at full load (2.5A). As can be seen from fig. 6(a), the current on the inductor L will reverse within the period of DT or (1-d) T, which creates the condition of soft switching of S 1. As can be seen from Figure 6(b) and Figure 6(c), both switches S 1 and S2 have realized ZVS. However, judging from the falling slope of voltage vds, the ZVS condition of S 1 ratio S2 is poor, which is the difference between strong tube and weak tube.
Fig. 7 shows the conversion efficiency of the converter under different load currents. The highest efficiency is 97. 1%, and the full-load efficiency is 96.9%.
4 conclusion
In this paper, a soft-switching strategy of Boost circuit is proposed: synchronous rectification plus inductor current reversal. In this scheme, according to the different soft switching conditions, the two switching tubes are divided into strong tube and weak tube. In the design, the inductance L should be determined according to the critical soft-switching condition of the weak transistor. Because soft switching is realized, the switching frequency can be designed to be relatively high. The inductance can be designed to be small, and the required inductance volume can also be relatively small (I-type magnetic core can usually be used). Therefore, this scheme is suitable for high power density and low output voltage.