“In the design of switching power supply, when the main power has multiple switching tubes, the drive must be isolated. For example, in multi-tube series flyback, dual-tube forward, LLC and other multi-switch tube topologies, the switching tube drive needs to be isolated. . At present, there are more mature bootstrap driver chips on the market to meet the design requirements, but the withstand voltage level of the driver chips is limited, and the high voltage can only reach about 600V.
In the design of switching power supply, when the main power has multiple switching tubes, the drive must be isolated. For example, in multi-tube series flyback, dual-tube forward, LLC and other multi-switch tube topologies, the switching tube drive needs to be isolated. . At present, there are more mature bootstrap driver chips on the market to meet the design requirements, but the withstand voltage level of the driver chips is limited, and the high voltage can only reach about 600V. In applications with higher input voltage levels, such as photovoltaic power supplies, SVG auxiliary power supplies, and other power products with an input voltage of 1500V or higher, the driver chip solution is no longer applicable, and only a magnetic isolation transformer drive circuit can be selected. Therefore, the design of the magnetic isolation drive circuit is extremely important.
Commonly used magnetic isolation drive scheme
The commonly used magnetic isolation drive circuit is shown in Figure 1 below.
Figure 1 Commonly used magnetic isolation drive circuit
Among them, the capacitor C1 is the DC isolation capacitor at the input end, and C3 is the equivalent input capacitor of the switch QM. The reference direction of the capacitor voltage is shown in Figure 1, and T1 is the magnetic isolation drive transformer. S1 is the output signal waveform of the pulse width modulation driver (PWM Driver), S2 is the waveform of the input terminal of the transformer, and S2 is the output waveform of the magnetic isolation driver. The working waveform of the circuit shown in Figure 1 is shown in Figure 2.
Figure 2 Schematic diagram of magnetic isolation drive waveform
Assuming that the driver’s output signal S1 has a period of T, a duty cycle of D, and an amplitude of VS1 in the steady state, and the input and output turns ratio of the transformer T1 is 1, then the input terminal DC isolation capacitor C1 in the steady state The voltage is D.VS1. When S1 is high level, S3 is also high level, and its amplitude is (VS1-VC1), that is, (1-D).VS1. When S1 is low, S3 is negative, and its amplitude is (-VC1), that is, D.VS1.
The specific derivation process is as follows:
It can be seen that the circuit of this isolation driving circuit is simple, and the driven MOS tube has a negative driving voltage, which has strong anti-interference ability. But it needs to be improved in that when the duty cycle D is larger, the high level amplitude (1-D) of S3. VS1 is smaller, which may result in insufficient QM driving voltage. In this way, this kind of driver circuit is not suitable for applications where the duty cycle changes greatly, that is, under application conditions with a large input ratio range, the driver circuit shown in Figure 1 will no longer be applicable.
Double direct magnetic isolation drive circuit
In order to solve the above-mentioned difficult problems, it is necessary to add a DC blocking capacitor on the secondary side of the magnetic isolation driving circuit shown in Fig. 1, as shown in Fig. 3.
Figure 3 Double DC magnetic isolation drive circuit
Compared with the magnetic isolation drive circuit in Figure 1, a secondary DC blocking capacitor C2 and a switching diode DR are added. In the steady state, the voltage VC1 on the DC blocking capacitor C1 at the input end is equal to D.VS1, and the VC2 on the DC blocking capacitor C2 at the output end is equal to D.VS1. The voltage reference direction is shown in Figure 3.
The working waveform of the circuit of Fig. 3 is shown as in Fig. 4. It can be seen that the voltage V3 of QM does not change with the change of the duty cycle after the steady state, which is suitable for the drive design of a wider range of input conditions.
However, in the circuit shown in Figure 3, the output signal of the pulse width modulation driver will suddenly disappear under certain special conditions, such as a sudden power failure at the input terminal or a sudden load change, that is, the drive signal S1 will be zero. Then the transformer T1 gradually enters saturation under the action of the DC blocking capacitor voltage (-DVS1) at the input, and the voltage on C2 is still DVS1. The amplitude of the input and output voltage on T1 gradually decreases from DV1. At this time, the voltage on QM will Start from 0 and gradually increase until it is equal to the voltage on C2. The schematic diagram of the waveform is shown in the red box in Figure 5.
Figure 5 Schematic diagram of the mis-conduction waveform of the double DC magnetic isolation drive circuit
It can be seen that under special conditions, the QM will have an out-of-control driving signal, which causes the switch tube to be turned on incorrectly, causing the main power transformer to saturate, and causing the product to explode.
Our optimized magnetic isolation drive scheme
In order to solve the misconducting problem of the double DC magnetic isolation drive circuit in Figure 3, our company has made a special discharge circuit for the secondary DC blocking capacitor C2. As shown in Figure 6, the primary side detection circuit and the secondary DC blocking capacitor discharge circuit are added. This circuit is an independent circuit invented by our company. Signal isolation by high-speed optocoupler effectively solves the problem of mis-conduction of the main power switch. The improved drive waveform is shown in the 2-channel PWM waveform in Figure 7. It can be seen that the voltage on C2 is fast when the drive is abnormally turned off. After the discharge is completed, the switch tube will not be abnormally turned on. Provide highly reliable drivers for our company’s ultra-high and ultra-wide voltage input series products, and enhance the comprehensive competitiveness of products.
Figure 6 Improved double direct magnetic isolation drive circuit
Figure 7: Waveform diagram of the improved double direct magnetic isolation drive circuit
With a magnetic isolation drive circuit made of a transformer, if only the primary side has a DC blocking capacitor, the secondary output voltage will decrease under the condition of a relatively large duty cycle, which cannot meet the requirements of the switch tube drive. When the secondary DC blocking capacitor is added, the problem of large duty cycle can be solved, but the discharge of the secondary DC blocking capacitor under abnormal conditions will cause the switch tube to be turned on abnormally, and the primary side will be saturated and the product will blow up. In order to solve the above problems, our company has added a secondary DC blocking capacitor discharge circuit, so that the magnetic isolation drive circuit is not affected by the duty cycle and perfectly solves the phenomenon of abnormal conduction of the switch under abnormal conditions. The design of the discharge circuit is simple and reliable, which greatly improves product reliability.